Introduction to Hydraulics

Hydraulic power converts the energy from pressurized fluid into force and motion. For this reason it is often also referred to as fluid power. Compressed air or pneumatics and steam are also considered part of fluid power as air and steam are considered fluids in the sense that they flow under pressure and can be utilized to produce force and motion. The advantage of hydraulic power is that hydraulic fluid is virtually incompressible, that is, it does not change volume when under pressure. This important characteristic is well used in hydraulic machinery to hold loads in position, to produce smooth continuous motion even under changing loads, and for safety systems.

The fluid used in hydraulics is usually oil though water is used in some applications. Hydraulic oil often has a number of chemical additives to prevent it from foaming, deteriorating over time, igniting under heat and pressure, and also to increase lubrication qualities.

The Secret To The Power of Hydraulics

One of the advantages of utilizing hydraulic power is that is can deliver tremendous forces in a small package. A typical 2 inch bore x 12 inch stroke hydraulic cylinder may weigh only 15 lbs and have an overlength of 18 inches and an outside diameter of 2-1/2 inches. Yet this small package can produce a thrust of over 6000 lbs (3 tons) of force when pressurized to 2000 psi which is a very common operating pressure. It will deliver this force continuously throughout the full stroke. If the load exceeds the power of the cylinder, it will simply stop. An overload will not damage the cylinder.

The secret to the remarkable strength of hydraulic equipment is the root formula that determines hydraulic strength: F = PA, where F= Output Force, P = System Pressure and A = Area, whether it is the area of a piston in a hydraulic cylinder or the area of a piston in a hydraulic pump.

What this hydraulic formula means is that a given force applied to a piston with a small area will produce a high pressure. When that pressure is then applied to the piston area of a large hydraulic cylinder, the force output is very large. Imagine you are on a dance floor and a 100 lb lady steps on your foot, it will not likely cause any pain, only embarrassment. But if that same 100 lb lady is wearing stilletto heeled shoes, the pain will be immense because her same weight, although small, is concentrated on a very tiny area of your foot. This is the essence of hydraulic power: producing high pressures by applying forces to small areas and then transmitting these high pressures to a hydraulic actuator where this high pressure is translated into powerful force.

Let's use the example of the most simple hydraulic system, a hand operated hydraulic piston pump supplying pressurized oil to a cylinder. If the human operator pushes down on the pump handle with 40 lbs of force (assuming no mechanical advantage due to leverage), and the pump piston has an area of 0.25 square inches, he will produce an output pressure of 160 pounds per square inch (psi) [F=PA or P=F/A=40 lbsf /0.25 square inches = 160 psi].

Now if this oil pressure is directed to a hydraulic ram with a 4 inch bore diameter, it will exert a force of 2000 lbs or 1 ton. The area of a 4 inch bore cylinder is the mathematical constant Pi times the square of a circles radius, therefore our cylinder has a piston are of 3.14x2x2=12.5 square inches. Plug that back into our root hydraulic formula F=PA=160psi x 12.5 square inches = 2000 lbs force. Thus, using hydraulics, a small human operator can lift a 1 ton object. Where is the disadvantage? The amount of oil supplied by the hand pump in our example is very small compared to the large volume of the 4 inch bore cylinder. Hence the human operator would have to pump the handle of the hydraulic pump many times to raise the one ton load an inch. If you add a 10:1 mechanical advantage to the handle of the above hand operated hydraulic pump, the output pressure would be increased 10 fold to 1600 psi and that same operator could lift 10 tons using the same cylinder!

If we substitute into the example above an electric motor or a gas engine turning a hydraulic pump in place of our human operator pulling the lever of a hand pump, we have the beginnings of a modern automated hydraulic system.

The power of hydraulic is further enhanced by the fact that it is is transmitted over short distances from one place to another. The motor and pump producing the power can be located remotely from where the hydraulic actuator is doing the actual work. For this reason, hydraulicly operated equipment appears to be very compact for its strength. Machine designers are given great flexibility in designing mechanisms when they are able to locate the source of hydraulic power outside of the actual work area.

Hydraulic equipment is very rugged and tolerant of harsh environments. The above cylinder will function at full capacity over and over in hot and hot weather, indoors and out of doors, in humidity, rain or snow, or in dry, dusty conditions. If the cylinder is equipped with a rod wiper, rod scraper or a protective rod boot, it will last even longer in these difficult conditions.

