Page 308 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 293

bond strength over conventional metal sprays and is less likely to "flake"
off in service.

Chrome Plating

This technique is used to restore piston rods to their original diameter,
but its success depends on the quality of the plating, which varies enor-
mously depending on the plater. Do not use this method unless the quali-
ty of the plating produced by a vendor is truly known.
The only chrome plating approved for piston rods is porous-chrome,
which is a reverse etching process. The depth of the pores in the chrome
produced by reverse etching is on the order of .OQ2"-.003" deep, and the
plating has approximately 40% porosity. Porosity on piston rods
enhances the ability to hold oil on the rod surface and provide adequate
lubrication.
Mote: Do not plate piston rods with hard chrome such as used on auto-
mobile bumpers.
For best results, the finished plating should have no more than .010"
radial thickness. Thicker plating is more susceptible to failure and should
be avoided. The finish on the rod diameter prior to plating should be 32
RMS or better to assure as smooth a plating as possible. When plating a
rod, it is important to avoid abrupt steps from the undersized area to the
adjacent area. Two basic precautionary procedures are possible:

1. Grind the entire rod undersize and plate on top of this undersized
area, allowing plate to fade to the undersized diameter outside the
packing travel area. In this case, it is necessary to polish the ends of
the plating to assure proper smoothness.
2. Grind a groove to the undersized diameter, then plate back to the
finished size. In this case, the ends of the groove should be reduced
to minimize stress.

Generally, the second method is not preferred because of the difficulty
in assuring that the final grind will be absolutely concentric to the origi-
nal rod diameter. It may, however, be preferred in situations where
greater than recommended plate thickness must be used.
With either method, after grinding undersize and prior to plating, the rod
should be carefully magnaflux inspected for grinding heat checks or other