Chain drives are used when precise speed ratios are required between the driver and driven shafts. A chain drive allows no slippage. An example is on a gearbox driving a conveyor. Chain drives are also used when a large amount of power is required through the drive. For example the drive on large rotating kilns.
Roller chain gets its name from the rollers that sit on the bushes of the chain link. The roller turns on the bush and so reduces rubbing friction with the sprocket teeth. Figure 1 shows the components of roller chain. Roller chain is available in numerous sizes and pitches. They come in single, double and triple stand configuration. Using multiple strand chain allows higher power transfer for the same diameter sprocket.
The speed ratio and power to be transferred usually decide the selection of a chain and sprockets. Close pitch for low power, high speed applications and large pitch for high power, low speed applications. The angle of contact between the chain and sprocket should be a minimum of 120o. The smaller sized socket should never have less than 19 teeth because of the high accelerations produced between the rollers and the teeth as they rotate.
Since chain will not easily break, using it increases the risk of breaking shafts or gearbox housings if an overload or jam occurs. When using chain drives it is necessary to protect against overload conditions. Overload protection can be in the form of motor current detection and cut-off of power, mechanical shear pins, which break when loads become excessive or slipping clutches within the drive sprocket, that are set to slip at a predetermined torque. The important thing is to protect the expensive components of the drive train if they become overloaded.
To prevent gearboxes being ripped off their feet orient them so the chain tension forces the gearbox to be pushed against its mounts. Cast iron gearboxes do not like to be in tension and a sudden overload can shear the housing off its feet. Once the feet on a cast gearbox are broken off, welding can never properly repair them.
Chains and sprockets are normally lightly lubricated. The lubrication must get into the pins, bushes and rollers of the chain. The sprocket teeth wear as the hardened rollers on the chain rub against them while the sprocket turns. Lubrication reduces the friction forces against the teeth and roller. The only time lubrication is not applied is in dusty environments. In dusty conditions the chain and sprocket are left dry otherwise the dust mixes with the lubricant and forms a grinding paste that wears the teeth away faster than if they ran dry.
The other way in which chains wear is by links stretching. Chains experiencing high loads or surging loads tend to gradually stretch. Usually the link pinholes elongate. As the chain lengthens, its pitch alters and it no longer meshes properly with the sprocket. When chains start jumping sprocket teeth it is likely the chain has stretched. Some stretch is acceptable (maximum 2%) and any slack is usually taken up by moving the shafts further apart or by using a jockey wheel on the slack side.
Change out all sprockets and chains in a drive train if a chain or sprocket needs replacing. If you use new parts with worn old parts they will quickly wear into the shape of the old part.
Mike Sondalini – Maintenance Engineer
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