The other related equipment in a hydraulic system is also similarly tough and forgiving of abuse. A properly designed and maintained hydraulic system will provide years of trouble free, hard working service.

The Components of a Hydraulic System

The Power Source

Hydraulic power is always a slave of another power source. This means that the hydraulic pressure and flow must come from another source of power such as electricity driving a motor or a mechanical engine such as a gas turbine or a diesel engine. A simple hydraulic system may utilize a human operated hand or foot pump to pressurize the oil. Emergency aircraft hydraulic systems sometimes derive power from a wind driven turbine lowered from the aircraft. In some simple low pressure systems, the oil can be pressurized by compressed air in air/oil reservoirs.

The Hydraulic Pump

The hydraulic fluid pressure and flow in most hydraulic systems, though, is usually produced by a hydraulic pump. The pump draws fluid, usually oil, from a reservoir or tank, pressurizes it and sends it to the hydraulic cylinder along pipes, tubes or hoses. The hydraulic pump receives its power from a motor, either an electric motor or an engine. The hydraulic pump, the driving motor and the oil reservoir are often in an assembled package called the Hydraulic Power Unit. The power unit can be mounted remotely from the cylinder(s) that it supplies.

Above: A typical Hydraulic Power Unit

Hydraulic pumps come in different designs including gear pumps, vane pumps, gerotor pumps and piston pumps. The most simple pumps are fixed displacement pumps which produce a set amount of pressurized output oil for each revolution of the pump's drive shaft. Variable volume pumps are able to produce a variable volume output of pressurized oil. These pumps sense the pressure on the output of the pump and adjust the volume being produced. This enables the variable volume pump to deliver a large volume of low pressure oil or a small volume of high pressure oil. These variable volume pumps are more efficient but also more complicated and expensive than fixed displacement pumps.

Hydraulic Circuits

In the most simple hydraulic system, the single acting circuit, the oil is fed directly into the cylinder on one side of the piston only. Thus the hydraulic cylinder moves in only one direction using hydraulic power. To return or retract the cylinder, the system pressure is relieved, or reduced to zero. This is often done by a bleed valve that dumps the oil back into the reservoir. The actuator then returns to its start point using an opposing force such as a mechanical spring or gravity.

Many hydraulic systems use a double acting circuit which alternately pressurizes both sides of the hydraulic piston to first extend the piston and then to retract it. This requires a valve called a directional control valve to be placed in the line between the pump and the cylinders.

Hydraulic Directional Control Valves

The valve valve in a double acting circuit directs the pressurized oil to one side of the cylinder while at the same time relieving the hydraulic pressure on the opposing side of the piston. The relieved oil is returned to the reservoir for reuse. When the cylinder is required to move in the opposite direction, the valve reverses the flow pattern going to the cylinder and the side that was once pressurized is now sent to the reservoir and the other side is now fed from the pump with pressurized oil. This valve is called a 4 way directional control valve because it provides 4 distinct flow paths for the oil to accomplish the double acting circuit function. The valve has 4 ports that are labelled P for Pump, T for Tank, A for the one side of the hydraulic cylinder and B for the other end of the cylinder. If the valve has two control conditions for advancing and retracting a cylinder it is called a 4 way, 2 position, directional control valve.

In most double acting hydraulic systems the 4 way valves actually have 3 separate control positions. At rest the valve sits in a center or neutral position. When the cylinder is required to advance the valve is moved to is advance position (usually called "A"). When the cylinder is required to retract the valve is moved to is retract position (usually called "B"). This valve type is called a 4 way, 3 position, directional control valve.

Often this center position holds both the cylinder ports closed or blocked while allowing the oil from the pump to be diverted directly to the oil reservoir when no work is being done. This is called the Tandem center valve. This type of valve will hold a hydraulic cylinder locked in position. The oil is unable to leave the cylinder from either side and it thus holds the load in place. Yet because the pump is unoaded and only pushing oil back to the tank in neutral, it is not drawing a large amount of power or trying to push oil against a blocked port.

Above: A Hydraulic Circuit with a Tandem Center Valve

Another type of valve has an Open center. In the neutral position, all of the ports are connected together including both cylinder ports, the supply port from the pump, and the port returning oil to the reservoir. This valve allows the cylinder to move when it is in the neutral position. A load attached to the piston rod may thus move if external forces are applied to it. A raised load will likely lower due to gravity with an Open center valve. The pump is unloaded and simply pushing oil into the tank.

