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A hydraulic pump, typically driven by an electric motor or internal-combustion engine, is the main power source of a hydraulic system. There are many different types of pumps available that deliver less than one gallon per minute (gpm) to several hundreds of gallons. Pressures typically range from 500 to 6,500 psi and some special designs can exceed pressures of 50,000 psi. Pumps are available in three basic designs: gear, vane, and piston.
Consist of two gears, usually of equal size, that mesh with each other inside a housing. As the gears rotate, liquid is drawn into the housing, compressed between gear teeth and housing, and delivered at pressure to the discharge port. Spur gears are used more than helical gears in these pumps.
Consists of an externally toothed gear that rotates inside and drives a larger internally toothed gear. As the teeth unmesh, a vacuum forms in the pocket between the teeth, drawing oil from the inlet. When the teeth engage, the action forces the oil out of the discharge port. A common configuration is a gerotor pump.
Typical vane pumps consist of a circular rotor mounted eccentrically in a circular cavity. As the rotor spins, vanes extend and retract to seal against the cavity surface (cam ring). Fluid is trapped between the vanes at the inlet, swept along by the vanes, and propelled through the outlet. They are efficient at speeds over 600 rpm, running below this will likely result in leakage and low efficiency.
Piston pumps are the most efficient type but they also cost more. They convert rotary motion of an input shaft to a reciprocating motion of one or more pistons. Fluid is drawn into and forced out of a chamber by the piston, with valves controlling flow direction.
Axial-Piston Pumps: Often use an angled cam or “swash plate” attached to the pump shaft. As the plate rotates, the pistons reciprocate, taking in fluid while moving toward the thin part of the plate and expelling it while approaching the thick end. Another configuration is called bent axis where the pistons are mounted at an angle to the drive shaft and allow the pistons and shaft to rotate. No swash plate is needed.
Radial-Piston Pumps: Converts rotary shaft motion into a radial reciprocating motion of the pistons. One type is driven by a rotating cam that runs through the center of the pump, driving the pistons.
Variable Displacement Pumps
These pumps are best used in hydraulic applications that call for high power (over 15 HP) in which flow varies over a wide range. They allow control of output flow. Vane pumps vary displacement by adjusting the position of the cam ring while piston pumps change axis angle or angle of the swash plate. Gear pumps typically change flow only by adjusting drive speed, which restricts them to fixed-displacement applications.
The main advantage of variable displacement pumps is lower power consumption, but they are generally not as efficient and cost more than fixed-displacement types. Fixed-displacement pumps should be used when: duty cycle is on-off, and the pump can be completely unloaded when not in use, and full flow from the pump is required under most operating conditions, even though load may vary.
Pressure Compensated Pumps
Only piston and vane pumps can be made to compensate for a pressure change during the pumping cycle. A pressure-compensated pump is designed to hold the same pressure on the outlet during a back-stroke as during the power stroke. This is usually accomplished by using a swash plate. Using load sensing remote controls in these systems can result in major energy savings.
Typical Rating Factors
- External Gear Pumps: Maximum pressure is approximately 4,500 psi
- Vane Pumps: Maximum pressure ranges from 2,000 to 4,000 psi
- Internal Gear Pumps: Maximum pressure ranges from 1,500 to 2,500 psi
- Piston Pumps: Maximum pressure is approximately 6,500 psi (although some can exceed 8,000 psi)
Content on this page was created using excerpts from the Power Transmission Handbook (5th Edition), which is written and sold by the Power Transmission Distributor’s Association (PTDA). The Power Transmission Handbook is just under 400 pages and is a valuable resource for anyone involved or interested in the power transmission industry.