Sunday 11 October 2015

Heat Treatment

1. INTRODUCTION
Heat treatment is any one of a number of controlled heating and cooling operations used to bring about a desired change in the physical properties of a metal. Its purpose is to improve the structural and physical properties for some particular use or for future work of the metal. There are five basic heat treating processes: hardening, case hardening, annealing, normalizing, and tempering. Although each of these processes brings about different results in metal, all of them involve three basic steps: heating, soaking, and cooling. The process of heat treating is the method by which metals are heated and cooled in a series of specific operations that never allow the metal to reach the molten state. The purpose of heat treating is to make a metal more useful by changing or restoring its mechanical properties. Through heat treating, we can make a metal harder, stronger, and more resistant to impact. Also, heat treating can make a metal softer and more ductile. The one disadvantage is that no heat treating procedure can produce all of these characteristics in one operation. Some properties are improved at the expense of others; for example, hardening a metal may make it brittle.

HEAT-TREATING THEORY
The various types of heat-treating processes are similar because they all involve the heating and cooling of metals; they differ in the heating temperatures and the cooling rates used and the final results. The usual methods of heat-treating ferrous metals (metals with iron) are annealing, normalizing, hardening, and tempering. Most nonferrous metals can be annealed, but never tempered, normalized, or case hardened. Successful heat treatment requires close control over all factors affecting the heating and cooling of a metal. This control is possible only when the proper equipment is available. The furnace must be of the proper size and type and controlled, so the temperatures are kept within the prescribed limits for each operation. Even the furnace atmosphere affects the condition of the metal being heat treated. The furnace atmosphere consists of the gases that circulate throughout the heating chamber and surround the metal, as it is being heated. In an electric furnace, the atmosphere is either air or a controlled mixture of gases. In a fuel-fired furnace, the atmosphere is the mixture of gases that comes from the combination of the air and the gases released by the fuel during combustion. These  gases  contain  various  proportions  of  carbon  monoxide,  carbon  dioxide,  hydrogen,  nitrogen, oxygen, water vapor, and other various hydrocarbons. Fuel-fired furnaces can provide three distinct atmospheres when you vary the proportions of air and fuel. They are called oxidizing, reducing and neutral.

2. STAGES OF HEAT TREATMENT
Heat treating is accomplished in three major stages:
Stage l- Heating the metal slowly to ensure a uniform temperature
 Stage 2- Soaking the metal at a given temperature for a given time and cooling the metal to room temperature
Stage 3- Cooling the metal to room temperature

2.1 HEATING STAGE
The primary objective in the heating stage is to maintain uniform temperatures. If uneven heating occurs, one section of a part can expand faster than another and result in distortion or cracking. Uniform temperatures are attained by slow heating. The heating rate of a part depends on several factors. One important factor is the heat conductivity of the metal. A metal with a high-heat conductivity heats at a faster rate than one with a low conductivity. Also, the condition of the metal determines the rate at which it may be heated. The heating rate for hardened tools and parts should be slower than unstressed or untreated metals. Finally, size and cross section figure into the heating rate. Parts with a large cross section require slower heating rates to allow the interior temperature to remain close to the surface temperature that prevents warping or cracking. Parts with uneven cross sections experience uneven heating; however, such parts are less apt to be cracked or excessively warped when the heating rate is kept slow.

2.2 SOAKING STAGE
After the metal is heated to the proper temperature, it is held at that temperature until the desired internal structural changes take place. This process is called Soaking. The length of time held at the proper temperature is called the soaking period. During the soaking stage, the temperature of the metal is rarely brought from room temperature to the final temperature in one operation; instead, the steel is slowly heated to a temperature just below the point at which the change takes place and then it is held at that temperature until the heat is equalized throughout the metal. We call this process Preheating. Following pre heat, the metal is quickly heated to the final required temperature. When apart has an intricate design, it may have to be preheated at more than one temperature to prevent cracking and excessive warping. For example, assume an intricate part needs to be heated to 1500°F for hardening. This part could be slowly heated to 600°F, soaked at this temperature, then heated slowly to 1200°F, and then soaked at that temperature.
Following the final preheat, the part should then be heated quickly to the hardening temperature of 1500°F. Nonferrous metals are seldom preheated, because they usually do not require it, and pre heating can cause an increase in the grain size in these metals.

2.3 COOLING STAGE
After a metal has been soaked, it must be returned to room temperature to complete the heat-treating process. To cool the metal, you can place it in direct contact with a Cooling medium composed of a gas, liquid, solid, or combination of these. The rate at which the metal is cooled depends on the metal and the properties desired. The rate of cooling depends on the medium; therefore, the choice of a cooling medium has an important influence on the properties desired. Quenching is the procedure used for cooling metal rapidly in oil, water, brine, or some other medium. Because most metals are cooled rapidly during the hardening process, quenching is usually associated with hardening; however, quenching does not always result in an increase in hardness; for example, to anneal copper, you usually quench it in water. Other metals, such as air hardened steels, are cooled at a relatively slow rate for hardening. Some metals crack easily or warp during quenching, and others suffer no ill effects; therefore, the quenching medium must be chosen to fit the metal. Brine or water is used for metals that require a rapid cooling rate, and oil mixtures are more suitable for metals that need a slower rate of cooling. Generally, carbon steels are water-hardened and alloy steels are oil-hardened. Non-ferrous metals are normally quenched in water.

HEAT COLORS FOR STEEL
During hardening, normalizing, and annealing, steel is heated to various temperatures that produce color changes. By observing these changes, you can determine the temperature of the steel. As an example, assume that you must harden a steel part at 1500°F. Heat the part slowly and evenly while watching it closely for any change in color. Once the steel begins to turn red, carefully note each change in shade. Continue the even heating until the steel is bright red; then quench the part. The success of a heat-treating operation depends largely on your judgment and the accuracy with which you identify each color with its corresponding temperature.


