Curbing the Killer Conveyors
A recent injury has highlighted the fact that using the wrong grease on a conveyor can turn it into a potential killer. Martin Slade of Renold Clutches & Couplings explains.
Recently a steel worker was injured when an inclined conveyor ran backwards, out of control, and the fluid coupling in the drive train exploded like a bomb, spreading shrapnel and hot hydraulic oil everywhere. It should never have happened. Accidents like this are completely avoidable, as investigators at the scene were to discover later.
The accident was caused by an unfortunate mistake during routine lubrication. It was a mistake that could have been avoided, and so preventing the same thing from happening again elsewhere is simplicity in itself. What it boils down to is a simple matter of knowing the difference between your MPs and your EPs, and we're not talking here about Members of Parliament. We're talking grease.
Maintenance engineers had used the wrong type of grease to lubricate the conveyor's most important safety device — the sprag clutch that allows it to run in one direction only. And that seemingly unimportant factor of using one grease rather than another was the sole cause of the horrific incident.
Inclined conveyors are commonly found at places like mines, quarries, cement works and steel works. Most of them tend to be extremely long and the weight of material on a conveyor at any one time can be enormous. If the drive train to a conveyor fails and the conveyor back-drives then it will accelerate under gravity until all the load on the conveyor has been dumped at the bottom.
But that's not all.
When a conveyor runs backwards it tends to run faster than it would normally run forwards. Let's imagine a conveyor that hauls coal up an incline at two metres per second. Running backwards, out of control, this same conveyor is likely to reach speeds of up to six metres per second, or more. Whilst these speeds are not a problem for the belting, the bearings or any of the other components on the low-speed side of the gearbox, it becomes a massive problem for everything on the other side, the high-speed side.
Most gearboxes in engineering are used for speed reducing functions but if you drive them the wrong way round, like when our conveyor back-drives, then the gearbox becomes a speed amplifier. This is fine until the conveyor's runaway speed exceeds the normal driving speed of the conveyor. Then there's a problem. Now, all of the components on the high-speed side of the gearbox are running above their intended operating speed. If the belt reaches three times its normal uphill speed then the components on the high-speed side of the gearbox will be running at three times their maximum.
A drive train for a conveyor will typically include an electric motor, a fluid coupling, a brake, a gearbox and a flexible coupling attached to the head shaft of the conveyor. The head shaft being the powered roller that turns the belt on the conveyor. A 200kW drive package, for example, is expensive, costing around £20,000, and so to control costs all these parts are sized to do their job and nothing else. In particular, the fluid coupling is sized to soft-start the conveyor and protect the motor/drive package from overloads. It is made from aluminium and is intended to run, in this case, at 1,500 to 1,800rpm. If it is run at speeds of up to 6,000rpm then the aluminium casing will be put under massive centrifugal loads sufficient to make the whole thing explode like a bomb, as in the unfortunate case referred to earlier.
To make sure things like this don't happen backstop clutches are fitted that only allow a shaft to turn in one direction. As soon as the shaft tries to go into reverse the sprags lock and the conveyor, with all its load, is instantly stopped. Backstops are basically sprag clutches that are mounted on one of the drive shafts and are anchored to the machine frame, allowing drive in the free direction only.
Sprag clutches are not dissimilar to roller bearings but with sprags, shaped like a figure eight, instead of rollers. The sprags will slip in one direction but stand up and lock in the other due to friction. Friction is quantified by the coefficient of friction and certain greases have additives such as molybdenum disulphide that reduce the coefficient of friction and encourage slipping. So, one way to make a sprag clutch fail really quickly is to fill it with the wrong type of grease. Then, when the drive train fails and the conveyor back-drives, the fluid coupling becomes a bomb just waiting to explode.
If a clutch is filled with a grease containing what is known as an extreme pressure (EP) additive then it will slip rather than hold. Clutch manufacturers recommend specific greases and oils that are quite basic in formulation.They employ simple mineral oils and non-complex lithium soaps. Greases of this nature are termed multi-purpose (MP).
It seems simple but the problem for maintenance engineers is that all the major lubricant suppliers have versions of MP grease that also have EP additives. Also, the names used by lubricant manufacturers are the same but for one letter, so we have, for example, names such as Renolit MP2 and Renolit EP2. Also, if you ask a supplier for an equivalent to say Mobilux No. 2 they are likely to offer Renolit EP2 rather than Renolit MP2 on the basis that they want to offer the best, most modern grease to their customers. All the lubricant companies act in the same way and so it's important to make sure the grease is a direct equivalent.
As well as grease with lurking EP additives there are other perils awaiting the unsuspecting maintenance fitter as well. Too thick a grease will cause the sprags to stick in a given position, perhaps held off the race. It may even form an impenetrable treacle-like layer on the sprag tracks preventing metal-to-metal contact. Conversely a grease with too low a viscosity will just flow out of the clutch leaving the over-running, active surfaces starved of lubricant leading to rapid wear.
Regular lubrication with the correct lubricant will ensure the long and reliable life of a backstopping clutch. Failure to use the correct grease can lead to rapid clutch failure, damage to drive equipment and will pose a very real danger to personnel. In fact, so serious is the issue that one manufacturer, Renold Clutches & Couplings, of Cardiff, now recommend using sealed-for-life clutches in safety critical backstopping applications as a matter of course.
Sealed-for-life clutches are just that. They're sealed for life and require no lubrication at all and are simply replaced at the end of their service life. Problem solved. Sealed-for-life clutches are fitted with high integrity lip seals to keep the grease, filled by the manufacturer, in place. The grease used is of aerospace quality to ensure long life and in many applications sealed-for-life clutches can run for over five years before they will need changing. If a clutch requires no lubrication then there is no possibility of it being filled with the wrong grease and the potential risk posed to plant, equipment and personnel is eliminated.
In the case of our steel worker, maintenance teams at the plant had been given one standard grease with which to lubricate everything, and the grease contained EP additives. Plant managers wanted to simplify the lubrication regime and wanted to eliminate using different greases for different components. In this case sealed-for-life clutches were the perfect solution as they helped to simplify the plant's maintenance schedule and improved safety at the same time.
Reproduced with the kind permission of Drives & Controls www.drives.co.uk
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