In its most general sense, a belt is a long strip made of a flexible material such as leather, fabric or rubber. In mechanical engineering, a belt is a flexible machine element that transmits power and motion between two or more shafts, running over pulleys. In countless machines, from car engines to industrial fans and from crushers to vibrating screens, belts carry the motor’s power to the driven equipment.
In this guide we cover what a belt is and how it works, belt types, V-belt profiles, pulley selection, correct tensioning and alignment, belt failures and the use of belts on mining equipment.
What Is a Belt and How Does It Work?
In a belt drive, as the driving pulley on the motor shaft turns, friction between the pulley and the belt moves the belt; the belt in turn transfers this motion to the driven pulley and therefore to the machine. The ratio between the pulley diameters determines the speed ratio: driving from a small pulley to a large one reduces speed and increases torque.
The main advantages of belt drives are:
- They damp shock and vibration, protecting the motor and the machine.
- They can transmit power over long distances between shafts.
- They can provide a degree of protection by slipping under overload (with V-belts).
- Installation and maintenance are relatively simple and cost is low.
- The speed ratio can easily be changed by changing pulleys.
The disadvantages are that an exactly constant ratio cannot be achieved because of slip (in friction belts), periodic tension checks are needed, and belts are affected by heat, oil and dust.
Belt Types
| Belt Type | Feature | Typical Use |
|---|---|---|
| Classical V-belt | Trapezoidal section; seats in the pulley groove with a wedging action | General industry, older drives |
| Narrow (wedge) V-belt | Higher power for the same width; compact drive | Crushers, screens, pumps, fans |
| Cogged (raw-edge) V-belt | Notched inner surface; flexible, works well on small pulleys, runs cooler | Drives with small pulley diameters |
| Banded (joined) V-belt | Several V-belts joined by a band across the top | Prevents belts turning over on vibrating and shock-loaded drives |
| Timing (synchronous) belt | Toothed; no slip, synchronous transmission | Camshaft drives on engines, precise positioning |
| Poly-V (ribbed) belt | Thin, multi-ribbed; high speeds and small pulleys | Automotive auxiliary drives, compact machines |
| Flat belt | Flat section; high speed, long distance | Older and special drives |
| Conveyor belt | Wide belt for carrying material | Belt conveyors (conveyor belt guide) |
V-Belt Profiles
V-belts are identified by profile letters that describe their cross-section:
- Classical profiles: Z, A, B, C, D, E — the further along the alphabet, the larger the section and the more power can be transmitted.
- Narrow profiles: SPZ, SPA, SPB, SPC (and 3V, 5V, 8V in similar standards) — transmit more power for the same pulley width; common on crushing and screening equipment.
Belt length is given as inside, outside or pitch (effective) length depending on the standard. In a designation such as “SPC 4000”, for example, the profile and length are stated together. When ordering, always state the profile, the length and the standard by which the length is given.
Pulley Selection
- Groove profile: The pulley groove must match the belt profile. Classical and narrow profiles have different groove dimensions.
- Minimum diameter: Every belt profile has a recommended minimum pulley diameter. A smaller pulley bends the belt excessively and shortens its life.
- Groove wear: In worn grooves the belt bottoms out, slips and heats up. Grooves should be checked with groove gauges.
- Mounting: Pulleys with taper lock bushes give easy installation and good centring. For bush types, see our shaft bush guide.
- Balance: High-speed pulleys must be balanced.
Correct Tensioning and Alignment
The great majority of belt failures are caused by incorrect tension and misalignment.
- Tension: The belt should be tensioned to the manufacturer’s values. In practice, deflection is measured by applying a set force at the middle of the span, or frequency-based tension meters are used. Too little tension leads to slip, heating and wear; too much tension overloads bearings and shafts.
- Check after running-in: New belts bed in and stretch during the first hours of operation; tension should be rechecked within the first day.
- Alignment: The angular and parallel alignment of the pulleys should be checked with a straightedge or a laser alignment tool.
- Replace as a set: On multi-belt drives, all belts should be replaced at the same time with belts from the same production batch (a matched set). Using old and new belts together results in uneven load sharing.
- Fitting: Belts must not be levered onto pulleys (with a screwdriver or bar); the motor should be moved to slacken the drive and the belt fitted freely.
