Function

It is a valid question, why one converts, in some modern, hydraulically supported systems, first of all, mechanical energy into electric energy (generator), then this into hydraulic energy. The latter is the job of this pump. The relatively complicated method is worthwhile and sometimes even delivers fuel savings. The electric pump is urgently necessary then, when its function should also be guaranteed when the internal combustion engine is switched off.The reasons for the fuel saving are relatively simple. In the case of the customary power steering, a pump is constantly driven. Although this pump only generates high pressures if the steering wheel is moved, it also nevertheless, consumes energy constantly, e.g., during long highway journeys. Apart from that, the space up front, in the engine compartment is limited for the ribbed belt drive. An electric pump can be installed, in principle, anywhere and is active only when required. Additionally, the steering control must only request pressure so often, that this drops below a certain value. Naturally, even more power-saving can be achieved by supporting the steering forces directly with an electric motor. In addition, the feed rate regulation is simpler.

Hydraulic pumps are evident in the automobile more and more often. They move convertible or folding roofs, the roofs of campers or numerous other movable car body parts. They are also used in the all-wheel-drive sector, e.g., for the activating of differential locking and/or for multiple-disc clutches. In fact, wherever the force of the largest, (as far as size and weight are concerned) electric motor is still not sufficient. An electric motor which drives a hydraulic pump can be smaller than one which substitutes for the complete hydraulic system.

How it works

The supply unit in the above figure is clearly divided, with the electric motor on the left, the pump in the middle, and the reservoir on the right. It can be assumed, that the motor has permanent magnets in the stator. The armature drives, at high RPMs and a low stroke, both pistons. Both have O-ring seals and move either to the right or to the left. Two check valves complete the relatively simple construction. With 12V-equipment, pressures of approx. 200 bar are possible, but no high transport capacity. The pump shown above is capable of developing approx. 70 bar pressure and 1.17 cm³ per armature rotation. 10/09