A V-belt drive that squealed on startup. The motor was 7.5 kW at 1450 RPM. The driven pulley was 200 mm diameter, running at 720 RPM. The belt was a standard SPA. The customer thought the belt was worn. It was new. The issue: the drive was sized for steady-state load, but the application was a stone crusher with shock load. The service factor was wrong. This is how I size V-belt drives.
The service factor
The basic V-belt rating is for steady-state load. Real applications have shock, starts, and varying loads. The service factor (SF) multiplies the motor power to get the design power:
P_design = P_motor × SF
| Application | Service Factor |
|---|---|
| Centrifugal pump, fan (steady) | 1.2 |
| Conveyor, compressor (mild shock) | 1.5 |
| Crusher, punch, reciprocating saw | 2.0-2.5 |
For the stone crusher: P_design = 7.5 × 2.0 = 15 kW. The belt was sized for 7.5 kW — half the required capacity. It slipped on startup because the belt couldn’t transmit the shock torque.
The belt length and center distance
The belt length is determined by the center distance and pulley diameters:
L ≈ 2C + π(D+d)/2 + (D-d)²/(4C)
Where C is center distance, D is large pulley diameter, d is small pulley diameter. For our drive: D = 200 mm, d = 100 mm (ratio 2:1), C = 500 mm. L = 2×500 + π(200+100)/2 + (200-100)²/(4×500) = 1000 + 471 + 5 = 1476 mm. I select the nearest standard belt length: 1500 mm (SPA belt). The center distance adjusts slightly to match.
The pulley diameter effect
Small pulleys bend the belt more. The belt rating drops when the small pulley is too small. For an SPA belt, the minimum recommended small pulley diameter is 90 mm. Our 100 mm pulley is fine. If I used a 75 mm pulley (to get a higher ratio in less space), the belt rating drops by 30%. The belt slips or wears prematurely. I always check the minimum pulley diameter in the manufacturer’s catalog.
What I changed
1. Increased to 3 belts. A single SPA belt at 15 kW design power needs a larger belt section (SPB). But I had space constraints. I used 3 SPA belts in parallel. Each belt carries 5 kW. The total capacity is 15 kW. The multi-belt drive fits the existing pulleys.
2. Increased center distance. The original center distance was 400 mm. I increased it to 550 mm. The longer belt wraps more around the small pulley (wrap angle increases from 165° to 172°). The wrap angle factor improves from 0.95 to 0.98. The belt transmits more power without slipping.
3. Added an idler. For shock loads, I add a spring-loaded idler on the slack side. The idler maintains belt tension as the belt stretches. Without it, the belt loosens after a few weeks and slips. The idler costs $50 and prevents a $500 belt replacement.
The tension check
After installation, I check belt tension with a tension gauge. The correct tension for an SPA belt is about 8-10 kg per belt (measured by deflecting the belt 1.6 mm at 10 kg force). Too tight: bearings wear. Too loose: belt slips and squeals. The squealing belt was undertensioned AND undersized. Fix both.
The factor I multiply: motor power × service factor (2.0 for crushers). Then select belt section and number of belts for that design power. The squealing belt wasn’t worn — it was sized for 7.5 kW steady-state on a 15 kW shock application. Add belts, increase center distance, and check tension.
