1. Collect the data
Everything else follows from these. Get them from the motor nameplate, the machine and the operator.
| Item | Why it matters |
|---|---|
| Motor power (kW) and speed (rpm) | Use the nameplate. Design to the motor rating, not the "usual" load, unless the drive is engineered for a measured load. |
| Driven machine and required speed | Sets the ratio and the service factor. |
| Hours per day | More hours → higher service factor. |
| Starting method | Direct-on-line high-torque motors and engines shock the belts at start-up. |
| Space limits: max pulley Ø, centre distance, pulley width | Guards, frames and shaft lengths limit what fits. |
| Shaft diameters and keyways | For the pulley bores or taper bushes. |
| Environment | Heat, oil, dust, water, wash-down (packhouses), outdoor sun (ozone), static-sensitive areas (anti-static belts). |
2. Service factor
The service factor raises the design power to cover shock loads, starting torque and long running hours. Typical values from industry practice:
- Normal start: AC motors started star-delta, by soft starter or VSD; normal-torque DOL motors; hydraulic couplings.
- Heavy start: high-torque DOL motors, single-phase motors, slip-ring motors on resistance start, internal combustion engines, frequent starts and stops.
- Add about 0.1 for idlers on the slack side and about 0.2 for idlers on the tight side or inside the drive, and for speed-up drives (ratio above 1:1.25 increasing). Use the maker's table for final figures.
3. Design power
4. Choose the belt section
Section selection charts plot design power against the speed of the small (faster) pulley. Because power = torque × speed, the chart is really choosing by torque on the small pulley:
| Indicative design torque (Nm) | Narrow wedge | Classical |
|---|---|---|
| up to ≈ 10 | SPZ | Z |
| ≈ 10 – 70 | SPZ | A |
| ≈ 30 – 230 | SPA | A / B |
| ≈ 120 – 750 | SPB | B / C |
| above ≈ 500 | SPC | C / D / E |
Bands overlap on purpose: near a boundary, try both sections and compare the number of belts and pulley widths.
5. Pulley sizes
- Choose the small pulley. Never below the section minimum (see sections). Bigger is better within the space and belt-speed limits: the belt bends less, power per belt rises and tension and shaft load fall.
- Large pulley = small pulley × ratio, corrected for 1–2 % slip, rounded to a standard diameter.
- Check belt speed: ideally about 10 to 30 m/s; at most about 30 m/s for classical and 40 m/s for wedge belts, and 30 m/s rim speed for grey cast-iron pulleys.
6. Centre distance and belt length
A good starting centre distance:
Calculate the belt length, pick the nearest standard length, then recalculate the actual centre distance:
Design the motor base for adjustment: about 2 % of belt length shorter to fit belts without forcing them, and about 3 % longer for take-up as belts bed in (the centre distance moves about half the length change).
7. Arc of contact
The belt grips the small pulley over its arc of contact θ. Below 180° the power per belt drops, which the correction factor Kα accounts for.
| (D − d)/C | 0 | 0.2 | 0.4 | 0.6 | 0.8 | 1.0 | 1.2 | 1.4 |
|---|---|---|---|---|---|---|---|---|
| θ (°) | 180 | 169 | 157 | 145 | 133 | 120 | 106 | 91 |
| Kα | 1.00 | 0.97 | 0.94 | 0.91 | 0.87 | 0.82 | 0.77 | 0.70 |
Below about 120° consider a longer centre distance, a smaller ratio, or a different drive.
8. Power per belt
The rated power of one belt depends on section, small pulley diameter, speed, ratio and construction (wrapped or cogged). It comes from the maker's power tables, which are specific to each belt range. This is the one step that cannot be done honestly from general formulas, so our calculators ask for it. Send us the drive details and we'll supply the rating from the manufacturer's tables.
The rating is then corrected for belt length with the factor KL (longer belts flex less often per minute and last longer: KL above 1; short belts below 1).
9. Number of belts
If the answer is just over a whole number (e.g. 3.04) a slightly larger small pulley often drops it to 3. More than about 8 to 10 belts suggests a bigger section.
10. Tension and shaft load
Each belt needs enough static tension to transmit its share of the power without slipping, plus an allowance for centrifugal force at speed:
On site you check tension with the force-deflection method (see installation). The drive's load on the shafts and bearings is approximately:
Check that the motor and machine bearings and shafts are rated for this load at the pulley's overhang.
Design rules of thumb
- Keep the ratio per stage at or below about 1:6 (1:7 maximum).
- Small pulley as large as space and belt speed allow.
- Mount pulleys as close to the bearings as possible.
- Prefer the driving side of the belt on the bottom on horizontal drives (the slack side on top increases arc of contact).
- Vertical-shaft drives and drives with the slack side underneath may need banded belts or an idler.
- Idlers: on the slack side, as close to the large pulley as possible, at least as large as the small pulley. An inside idler reduces arc of contact; a back-side idler bends the belt backwards and shortens its life.
- Specify matched sets for multi-belt drives, and replace the whole set at once.
- Design guards in from the start (South African Driven Machinery Regulations, 2015).
Worked example
Duty: a 15 kW, 1 460 rpm, 4-pole motor started star-delta drives a belt conveyor (uneven load) at 730 rpm, 14 hours a day. Space is generous.
- Service factor: medium duty, 10–16 h, normal start → 1.2.
- Design power: 15 × 1.2 = 18 kW.
- Design torque on the motor pulley: 9 550 × 18 / 1 460 = 118 Nm → indicative section SPA (SPB also possible).
- Pulleys: SPA minimum ≈ 90 mm; choose 118 mm. Ratio 2:1 with 1 % slip → 233.6 mm → standard 236 mm. Driven speed: 1 460 × 118 / 236 × 0.99 = 723 rpm.
- Belt speed: π × 118 × 1 460 / 60 000 = 9.0 m/s: fine, but on the low side; a larger pulley pair would carry more per belt.
- Centre distance: start at max(236, (236 + 3×118)/2) = 295 mm → L = 1 157 mm → standard SPA 1180 → actual C = 306 mm.
- Arc: θ = 180 − 2·asin(118/612) = 157.8°, Kα = 0.945.
- Rating: suppose the maker's table gives 4.5 kW per SPA belt at 118 mm and 1 460 rpm, KL = 1.0.
- Belts: 18 / (4.5 × 0.945 × 1.0) = 4.23 → 5 belts. Re-run with a 140 mm motor pulley and 280 mm driven pulley: belt speed rises to 10.7 m/s, the rating per belt rises, and the count usually falls. That's the trade-off to look at.
- Tension: T = 500 × (2.5 − 0.945)/0.945 × 18/(5 × 9.0) + 0.12 × 9.0² ≈ 338 N per belt; deflection 1/64 of the span with ≈ 21 N per belt (≈ 27 N for new belts). Shaft load ≈ 2 × 5 × 338 × sin(78.9°) ≈ 3.3 kN.
Run your own in the drive designer
When not to use belts
- Very low speed, very high torque (below about 5 m/s belt speed, belts need huge tension): consider chain or a gearbox.
- Exact positioning with no backlash at all: a timing belt may do; otherwise gears or direct drive.
- Explosive atmospheres: belts must be anti-static (conductive) and the installation must meet the hazardous-area requirements.
- Temperatures beyond the belt compound's rating, or heavy oil contamination that can't be guarded against.
Not sure? Ask us for a free drive check.