Conveyor pulleys and lagging: the pulley positions, how a drive pulley grips the belt, choosing lagging, how pulleys fail and what to inspect
Conveyor pulleys and pulley lagging: drive, head, tail, snub, bend and take-up pulleys and what each sees, how a drive pulley grips the belt through friction and wrap angle, plain, grooved, ceramic and polyurethane lagging and where each suits, how pulleys fail from shell weld fatigue to locking assembly slip, and what to inspect and record. With a drawing of the belt path, drive wrap and a pulley section.
Key takeaways
- A conveyor has a pulley at every change of direction: drive, head, tail, snub, bend and take-up. Each sees a different tension and a different kind of dirt, and is specified and maintained for it.
- A drive pulley grips the belt by friction. The tension it can transmit before slipping rises exponentially with the friction coefficient and the angle of wrap, which is what lagging and snub pulleys are for.
- Rubber lagging suits most pulleys, grooved rubber sheds water on drive pulleys, and ceramic lagging gives the most grip and life on wet or heavily loaded drives.
- Pulleys mostly fail by fatigue cracking at the shell to end disc weld, shaft fatigue, locking assembly slip, lagging loss and build-up. Many are found only when the lagging is stripped.
- Inspect lagging against a wear limit, check welds when the lagging is off, and record pulley condition against each pulley's tag so failures can be traced to a position or a design.
A long overland conveyor trips on drive slip every wet morning. The belt tension is right, the take-up weight is right, and the drive pulley's diamond grooved lagging has been worn smooth for months. Nobody had a reason to look at it, because the pulley was not on the inspection list, and the lagging was not in the CMMS as something with a wear limit.
This guide covers conveyor pulleys and lagging: the pulley positions and what each does, how a drive pulley grips the belt, choosing lagging, how pulleys fail, and what to inspect and record. It sits beside the guide to conveyor belt tracking and maintenance, which covers the belt and idlers, and the conveyor capacity calculator. Friction values are illustrative, and pulley design belongs to the manufacturer and the conveyor designer.
Where the pulleys are
Every change in the belt's direction needs a pulley. On a typical conveyor the drive sits at the head, with a snub pulley just behind it to increase the wrap, a gravity take-up in a loop of the return belt with its bend pulleys, and a tail pulley at the loading end. Each position sees a different tension, a different side of the belt and a different amount of dirt, and the pulley and its lagging are chosen for it.
A pulley is a steel shell welded to two end discs, with hubs clamped to a shaft by locking assemblies, running in bearings at each end. The face is covered with lagging. The shaft is designed for the bending and torque at its position, and the shell and end discs for the belt tension wrapped around them.
| Pulley | Job | What matters |
|---|---|---|
| Drive pulley | Transmits the drive torque to the belt by friction, usually at the head | Highest torque and tension, lagged for grip |
| Head pulley | Turns the belt at the discharge, where it is not also the drive | Lagged where build-up or tracking needs it |
| Tail pulley | Turns the belt at the loading end | Faces the dirty side of the belt, prone to build-up and spillage damage |
| Snub pulley | Increases the wrap of the belt on the drive pulley | Close to the drive, so it sees high tension |
| Bend pulley | Changes the belt's direction, as in a take-up or a tripper | Size set by the belt's minimum bend diameter |
| Take-up pulley | Keeps the belt tension, by gravity weight, screw or winch | Moves through its travel as the belt stretches and starts |
How a drive pulley grips the belt
A drive pulley pushes the belt by friction alone. The difference between the tight side tension and the slack side tension is what moves the load, and the most the pulley can hold before slipping is set by the capstan relation: the ratio of tight to slack tension is e to the power of the friction coefficient times the wrap angle in radians. Both are exponential, so small changes have large effects.
There are three ways to stop a drive slipping: more slack side tension from the take-up, more wrap from a snub pulley or a second drive pulley, or more friction from the lagging. More tension costs belt and pulley life. More wrap and better lagging cost far less, which is why both are standard on any drive with real load.
| Pulley face | Ratio at 180° | At 210° | At 240° |
|---|---|---|---|
| Bare steel, μ about 0.25 | 2.19 | 2.50 | 2.85 |
| Rubber lagged, μ about 0.35 | 3.00 | 3.61 | 4.33 |
| Ceramic lagged, μ about 0.40 | 3.51 | 4.33 | 5.34 |
Worn drive lagging is a tension problem in disguise. Restore the grip before adding take-up weight.
Choosing lagging
Most non-drive pulleys need only plain rubber lagging to protect the shell and reduce build-up. Drive pulleys need grip, and how much depends on the duty and how wet it gets. Grooved rubber sheds water from the contact. Ceramic tiles set in rubber give the most grip and the longest life on wet, heavily loaded or long conveyors. Bolt-on or weld-on strip lagging can be replaced in place during a shutdown, which saves taking the pulley out.
| Lagging | Suits | Notes |
|---|---|---|
| Plain rubber | Non-drive pulleys, dry conditions | Protects the shell, little extra grip |
| Grooved rubber, diamond or herringbone | Drive pulleys in damp conditions | Grooves shed water, herringbone runs in one direction only |
| Ceramic | Drive pulleys in wet, muddy or high tension duty | Highest grip and longest life, costs more, harder on the belt cover if slipping |
| Polyurethane | Where abrasion or chemical attack is high | Long wearing, bonded or bolted |
| Weld-on or bolt-on strips | Fast replacement without removing the pulley | Change in place during a shutdown |
How pulleys fail
A pulley shell bends once per revolution as the belt tension wraps around it, millions of times a year. The bending stress concentrates at the weld between shell and end disc, and that is where most pulley cracks start. Turbine or T-section end discs move the weld away from the peak stress and are common on high tension pulleys. Shafts fail by fatigue at changes of section. Locking assemblies slip when their bolts lose torque.
