Piston Rod Runout

Piston rod runout is a measure of how well the cylinder is lined up with the frame and, also, how well the internals of the compressor (piston, rod, crosshead) are lined up with each other and the frame and cylinder combination. This runout is checked and adjusted with all parts "cold"; the ambient temperature should be in the 60°-68°F range.

Prior to performing runout measurements, cylinder and support, cylinder frame head, and doghouse nuts should be properly set or torqued. Oil scraper and packing should be loosened to prevent binding. Runout should be set during erection; followup measurements are needed whenever regrouting, major parts replacement, overhaul, etc., are performed.

The following procedure is a guide for obtaining and interpreting runout readings using dial indicators.

• Initial setup. Use a dial indicator with facilities to be securely fastened to a non-moving surface in or on the housing. The compressor piston is to be set at its maximum outboard stroke by barring the unit. The dial indicator traveling pin is to be positioned anywhere along the rod where it is convenient.

To measure horizontal runout, set the pin on the horizontal centerline of the piston rod. For vertical runout, the vertical centerline is used. Both vertical and horizontal runout are extremely important and both are to be measured.

When setting the traveling pin in the desired position, depress the dial indicator so the needle makes at least one revolution, lock the indicator neck, and set the dial face so that the needle reads zero.

• Reading the runout. Bar the unit over 360° (one complete revolution). The dial will go to a limit when the unit has been barred 180° (the piston should be at its maximum inboard stroke). This reading, whether plus or minus, is the total runout. The unit is then barred over the remaining 180° (piston is now at its maximum outboard stroke) to ensure that the needle returns to the zero setting. If the needle does not return to zero, redo the measurement after checking the indicator arrangement for tightness and making sure it is well secured at its base.

• Vertical runout. This reading generally indicates how well the compressor piston and rod are in line with the cylinder and frame. We like to see a cold runout of a minus on the piston end. The expansion of the piston while in service is thus anticipated.

• A positive reading means that the piston is high in relation to the crosshead, and a minus means that the piston is low, as desired, in relation to the crosshead. If the piston is high, or if it is lower than the desired limit, adjustment must be made in accordance with the compressor manufacturer's instruction manual.

Figure 4-10 shows allowable piston rod runout (vertical and horizontal) as a function of cylinder running clearance (Cooper-Bessemer). Cylinders may be operated if they fall within these limits. If, after a period of time, runout exceeds allowable, this is an indication that wear has occurred and maintenance is required.

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  • leda
    How are piston rod run outs measured?
    3 years ago
  • priscilla
    How to set bar end runout dial?
    3 years ago
  • Susanna
    How do check piston rod runout?
    3 years ago

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