Table 2.1—Heat Colors for Steel



Table 2.2—Approximate Soaking Periods for Hardening, Annealing, and Normalizing Steel






3. TYPES OF HEAT TREATMENT
Four basic types of heat treatment are used today. They are annealing, normalizing, hardening, and tempering.  The  techniques  used  in  each  process  and  how they  relate  to  Steelworkers  are  given  in  the  following paragraphs.

3.1 ANNEALING
In general, annealing is the opposite of hardening, anneal metals to relieve internal stresses, soften them, make them more ductile, and refine their grain structures. Metals are annealed to relieve internal stresses, soften them, make them more ductile, and refine their grain structures. Metal is annealed by heating it to a prescribed temperature, holding it at that temperature for the required time, and then cooling it back to room temperature. The rate at which metal is cooled from the annealing temperature varies greatly. Steel must be cooled very slowly to produce maximum softness, This can be done by burying the hot part in sand, ashes, or some other substance that does not conduct heat readily (packing), or by shutting off the furnace and allowing the furnace and part to cool together (furnace cooling).

3.2 NORMALIZING
Normalizing is a type of heat treatment applicable to ferrous metals only. It differs from annealing in that the metal is heated to a higher temperature and then removed from the furnace for air cooling. The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining. Stress, if not controlled, leads to metal failure; therefore, before hardening steel, we should normalize it first to ensure the maximum desired results. Usually, low-carbon steels do not re-quire normalizing; however, if these steels are normalized, no harmful effects result. Castings are usually annealed, rather than normalized; however, some castings require the normalizing treatment. Table 2.2 shows the approximate soaking periods for normalizing steel. Note that the soaking time varies with the thickness of the metal. Normalized steels are harder and stronger than annealed steels. In the normalized condition, steel is much tougher than in any other structural condition. Parts subjected to impact and those that require maximum toughness with resistance to external stress are usually normalized. In normalizing, the mass of metal has an influence on the cooling rate and on the resulting structure. Thin pieces cool faster and are harder after normalizing than thick ones. In annealing (furnace cooling), the hardness of the two are about the same. Annealing consists of heating a metal to a specific temperature, holding it at that temperature for a set length of time, and then cooling the metal to room temperature.

3.3 HARDENING
The hardening treatment for most steels consists of heating the steel to a set temperature and then cooling it rapidly by plunging it into oil, water, or brine. Most steels  require  rapid  cooling  (quenching)  for  hardening but  a  few  can  be  air-cooled  with  the  same  results. Hardening increases the hardness and strength of the steel, but makes it less ductile. Generally, the harder the steel, the more brittle it becomes. To remove some of the brittleness, you should temper the steel after hardening. Many nonferrous metals can be hardened and their strength increased by controlled heating and rapid cooling. In this case, the process is called heat treatment, rather than hardening. To harden steel, you cool the metal rapidly after thoroughly soaking it at a temperature slightly above its upper critical point. The approximate soaking periods for hardening steel are listed in table 2-2. The addition of alloys to steel decreases the cooling rate required to produce hardness. A decrease in the cooling rate is an advantage, since it lessens the danger of cracking and warping.

3.4 TEMPERING

After the hardening treatment is applied, steel is often harder than needed and is too brittle for most practical uses. Also, severe internal stresses are set up during the rapid cooling from the hardening temperature. To relieve the internal stresses and reduce brittleness, you should temper the steel after it is hardened. Tempering consists of heating the steel to a specific temperature (below its hardening temperature), holding it at that temperature for the required length of time, and then cooling it, usually instill air. The resultant strength, hardness, and ductility depend on the temperature to which the steel is heated during the tempering process. The purpose of tempering is to reduce the brittleness imparted by hardening and to produce definite physical properties within the steel. Tempering always follows, never precedes, the hardening operation. Besides reducing brittleness, tempering softens the steel. That is un-avoidable, and the amount of hardness that is lost depends on the temperature that the steel is heated to during the tempering process. That is true of all steels except high-speed steel.  Tempering increases the hardness of high-speed steel. 

Hydraulic Ram

1. INTRODUCTION
A hydraulic ram pump is a water pump powered by water with a height difference. In areas where natural flows exist with a height difference of the water over a small distance, hydraulic ram pumps can be used to transport water to higher grounds without using electricity or fuel. The hydraulic ram uses the water hammer effect to develop pressure that allows a portion of the input water that powers the pump to be lifted to a point higher than where the water originally started. Apart from the kinetic energy of the water, no other source of power is needed.
The hydraulic ram pump was invented in 1772 and widely used in the 19th century, but was side-tracked by the advent of the coal-powered steam engine and later by diesel powered pumps. In recent years the hydraulic ram pump has seen a renewed interest, because it is powered by sustainable energy, and can be produced locally.
A hydraulic ram pump is powered by a body of water flowing downhill with a height difference. A general rule of thumb is that the water can be pumped 30 times as high as the available drive head (the height difference of the water driving the pump). So a head of 1 m can be used to pump up water to ~30m, while a 7 m head can pump water up to 210 m.
The capacity of a hydraulic ram depends on the scale of the pump, which is often measured in the diameter of the tube delivering the water to the pump. Pumps exist in the range 1" up to 5".
With height difference, the actual difference in vertical height is meant, not the length measured along the slope.

Advantages
Disadvantages
- Uses renewable energy sources. 

