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Rod motion (figure 1) is typically gathered using a proximity probe to measure the degree-by-degree gap between the compressor frame, and the piston rod as it travels from TDC to BDC and back. Most useful information comes from the probe mounted in the vertical axis between the pressure packing case, or cylinder crank-end head, and the piston rod. The probe must be mounted using a rigid mount free of any low frequency resonances. In some cases, additional information may be beneficial from a probe mounted in the horizontal axis, i.e., for vertical compressors.
The following is a list of information that can be obtained:
Rod Drop is how much the gap has changed over time. This requires a permanently mounted probe. The things that can wear and affect this gap are the cylinder bore, the piston or rider band, the crosshead slide and the crosshead shoes. This can be accomplished by recording the instantaneous gap at a particular crank-angle degree when it is assumed that the vertical movement would be at its minimum. The other method is to average the gap for the full crankshaft revolution.
Rod Runout is the degree- by-degree measurement of the rod motion data for 360 degrees of crankshaft rotation. This information can be obtained from a permanently mounted or temporarily mounted probe. Everything that the proximity probe sees is displayed (maximum peak to peak movement, alignment issues, surface wear and irregularities, and coating and machining processes. It is a valid assumption that the piston stays in contact with the bottom of the cylinder bore. The crosshead, however, may move up and down (figure 2) so the most significant and gradual changes in rod runout over time are due to cylinder or piston surface wear or from an increase in crosshead shoe clearances. A rapid change in rod runout may be from a lubrication issue or mechanical component failure and should necessitate action to stop the machine.
Cylinder Alignment is the comparison of the measured gap at TDC and BDC piston positions. This information can also be obtained from a permanently mounted or temporarily mounted probe. The information gathered will be very similar to that obtained with a dial indicator when making manual cylinder alignment checks. A reasonable assumption is that the piston and crosshead will be fully sitting on their supporting surfaces at this point. Careful consideration must be made when establishing the display scales so as not to get confused about which is higher (cylinder or crosshead) as indicated in figures 1 and 2.
Rod Wear is the degree-by-degree comparison of the vertical probe gaps from two vertically mounted probes (top and bottom). I have had reasonable success using a single portable probe and taking both a top and bottom runout measurement and bringing the TDC ends of the two data points together and comparing the mirror images. The dynamic measurements agreed usually within .001 inch with micrometer measurements.
Surface Irregularities
Gap changes because of rod surface wear, scratches, machining or coating processes.
Happy monitoring.
Regards,
Warren
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