RPM and feed speed calculator

WORKED FIELD EXAMPLE · HYPOTHETICAL

The motor turns, but why is the axis feed slower?

Consider a ball-screw axis with a 10 mm/rev lead and a 2:1 mechanical reduction. Calculate the expected feed at 1,500 motor RPM, then compare it with a measured steady speed. This is a hypothetical calculation exercise, not a reported repair on a particular machine.

1. Enter the reference conditions

Select Korea · SI in the country selector and press the calculator’s example/reset button. Use RPM → feed, motor speed 1,500 RPM, lead 10 mm/rev, reduction ratio 2 and travel distance 300 mm. In US units, the equivalent lead is approximately 0.393701 in/rev and distance is 11.8110 in.

2. Interpret the result

The screw turns at 1,500 ÷ 2 = 750 RPM. Feed is 750 × 10 = 7,500 mm/min, or 125 mm/s. The theoretical time for 300 mm at constant speed is 2.4 seconds. A start-to-stop timed move includes acceleration and deceleration and cannot be compared directly with this constant-speed time.

3. What if measured feed is 5,250 mm/min?

Suppose a steady-speed measurement gives 5,250 mm/min. Change to feed → RPM and enter 5,250 as target feed, retaining the same lead and ratio. The corresponding motor speed is 1,050 RPM.

Feed is 30% below the reference in this example. This alone does not identify a fault. Compare commanded and actual motor speed first, then check that the entered lead and mechanical ratio match the axis.

Checks before drawing a conclusion

• Nameplate rated RPM may differ from actual running RPM. Identify what the controller or drive display represents.
• Check feed override and speed-limit settings. Compare measurements from a steady-speed segment.
• Keep electronic gearing and pulse settings separate from the mechanical reduction ratio used here; equal numbers do not imply equal meanings.
• If motor RPM matches but feed does not, recheck lead, transmission ratio and measurement units. Slip or coupling condition requires separate inspection.

Keep hands clear of rotating shafts and couplings. Follow the manufacturer’s stop and energy-isolation procedures before internal inspection. Do not increase operating speed or protection settings solely from this calculation.

Rotation to linear travel through a ball screw

This calculator models a motor driving a screw through a reduction stage. Lead is linear travel per screw revolution. Reduction ratio i is motor RPM divided by screw RPM: enter 1 for direct drive and 2 for a 2:1 reduction. A positive ratio below 1 represents speed increase.

Formula

Screw RPM = motor RPM ÷ i. Feed v(mm/min) = motor RPM × lead(mm/rev) ÷ i. Reverse calculation: motor RPM = v × i ÷ lead. Divide mm/min by 60 for mm/s. Constant-speed travel time is distance ÷ speed × 60 seconds.

Worked example: 1,500 RPM and 10 mm lead

Motor speed 1,500 RPM, lead 10 mm/rev and reduction ratio 2 give screw speed 750 RPM. Feed is 7,500 mm/min = 125 mm/s ≈ 295.2756 in/min. Constant-speed travel over 300 mm takes 2.4 seconds.

Work backward from target feed

For target feed 3,000 mm/min, lead 5 mm/rev and ratio 3, required motor speed is 3,000 × 3 ÷ 5 = 1,800 RPM. Screw speed is 600 RPM. Choose reverse mode to reproduce this result.

Compare screw leads

At 1,500 motor RPM and 2:1 reduction, leads of 5, 10 and 20 mm give 3,750, 7,500 and 15,000 mm/min respectively. Larger lead is not always preferable: required torque, resolution and permissible speed also matter.

Lead versus pitch

Lead is the distance traveled per screw revolution. Pitch is the spacing between adjacent threads. They match for a single-start screw; for a multi-start screw, lead = pitch × number of starts. Use the manufacturer-specified lead.

Scope and limitations

This is a kinematic calculation for ball screws and lead screws. It does not directly calculate cutting feed per tooth, belt or rack-and-pinion travel, required motor power or load torque. Travel time omits acceleration, deceleration and dwell, so actual cycle time can be longer.

Is the calculated RPM permissible?

The result does not establish a safe operating speed. Check screw critical speed, DN limits, support arrangement, and motor and reducer ratings against manufacturer data.

Can speed be zero?

Yes. Zero RPM gives zero feed, and positive-distance travel time is shown as undefined. Lead and reduction ratio must remain positive.

Formula references

THK — Studying the driving motor

Other calculators