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Reciprocating Machines


The most common types of reciprocating machines are piston pumps and compressors and internal combustion engines. In all these machines, the piston rate (usually 1X) is dominant, along with the firing rate for 4-cycle engines. Vibration levels as high as 125 VdB (1.0 inches per second peak) are not uncommon for healthy machines such as these. The analyst must judge the machine condition by comparison to previous levels rather than applying absolute reference levels.

Many reciprocating engines have turbo chargers, and they are diagnosed like other rotating turbines and compressors. Camshaft gear problems are also common, and can be seen by looking for the tooth mesh frequency. If the engine has a torsional vibration damper on the shaft, it can fail, greatly increasing vibration at the frequency of the first crankshaft torsional vibration mode. This frequency must be obtained from the engine maker.

Variable displacement piston pumps are much smoother than compressors, and lend themselves well to vibration analysis. If harmonics of the piston rate are present in significant levels, it usually indicates a piston drive linkage problem. A very prominent tone at piston frequency fundamental may indicate a worn spot on the wobble plate.


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