Hydraulic Pumps

Introduction

Previous lesson in this unit have explained what may have appeared to be unimportant parts of a hydraulic system. However, they are all really important if the system is to function properly, and this lesson describes one of the most important parts of the hydraulic system – the pump; and explains its operating characteristics.

Pumps are described according to their different types of construction, operating characteristics, and design features. From this lesson you will learn important maintenance and installation requirements of hydraulic pumps and boosters. When you know how pumps are constructed and how they operate you will be able to troubleshoot the system quickly and with confidence.

Pump Classification

Pumps are used in hydraulic system to convert mechanical energy into hydraulic energy. More specifically, a pump converts the kinetic energy of a rotating shaft into the kinetic energy of fluid flow. The fluid flow also has potential energy that allows it to overcome the resistance of the system to fluid flow. Remember that a pump produces flow, not pressure. Hydraulic pressure is caused by the resistance of the hydraulic system to fluid flow.

When operating, a hydraulic pump creates a partial vacuum at its inlet, permitting the atmospheric pressure in the fluid reservoir to push the hydraulic fluid through the inlet strainer and line into the pump. The pump then transfers this fluid to its outlet and into the hydraulic system as shown in Fig. 5 – 1. As the fluid leaves the pump it encounters the back pressure in the system. This back pressure is built up by the pressure regulating valve, the system work load, and also by flow losses in the hydraulic tubing.

Pumps are classified on the basis of the physical arrangement of their pumping mechanism and their basic principle of operation. Pumps classified on principles of operation include POSITIVE displacement and NON POSITIVE displacement types. Positive displacement pumps are equipped with a mechanical seal (gear, vanes, or impellers) between the inlet and outlet, which reduces internal leakage or slippage. Therefore, the output of positive displacement pumps is almost unaffected by variations in system pressure.

Non – positive displacement pumps do not have a positive internal seal against leakage or slippage. Because of this slippage, the delivery of these pumps is reduced as the back pressure of a system is increased. However, the non – positive displacement pumps deliver a continuous flow, while the positive displacement pumps deliver a pulsating flow. These pulsations are small and can be smoothed out by the accumulator or the system piping. Virtually all hydraulic system pumps are positive displacement pumps.

Positive displacement pumps have either a FIXED or VARIABLE DISPLACEMENT. The volume or gpm (gallon per minute) of a fixed displacement pump can be changed only by changing the speed of the pump, because the physical arrangement of the pumping mechanism cannot be changed.  (This does not mean that the flow in other portions of the system cannot be adjusted by valves)

The flow of a variable displacement pump can be changed by changing the physical arrangement of the pumping mechanism with a built – in controlling device. This device often functions in response to system back pressure or other signals. Variable displacement pumps are more complex than fixed displacement pumps and, therefore, cost more. In addition, the internal efficiency of a variable displacement pump is lower than that of fixed displacement pump. This is offset somewhat by the higher overall efficiency of a system powered by a variable displacement pump.

Most positive displacement pumps are classed as ROTARY pumps. This is because the assembly that transfers the fluid from the pump inlet to the outlet has a rotating motion. Rotary pumps are further classified according to the type of element, or part that transfers the fluid , such as gears, vanes or screw.

A different positive displacement pump is the PISTON pump. This pump uses a reciprocating or back – and – forth motion of the piston, alternately to trap fluid on the pump inlet side, and to discharge the fluid on the pump outlet side. A RADIAL PISTON pump has a revolving assembly with several piston assemblies built into it, and can be classified as a rotary pump. Several types of piston pumps will be discussed later in this lesson.