Worth looking below the belt
01 September 2014
The traditional cure for an inefficient belt drive has been to simply replace the worn belt with a new one. However, Mark Maher, technical manager at ERIKS UK, argues that the repair or replacement of a pulley is a more effective way to resolve the problem.
Belt drives, which operate through friction between belts and pulleys, are commonplace in many industrial applications, ranging from fans, pumps and compressors, to conveyors, mills and crushers.
However, when a belt drive either appears to be operating well below its intended efficiency, or fails completely, more often than not the belt is simply replaced whilst the pulley condition is overlooked. Remarkably, as many as 50% of new belts are actually fitted to worn pulleys.
A pulley is just as susceptible to wear and tear as it goes through the rigours of everyday life, yet in many cases its condition passes a maintenance manager by. However, determining a pulley’s condition is not a difficult task, nor does it require any expensive machinery or tools to undertake.
More often than not, a simple tool to measure the groove gauge such as the Fenner Pulley Groove Gauge – which can fit on a key ring – is enough to determine whether a pulley needs to be replaced. Simply clean off a section of the pulley and insert the appropriate width gauge profile into the groove and if there is more than 5% wear on the pulley’s pitch-line then it needs to be replaced.
Another good rule of thumb to bear in mind is that when the gauge is inserted, there should be no visible gap on either side.
In reality, actually replacing a worn belt and then ignoring the condition of the pulley will not only do little to improve the efficiency of a drive, it will actually reduce the life of the new belt as one of the most noticeable effects of pulley groove wear is rapid belt deterioration.
From a financial perspective, poor drive efficiency will undoubtedly impact on both overall plant production and energy bills. To put it into context, a 37kW electric motor running continuously at an average electricity tariff of £0.08kWh could cost £28,500 per year to run if fully loaded. Poor pulley groove condition can easily waste 10% of the energy input into the unit, amounting to £2800 per annum of the energy being absorbed by the worn belt drive, rather than being transmitted to the load. However, in this context, replacement pulleys for this system would have a payback period of less than two months.
Prevention rather than cure
Whilst it is inevitable that pulley damage will occur, there are a number of actions a maintenance manager can investigate to increase a pulley’s operation life. Correct installation is key as 70% of belt friction drives are actually incorrectly fitted.
In the first instance, the maintenance manager must ensure that the most appropriate drive has been chosen. A correctly selected drive uses the fewest number of belts or the minimum belt width required. Doing so will reduce loading on machine bearings which in turn will increase the system’s lifespan and reduce the potential for both machine downtime and mechanical failure.
Once the correct drive has been chosen, it then must be installed effectively. Alignment is key and there are various ways of testing this, from simply holding a taught length of string across the pulley faces in different positions, to probably the most effective way – a laser alignment gauge. Similarly, a belt tension indicator can be used to monitor accurate belt tension, which can reduce pulley groove wear and prevent belt slippage.
Given the frequency of belt drives in industrial applications, it is vitally important that maintenance managers are aware of the need to monitor both belt and pulley condition. Ignoring the pulley component will not only shorten the operational life of a new belt, but will continue to affect a system’s operational efficiency and a plant's annual energy bills. Attention must be paid equally to the pulley and the belt; only then will you truly be able to reap the rewards of an optimised and highly efficient belt drive system.