MRO Magazine

The Backup Plan: Make conveyor blockages a less tense situation


May 2, 2012
By PEM Magazine

Two fatalities in just one year — related to the same piece of equipment — led to important improvements in worker protection. Arguably, these did not go far enough, but one solution may help provide an extra boost of safety.

Mining, as anyone who works within it knows only too well, can be a dangerous industry. It is not just underground and at the coal face where serious (and even fatal) accidents can occur. There is one piece of equipment in particular that has led to serious accidents: the inclined conveyor. But a solution from Renold Couplings promises to alleviate the risk, and Rio Tinto is just one company that has taken advantage of the innovation to protect its workers.

The mining industry would find it hard to function without the inclined conveyor to transport coal. However, in the event of a loss of power, their design means that they can run backwards, out of control, if in a loaded condition. For that reason, for many years, these systems have been fitted with backstop sprag clutches. These work by allowing a shaft to rotate in one direction as the sprags slip. But if the shaft tries to turn the other way — as it will do if the conveyor begins to run backwards — the sprags immediately stand up and lock.

These same safety components are also fitted to theme park rides to prevent cars from running downhill in the event of a power failure.


Although this effectively solves the problem it was designed to, another associated problem can also occur, for which it is not only no help but is also actually the direct cause.

If a blockage occurs between the lower belt and the return end shaft, the conveyor will stall. The backstop sprag clutch then comes into operation to prevent the belt from running backwards — but as a result, tension builds in the upper belt, because the slack in the lower belt can’t feed back through the drive.

The natural reaction of the operator is to remove whatever is causing the obstruction to get the conveyor back in operation as quickly as possible. But if the tension in the upper belt isn’t released first, the sudden release that occurs when the blockage is removed will cause it to pull the lower belt rapidly around the return roller with huge force. This force can be so powerful that it has been known to cause the load on the top belt to explode in all directions, and has even pulled the worker removing the blockage into the conveyor.

One answer, developed by Renold, is the tension release and torque limiting safety holdback sprag clutch, which has been designed to enable the extreme tension in the upper belt to be released in a controlled way through controlled friction slippage of the sprag clutch element of the backstop.

With the tension released in this controlled manner, maintenance work can be carried out to remove the blockage and get the conveyor up and running again, quickly and safely.

This is an edited article provided by Renold Couplings. For more information, visit