How do I determine drive torque and power?

The decisive calculations for selecting the right motor for your screw jack.

Once the gearbox size has been defined on the basis of load capacity, the drive torque and drive power are determined. These quantities are crucial for correct motor selection: an undersized motor will not move the load reliably or will overheat; an oversized motor makes the solution more expensive and increases energy consumption.

Calculating the required drive torque (T)

The drive torque is the torque at the input/worm shaft that is required to move the load (unit: Nm). The following are used for the calculation:

• Axial load on the screw (F) – in N
• Screw lead (p) – travel per revolution (e.g. mm/rev)
• Overall efficiency of the screw jack (η₍tot₎) – as a decimal value (value from data sheet/tables)
• No-load torque (T₀) – in Nm (friction torque of the gearbox without external load; tabulated value)

Important parameters explained:

• Overall efficiency (η₍tot₎): depends on the drive type; TR typically lower, KGT very high. Take exact values from the ZIMM data sheets.
• No-load torque (T₀): torque required to move the gearbox without external load (overcoming internal friction); tabulated value.
• Note – starting torque: The calculated torque applies to continuous operation. The starting/breakaway torque – especially for TR after standstill – can be higher. The motor must be able to provide this briefly.

Calculating the required drive power (P)

Drive power indicates how much work the motor has to perform per unit of time. Unit: kW. It depends on the drive torque and the drive speed. The formula for the calculation is:

• Input values:
  • Required drive torque (T) – in Nm (from the previous calculation)
  • Drive speed (n) – in rpm at the worm shaft (results from the desired lifting speed and the screw lead; take any gear ratio into account if applicable)
• Power reserve:
  • For a safe design, it is recommended to allow for a reserve of approx. 30% over the calculated power when selecting the motor (among other things for starting torque and mains voltage fluctuations).