Planetary Gearbox Selection: Beyond Power & Ratio A gearbox enquiry usually begins with a power figure and a required output speed. Those are useful starting points, which lead to further questions about the application. A suitable selection also depends on how the machine operates, how the gearbox connects to it and the required lifetime of the gearbox. Two concrete pumps can illustrate the difference. Both require a planetary gearbox, yet their drive arrangements, output loads and operating lives lead to substantially different selections. Understanding those differences is where application engineering contributes to the specification. Why planetary gearing? Planetary gearing uses planet gears arranged around a central sun gear, working within a surrounding ring gear. In a typical reduction arrangement, the sun gear provides the input, whilst the planet carrier delivers a lower output speed and higher output torque according to the reduction ratio. Sharing the transmitted load across several planet gears enables high torque capacity within a compact assembly. However, that capability must be assessed against the requirements of the complete installation. Start with the operating conditions The first worked example concerns a hydraulic motor driving a concrete pump through a gearbox and chain drive. The hydraulic motor runs at 150 rpm. With a geometric displacement of 154 cm³, a pressure drop across the motor of 175 bar and the example’ stated efficiency of 92%, the calculated motor output torque is approximately 394.6 Nm. The required gearbox output speed of 30 rpm gives a calculated reduction ratio of 5:1 and an output torque of approximately 1,973 Nm. The specified operating life is 4,000 hours, based on intermittent operation over ten years. The second example uses an electric motor with a variable frequency drive, or VFD. Its input speed is 1,430 rpm, while the required output speed is just 4 rpm. The calculated ratio is therefore 357.5:1, with an output torque of approximately 22,681 Nm. This calculation is based on a transmitted motor power of 9.5 kW, equivalent to approximately 63.4 Nm at the gearbox input. The installation is required to run for 6,000 hours per year over twenty years, giving a required life of 120,000 hours. The description “concrete pump” tells us the general application. It does not establish the gearbox specification.
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