- If properly designed, can be produced and maintained locally. 
- Very effective in mountainous areas
- Water with height difference is needed



2. PRINCIPLE OF OPERATION
A hydraulic ram has only two moving parts, a spring or weight loaded "waste" valve sometimes known as the "clack" valve and a "delivery" check valve, making it cheap to build, easy to maintain, and very reliable. In addition, there is a drive pipe supplying water from an elevated source, and a delivery pipe, taking a portion of the water that comes through the drive pipe to an elevation higher than the source.

2.1       Sequence of operation



Figure: Basic components of a hydraulic ram:
1. Inlet – drive pipe
2. Free flow at waste valve
3. Outlet – delivery pipe
4. Waste valve
5. Delivery 
check valve
6. Pressure vessel
A simplified hydraulic ram is shown in Figure. Initially, the waste valve is open, and the delivery valve is closed. The water in the drive pipe starts to flow under the force of gravity and picks up speed and kinetic energy until the increasing drag force closes the waste valve. The momentum of the water flow in the supply pipe against the now closed waste valve causes a water hammer that raises the pressure in the pump, opens the delivery valve, and forces some water to flow into the delivery pipe . Because this water is being forced uphill through the delivery pipe farther than it is falling downhill from the source, the flow slows; when the flow reverses, the delivery check valve closes. Meanwhile, the water hammer from the closing of the waste valve also produces a pressure pulse which propagates back up the supply pipe to the source where it converts to a suction pulse that propagates back down the pipe.
This suction pulse, with the weight or spring on the valve, pulls the waste valve back open and allows the process to begin again. A pressure vessel containing air cushions the hydraulic pressure shock when the waste valve closes and it also improves the pumping efficiency by allowing a more constant flow through the delivery pipe. Although, in theory, the pump could work without it, the efficiency would drop drastically and the pump would be subject to extraordinary stresses that could shorten its life considerably. One problem is that the pressurized air will gradually dissolve into the water until none remains. One solution to this problem is to have the air separated from the water by an elastic diaphragm (similar to an expansion tank); however, this solution can be problematic in developing countries where replacements are difficult to procure. Another solution is to have a mechanism such as a snifting valve that automatically inserts a small bubble of air when the suction pulse mentioned above reaches the pump. Another solution is to insert an inner tube of a car or bicycle tire into the pressure vessel with some air in it and the valve closed. This tube is in effect the same as the diaphragm, but it is implemented with more widely available materials. The air in the tube cushions the shock of the water the same as the air in other configurations does.

2.2 Efficiency

Typical energy efficiency is 60%, but up to 80% is possible. This should not be confused with the volumetric efficiency, which relates the volume of water delivered to total water taken from the source. The portion of water available at the delivery pipe will be reduced by the ratio of the delivery head to the supply head. Thus if the source is 2 meters above the ram and the water is lifted to 10 meters above the ram, only 20% of the supplied water can be available, the other 80% being spilled via the waste valve. These ratios assume 100% energy efficiency. Actual water delivered will be further reduced by the energy efficiency factor. In the above example, if the energy efficiency is 70%, the water delivered will be 70% of 20%, i.e. 14%. Assuming a 2 to one supply head to delivery head ratio and 70% efficiency, the delivered water would be 70% of 50%, i.e. 35%. Very high ratios of delivery to supply head usually result in lowered energy efficiency. Suppliers of rams often provide tables giving expected volume ratios based on actual tests.

2.3 Drive and delivery pipe design

Since both efficiency and reliable cycling depend on water hammer effects, the drive pipe design is important. It should be between 3 and 7 times longer than the vertical distance between the source and the ram.
Commercial rams may have an input fitting designed to accommodate this optimum slope. The diameter of the supply pipe would normally match the diameter of the input fitting on the ram, which in turn is based on its pumping capacity. The drive pipe should be of constant diameter and material, and should be as straight as possible. Where bends are necessary, they should be smooth, large diameter curves. Even a large spiral is allowed, but elbows are to be avoided. PVC will work in some installations, but steel pipe is preferred, although much more expensive. If valves are used they should be a free flow type such as a ball valve or gate valve.
The delivery pipe is much less critical since the pressure vessel prevents water hammer effects from traveling up it. Its overall design would be determined by the allowable pressure drop based on the expected flow. Typically the pipe size will be about half that of the supply pipe, but for very long runs a larger size may be indicated. PVC pipe and any necessary valves are not a problem.

2.4  Starting operation

A ram newly placed into operation or which has stopped cycling must be started as follows. If the waste valve is in the raised (closed) position, which is most common, it must be pushed down manually into the open position and released. If the flow is sufficient, it will then cycle at least once. If it does not continue to cycle, it must be pushed down repeatedly until it cycles continuously on its own, usually after three or four manual cycles. If the ram stops with the waste valve in the down position it must be lifted manually and kept up for as long as necessary for the supply pipe to fill with water and for any air bubbles to travel up the pipe to the source. This may take a minute or more. Then it can be started manually by pushing it down a few times as described above. Having a valve on the delivery pipe at the ram makes starting easier. Close the valve until the ram starts cycling, then gradually open it to fill the delivery pipe. If opened too quickly it will stop the cycling. Once the delivery pipe is full the valve can be left open.

2.5  Common operational problems

Failure to deliver sufficient water may be due to improper adjustment of the waste valve, having too little air in the pressure vessel, or simply attempting to raise the water higher than the level of which the ram is capable. The ram may be damaged by freezing in winter, or loss of air in the pressure vessel leading to excess stress on the ram parts. These failures will require welding or other repair methods and perhaps parts replacement.
It is not uncommon for an operating ram to require occasional restarts. The cycling may stop due to poor adjustment of the waste valve, or insufficient water flow at the source. Air can enter if the supply water level is not at least a few inches above the input end of the supply pipe. Other problems are blockage of the valves with debris, or improper installation, such as using a supply pipe of non uniform diameter or material, having sharp bends or a rough interior, or one that is too long or short for the drop, or is made of an insufficiently rigid material. A PVC supply pipe will work in some installations but is not as optimal as steel.




