Air Motor Duty Selection Worksheet

Route: /engineering-tools/air-motor-duty-selection-worksheet/ (proposed until route reconciliation). The governing motor choice must come from manufacturer torque-speed, power and air-consumption data at the stated inlet pressure.

1. Purpose and Engineering Decision

Air motors are selected by the mechanical work they must deliver at the shaft: speed, torque, starting requirement, duty cycle and allowable operating pressure. A motor cannot be selected correctly from air consumption alone, because two motors can consume similar airflow while producing very different torque-speed characteristics.

The worksheet keeps working torque, starting torque, stall torque and free speed separate.

2. Core Mechanical Relationship

P_mech = 2 x pi x n x T / 60

Where P_mech is shaft power in watts, n is rotational speed in revolutions per minute and T is torque in newton-metres. Equivalent rearrangements are:

• T = 60 x P_mech / (2 x pi x n)
• n = 60 x P_mech / (2 x pi x T)

These equations are exact for rotational mechanical power when consistent SI units are used.

3. Required Asset - Air Motor Duty Selection Worksheet

4. Working Point Must Come from the Torque-Speed Curve

Pneumatic motors generally produce different torque at different speeds. The required operating point must therefore be checked on the exact motor's torque-speed curve at the intended supply pressure.

5. Starting Torque, Running Torque and Stall Torque Are Different

6. Torque Margin

Running Torque Margin = T_motor_at_target_speed - T_required

Starting Torque Margin = T_motor_start - T_required_start

A positive running margin does not prove that the motor can start the load, and a high stall torque does not prove acceptable efficiency or speed stability.

7. Gearbox / Reduction Boundary

If gearing is used between the motor and the driven load, the worksheet may translate shaft duty through the gear ratio and an explicit mechanical efficiency.

n_motor = n_load x Gear Ratio

T_motor_required = T_load / (Gear Ratio x eta_gear)

Gearbox efficiency must be entered from a defensible source or shown as an explicit preliminary assumption.

8. Available Pressure Is Pressure at the Motor

Motor performance should be checked at the pressure actually available at the motor inlet while air is flowing. Compressor discharge pressure can be higher than motor inlet pressure because of dryers, filters, regulators, valves, piping, hoses and couplings.

9. Air Consumption Must Be Model-Specific

Air consumption is not calculated from shaft power alone. Pneumatic-motor efficiency and internal flow characteristics vary by design and operating point.

10. Mechanical Efficiency Check

eta_overall = P_mechanical_out / P_pneumatic_input

Where pneumatic input power is available from a verified thermodynamic calculation or OEM data, an efficiency screen may be shown.

11. Worked Validation Case A - Torque and Speed to Power

Synthetic validation case: a mixer requires 18 N.m at 900 rpm during normal operation.

P = 2 x pi x 900 x 18 / 60 = 1.70 kW

The required mechanical shaft power is approximately 1.70 kW. The candidate motor must still be checked for at least 18 N.m at 900 rpm on its OEM curve.

12. Worked Validation Case B - Power and Speed to Torque

Synthetic validation case: the driven load requires 2.2 kW at 1,200 rpm.

T = 60 x 2200 / (2 x pi x 1200) = 17.51 N.m

Required running torque is approximately 17.51 N.m at 1,200 rpm.

13. Worked Validation Case C - Gear Reduction

Synthetic validation case: load requires 40 N.m at 300 rpm. A 3:1 reduction gearbox is proposed with a documented efficiency of 0.92.

14. Worked Validation Case D - Starting Torque Gate

Synthetic validation case: required normal running torque is 12 N.m, but the machine needs 24 N.m breakaway torque at start. Candidate Motor A provides 16 N.m starting torque; Candidate Motor B provides 28 N.m.

This case shows why normal power alone is not enough. Starting torque can govern the motor size.

15. Duty Cycle and Thermal / Wear Considerations

Continuous, intermittent and frequent-start duties may require different motor sizing and lubrication arrangements.

16. Reversing Duty

Reversible air motors may be attractive for winches, positioning and maintenance equipment, but direction changes can create transient torque and control demands. The worksheet therefore records reversal frequency and whether the load can reverse under torque.

17. Speed Control

Air-motor speed may be influenced by inlet pressure, throttling, flow control and load. Where speed accuracy matters, the control method and feedback requirement should be stated.

18. Supply-Air Quality

Motor OEM requirements should govern filtration and lubrication. The broader compressed-air source material reinforces the point-of-use principle: filter where necessary, regulate intelligently, and lubricate only where specified.

19. Multiple Motor Demand

Q_total = Sum(Q_motor,i x Simultaneity_i)

Each motor's air consumption should come from its own model-specific operating point. Plant demand then depends on how many motors can operate simultaneously and for how long.

20. Input Model

21. Validation and Guardrails

22. PASS / HOLD / FAIL Logic