Yet another common valve configuration employs a Closed center position. In the neutral center position all the valve ports are blocked. Oil is not able to move to or from the cylinder. This will hold a cylinder load firmly in position. At the same time, however, the pump, if left running, will be supplying oil pressure to a blocked valve. This may be acceptable if the pump being used is a variable volume pump that can sense the lack of oil flow and idle in a no flow condition. In a hydraulic system using a fixed displacement pump, one that produces a fixed volume of oil for each revolution from the drive motor, another method of returning the oil to the reservoir must be in place. This is often provided by the system pressure relief valve. The unused oil flow will thus be forced to return to the oil reservoir over the relief valve. While this works, it is very noisy and energy inefficient.

The Regeneration Center valve has a special neutral condition wherein the pump and both cylinder ports and connected together but the tank port is closed. This produces a condition where the oil from the rod end of the cylinder is sent back to the cap end of the cylinder as the cylinder extends. Thus the oil is said to regenerate. This results in a very rapid cylinder advance speed. The drawback is that the output force produced by the cylinder in this configuration is much reduced. This is because the pump pressure is acting on both sides of the piston at the same time. The output force is therefore equivalent to the difference in the area of the two sides of the piston. This, in fact, works out to be the same as the area of the piston rod. This area becomes the effective thrusting area on the cap end of the cylinder. The force output is expressed in the formula F = P x Ar where F = output force, P = oil pressure and Ar = the piston rod area. The regeneration circuit is often used when a cylinder must advance quickly through a long portion of it's stroke without a load before it is required to kick into a full output force mode. This circuit enables a designer to build a smaller hydraulic power unit, reduce capital expense, and reduce machine cycle time.

There are a number of other valve types available for hydraulic circuits including some that are variations of the types described above. Whatever valve is used, it must be sized large enough to accomodate not just the flow of oil produced by the pump but also by the return flow of oil from the cylinder which may be much larger than the pump flow.

Directional control valves may be actuated by a large variety of methods both manual and automated. Manual methods of operating directional valves often involve levers but may also include foot pedals and buttons. Automated hydraulic equipment often uses electrical solenoid operators to actuate valves, but may also use hydraulic pressure or air pressure (called pilot operators) or mechanically operated mechanisms such as tappets and springs. Three position manual valves may be designed to "stick" in all three positions (called "detented valves") or they may spring to the center neutral position when the handle is released. Three position solenoid operated valves will spring to the center neutral position when electric power is released from the solenoids.

The are a wide variety of other types of control valves used in hydraulic circuits to control flow and pressure under different circumstances. These are often mounted in conjunction with directional control valves. Many offer safety functions. Others maintain consistent flow under changing loads. These auxilliary valves include Pilot Operated Check Valves, Pressure Reducing Valves, Sequence Valves, Counterbalance Valves, and Flow Control Valves.

Hydraulic valves may be supplied as individual stand-alone units or they may be designed to mount into a custom hydraulic manifold with a number of other valves. These may be in the form of what are called "cartridge" style valves. Still other valves are in the form of a "sandwich" style that can be stacked vertically onto a subbase manifold.

The Hydraulic Oil Reservoir

Before hydraulic oil is used by a hydraulic system it is contained in a holding tank called the oil reservoir. The reservoir is an enclosed steel or plastic tank where the oil is allowed to rest. While the idea of inanimate oil resting may seem odd, it is in fact true. Oil in a hydraulic system comes under extreme stresses from high pressures, flowing at high speeds, heat, and contamination.

The oil reservoir enables the oil to cool by radiating through the sides of the tank. In some high temperature applications a refrigeration circuit is placed in the oil reservoir to assist in cooling the oil. On the other hand, in a very low temperature application, a heater may be installed in the reservoir to warm up the oil so it is the proper viscosity for use. Contaminants, such as dirt, water, and metal filings, are allowed to settle to the bottom of the tank while the oil is a rest. Any foam produced will dissipate.

The reservoir must be sized large enough that it contains enough oil to supply the oil volume for the entire hydraulic system. This may be a considerable amount of oil if the system is driving very large bore and long stroke hydraulic cylinders. In addition, if the system is used on a continuous basis, such as in a production machine being operated 24/7, the oil reservoir should be large enough for adequate cooling.

