Troughed Pulley Conveyors
Lagging is the term used to describe the application of a coating, cover or wearing surface which is sometimes applied to pulley shells.
Lagging is often applied in order to extend the life of the shell by providing a replaceable wearing surface or to improve the friction between the belt and the pulley. Notably drive pulleys are often rubber lagged for exactly this reason.
This section provides some useful information regarding the different types of lagging as well as some technical details and recommendations for lagging of pulleys.
1. Types of Lagging
There are primarily three categories of lagging which are used on pulleys and these are described below :-
a) Rubber lagging
Rubber lagging is applied to pulley shells in order to improve the friction between the pulley and the belt.
Conveyor drive pulleys are often supplied with diamond grooved lagging as indicated in the adjacent sketch.
'Diamond grooved lagging' refers to the pattern in the lagging and this pattern, like with vehicle tyres, enhances the grip or friction even if the belt surface is dirty.
A number of variations to diamond-grooved lagging are available including for example, chevron-lagging.
'Plain' lagging can also be applied to pulley shells as indicated adjacent.
The application procedure is much the same as with diamond grooved lagging however, the pattern is absent.
Plain lagging is applied to pulleys which are not drive pulleys for example tail pulleys, take-up pulleys, etc. but which require good traction between belt and pulley for improved belt tracking capability.
This is especially the case where the belt surface is expected to be dirty or wet, which reduces friction and can result in less 'control' over the belt's behaviour.
Rubber lagging is a relatively 'soft' liner (usually approximately 40 to 60 shore) which is resilient to wear but at the same time offers good traction. As such rubber lagging does not damage the belt however its operational life is limited and pulleys must be re-lagged on a routine maintenance programme.
Rubber lagging material is manufactured in standard widths in either 'plain' or 'diamond grooved' pattern and is vulcanized to the pulley shell to prevent the lagging from peeling off of the shell.
b) Ceramic lagging
Ceramic lagging or lining of a pulley is used in instances where the pulley operates in extremely aggressive conditions.
An example of such a condition is the pulleys on a bucket elevator, where the pulleys operate within the enclosed elevator housing and material cannot be prevented from becoming trapped between pulley shell and belt.
While rubber lagging may assist with belt tracking in this application, the wear rate would be excessive and thus a longer-lasting lagging is essential.
Ceramic lagging offers better wearing properties and thus, in spite of the aggressive environment, an extended lagging 'life'. Ceramics are however brittle and should large, hard objects be trapped between the belt and pulley, localized areas of the ceramic liner can be cracked and flake off of the pulley. This can result in belt training problems.
c) Replaceable Strip lagging
In applications where the required duty and/or the availability of a conveyor is such that down-time for maintenance is at a premium, pulley lagging can often be overlooked.
As an alternative method to vulcanized or ceramic lagging 'strip' lagging shown adjacent, is a term given to the application of either rubber or ceramic strips to a pulley.
This lagging comprises a series of steel support strips which are welded or bolted to the pulley shell.
The lagging material, which is manufactured in either rubber or ceramic strips, is then slid into the steel retaining strips to provide a comprehensive lagging on the pulley.
The advantages of this type of lagging lies in the fact that 're-lagging' of the pulley can be performed in a site workshop and in some instances, in-situ. This reduces downtime and improves availability of the conveyor.
One disadvantage of this lagging method lies in the fact that the reduced lagging surface area leads to an increased wear rate and thus increased re-lagging frequency.
2. Lagging Specifications
Rubber pulley lagging is usually specified in terms of its type, hardness, thickness and finish as follows :-
a) Lagging Type and Finish
The type of lagging to be used is specified on the basis of the application, duty and required life of the lagging as outlined above.
In general, the type of lagging used on troughed belt conveyors is indicated in the table below :-
Type of Lagging Diamond
Drive X X Tail X X X Bend X X Take-up X X Snub X X
Ceramic-type lagging is not usually applied to troughed belt conveyor pulleys, neither is strip lagging.
b) Lagging Hardness
The hardness of the rubber lagging to be used on a conveyor is generally dependant on the pulley location.
Pulleys which come into contact with the dirty- or carrying-side of the belt including snub pulleys and bend pulleys, usually employ lagging with a hardness of 35 to 45 Shore 'A'.
Drive, take-up and tail pulleys should use lagging with a Shore 'A' hardness of between 55 and 65.
c) Lagging Thickness
Rubber lagging thickness varies extensively. Commonly used lagging thicknesses are as follows :-
Drive pulleys, grooved lagging
: 10 to 12 mm.
Other pulleys, plain lagging
: 6 to 10 mm.
3. Useful Tips for the Care and Maintenance of Pulley Lagging
Like all equipment, the life expectancy of rubber lagging is dependant on the operational environment, the quality and frequency of maintenance and the manner in which the equipment is used.
a) Correct operation of the conveyor
Notwithstanding the above, it is possible to achieve the optimum life from pulley lagging by observing the following basic pointers :-
Ensure that a conveyor's take-up unit applies the correct load to the belt to prevent belt slip on the drive pulley(s) on startup.
Do not attempt to reduce belt slip by throwing sand or similar material into the pulley feed/nip point. This accelerates wear of the lagging.
Do not over-fill the conveyor as this will necessitate additional torque to start and run the belt, which may result in belt slip over the drive pulley.
Install and maintain a belt plough on the return belt, immediately ahead of the tail pulley. This will minimise the opportunity for lumpy material to be trapped between the belt and pulley, which results in localized damage to the lagging and belt.
Ensure that pulleys are correctly aligned so that the wear pattern is uniform across the pulley.
In the case of screw take-up units, ensure that the take-up is correctly adjusted to provide adequate belt tension under all operating conditions.
Prevent or address material spillage onto the return belt so that pulleys are in contact with a clean belt as far as possible.
Install and maintain belt scrapers to minimise material carry-over, which is deposited onto bend and snub pulleys.
If a drive pulley continues to slip on startup for no apparent reason, consult the designer to confirm the conveyor design parameters and start-up philosophy. Do not ignore the slippage!
b) Correct design and component selection
The designer should ensure that the correct pulley diameter is used so as to prevent over-stressing of the belt and lagging.
Select the appropriate type and hardness of lagging to be used depending on whether the pulley is a drive, tail, take-up pulley and so on.
Use grooved lagging on drive pulleys which assists with traction, expels moisture and is less likely to slip when dirt has become trapped onto the return belt.
Follow recognized conveyor design procedures and equipment sizing guidelines when designing a conveyor.
c) Maintenance measures
Inspect and monitor the condition of pulley lagging regularly.
Repair local damage to lagging before the damage propagates to other areas.
Ensure that re-lagging of pulleys is done by experienced personnel to maximize the useful life of the lagging.
Make a spare pulley available so that a damaged pulley, or damaged lagging can be removed from service and rectified or refurbished correctly and in a controlled environment.