Pulley RPM & Speed Ratio Calculator
Calculate driven RPM, speed ratio, and belt speed
Driver pulley से driven pulley ki RPM और belt speed निकालें
Pulley RPM and speed ratio follow one rule: belt speed at both pulleys is identical, so N_driver × D_driver = N_driven × D_driven, where N is RPM and D is pitch diameter. Rearranged: driven RPM = driver RPM × (driver Ø / driven Ø). A 100 mm motor pulley spinning at 1440 RPM driving a 300 mm pulley gives 1440 × (100/300) = 480 RPM at the output — a 3:1 speed reduction with a matching 3× torque gain. Belt linear speed is v = π × D × N / 60 in m/s (with D in metres). For typical industrial drives, keep belt speed below 25 m/s for classical V-belts (A, B, C) and 35 m/s for narrow sections (SPZ, SPA, SPB). This calculator returns driven RPM, speed ratio, and belt speed — the three numbers you need when sizing a compressor, pump, grinder, or conveyor drive.
Calculator
Motor side
Load side
4-pole: 1440 · 2-pole: 2880 · 6-pole: 960
Formulas: N_driven = 1440 × (100 / 300) = 480 RPM. Belt speed v = π × D_driver × N / 60000 = 7.54 m/s. Torque at driven shaft ≈ 3.00× driver torque (minus belt losses).
ƒFormula
N_driven = N_driver × (D_driver / D_driven) | v_belt = π × D × N / 60 (m/s)N is pulley RPM, D is pulley pitch diameter. Since the belt does not stretch, linear speed at both pulleys is identical: N × D is a constant. Rearranging gives the output RPM formula. Belt linear speed v uses D in metres and N in RPM — result in m/s. Maximum safe belt speed: 25 m/s for A/B/C classical, 35 m/s for SPZ/SPA/SPB narrow.
Units: RPM: revolutions per minute · D: millimetre (or metre for v calculation)
Pulley RPM calculation kaise karein?
Pulley drive mein ek basic rule hai — belt ki speed dono pulleys pe same rehti hai. Iska matlab: N_driver × D_driver = N_driven × D_driven. Yaani: agar motor pulley chhota hai aur driven pulley bada, toh driven pulley slow chalegi (speed reduction). Agar ulta hai, toh speed badh jayegi. Formula: driven RPM = motor RPM × (motor pulley Ø / driven pulley Ø). Belt ki linear speed v = π × D × N / 60 m/s. Zyada speed (above 25 m/s) se classical V-belts pe slip aur heating hoti hai.
Worked Examples
Sizing a water pump drive
Problem: A 3 HP motor at 1440 RPM drives a water pump that needs 720 RPM. Motor pulley 100 mm — what driven pulley?
Solution: Need 2:1 reduction: driven Ø = 100 × (1440/720) = 200 mm. Use a 200 mm pulley on the pump. Speed ratio 2:1 means output torque is 2× the motor torque (minus belt losses).
Step-up drive for a high-speed grinder
Problem: A 1440 RPM motor drives a grinder requiring 3600 RPM. Motor pulley 200 mm — driven pulley?
Solution: Need 1:2.5 step-up: driven Ø = 200 × (1440/3600) = 80 mm. Belt speed on motor pulley = π × 0.2 × 1440/60 = 15.1 m/s — still safe. But verify the small pulley (80 mm) is at least 3× the belt section thickness to avoid fatigue.
Belt speed check for high-speed compressor
Problem: Motor 2880 RPM, pulley 150 mm. Is belt speed safe for A-section?
Solution: v = π × 0.15 × 2880 / 60 = 22.6 m/s. Below the 25 m/s classical V-belt limit — safe. For peace of mind, switch to SPA narrow section which handles up to 35 m/s.
Common Motor RPM & Pulley Ratios — Quick Reference
| Motor RPM | Motor Pulley Ø | Ratio → Driven RPM | Belt Speed (m/s) |
|---|---|---|---|
| 1440 (4-pole) | 80 mm | 1:1 → 1440 | 6.0 |
| 1440 | 100 mm | 1:2 → 720 | 7.5 |
| 1440 | 100 mm | 1:3 → 480 | 7.5 |
| 1440 | 150 mm | 1:2 → 720 | 11.3 |
| 1440 | 200 mm | 2:1 step-up → 2880 | 15.1 |
| 2880 (2-pole) | 100 mm | 1:1 → 2880 | 15.1 |
| 2880 | 150 mm | 1:2 → 1440 | 22.6 |
| 2880 | 200 mm | 1:3 → 960 | 30.2 ⚠️ narrow only |
| 960 (6-pole) | 100 mm | 1:1 → 960 | 5.0 |
| 960 | 150 mm | 1:2 → 480 | 7.5 |
Indian 50 Hz motors: 2-pole = 2880 RPM, 4-pole = 1440 RPM, 6-pole = 960 RPM, 8-pole = 720 RPM. Minimum recommended small pulley Ø: ≥ 3× belt thickness for classical, ≥ 2× for narrow.
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