The reservoir tank should have a visual level gauge to enable maintenance personnel to observe the level of oil volume. It should also have a thermometer to enable maintenance personnel to observe the oil temperature. A breather filter should be installed to allow filtered air to enter and leave the tank when the level rises and falls as oil is used when cylinders extend and retract. A large access door should be built into the side of the tank to enable the reservoir to be inspected and cleaned periodically.

Oil from the reservoir should be tested periodically as it can age and lose its chemical properties. The special chemical additives can evaporate. Chemical contaminants can pollute the oil causing it to become corrosive. Mechanical contaminants including biologics can infest the oil. Regular testing of the oil in the reservoir will reveal any such problems and may require a complete replacement of the system oil volume.

Hydraulic Oil Filters

Before the oil is drawn from the reservoir into the pump, it usually passes through a filtration system. The suction line of the pump should draw the fresh oil from near the bottom of the reservoir in a location away from the disturbed oil surrounding the oil return line. The suction line often is equipped with a coarse strainer (typically 20 mesh) to prevent large particles such as metal filings, sand and dirt from becoming entrained in the oil flow to the pump.

A second, finer filter, called the return line filter (typically 20 micron) should also be mounted on the return line running back to the reservoir. The return line filter is usually a spin-on/spin-off cannister style filter that enables the filter element to be quickly and easily changed. A good return line filter system includes a differential pressure indicator that displays the pressure drop falling across the filter with flowing oil. As the filter becomes clogged with trapped contaminants, the pressure drop increases. This will indicate to maintenance personnel when the filter element should be changed. Additionally, a spring loaded bypass valve located in the filter head will open should the pressure drop across the filter exceed a certain limit. The idea being that this is better than having the filter element explode under high differential pressure. The return line filter must be sized to meet the maximum oil flow of the hydraulic system. This may be much larger than the oil flow produced by the system pump if cylinders with large piston rod diameters are installed in the system.

Hydraulic Relief Valves

A pressure relief valve should be installed on the pressure side of the hydraulic pump even if the pump is a variable volume pressure compensated unit. The relief valve should be set slightly above the maximum designed pressure of the system. Thus, in the event of a system failure, or if pressure spikes occur, the relief valve will open and dump the excess pressure back to the reservoir. It will protectect the other components in the system from over-pressurizing.

Pressure relief valves are often supplied with a tamper resistant adjustment to prevent unauthorized personnel from adjusting the pressure relief setting which could prove to be a serious safety issue.

Hydraulic Fittings, Hoses, Pipes and Tubes

Hydraulic power is transmitted from the hydraulic power unit to the actuators via hoses, pipes and tubes. These are often referred to collectively as the "hydraulic lines". The hydraulic lines must be sized to handle both the pressure that is produced within the hydraulic system and the volume of fluid flow being produced.

Because many hydraulic cylinders are required to move through an arc when they are pushing on their load, flexible hydraulic hoses must be used at some point along the lines from the valves to the cylinder ports. Specially designed high pressure hydraulic fittings adapt from the flexible hose to the ports of the cylinders. Although hoses are flexible, a wise machine designer selects hydraulic fittings that will minimize the amount of flexing that the hydraulic hose will see in service. Excessive flexing and rubbing against machinery will abrade the outer surface of hoses and cause premature failure. Hoses are often covered with protective sleeves in applications where they may rub against other equipment.

Hydraulic hoses and fittings must be installed to prevent premature failure

Hydraulic lines may also be manufactured using threaded pipes or tubing connected with compression fittings. These are not subject to the same problem of wear as flexible hoses but they may be subject to leakage or failure due to vibration or external mechanical forces exerted on their structure. Fixed hydraulic pipes and tubes should be properly supported and protected from vibration, flexing, and mechanical loading.

Hydraulic Accumulators

Hydraulic accumulators are often added to a hydraulic system to absorb shock pressures or to provide a stored volume of pressurized fluid. An accumulator is a pressurized vessel that has a charge of high pressure gas acting against a volume of oil. The gas is often high pressure nitrogen. The gas must be separated from the oil to prevent it from ever becoming entrained in the rest of the hydraulic system. The separation of gas and oil my be accomplished by a flexible rubber membrane as in a bladder style accumulator. Another method uses a metal piston much like a hydraulic cylinder but without a piston rod attached to the piston.