3. HOW A HYDRAULIC RAM PUMP WORKS

Here's a simplified version of how the hydraulic ram pump actually works, step-by-step:

Water (blue arrows) starts flowing through the drive pipe and out of the "waste" valve (4 on the diagram), which is open initially.  Water flows faster and faster through the pipe and out of the valve.
        
At some point, water is moving so quickly through the brass swing check "waste" valve (4 on the diagram) that it grabs the swing check's flapper, pulling it up and slamming it shut.  The water in the pipe is moving quickly and doesn't want to stop.


All that water weight and momentum is stopped, though, by the valve slamming shut.  That makes a high pressure spike (red arrows) at the closed valve.  The high pressure spike forces some water (blue arrows) through the spring check valve (5 on the diagram) and into the pressure chamber.  This increases the pressure in that chamber slightly.  The pressure "spike" the pipe has nowhere else to go, so it begins moving away from the waste valve and back up the pipe (red arrows).  It actually generates a very small velocity backward in the pipe.

As the pressure wave or spike (red arrows) moves back up the pipe, it creates a lower pressure situation (green arrows) at the waste valve.  The spring-loaded check valve (5 on the diagram) closes as the pressure drops, retaining the pressure in the pressure chamber.


At some point this pressure (green arrows) becomes low enough that the flapper in the waste valve falls back down, opening the waste valve again. 
 






Most of the water hammer high pressure shock wave (red arrows) will release at the drive pipe inlet, which is open to the source water body.  Some small portion may travel back down the drive pipe, but in any case after the shock wave has released, pressure begins to build again at the waste valve simply due to the elevation of the source water above the ram, and water begins to flow toward the hydraulic ram again.
Water begins to flow out of the waste valve and the process starts over once again.
Steps 1 through 6 describe in layman's terms a complete cycle of a hydraulic ram pump.  Pressure wave theory will explain the technical details of why a hydraulic ram pump works, but we only need to know it works. The ram pump will usually go through this cycle about once a second, perhaps somewhat more quickly or more slowly depending on the installation. 

Each "pulse" or cycle pushes a little more pressure into the pressure chamber.  If the outlet valve is left shut, the ram will build up to some maximum pressure (called shutoff head on pumps) and stop working.

Hydraulic Intensifier

1. INTRODUCTION
A hydraulic intensifier is a device which is used to increase the intensity of pressure of any hydraulic fluid or water, with the help of the hydraulic energy available from a huge quantity of water or hydraulic fluid at a low pressure. Know about the components and construction of intensifiers. A hydraulic intensifier is a hydraulic machine for transforming hydraulic power at low pressure into a reduced volume at higher pressure. A hydraulic intensifier is a device which is used to increase the intensity of pressure of any hydraulic fluid or water, with the help of the hydraulic energy available from a huge quantity of water or hydraulic fluid at a low pressure. These devices are very important in the case of hydraulic machines, mainly hydraulic presses, which require water or hydraulic fluid at very high pressure which cannot be obtained from the main supply directly. In most of the hydraulic machinery used, the usual pressure of 80 to 100-psi may not be sufficient to operate certain spool valves and other mechanisms. To cater to the need for a high pressure requirement for a comparatively short period of time, pumps and accessories are definitely not the solution. But the substitute can be hydraulic intensifiers which can increase the pressure from 100 psi to 40,000 psi, using small volumes of fluid. There are different types based on the medium of hydraulic fluids used and the number of strokes used to intensify to the desired pressure. They are single-stroke, differential cylinder intensifiers, oil-oil intensifiers, air-air intensifiers, and oil-air intensifiers. Recent developments are so vast that huge pressures are achieved by using combinations of the above types. An intensifier heightens the intensity of the meaning of an item. A hydraulic intensifier is a hydraulic machine for transforming hydraulic power at low pressure into a reduced volume at higher pressure. It increases the intensity of pressure of the liquid by utilizing the energy of a larger quantity of liquid at low pressure. Such a machine is constructed mechanically by connecting two pistons, each working in a separate cylinder of different diameter. This concept is developed from Pascal’s law for incompressible fluid. If the diameters of the pistons are different, the hydraulic pressure in each cylinder will vary with the area ratio of the pistons, the smaller piston giving rise to higher pressure intensity than the larger piston pressure intensity. The increase in the intensity of pressure is generally required when the liquid supplied by the pump does not possess the required intensity of pressure. The hydraulic intensifier is very important in the case of hydraulic machines, mainly hydraulic presses, which require water or hydraulic fluid at a very high pressure which cannot be obtained from the main supply directly. High pressure metal hydro-forming requires 20,000 psi or 1379 bar [3]. Definitely this pressure will require a massive construction of pump. But if there is a HPI, it is possible to raise the pressure by using a reasonable size of pump. A machine which has come into general use very rapidly in the last few years is the high-speed forging press for casting and forming heavy and complicated shapes. The next broad step after the direct pump-driven press was the hydraulic intensifier which made it possible to raise the pressure which cannot be achieved directly by pumping action. To press two metals sheet adjacently and to lifting heavy load, as for example, bridge slab, it requires a device which is capable of heavy load carrying capacity and smooth operation. Again, it is hydraulic jack which possesses the capabilities of smooth operation and heavy load carrying behavior. But for its proper functioning it is necessary to supply the compressed fluid at high pressure and this can be done by using a HPI. Hydraulic intensifier is used in constructing water cutting jet machine. It is also used in mining and construction firms. The objectives of the present work is to design and construct a automatic controlled reciprocating HPI which can maximize pressure 6.25 times the input pressure range of maximum 5 bar. In existing rotary type HPI critical intensification ratio is 2.5 and its efficiency is 45%.







