FLUID POWER HANDBOOK
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HYDRAULIC
PUMPS Hydraulic pumps are used in literally every single hydraulic power transmission system. A hydraulic pump is the device that converts mechanical energy into hydraulic energy, which is a combination of pressure and flow. A hydraulic pump can be any device that you can input force into to create pressure, which in turn creates flow.
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Most hydraulic pumps have a mechanical input from an internal combustion engine or electric motor. These prime movers input their mechanical power to the hydraulic pump in a rotational fashion. The input shaft of the pump will be connected to gears, vanes or pistons of the hydraulic pump, where they will rotate or reciprocate to transfer pressure (force) to the hydraulic fluid. As long as the force (pressure) created by the pump is high enough, flow will occur at a rate dictated by the displacement volume of the pump and the speed at which it rotates. These pumps, also called positive displacement pumps, have a small clearance between rotating and stationary parts. A specific amount of fluid is delivered to the system for each revolution. Positivedisplacement pumps can be further divided into two categories: fixed- and
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variable-displacement. Fixed-displacement pumps provide a single, specific volume displacement per revolution. In variabledisplacement pumps, displacement per cycle can vary from zero to maximum volumetric capacity. Some of the more widely used types of positive-displacement pumps are gear, piston and vane. Gear pumps can be either internal or external styles. External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. They feature decent mechanical and volumetric efficiency, compact dimensions and low weight/power ratio. Of the three common types of positive displacement pumps, gear pumps are the least efficient; their appeal is low cost and simple design. External gear pumps can be equipped with straight spur (the most common type), helical or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump. Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur
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7/15/19 1:37 PM