Belt Failures and Their Causes
| Symptom | Possible Cause | Remedy |
|---|---|---|
| Squealing, chirping | Slip; low tension, worn groove, overload | Adjust tension, check pulley |
| Glazing and hardening of the belt sidewalls | Heat from slipping | Tension, pulley groove, belt replacement |
| Belt breakage | Shock load, foreign material, levering during fitting | Fitting method, guard, root cause |
| Belt turning over in the groove or jumping off | Misalignment, vibration, worn groove | Alignment, use of banded belts |
| Cracks | Small pulley, ageing, heat | Correct pulley diameter, suitable belt type |
| Belts at different tensions | Mixed set, different lengths | Matched set |
| Oil contamination and swelling | Oil leak | Eliminate the source of the leak |
Belts on Crushers and Screens
Belt drives are common on crushing and screening equipment because they damp shock loads and protect the motor when uncrushable material enters the crusher.
- Jaw crushers: The motor drives the flywheel–pulley through V-belts. Because of heavy shock loads, narrow-profile belts and correct tension are important (flywheel guide).
- Cone crushers: On many models the countershaft is belt-driven.
- Impact and vertical shaft crushers: Multiple V-belts are used for high-speed, high-power transmission. On VSI crushers with twin-motor drives, the belt sets must be at equal tension.
- Vibrating screens: Because the screen body vibrates, the belts flex constantly; banded V-belts and a suitable motor position are preferred.
- Conveyors and pumps: Belt or gearbox drives are used (gearbox guide).
To prevent motor bearings from suffering from excessive belt tension, see also the recommendations in our electric motors guide.
Tips for Longer Belt Life
- Use belt guards; keep stones and dust out.
- Store belts in a cool, dry place away from sunlight, without kinking them.
- Check pulley grooves at every belt change.
- Measure tension periodically and always recheck it after the first run.
- Prevent contact with oil and chemicals.
- Record belt change dates and service lives.
Frequently Asked Questions
What does a belt do?
A belt transfers the rotary motion and power of the motor to the driven machine through pulleys; it also helps change the speed ratio and damp shocks.
What is the difference between a V-belt and a timing belt?
A V-belt transmits power by friction and can slip slightly; a timing belt does not slip thanks to its teeth and provides synchronous motion.
Why do V-belts break?
The main causes are excessive or insufficient tension, misalignment, worn pulley grooves, foreign material, shock loads and incorrect fitting.
Can belts be replaced one at a time?
On multi-belt drives, all belts should be replaced as a matched set; otherwise the load is not shared evenly.
What causes belt noise?
Usually slip. Tension, the pulley groove and alignment should be checked.
Belt Pulley Types
| Pulley Type | Feature |
|---|---|
| Pilot-bore pulley | Machined to suit the shaft; keyed connection |
| Taper lock bush pulley | Secured to the shaft with a bush; easy fitting and removal |
| Multi-groove V pulley | For multiple V-belts |
| Variable-diameter pulley | Speed ratio can be changed |
| Timing pulley | Toothed profile for toothed belts |
| Poly-V pulley | For thin multi-ribbed belts |
Factors Affecting Belt Prices
- Belt profile (classical, narrow, cogged, banded),
- Length,
- Belt construction and cord material,
- Properties such as heat, oil and static resistance,
- Matched-set requirement,
- Brand and quality class.
Using quality belts on heavy-duty drives such as crushers and screens reduces stoppages for belt changes and the load on motor and pulley bearings.
Belt Tension Check Steps
- Stop the machine, lock it out and remove the belt guard.
- Visually inspect the condition of the belts and pulley grooves.
- Measure deflection by applying the force recommended by the manufacturer at the middle of the span, or use a frequency-type tension meter.
- If necessary, adjust tension by moving the motor slide base; make sure the motor moves parallel.
- Recheck pulley alignment.
- Refit the guard and start the machine.
Glossary
- Pulley (sheave): The grooved wheel on which the belt runs.
- Profile: The cross-section size of the belt (e.g. SPB, SPC).
- Pitch length: The effective length of the belt.
- Matched set: A set of belts with very close length tolerances.
- Deflection: How far the belt deflects under a given force.
More Questions
What happens if a V-belt breaks?
The driven machine stops; on multi-belt drives the remaining belts become overloaded and may soon break too. For this reason the whole set should be replaced along with the broken belt.
How is the belt pulley ratio calculated?
The speed ratio is approximately equal to the ratio of the driven pulley diameter to the driving pulley diameter.
How long can belts be stored?
Under suitable conditions (cool, dry, out of sunlight) they can be stored for a long time, but the manufacturer’s shelf-life recommendations should be followed.
CSP Mühendislik Support
CSP Mühendislik supplies V-belts, banded belts, pulleys and taper lock bushes for crushers, screens, conveyors and pumps, and provides service support for the alignment and maintenance of belt drives. See our crusher spare parts page for crusher drive parts and our vibrating screen spare parts page for screens; send the belt profile and length via our WhatsApp line for a quote.