2. THEORETICAL STUDY

Hydraulic Pressure Intensifier is a mechanical device which is used for increasing the intensity of pressure of the liquid by utilizing the larger quantity of liquid at low pressure. Often hydraulic machines such as press, etc., require liquid at high pressure which may not be directly available from a pump. It can, however, be provided by introducing an intensifier between the pump and machine. It consists of several kinds of mechanical and electrical equipment.
2.1 Classification
Basically there are only two types of hydraulic intensifier namely single action and double action intensifier. These two principal types of hydraulic intensifier have been modified in so many ways as per requirements of industry. Some of them are described as follows: Classification based on body construction of Hydraulic intensifier:
2.1.1 Tie-Rod Construction
This type of construction is most widely used in industry. ISI standard also generally refers to one of this type of construction. As all the components are only machined and assembled together and not welded. Hence planning manufacturing, quality control assembly and maintenance are more convenient than other types of construction. As long as tie-rods are used to hold the entire components together, special care is required to tighten them and safe-guard against loosening in operation.
2.1.2 Threaded Construction
This construction is similar to tie-rod construction, but more compact, stronger, and requires more accuracy and care in manufacturing and quality control. In this design, both ends are assembled with cylinder-tube by threading, as shown in following design. These are used for medium to heavy-duty operation, and widely used in earth-moving purpose respectively.
2.1.3 Bolted Construction
This type of construction involves welding of flanges to cylinder tube, and bolting of end cover to the welded flange. Similar to tie rod construction these are also designed and manufactured as standard hydraulic component and widely used in industry.
2.1.4 One Piece-Welded Cylinder
Similar to shock absorber, in this design the end-covers and cylinder tube are welded together. These are economical but cannot be repaired.
These are used for low pressure agriculture machinery application. Figure 4 shows One Piece-Welded HPI.
2.1.5 Custom Build HPI
In this type of cylinder, various type of construction is mixed together to suit the requirement. One of the most widely used combinations is welded cap-end cover, bolted head-end cover with front tube flange mounting. In case of high capacity cylinder when it is steel cast or machined from solid steel forging, then end cover and front flange may be integral part of cylinder tube. Cylinder with this type of construction is widely used in hydraulic press.
2.2 Main Parts
A hydraulic pressure intensifier consists of several kinds of mechanical and electrical components. There are two main parts in the hydraulic intensifiers to be noted. These are Piston and Cylinder.
2.3 Working Principle of HPI
The working principle of HPI is described below:


  1.     Oil is forced into the right half of the hydraulic cylinder.
  2. The piston-plunger assembly moves to the left. Oil is displaced out of the left half of the hydraulic cylinder and the water in the left high pressure cylinder is pressurized.
  3.  The plunger moves to the left.
  4.   Once pressure has begun to build, the high pressure water is forced out of the intensifier through the center of the check valve.
  5.  While the piston-plunger assembly is moving to the left, it is also allowing fresh water to flow into the right high pressure cylinder through the inlet holes of the check valve.
  6.   When the plunger-piston assembly has reached the end of its stroke to the left, the right high pressure cylinder is now full of water.
  7.    The directional control valve receives a signal via a proximity sensor near the piston to reverse the flow of hydraulic oil. Oil is now forced into the left half of the hydraulic cylinder and the piston moves to the right.
  8.   Oil is displaced out of the right half of the hydraulic cylinder while the water in the right high pressure cylinder is pressurized by the right plunger. Such a machine may be constructed by mechanically connecting two pistons, each working in a separate cylinder of a different diameter. As the pistons are mechanically linked, their force and stroke length are the same. If the diameters are different, the hydraulic pressure in each cylinder will vary in the same ratio as their areas: the smaller piston giving rise to a higher pressure. As the pressure is inversely proportional to the area, it will be inversely proportional to the square of the diameter.




3. OPERATION
The working volume of the intensifier is limited by the stroke of the piston. This in turn limits the amount of work that may be done by one stroke of the intensifier. These are not reciprocating machines (i.e. continually running multi-stroke machines) and so their entire work must be carried out by a single stroke. This limits their usefulness somewhat, to machines that can accomplish their task within a single stroke. They are often used where a powerful hydraulic jack is required, but there is insufficient space to fit the cylinder size that would normally be required, for the lifting force necessary and with the available system pressure. Using an intensifier, mounted outside the jack, allows a higher pressure to be obtained and thus a smaller cylinder used for the same lift force. Intensifiers are also used as part of machines such as hydraulic presses, where a higher pressure is required and a suitable supply is already available.

Some small intensifiers have been constructed with a stepped piston. This is a double-ended piston, of two different diameters, each end working in a different cylinder. This construction is simple and compact, requiring an overall length little more than twice the stroke. It is also still necessary to provide two seals, one for each piston, and to vent the area between them. A leak of pressure into the volume between the pistons would transform the machine into an effective single piston with equal area on each side, thus defeating the intensifier effect. A mechanically compact and popular form of intensifier is the concentric cylinder form, as illustrated. In this design, one piston and cylinder are reversed: instead of the large diameter piston driving a smaller piston, it instead drives a smaller moving cylinder that fits over a fixed piston. This design is compact, and again may be made in little over twice the stroke.
It has the great advantage though that there is no "piston rod" and the effective distance between the two pistons is short, thus permitting a much lighter construction without risk of bending or jamming. In the example illustrated, the two pistons are approximately 1:2 ratio in diameter, giving a 1:4 increase in pressure. Note that it is the diameter of the effective piston, i.e. the seal diameter that matters. The cylinders here are relieved beyond the seal and are of greater diameter, for easy running. Although the moving cylinder's bore is around ¾ of the outer diameter, not ½, it is its seal diameter that matters, not its internal clearance bore.

