How to Select a Torque Limiter for a Twin Screw Extruder Drive System

A torque limiter helps protect a twin screw extruder drive system when an abnormal load occurs. However, choosing a unit by motor power or shaft diameter alone can leave important operating requirements unchecked.

A suitable torque limiter must transmit normal operating loads, disconnect at the intended overload threshold and fit the mechanical installation. Its speed rating, reset method and monitoring arrangement also need to match the application.

The starting point is to understand the drive system and its allowable loads—not simply to choose the highest available trip torque.

First, identify where the torque limiter will be installed.

In many twin screw extruder drive systems, it is positioned between the motor and the gearbox input. Selection must therefore use the torque and speed at that location.

Do not directly apply the gearbox’s screw-side output torque to a motor-side coupling. The reduction ratio changes both speed and torque, and gearbox efficiency must be considered when converting between the two sides.

Provide the supplier with a drive arrangement drawing showing the motor, torque limiter, gearbox and connecting shafts. This helps establish which components the limiter is intended to protect.

For a rotating shaft, nominal torque can be calculated as:

T = 9550 × P ÷ n

Where:

• T = torque in Nm.
• P = mechanical power transmitted at that shaft in kW.
• n = rotational speed at the same shaft in rpm.

For example, 110 kW transmitted at 1,480 rpm corresponds to approximately 710 Nm.

This calculation provides a reference operating point. It does not establish the correct trip torque.

Motor nameplate power represents rated output, while actual operating torque depends on loading and drive control. For variable-speed operation, review the motor and inverter torque characteristics across the operating range. Do not assume rated power remains available at every speed.

A motor’s pole count is useful preliminary information, but actual rated speed and maximum operating speed should also be supplied.

Trip torque is the threshold at which the limiter disengages.

If the setting is too low, acceptable startup or process load variations may cause unwanted trips. If it is too high, the drive may exceed an allowable component load before the limiter operates.

Review:

• Normal operating torque.
• Permitted startup and acceleration loads.
• Expected process fluctuations.
• Allowable loads of the protected components.
• Trip setting tolerance and dynamic response.

There must be a workable margin between acceptable operating loads and the protection limit. If these overlap, the drive or process needs further review.

Avoid applying a universal multiplier to motor torque. The final setting should be confirmed against the actual drive data and component limits. A catalogue’s maximum trip torque describes a product’s capacity; it is not automatically the recommended setting.

Torque limiters differ in what happens after an overload.

Some designs continue transmitting limited torque through slipping. Others disengage and interrupt transmission. Within disengaging designs, some automatically re-engage, while others remain disconnected until deliberately reset.

For an extruder application requiring separation until the fault has been investigated, a fully disengaging design with controlled reset is worth considering. Manufacturers offer extruder-specific designs combining overload disconnection with a flexible coupling. 

Specify the required behavior clearly:

• Complete disconnection or controlled slipping.
• Manual or automatic re-engagement.
• Permitted relative speed after disengagement.
• Permitted duration of disengaged operation.

A free-running design still has operating limits. Confirm these rather than assuming it can rotate indefinitely after a trip.

Torque capacity alone does not establish mechanical suitability.

Confirm the maximum operating speed, including the highest speed permitted by the inverter. Check the selected coupling assembly’s speed rating and any balancing requirements.

For both shaft connections, provide bore diameter, engagement length, connection type and applicable tolerances. A bore that physically fits does not by itself confirm adequate torque transmission.

Also check:

• Available axial and radial space.
• Shaft-end spacing.
• Overall coupling dimensions.
• Access for installation and reset.
• Access for future maintenance.

Where a flexible coupling is included, verify its torque rating and permitted misalignment. Its flexibility accommodates specified movement; accurate shaft alignment remains necessary.

A disengagement signal allows the control system to stop the motor and indicate an overload.

Mechanical or contactless switches can monitor the movement associated with torque limiter disengagement.

For a proximity switch arrangement, confirm the sensing target, mounting position, sensing gap and electrical compatibility. The detection arrangement must match the selected limiter.

Define the reset procedure as part of selection. Operators need access to the reset mechanism, and the system should prevent unexpected restarting during inspection or reset.

After installation, verify the monitoring and reset functions according to the equipment instructions.

Prepare the following information before requesting a quotation:

If trip torque is unknown, provide the available drive data and identify the missing information. Selecting a setting requires more than motor power alone.

TIANYU offers modular-type and ball-detent torque limiters for extruder drive applications, with overload disconnection and manual reset.

Send your motor specifications, gearbox input shaft dimensions and installation drawing to discuss a suitable coupling arrangement and trip torque range. If an existing limiter trips frequently, include its current setting and operating conditions so the cause can be reviewed before increasing the threshold.