Many of these failures are found only when the lagging is stripped and the welds can be seen, which makes a lagging change the right time for a crack inspection. A failed pulley takes the conveyor down for as long as it takes to find a spare, so high tension pulleys are often critical spares with a pulley built and held in the store.
| Failure | Cause | What it looks like |
|---|---|---|
| Shell cracking at the end disc | Fatigue at the shell to end disc weld from bending with every revolution | Cracks along the weld line, often found when the lagging is stripped |
| Shaft failure | Fatigue at a change of section or at the hub, sometimes after a bearing problem | Usually sudden, and often traced to design or a past repair |
| Locking assembly slip | Under-torqued or loosened clamping bolts, or contaminated faces | Scoring marks on the shaft, the hub moving on the shaft |
| Lagging wear and loss | Abrasion, slip, poor bonding, material trapped between belt and pulley | Worn smooth, delaminated or missing tiles, grooves worn away |
| Build-up | Carryback and sticky material on the pulley face | Belt mistracking, belt cover damage, vibration |
| Bearings | Seal failure, contamination, misalignment, poor lubrication | Heat, noise, vibration, grease escaping |
Inspecting and recording
Each pulley is an asset with its own tag, position, drawing, bearing and locking assembly details, lagging type and wear limit. Recording inspections against that tag, rather than against the conveyor as a whole, is what lets a site see that one position keeps cracking or one lagging type keeps wearing out. The asset tagging and equipment maintenance log guides cover how.
| Check | What to look for | When |
|---|---|---|
| Lagging | Thickness and condition, groove depth, missing or cracked tiles | Shutdowns, against a wear limit |
| Shell and end discs | Cracks at the welds, shell wear, distortion | When lagging is changed, by visual and dye penetrant or ultrasonic testing |
| Locking assemblies | Bolt torque, witness marks between hub and shaft | Shutdowns, and after any slip |
| Bearings | Temperature, vibration, seals, lubrication | Routine rounds and condition monitoring |
| Cleaners and build-up | Carryback, cleaner blade wear, material on the pulley face | Daily rounds on high-risk pulleys |
| Alignment | Pulley square to the belt and level, take-up running true | After any pulley change and when the belt mistracks |
Common mistakes
- Adding take-up weight to stop drive slip instead of replacing worn lagging.
- Lagging with no wear limit and no inspection on the list.
- Replacing a cracked pulley with the same design in the same position, so it cracks again.
- Not checking the welds when the lagging is off.
- Locking assembly bolts never checked for torque after installation.
- Pulleys recorded against the conveyor rather than their own tags, so patterns are lost.
- No spare for a long lead time drive or high tension pulley.
Common questions
What are the types of conveyor pulley?
Conveyor pulleys are named by position and job: the drive pulley transmits the drive to the belt, the head pulley turns the belt at the discharge, the tail pulley turns it at the loading end, a snub pulley increases the wrap on the drive pulley, bend pulleys change the belt's direction, and the take-up pulley keeps the belt tensioned. Wing pulleys, with slats instead of a solid face, are sometimes used at the tail to shed material.
What is pulley lagging?
Pulley lagging is a covering bonded or bolted to the face of a conveyor pulley. It increases the friction between belt and pulley, sheds water and material, protects the steel shell from wear, and reduces build-up. Common types are plain rubber, grooved rubber in diamond or herringbone patterns, ceramic tiles set in rubber, and polyurethane.
When should ceramic lagging be used?
Ceramic lagging is used on drive pulleys where grip matters most: wet, muddy or icy conditions, high belt tension, long or heavily loaded conveyors, and where rubber lagging wears quickly. It gives a higher friction coefficient and a longer life than rubber, at a higher cost. If the pulley slips on ceramic lagging, it can wear the belt's bottom cover, so the cause of the slip still needs fixing.
How does wrap angle affect a drive pulley?
The ratio of tight side to slack side tension a drive pulley can hold before the belt slips is e to the power of the friction coefficient times the wrap angle in radians. Increasing the wrap from 180 to 210 degrees with rubber lagging raises the ratio from about 3.0 to about 3.6, which is why snub pulleys are used. Lagging that raises the friction coefficient has the same effect.
Why do conveyor pulleys crack?
Most pulley cracking is fatigue. The shell bends once per revolution under belt tension, and the stress concentrates where the shell is welded to the end discs. Over millions of cycles, cracks grow from the weld. Pulley designs that move the weld away from the highest bending stress, such as turbine or T-section end discs, are used on high tension pulleys for that reason. Overloading, undersized shafts and poor weld quality speed it up.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- CEMA publications, including belt conveyor and welded steel pulley standards (Conveyor Equipment Manufacturers Association)
- AS 1403-2004 Design of rotating steel shafts (Standards Australia)
- AS/NZS 4024.3610:2015 Safety of machinery, conveyors, general requirements (Standards Australia)
Related case studies and tools
- Conveyor Capacity Calculator (tool)
Related reading
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