Summary

There is a wide range of available hydrostatic pumps and motors from the market and the purpose of the article is to describe the operating principles and features of the most commonly used types. The formulate that are used for determining the performance of pumps are presented and some of the major parameters that can be used as a basis for comparison are outlined as a background for the selection process. However, because of the wide variety of the types of units that are available it is impossible to generalise on the selection process in any given application.

Commonly machine builders and users have preferences for particular types of pumps that are based on experience with particular applications which are determined by factors such as the system function, its control, servicing aspects, enviornmental features, life expectancy, duty cycle and type of fluid to be used. The designer needs to be aware of the relative performance of the difference types and how this knowledge can be utilised in the selection process to suit a particular application.

1. Introduction

Power transmission pumps in fluid power systems are usually hydrostatic or positive displacement units, which convert mechanical power into fluid power, the most common types being gear pumps, vane pumps and piston pumps. In these pumps fluid is transfered through the machine in discrete volumes e.g. a gear tooth cavity. The pump size and speed determines the fluid flow rate.

Hydrostatic pumps are sources of flow so that when they are connected to a hydraulic motor, the outlet pressure will rise so that the flow can cause the motor to rotate against the load torque. Hydrostatic motors convert fluid power into mechanical power so that rotation of the output shaft can take place against an opposing torque load. Generally speaking pumps can be run as motors but a number of factors influence this possibility, some of which are:

Not all pumps are reversible in direction of rotation because of their internal and external sealing arrangements.
Pumps are designed to operate at relatively high speeds and can be inefficient at low speeds particularly during starting.
Motor applications often require significant shaft side load capacity. Pump rotating components are generally not designed to carry such shaft side loads and consequently cannot be directly coupled to the output drive where side loading exists.