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What is Bumping Clearance And Its Importance?

Bumping clearance is the clearance between piston top and cylinder cover with piston at TDC position. Bumping clearance is checked to ensure efficient and safe operation of compressor.

  • Too small a bumping clearance may result in piston bumping against cylinder cover resulting in sever damage.
  • To large a bumping clearance would mean that more high pressure air trapped above piston at end of compression or delivery stroke has to be expanded back to lower pressure before fresh air from suction valve is introduced. Thereby reducing net air introduction in cylinder each stroke. Hence volumetric efficiency decreases, therefore less air throughput, more running time of the compressor.
Bumping clearance is optimized, normally it is 0.5% cylinder bore.

How to Adjust Bumping Clearance:
Practically , in multistage single crank air compressor, bumping clearance is adjusted by:
  1. LP stage: Adding or removal of shim/ gasket of varying thickness between cylinder cover and cylinder block.
  2. HP stage: Adding or removal of foot liners of varying thickness to palm of connecting rod. After this, LP stage bumping clearance is checked and adjusted accordingly by method given in point 1 above.
Addition  or removal of shims of foot liner only for adjusting bumping clearances is not enough. In multistage single crank air compressor employing tandem piston, if LP stage bumping clearance is reduced by addition of foot liner to connecting rod, it will increase HP stage bumping clearance.

Similarly if HP stage bumping clearance is reduced by removing foot liners from connecting rod, LP stage bumping clearance will increase.

Bottom end bearing adjustment does not affect bumping clearance. Piston rests on top half of bottom end bearing, when at TDC. Addition of shims, to adjust bearing clearance does not effectively lift up the piston. Thereby, not affecting bumping clearance.


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