Friday, 1 June 2012

Chain drives

Chain drives
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Chain drives

 Rotation of camshafts in an engine may be by gears or by chain turned by the main crank. The disadvantage of using gears is difficulty in alignment, lubrication and disadvantage to wear from foreign materials as well as their increased cost.The disadvantage of chains is the requirement for tensioning and their finite life. Although for large installations this can be very long.
Wear on the chain pins, bushes as well as the chain sprockets can all lead to a slackening off of the chain. This can lead to 'slap' and changing of cam timing.This alters the leads of the fuel p[umps and exhaust valves.. The degree of angular displacement by checked using a manufacturer supplied poker gauge.
Chain damage occurs if the chain is too tight or too slack and the result is fatigue cracking of the links. If the tension is too tight, then this adds to the working stress of the chain. Insufficient tension leads to 'slap' with resultant damage to chain and rubbing strips. Vertical misalignment of the sprockets means rubbing at the side plates resulting in reduction of thickness and possible failure.
 a very simple chain link
Chain stretch and hence reduction in tension can be accounted for by movement of a tensioning wheel. The tension usually being checked by movement to and fro at the centre of the longest free length.
Max. is about 1 chain pitch.
Recommended limit on stretch is about 1.5 to 2%, if max. movement of the tensioned is reached before the chain has reached its max. stretch then a pair of links may be removed. When max. stretch is reached, or if the chain shows signs of damage then the chain should be replaced.
The simplest method is to break the old chain and attach the new chain to it. The engine is then turned and as the old chain is paid off, the new chain can be paid in. This maintains approximately the correct timing, the tension of the chain can then be set.
Final adjustment of the timing can be made following manufacturers instructions, this generally means turning the engine until No1 is at top dead, then checking by us of pointer gauges the position of the cam.
The cam drive is adjustable and can be slackened off, by hydraulic means on large modern engines, the section of cams can then be turned relative to the crankshaft angle and the timing restored.
The chains are lubricated by the injection of a jey of oil between the chain wheels and the chain rollers just before the rollers are about to engage the wheel. Thereby an oil cushion is formed to dampen the impact
A question asked by an examiner was to explain the polygon of forces with respect to chain drive. This refers to the forces acting on the chain links as they pass over the chain wheel Some of these forces are; Bending moment on the link as it travels around the sprocket
Stress changes on the link as it passes from the driving side to the driven
Tensioning of the chain dependent on the number of links between the sprockets varying i.e. related to the pitch
Centrifugal forces acting on the links


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