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Article # 0004
Hydraulic Power Systems
By Tom Cox, P.E.
Hydraulic power systems are all around us, but if you ask the average person about hydraulic power, they will think of hydroelectric power. When engineers discuss hydraulic power systems, we generally think of a closed system consisting of a hydraulic fluid reservoir, a pump, control valves, tubes/hoses, and a hydraulic cylinder or motor. Hydraulic power systems are on most automobiles in the form of the power steering system. Modern agricultural and industrial tractors would be nearly worthless without a hydraulic power system to operate the backhoes, buckets, plows, and planters attached to them.
For a hydraulic power system to function, there must be a ready supply of hydraulic fluid. The purpose of the reservoir is to provide this supply. Reservoirs are available that vent to the atmosphere or are pressurized. Sizes range from a few ounces to hundreds of gallons. Generally, there are at least three ports or openings consisting of an outlet, an inlet, and a fill opening. Some also include a separate opening or openings for instrumentation. Air-over-hydraulic systems supplement or eliminate the hydraulic pump in favor of air pressure. These are useful where high shock loads are imposed on the system.
The heart of any hydraulic system is the pump. Hydraulic pumps are available in a variety of forms and capacities from a manual piston pump to self-contained “power-packs” that combine a pump, motor, and, often, a reservoir.
The manual pumps are useful in circumstances requiring maximum portability or very low flow rates. These pumps are relatively light in weight and typically include a small reservoir. They can develop pressures up to about 3000 psi. Typical flow rates are in fractional fluid ounces per stroke. This type of pump is found on the typical “bottle jack” and on many small hydraulic presses. They are also on the manual “port-a-power” type pumps.
Applications requiring a low starting torque frequently utilize a rotary vane pump. This type of pump generally requires a minimum rpm speed before the pump delivers significant flows or pressures. Typical maximum pressures range from 1500 to 4000 psi range with flow rates of 1 to 20 gpm.
The most common hydraulic pump is the gear pump. These workhorses are available in a multitude of sizes, mounting and drive options. Common maximum pressures again range from 1500 to 4000 psi and flow rates of 4 to 60 gpm are common. Rotary vane pumps and gear pumps may be driven by belts, shafts, or even be incorporated into the motor/engine.
Power packs typically combine the drive motor and pump into a single package. They are generally available with electric motors in 12-24 volt DC, 110-240 volt AC or with a combustion engine. These units are used to power hydraulically operated presses and other equipment on the shop floor or to add hydraulic power capabilities to vehicles. Maximum pressures are typically 500-2500 psi with flow rates between 0.5 and 5 gpm.
Manual control valves or spools are available in three main types: closed center, open center, and open center - power beyond. The closed center type blocks all flow when the spool is in the center or neutral position. When the spool is shifted, flow is enabled from the inlet port to a work port and exhaust oil is allowed through the outlet port. Open center types allow the flow of oil directly from the inlet port to the outlet port while the spool is in the center position. When the spool is shifted, the function is identical to the closed center type. Open center – power beyond types function the same as the open center types except the high-pressure inlet exits to a power beyond port rather then returning to the reservoir until the spool is shifted.
Manual spools may be stacked to provide control over multiple cylinders/motors while using a single inlet and outlet port.
Pilot operated valves function by using a relatively low-pressure flow to trigger the movement of the spool. These valves are primarily useful to lower the actuating force for the operator or occasionally for electromechanical controls.
Flow dividers are used to equalize flow to multiple circuits or to prioritize a circuit. These controls range in complexity from simple manifolds to precision gear-type flow splitters driven by servomotors or the pump drive motor.
Steering valves provide proportional control needed for actuating steering cylinders, or whenever proportional control is required. These valves typically require a rotational input from the operator.
Cushion valves are installed between the control valve and cylinder/motor. These valves allow a bypass of fluid between the extend/retract or inlet/outlet in order to relive shock, surges, or overpressures in the system. The relief pressure is generally adjustable by the operator.
