How to Select a Gearbox for a Screw Extruder: Torque, Ratio and Service Factor
Quick answer: Select an extruder gearbox from required screw speed and ratio, calculate output torque from power and output speed, apply a suitable service factor, then verify thermal capacity and axial thrust. Matching motor kW alone is not sufficient.
Selecting an extruder gearbox requires more than matching motor power. The key engineering inputs are torque, ratio, duty cycle, service factor, thermal capacity, axial thrust and the screw configuration.
For KNGEAR product specifications and quotations, visit our Extruder Gearbox page. This article is the technical selection guide.
1. Define the Required Ratio
Gear Ratio = Motor Speed / Screw Speed
Example: 1500 rpm motor / 50 rpm screw speed = 30:1.
2. Calculate Output Torque
Torque (N·m) ≈ 9550 × Power (kW) / Output Speed (rpm)
Example: 132 kW at 50 rpm gives approximately 25,212 N·m or 25.2 kN·m.
3. Apply an Appropriate Service Factor
| Operating Condition | Typical Service Factor |
|---|---|
| Stable plastic extrusion | 1.3–1.6 |
| Compounding / filler-rich material | 1.6–2.0 |
| Pelletizing and recycling | 1.8–2.3 |
| Frequent starts or high jamming risk | 2.0–2.5+ |
A calculated torque of 25.2 kN·m with a service factor of 2.0 requires a gearbox rated for approximately 50.4 kN·m or more.
4. Check Thermal Capacity
Continuous extrusion duty can overheat a gearbox even when its mechanical torque rating is sufficient. Confirm thermal rating, oil temperature, ambient temperature and cooling capacity. Higher-power applications may require forced oil circulation and an external heat exchanger.
5. Confirm Axial Thrust
Screw extruders can generate significant axial load. Confirm whether the gearbox incorporates a thrust-bearing system or whether thrust is carried by a separate unit. Axial capacity should be reviewed together with screw and process data.
6. Single-Screw vs Twin-Screw Extruder Gearboxes
The screw arrangement affects the gearbox architecture, not just the required torque. A single-screw extruder normally has one output shaft and a relatively direct load path. Twin-screw extruders may require two closely spaced output shafts, torque splitting and a more complex internal gear arrangement.
| Selection Item | Single-Screw | Twin-Screw |
|---|---|---|
| Output arrangement | One screw shaft | Two synchronized screw shafts |
| Torque distribution | Single output path | Torque must be divided between two outputs |
| Shaft spacing | Usually less restrictive | Center distance can strongly influence gearbox design |
| Axial thrust | Must be checked for the process duty | Must be checked for both screw and thrust-bearing arrangement |
| Thermal duty | Depends on power, speed and operating time | Often needs close review because of high torque density and continuous compounding duty |
For twin-screw applications, provide the screw center distance, rotation direction, required output speed, torque per screw and thrust data whenever available.
7. Information to Provide
- Motor power and speed
- Required screw speed
- Continuous and peak torque
- Single-screw or twin-screw configuration
- Screw center distance for twin-screw applications
- Axial thrust
- Duty cycle and starts per hour
- Mounting and shaft arrangement
- Lubrication and cooling requirements
- Existing drawings for replacement projects
8. Common Selection Mistakes
- Selecting only from motor power
- Ignoring peak load and service factor
- Ignoring thermal capacity
- Not confirming axial thrust
- Not providing twin-screw center distance and output arrangement
- Using inadequate lubrication or cooling
- Failing to check mounting and interface dimensions
From Selection Guide to Gearbox Specification
Once ratio, torque, service factor, thrust, thermal capacity and interfaces are defined, the data can be converted into a practical gearbox specification.
See KNGEAR Extruder Gearboxes for the product page, or visit our Plastic & Rubber Extrusion Gearboxes industry page for application information.
Contact KNGEAR or email info@kngear.com for an engineering review.
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