They look like creatures from prehistoric times. However, these gigantic bucket wheel excavators are “creations” of great engineering skill. In opencast mining, the excavators dig into the earth with enormous bucket wheels. Scoop by scoop, they remove tons of earth to one side or convey brown coal or other mineral resources. The bucket wheels are driven hydraulically by radial piston motors.

In the radial piston motor, each piston extends to turn the radial element one step further as it follows the cam track. Once a piston reaches the top dead centre point, it is released. It can then move to the bottom dead centre point. Shortly after reaching this position, it is again extended by hydraulic means.

Radial piston motors offer the following advantages:

• simple shaft fitting

• no gearbox required

• high torque with very low space requirement

• high breakaway torque

• good characteristics at low speeds

• low starting friction

• no stick-slip at low speeds

• robust design, low maintenance requirements

• overall efficiency 85%

The good characteristics of these motors are also put to use in mixers for rubber production and in shredder plants as well as in drying drums and sugar cane mills.

Bearings in the head of each piston transmit the torque to the output shaft. Despite the large dimensions of such motors, there is always a lack of space at these points. The bearings must therefore have high load carrying capacity while requiring very little space. The low speeds due to the slow running of the motor place a particular type of loading on the bearings. Of course, the bearings must also have a long rating life and low maintenance requirements, since any downtime on these enormous machines is very expensive.