Relief valves are generally intended to protect the entire system from overpressure by venting directly from the pump back to the reservoir. Relief valves may also be incorporated into individual circuits in order to protect a specific cylinder/motor.
Hydraulic cylinders are available in a nearly infinite range of sizes and with assorted end connections. The generic size specification is simply the nominal bore and stroke dimensions. The popular end connections include clevis, single eye, pin hole, threaded, crosstube, and trunnion. The end connections may be mixed as required to match the application.
There are two major types of hydraulic cylinders: single acting and dual acting. Single acting cylinders are under power in one direction only and require gravity, springs, or other methods to return the cylinder to normal position. Single acting cylinders are typically found on lifts and presses.
Double acting cylinders use hydraulic pressure to both extend and retract the cylinder. Besides the obvious difference of providing power in both directions, double acting cylinders do not require as large a reservoir as a single acting cylinder of the same size.
Cylinders are available as a welded or tie rod assembly. Welded cylinders of a given bore size have the advantage of presenting a smaller outside diameter than tie rod cylinders. Welded cylinders tend to be more expensive than tie rod types for short stroke lengths but as the length increases, welded cylinders are less expensive due to the savings in material for the tie rods.
Specialty cylinders are also available. These include telescoping cylinders with multiple stages allowing a very long stroke with a short retracted length and double rod cylinders that operate in two directions.
Hydraulic motors are used to produce rotary motion. The primary advantage to using a hydraulic motor is the elimination of a conventional drive train. They can also be used in hazardous or enclosed locations where a conventional combustion engine would not be acceptable. Hydraulic motors also have an advantageous ratio of size/weight to power output.
There are three commonly used fitting types used in hydraulic systems, SAE, JIC, and NPT. Theses types are not interchangeable but are similar enough to cause confusion to the uninitiated. As a general rule, if there is an O-ring then the fitting is SAE, if there is a conical mating surface, then the fitting is JIC, and if the threads are tapered then the fitting is NPT. Many systems use a combination of fittings.
The plumbing between the assorted components includes pipe, tube, and/or hoses. Steel pipe or tube is generally the most economical for long runs. Temporary or field installed lines as well as the final portion to cylinders or motors typically use hoses. In any case, the lines must be able to withstand the high pressure of the hydraulic power system.
Many hydraulic power systems may power various attachments. Quick couplers allow a simple and fast means of making the connections when changing between the attachments. As with the fittings, quick couplers come in a variety of types and styles and are rarely interchangeable.
About the Author
Tom Cox owns an engineering consulting firm and this site.
Article # 0004 TEST QUESTIONS:
1. What is the purpose of the reservoir in a hydraulic power system?
To provide a mounting surface for the other components.
To hold cooling water
To provide a supply of hydraulic fluid.
All of the above
2. What is the most common type of hydraulic pump?
rotary vane
reciprocating piston
diaphragm
gear pump
3. Which of the following are typical components of a hydraulic power system?
a pump
a reservoir
control valves
All of the above
4. Which of the following is not a main type of manual control valve?
open center
closed center
reciprocating ball
All of the above
5. What type of control may be used to equalize flow to multiple circuits?
cushion valve
flow divider
flow metering valve
All of the above
6. Which of the following are popular end connections for hydraulic cylinders?
clevis
trunnion
crosstube
All of the above
7. What are the two major types of hydraulic cylinders?
open center and closed center
single acting and dual acting
tie rod and threaded packing gland
All of the above
8. Which of the following is a commonly used type of fitting in hydraulic systems?
Whitworth
JIC
SI
None of the above
9. What is the the advantage of a telescoping cylinder?
A very long stroke with a short retracted length.
They provide a clear view of distant objects.
They are extremely durable and resistant to leakage.
All of the above
10. What dimensions are needed to describe the generic size of hydraulic cylinder?
The retracted and extended lengths and the nominal bore.
The nominal bore and stroke.
The volume and the stroke.
The nominal rod diameter and the retracted length.
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