The inclusion of an accumulator in a hydraulic system will protect it from shock pressures. The accumulator in this case acts as a cushion and absorbs the pressure spikes.

The oil volume in an accumulator can be used to provide oil volume if there is a sudden short high oil flow requirement that exceeds the flow capacity of the system hydraulic pump. This ability allows a designer to keep down the size, cost and power requirements of a hydraulic system if it will see peak flow capacity only for short periods.

The stored oil volume in a hydraulic accumulator may also be used to provide an emergency source of hydraulic power should the system pump fail. In such a failsafe application the volume of stored oil must be sufficient to extend or retract the cylinders into the necessary emergency positions.

Hydraulic Actuators

Hydraulic cylinders are the muscles of many machines. They are produced in sizes ranging from miniature units smaller than a human finger to massive actuators capable of exerting hunders or even thousands of tons of force. For a more detailed discussion of hydraulic cylinders, their design, construction, advantages, and use, see the separate tutorial Introduction to Cylinders.

A special kind of hydraulic cylinder is the rotary actuator. This mechanism converts the linear motion from a hydraulic cylinder into a recriprocating rotary motion or torque. This is often accomplished using a rack and pinion mechanism. Outputs of 90, 180, 270, 360, and 720 degress of rotations and even more are possible using these devices.

Another kind of hydraulic rotary actuator uses a rotating vane attached to the output shaft. This kind is usuallu restricted to a maximum of 280 degrees of roatation.

Cylinders are not the only form of transmitting or converting hydraulic power. Hydraulic motors use hydraulic oil flow to produce a continuous rotational motion. In fact, hydraulic motors could be described as a pump working in reverse. Rather than converting shaft rotation into hydraulic pressure, the hydraulic motor converts hydraulic flow and pressure into continuous output shaft rotation.

Hydraulic motors, like cylinders, pack an enormous amount of power and torque into a very compact package. They also have a few advantages over electric motors. They have the advantage of producing high torque in very low speed ranges. They are also unharmed if overloaded. Unlike an electric motor which enters a high current condition when overloaded or stalled, the hydraulic motor will simply stop until it is able to overcome the load again. It is unharmed whereas an electric motor may burn out.

Machine Structure

Hydraulic actuators are very powerful and the associated structure of any machinery must be designed to withstand the forces produced. Many machine designer has watched in horror on a machine start up as a hydraulic cylinder extended through its stroke and bent the very structure it was pushing. Associated structural components must be designed with hydraulic forces in mind and incorporate a good margin of safety.

Position Sensors and Feedback Transducers

Although the science of hydraulics moves ahead much more slowly than electronics or computers, modern hydraulic equipment manufacturers have made a number of interesting technology advances in recent decades by interfacing hydraulics with electronics by using electronic sensors and position feedback controls.

One innovation involves installing electrical limit switches into the heads of the cylinders to provide a signal when the cylinder reaches the end of its stroke. More sophisticated analogue and digital sensors including linear potentiometers and digital displacement encoders can also be mounted externally on a cylinder so that a variable indication of the cylinder stroke position is provided.

Sensors and controls are often integrated into the internal structure of a cylinder. An example of this is a cylinder that incorporates a linear velocity and displacement transducer (LVDT). The LVDT is installed in the read head of the cylinder and has a probe that extends through the piston and into a hollow piston rod. A continuous electronic position feedback signal is generated that can be fed from the cylinder to a computerized controller.

Pressure and Flow Considerations

The hydraulic pressure seen in any hydraulic system may be considerably more than the theoretical pressure produced by the pump. This is because in many systems external loads pushing or even slamming against a hydraulic cylinder may cause pressure spikes to ripple back through the whole system. This is the root hydraulic equation F=PA now working in reverse against the system due to external forces creating pressures, P=F/A. Imagine a large rock weighing several tons falling against the bucket of an excavator on a construction site. The force pulls the bucket cylinders down by their piston rods. This external force is pushing against the small area of the rod end of the cylinders. A sudden 4000 psi pressure is produced for an instant of time. These are called shock pressures. They may be considerably higher than system pressure.

The flow of oil that hydraulic lines will encounter are often much more than the volume of oil flow produced by the system hydraulic pump. This is the case, for instance, when a hydraulic cylinder is retracted. The pump flow fills the rod end

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