How to use this calculator
- Classify the driven machine. Match it to the class with the most similar example machines — when between classes, take the rougher one.
- Classify the driver. Standard AC induction and shunt DC are normal torque; design C/D, single-phase, slip-ring and engines are high torque.
- Set the duty. Hours per day of actual running, not shift length.
- Size on design power. Belts, sheave diameters and belt counts come from P × SF in the manufacturer rating tables.
How it works
Belt ratings are catalog numbers measured under smooth load. Real drives see torque pulses from the driven machine, rough starts from the driver, and fatigue proportional to hours — the service factor folds all three into one multiplier:
design power = motor power × SF (SF = 1.0 … 1.8)
Selection then proceeds on design power: belt cross-section, sheave diameters and the number of belts all come from the manufacturer rating tables at that power. Skipping the factor is the classic reason a "correctly sized" drive throws belts in month three.
The neighbors in the same workflow: the V-belt length calculator for the geometry, the belt tension calculator for installation tension, and the pulley calculator for the speeds the drive must deliver.
Worked example
Verified against the live calculator
A 5 hp (3.73 kW) normal-torque AC motor driving a
medium-duty machine tool on a normal 8-10 h day takes
SF = 1.2: design power 6 hp (4.5 kW). Move
the same motor to a hammer mill (heavy class) running
two shifts and SF jumps to 1.4 — 7 hp of
design power — and on an engine drive it would be 1.6. Same nameplate,
60% more belt: the factor, not the motor, sizes the drive.
Reference data
The classic table this calculator implements:
| Class | Example driven machines | Normal torque (int/norm/cont) | High torque (int/norm/cont) |
|---|---|---|---|
| Light | Agitators, centrifugal pumps/compressors, blowers, fans ≤10 hp, light conveyors | 1.0 / 1.1 / 1.2 | 1.1 / 1.2 / 1.3 |
| Medium | Machine tools, generators, line shafts, fans >10 hp, belt conveyors, presses, printing, rotary PD pumps, screens | 1.1 / 1.2 / 1.3 | 1.2 / 1.3 / 1.4 |
| Heavy | Piston compressors/pumps, bucket elevators, hammer mills, PD blowers, pulverizers, sawmill/woodworking, textile | 1.2 / 1.3 / 1.4 | 1.4 / 1.5 / 1.6 |
| Severe | Crushers, ball/rod/tube mills, hoists, rubber calenders/extruders/mills | 1.3 / 1.4 / 1.5 | 1.5 / 1.6 / 1.8 |
Source: Classic industry V-belt service factor table (Gates-style), cross-checked 2026-06-10 against two independent published copies; manufacturer manuals vary ±0.1 in some cells and add idler/environment adders — the drive supplier's manual governs.
Frequently asked questions
What is a belt drive service factor?
A multiplier (1.0-1.8) applied to the motor nameplate power before selecting belts and sheaves: design power = motor power × SF. It covers the shock, torque peaks and duty hours the nameplate number does not — a 5 hp motor on a hammer mill running two shifts needs a drive rated for 7.5 hp, not 5.
How do I choose the service factor?
Three questions: how rough is the driven machine (light/medium/heavy/severe class, by example machine lists), how rough is the driver (normal-torque AC vs high-torque motors and engines), and how many hours per day (3-5, 8-10, 16-24). The table cell at that intersection is the factor.
Why do engines need a higher service factor than motors?
Torque smoothness. Standard AC induction motors deliver nearly steady torque; single-cylinder engines, design C/D motors and slip-ring machines hit the belt with torque pulses and harder starts, so every machine class carries roughly +0.1 to +0.2 for high-torque drivers.
Do all manufacturers use the same table?
Almost — the classic 4-class structure is industry-wide, but published values differ by about ±0.1 in a few cells, and manuals add adders for backside idlers, speed-up drives and harsh environments. Use this for screening and the drive supplier's manual for the production selection.
Method & assumptions
- Classical V-belt drives; synchronous belts, chains and gears use their own tables.
- Adders not included here (apply per the manual): backside idlers, speed-up drives, dusty/hot/oily environments, frequent starting.
- Severe choke-prone loads may need sizing on motor breakdown torque; soft starters and VFD ramps can justify the gentler driver column.
- Screening table — the manufacturer's current manual governs production drive selection.