Sometimes the complexity and magnitude of a job suggest that a system of separate hydraulic components would be difficult to put into action. In such a case, a power unit consisting of a reservoir, pump, drive motor, filter, heat exchanger, and basic system valving could be used. The pump includes integral controls, perhaps an auxiliary supercharge pump, and relief valves. The reservoir includes inlet filter, all necessary line connections, fluid level and temperature gages, lifting lugs, and observation and clean-out covers. This type of power unit is a fine example of operational convenience.

Manufacturers agree that an assembled unit is more expensive than a combination of components purchased separately. But they suggest that the additional cost is offset by the fact that the user need not select the components, design the system, build it, or troubleshoot it. They point out that hydraulic power units with compatible components, supplied by one manufacturer for a specific system, usually provide maximum system efficiency, controllability, and durability, with minimum noise.

The heart of a system is one or more pumps. For simple push-pull applications or constant power drive, a fixed delivery, one-way pump may be adequate. In applications requiring more complex functions, a variable-displacement unit is probably necessary. In either case, the system should have adequate reserve capacity.

Although most pump controls merely select the output volume or flow, other functions can be provided. These include controls of generative and regenerative direction of flow, acceleration, and deceleration of flow, zero flow, limiting pressures, constant power, automatic decompression, and automatic interlocks.

Many control modes are possible: Mechanical, hydraulic, pneumatic, electric, and electrohydraulic controls are available. The simplest direct control is a hand-operated screw. Mechanical extensions or electric pilot motors with geared head, brakes, and limit switches provide remote operation. Low-pressure hydraulic control circuits or low-power electrical circuits permit remote selection of multiple volumes. And hydraulic servomotor controls can be used to provide automatic control of high accuracy.

Any type of drive can be supplied with a power unit: electric motor, diesel engine, gas turbine, line shaft, or others. The most widely used drive is an electric motor directly coupled to the pump through a flexible coupling.

To design and build a power unit, the project engineer must know, among other things: 1. Type of machine or application. 2. Work to be performed. 3. Maximum force and working pressure required. 4. Minimum and maximum working speeds. 5. Size and weight of machine moving parts. 6. Type and accuracy of control and functions. 7. Type of service cycle. 8. Type of input power available to power unit. 9. Space available. 10. Heat-exchange method required. 11. Noise specifications. 12. Any unusual conditions of the application, special requirements, or environmental conditions.