Input values
Check torque, power, speed, and thermal limits
Enter ratings from the motor, load, and gearbox data sheet. Use consistent manufacturer conditions for every limit.
Formula used
Core gearbox calculations
Use the gear ratio as input speed divided by output speed.
Input power (kW) = 2 × π × Input speed (rpm) × Input torque (N·m) ÷ 60,000
Output speed (rpm) = Input speed (rpm) ÷ Gear ratio
Actual output torque (N·m) = Input torque × Gear ratio × Efficiency
Design output torque (N·m) = Actual output torque × Service factor
Output power (kW) = Input power × Efficiency
Constraint utilization (%) = Calculated demand ÷ Allowable rating × 100
Efficiency is entered as a percentage. The calculator converts it to a decimal before calculating torque and power.
How to use this calculator
Run a practical constraint check
- Enter motor or prime mover torque and speed.
- Enter the gearbox reduction ratio and expected efficiency.
- Add a service factor for load severity and duty.
- Copy allowable torque, power, speed, and thermal ratings.
- Select calculate to show results above the form.
- Check every utilization value before choosing the gearbox.
- Download the CSV or print the result for records.
Example data
Sample gearbox constraint review
| Input or result | Example value | Meaning |
|---|---|---|
| Input torque | 120 N·m | Torque supplied by the motor. |
| Input speed | 1,450 rpm | Motor shaft rotational speed. |
| Reduction ratio | 12 : 1 | Speed reduction and torque multiplication. |
| Efficiency | 94% | Allowance for internal gearbox losses. |
| Service factor | 1.35 | Extra allowance for operating severity. |
| Design output torque | 1,827.36 N·m | Torque compared with the allowable rating. |
| Output power | 17.128 kW | Power after gearbox efficiency losses. |
| Output speed | 120.83 rpm | Expected shaft speed after reduction. |
Gearbox selection guidance
Understanding torque and power limits
Gearboxes convert rotational speed into useful torque. They also introduce losses. Good selection requires more than choosing a ratio. The drive, load, duty cycle, and environment must agree. This calculator combines those factors in one clear check. It estimates operating values before equipment is selected or tested.
Input torque is the twisting effort arriving at the gearbox. Input speed is shaft rotation in revolutions per minute. Their product determines input power. A reduction ratio slows the output shaft. It also increases available output torque. Higher ratios create more torque multiplication. They may also increase size, cost, backlash, and heat.
Efficiency represents power remaining after internal losses. Gear tooth friction, bearings, seals, lubricant movement, and churning consume energy. A realistic efficiency value is important. Ideal gear calculations can overstate available torque. Manufacturers normally publish efficiency ranges for specific units. Use the lower realistic value when duty is severe.
Service factor accounts for uncertain or demanding operation. Shock loads, starts, reversals, vibration, and long operating hours increase demand. Multiply calculated output torque by the service factor. Then compare that design torque with allowable gearbox torque. This avoids selecting a unit that only works under perfect conditions.
Power capacity is a separate limit. A gearbox can have sufficient torque capacity yet exceed its power rating. High speed creates this problem. Thermal capacity also matters. Continuous losses become heat inside the housing. When heat cannot leave safely, lubricant quality falls. Bearings and seals may wear faster. Thermal limits protect long duration performance.
Output speed must remain below the gearbox limit. Excessive speed can raise noise, vibration, churning losses, and bearing stress. Compare calculated output rpm with the stated maximum. Speed checks are especially important when a motor spins quickly. A low ratio may leave the output speed too high.
The calculator reports capacity utilization for torque, power, speed, and thermal demand. Values at or below one hundred percent pass the stated constraint. Lower utilization leaves more reserve. A positive headroom value is useful. It shows how much capacity remains. The smallest margin identifies the controlling constraint.
Use consistent units throughout the calculation. Torque should be in newton metres. Speed should be in revolutions per minute. Power capacities should be in kilowatts. Verify that ratings use the same operating conditions. Ambient temperature, mounting position, lubrication, and duty cycle can change published limits.
Treat the result as an engineering screening tool. It does not replace manufacturer data or a detailed drive analysis. Check shaft loads, overhung loads, torsional vibration, gear life, lubrication, and safety requirements separately. Review startup conditions carefully. The highest transient torque may control the final selection. Document assumptions before approving the gearbox.
Constraint checks should not be averaged. One exceeded rating can invalidate the design. Choose the next larger gearbox when a critical limit is surpassed. Recalculate after changing the ratio, motor speed, or duty assumption. Confirm the final unit with the supplier.
Frequently asked questions
Gearbox torque and power questions
1. What does the gear ratio mean?
The reduction ratio compares input speed with output speed. A 12:1 ratio means the input turns twelve times for one output turn. In an ideal system, it also multiplies torque by twelve.
2. Why is a service factor used?
A service factor raises calculated torque for uncertain duty. It helps account for shock, starts, vibration, reversals, and long operating hours. Use the factor recommended for the actual driven machine.
3. Does efficiency change the torque result?
Yes. Internal losses reduce delivered torque. This calculator multiplies ideal output torque by the efficiency decimal. A 94% efficiency keeps 94% of ideal torque available at the output.
4. Can torque pass while power fails?
Yes. A gearbox may tolerate the calculated torque but still exceed its rated power. This can happen at high speed because power increases with both torque and rotational speed.
5. What is thermal capacity?
Thermal capacity is the continuous power a gearbox can handle without excessive temperature. It reflects heat generation and cooling. It can be lower than the mechanical power rating under difficult conditions.
6. What happens when output speed is too high?
High output speed can increase noise, bearing load, lubricant churning, vibration, and heat. Keep the calculated output speed below the manufacturer maximum for the gearbox and its installation.
7. Should I enter peak torque or continuous torque?
Enter the torque that matches the rating being checked. Use peak torque for transient checks. Use continuous torque for continuous ratings. Review both conditions when the application has frequent starts or shock loads.
8. Is actual output torque the same as design torque?
No. Actual output torque includes gear ratio and efficiency. Design torque also includes the service factor. Design torque is normally the value compared with the allowable gearbox torque rating.
9. Do ratings remain unchanged at every ambient temperature?
No. Ambient temperature, mounting position, lubrication, cooling airflow, and duty cycle can change usable capacity. Check the manufacturer documentation for corrections that apply to the installation.
10. Does this calculator check shaft and bearing loads?
No. It checks torque, power, output speed, and thermal constraints. Confirm radial loads, axial loads, overhung loads, torsional vibration, shaft strength, and coupling limits separately.
11. Can I save the calculated results?
Yes. After calculating, use Download CSV for a spreadsheet-friendly file. Use Print / Save PDF to create a printable record with the entered values and calculated constraint results.