The celebrated mechanical engineer Harry Ricardo began his career by working in his grandfather, Alexander Rendel's, civil engineering practice. At the time they were involved in the construction of bridges in India, which required hydraulic lifting, hoisting and riveting equipment. As the existing transport infrastructure was poor, all plant used on site needed to be lightweight and easily portable. Machines also needed to be connected to their hydraulic power source by flexible tubing, which limited their working pressure to around 500 psi. At this time, modern shipyard equipment was using pressures of up to 2000 psi. This high-pressure equipment was smaller and lighter than the bulkier low-pressure variety, a desirable feature for this construction work. Ricardo's innovation was to specify the use of portable hydraulic intensifiers for these tools, permitting the use of the improved high-pressure form, even where their supply was at low-pressure, through flexible hose. These intensifiers were so successful that eventually several hundred were supplied and used.

Saturday 10 October 2015

Hydraulic Press

1. INTRODUCTION

A hydraulic press is a device using a hydraulic cylinder to generate a compressive force. It uses the hydraulic equivalent of a mechanical lever, and was also known as a Bramah press after the inventor, Joseph Bramah, of England. He invented and was issued a patent on this press in 1795. As Bramah (who is also known for his development of the flush toilet) installed toilets, he studied the existing literature on the motion of fluids and put this knowledge into the development of the press. The hydraulic press depends on Pascal's principle: the pressure throughout a closed system is constant. One part of the system is a piston acting as a pump, with modest mechanical force acting on a small cross-sectional area; the other part is a piston with a larger area which generates a correspondingly large mechanical force. Only small-diameter tubing (which more easily resists pressure) is needed if the pump is separated from the press cylinder.
Pascal's law: Pressure on a confined fluid is transmitted undiminished and acts with equal force on equal areas and at 90 degrees to the container wall.
A fluid, such as oil, is displaced when either piston is pushed inward. Since the fluid is in compressible, the volume that the small piston displaces is equal to the volume displaced by the large piston. This causes a difference in the length of displacement, which is proportional to the ratio of areas of the heads of the pistons given that volume = area X length. Therefore, the small piston must be moved a large distance to get the large piston to move significantly. The distance the large piston will move is the distance that the small piston is moved divided by the ratio of the areas of the heads of the pistons. This is how energy, in the form of work in this case, is conserved and the Law of Conservation of Energy is satisfied. Work is force applied over a distance, and since the force is increased on the larger piston, the distance the force is applied over must be decreased.
A hydraulic press is the cause of death for the Terminator in the film of the same name, as well as Andre Delambre in The Fly. The room featured in Fermat's Room has a design similar to that of a hydraulic press. Boris Artzybasheff also created a drawing of a hydraulic press, in which the press was created out of the shape of a robot.
The hydraulic press is one of the oldest of the basic machine tools. In its modern form, is well adapted to press work ranging from coining jewelry to forging aircraft parts. Modern hydraulic presses are, in some cases, better suited to applications where the mechanical press has been traditionally more popular.

2. FUNDAMENTALS

Advantages of Hydraulic Presses
The mechanical press has been the first choice of many press users for years. The training of tool and die makers and manufacturing engineers in North America has been oriented toward applying mechanical presses to sheet-metal press working. Modern hydraulic presses offer good performance and reliability. Widespread application of other types of hydraulic power equipment in manufacturing requires maintenance technicians who know how to service hydraulic components. New fast acting valves, electrical components, and more efficient hydraulic circuits have enhanced the performance capability of hydraulic presses.

Factors that may favor the use of hydraulic presses over their mechanical counterparts may include the following:
1. Depending on the application, a hydraulic press may cost less than an equivalent mechanical press.
 2. In small lot production where hand feeding and single stroking occurs, production rates equal to mechanical presses are achieved.
3. Single stroking does not result in additional press wear.
4. Die shut heights variations do not change the force applied.
5. There is no tonnage curve derating factor.
6. Forming and drawing speeds can be accurately controlled throughout the stroke.
7. Hydraulic presses with double actions and or hydraulic die cushions are capable of forming and drawing operations that would not be possible in a mechanical press.

Example of a Gap-Frame Hydraulic Press:
Like the mechanical press, hydraulic presses deliver a controlled force to accomplish work. The style of the press frame and the hydraulic components vary depending on the intended use. Figure illustrates a gap-frame or C-frame hydraulic press. The bolster and ram provide a surface to mount tooling. The ram is actuated by a large hydraulic cylinder in the center of the upper part of the frame. Additional alignment is provided by two round guide rods. The motor drives a rotary pump, which draws oil out of the reservoir housed in the machine frame. The control system has electrically-actuated valves which respond to commands to advance and retract the slide or ram. A pressure regulator is either manually or automatically adjusted to apply the desired amount of force.


Unique Features of Hydraulic Presses:
In most hydraulic presses, full force is available throughout the stroke. Figure illustrates why the rated force capacity of a mechanical press is available only near the bottom of the stroke. The full force of a hydraulic press can be delivered at any point in the stroke. This feature is a very important characteristic of most hydraulic presses. Deep drawing and forming applications often require large forces very high in the press stroke. Some mechanical presses do not develop enough force high enough in the downward stroke to permit severe drawing and forming applications such as inverted draw dies to be used without danger of press damage.
Another advantage is that the stroke may be adjusted by the user to match the requirements of the job. Only enough stroke length to provide part clearance is required. Limiting the actual stroke will permit faster cycling rates and also reduce energy consumption. The desired pre-set hydraulic pressure provides a fixed working force. When changing dies, different shut heights do not require fine shut height adjustment. Different tool heights or varying thicknesses of material have no effect on the proper application of force.
The availability of full machine force at any point in the stroke is very useful in deep drawing applications. High force and energy requirements usually are needed throughout the stroke. The ram speed can also be adjusted to a constant value that is best for the material requirements.

Built-in Overload Protection:
The force that a hydraulic press can exert is limited to the pressure applied to the total piston area. The applied pressure is generally limited by one or more relief valves. A mechanical press usually can exert several times the rated maximum force in the event of an accidental overload. This extreme overload often results in severe press and die damage. Mechanical presses can become stuck on bottom due to large overloads, such as part ejection failures or diesetting errors. Hydraulic presses may incorporate tooling safety features. The full force can be set to occur only at die closure. Should a foreign object be encountered high in the stroke, the ram can be programmed to retract quickly to avoid tooling damage.

Lubrication:
Hydraulic presses have very few moving parts. Those parts that do move, operate in a flood of pressurized oil, which serves as a built-in lubrication system. Should leakage occur, it is usually caused by the failure of an easily repairable part such as the ram packing, or a loose fitting. Hydraulic presses having guide rods or gibbing, may require a different lubricant than the hydraulic fluid. The same type of metered or recirculating lubrication systems used on mechanical presses are used in such cases.

Large Force Capacity:
Mechanical presses with high force capacities are physically much larger than their hydraulic counterparts. Few mechanical presses have been built with force capacities of 6.000 tons (53.376 mN) or more. Higher tonnages or more compact construction is practical in hydraulic presses. Hydraulic presses for cold forging are built up to 50,000 tons (445 MN) or greater force capacity. Some hydraulic fluid cell presses have force capacities over 150,000 tons (1,334 mN). Figure 3 illustrates how two pistons having different diameters both deliver 75 tons (667 kN) of force. The force developed by a hydraulic piston is the product of the area of the piston times the applied pressure.

Force Depends on Hydraulic Pressure and Piston Area:
75 tons (667 kN) of force can be achieved by applying 5,300 psi (36,538 kPa) to a 6-inch (152.4 mm) diameter piston. The same 75 tons (667 kN) of force is achieved by applying 1,910 (13,168 kPa) to a 10-inch (254 mm) diameter piston. There is no set rule on the best peak operating pressure for a press design. Obviously, higher pressures permit the use of more compact cylinders and smaller volumes of fluid. However, the pumps, valves, seals, and piping are more costly because they must be designed to operate at higher pressure.

 Advantages of Adjustable Force:
The force of a hydraulic press can be programmed in the same way that the movements of the press are preset. In simple presses, the relief valve system that functions to provide overload protection may also serve to set the pressure adjustment. This allows the press to be set to exert a maximum force of less than press capacity. Usually there is a practical lower limit, typically about 20% of press capacity. At extremely low percentages of force capacity, a stick-slip phenomena known as stiction in the cylinder rod and piston packing can cause jerky erratic action. Programmable controllers are a feature of many modern hydraulic presses. The correct pressure together with ram travel and other parameters is stored in memory by job number and automatically preset by the die setter. For deep drawing operations, the blank holder or hydraulic die cushion force can be varied through the press cycle for best results.

Press Construction Depends on the Type of Work Performed:
The bed size, stroke length, speed, and tonnage of a hydraulic press are not necessarily interdependent. Press construction depends upon the amount of total force required and the size of dies to be used.


3. GUIDE LINES FOR PRESS SELECTION

Bed size does not directly relate to press force capacity. Both if these illustrations show presses that use the tie rods for ram guiding which is suitable for jobs that do not produce lateral or side loads. Hydraulic presses are available in many types of construction which is also true of mechanical presses. There are many factors to consider when deciding between a hydraulic and mechanical press. These include stroke length, actual force requirements, and the required production rate.

Long Stroke Lengths Can be an Advantage:
Since the stroke length can be fully adjustable, long stroke lengths provide for ease of setup and flexibility of application. The full stroke may be used to open the press up for the installation of dies. In production, the stroke length can be set as short as possible to provide for stock feeding and part ejection while maximizing stroking rates.

Hydraulic Press Speeds:
Most press users are accustomed to describing press speeds in terms of strokes per minute. Speed is easily determined with a mechanical press. It is always part of the machine specifications. The number of strokes per minute made by a hydraulic press is determined by calculating a separate time for each phase of the ram stroke. First, the rapid advance time is calculated. Next the pressing time or work stroke is determined. If a dwell is used that time is also added. Finally the return stroke time is added to determine the total cycle time. The hydraulic valve reaction delay time is also a factor that should be included for an accurate total time calculation. These factors are calculated in order to determine theoretical production rates when evaluating a new process. In the case of jobs that are in operation, measuring the cycle rate with a stopwatch is sufficient. Most hydraulic presses are not considered high speed machines. In the automatic mode, however, hydraulic presses operate in the 20 to 100 stroke per minute range or higher. These speeds normally are sufficient for hand fed work. The resulting production rate speeds are comparable to that of mechanical OBI and OBS presses used single stroking applications. Here, there is no additional clutch and brake wear to consider in the case of the hydraulic machine.

Force Requirements:
When choosing between a mechanical or hydraulic press for an application a number of items should be considered. The force required to do the same job is equal for each type of press. The same engineering formulas are used.
There is always a possibility that an existing job operated in a mechanical press requires 20 to 30 % more force than the rated machine capacity. The overloading problem may go unnoticed, although excessive machine wear will result. If the job is placed in a hydraulic press of the same rated capacity, there will not be enough force to do the job. Always make an accurate determination of true operating forces to avoid this problem.

Machine Speed:
The forming speed and impact at bottom of stroke may produce different results in mechanical presses than their hydraulic counterparts. Each material and operation to form it has a optimal forming rate. For example, drop hammers and some mechanical presses seem to do a better job on soft jewelry pieces and jobs where coining is required. In some cases, a sharper coined impression may be obtained at a rapid forming rate. In deep drawing, controllable hydraulic press velocity and full force throughout the stroke may produce different results. Often parts that cannot be formed on a mechanical press with existing tooling can be formed in a hydraulic press that has controllable force throughout the press stroke and variable blank holder pressure as a function of the ram position in the press stroke.

The Type of Press Frame:
Like the mechanical press, open gap-frame machines provide easy access from three sides. Four-column presses such as the machine shown in Figure 6 insure even pressure distribution provided that there is little or no off-center loading. Some two piston presses similar to the design shown in Figure 6 feature a system of linear position transducers and servo valves to vary the force to each piston in order to maintain the ram level with the bed. How well such a system works depends on the accuracy of the position sensors and reaction speed of the servo valving. If snap through energy release is involved, the servo system may not be able to react quickly enough to prevent ram tipping that may be harmful to the press, tooling and process. Straight side presses such as that illustrated in Figure 7 are much better able to withstand off-center loading and snap through energy release than the type shown in Figure 6. Quality features to look for in a press designed for severe work when ram tipping is to be minimized are a single piston design together with a large ram or slide with long guiding and eight point gibing. However, loading should be carefully balanced and cutting dies timed to minimize snap through shock to the best extent possible in any press working operation.


4. HYDRAULIC PRESS LIMITATIONS

The fastest hydraulic press is slower than a mechanical press designed for high-speed operation. For example, the high speeds together with short stroke and feed progressions used for electrical terminal production favor the use of mechanical presses.

Stroke Depth Control:
 While hydraulic presses are available with an reasonably accurate built-in method of stopping the down stroke, generally stop or bottoming blocks must be provided in the tooling. Under production conditions, stroke depth typically can be controlled to within 0.020-inch (0.51 mm), even though readout devices with higher resolution may be provided on the machine. Hydraulic presses are often provided with controls to reverse the machine at a pre-set pressure. This feature used in conjunction with stop or bottoming blocks in the die, can result in excellent part uniformity.

Shock after Breakthrough in Blanking:
Problems with snap-through energy release are common to both mechanical and hydraulic presses. Damage to the hydraulic press structure may result. Severe snapthrough shock can damage lines, fittings, valves, and the press electrical controls. Presses that are robustly constructed have lower deflection and are preferred for heavy blanking applications. Snap through energy release is directly proportional to the amount of machine deflection at the moment of breakthrough when blanking.

Arresting Snap-Through Energy:
Some hydraulic press manufacturers build snap-through arresting devices into hydraulic presses used for heavy blanking applications. Hydraulic damping cylinders on each corner of the machine arrests snap through energy. Hydraulic snap-through arrestors are also available as add-on devices for retrofitting to existing mechanical and hydraulic presses. Hydraulic dampers are effective snap-through arresting devices on both mechanical and hydraulic presses. While the dampers are an effective solution, they add to equipment cost, may require time-consuming adjustment for different jobs and increase energy consumption. Snap-through energy control should be achieved through good die timing wherever possible.

5. MODERN DEVELOPMENT IN HYDRAULIC PRESSES

 As hydraulic presses continue to improve, there is no doubt that they will play an increasingly significant role in industrial production. Hydraulic presses are already in far more widespread use in Europe than in North America.

Response Time and Precise Control:
Hydraulic press speeds have increased over the last few years. Hydraulic component manufacturers have developed new valves with higher flow capacities, faster response time, and precise flow control capability. But unless a radically different hydraulic circuit design is developed, it is unrealistic to predict that hydraulic press speeds will overtake the mechanical press.

Feeders and Auxiliary Equipment:
 With the exception of high speed operations, mechanical press crankshaft-driven feeders are seldom specified for new installations. Today, hydraulic presses use the same roll feeders, and other auxiliary equipment designed for mechanical presses. Actuation is by one or more microprocessor-based programmable controllers. Important features of such systems are easy programming and multiple job memory capability.

Programming the Press:
Modern control systems permit the press sequence to be programmed for each job. Based on job memory parameters the correct pressure, stroke length, speed and dwell time, retraction force can be set-up quickly.

Safety, Human Engineering and Ergonomics:
Important improvements on all types of presses are continuing to increase the comfort and safety of the operator. Better illumination, quieter machines, comfortable work positions, semi unattended operation, and provisions for simplified machine adjustments, all add to operator comfort and increased productivity. Hydraulic presses are increasingly specified for production applications where mechanical presses were once used almost exclusively. The proper selection and use of the machine can be enhanced by a greater understanding of the characteristics of a hydraulic press. The manufacturing engineer should view the press as only one part of a total system which includes tooling, part feeding, personal protection, and part unloading equipment.

Programmable Hydraulic Blankholder Force Control:
Hydraulic die cushions are used on both mechanical and hydraulic presses. They have several advantages when compared to an air cushion.
These include:
1. Much larger forces can be obtained in the same press bed space.
2. Timed cushion lock-down or return delay: this feature is used to avoid deforming the part as the press opens.
3. The ability to control the instantaneous cushion pressure with a servo valve. This feature can be used to optimize the blankholder force as a deep drawing operation is in progress. By controlling the hydraulic die cushion pressure with a servo valve, optimization of blankholder force can be achieved. Typically the pressure of air-actuated die cushions increases 10% or more between initial contacts to the end of travel. A pressure increase of up to 40% is typical for self-contained nitrogen cylinders and some manifold systems. Metal movement on the blankholder may be severely retarded at the end of the forming cycle by this pressure increase. The result may be failure due to fractures. A programmable hydraulic die cushion can optimize blankholder forces through the forming sequence.