Page 310 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 295

make use of the higher stress levels possible with rolled threads and the
unauthorized use of cut threads could cause premature failure of the rod
with disastrous results.
If cut threads have been deemed adequate on a given job, careful man-
ufacture is still essential. The tool bit must be properly ground, so that no
pulls or tears occur. It must be correctly rounded at the bottom or root of
the thread. Obviously, the pitch diameter must be proper.

TIGHTENING PISTON NUTS ON PISTON RODS

It is important to tighten the piston nut to the proper torque to prevent
loosening of the piston on the rod. Loosening could result in damage to
the piston, or could cause stretching of the piston rod at the thread end
beyond the elastic limit of the material.
Before tightening is attempted, an adequate means of holding the pis-
ton rod/piston assembly must be available. Figure 4-36 showed one such
arrangement with V-blocks to hold the rod.
If the piston rod has been coated or plated, it must be held in the area
that has not been coated. The stress caused by tightening of the V-blocks
could easily cause cracking of the coating or the plating.
In addition to adequate V-blocks to prevent the piston rod from turning
when tightening or loosening the piston nut, the piston must be properly
supported to prevent bending of the piston rod.
Caution: Never attempt to tighten a piston nut with piston rod assem-
bly in the cylinder and fastened to crosshead. Always tighten the piston
nut before installation of assembly in cylinder.
Again, under no circumstances should a pipe wrench be used to hold
piston rod. Note also that copper-based compounds (such as Pel-Pro)
should never be used on threads since galling may result from this practice.

TORQUE VALUES FOR PISTON NUTS

Proper tightening of piston nut to the piston rod includes torquing the
nut to the proper value. Approved torquing procedures include the use of
a torque wrench or the "stretch" method.
Table 4-8 includes torque values for various size piston rod thread
sizes. Instructions for tightening and determination of pre-stress value
were given earlier in this chapter (see section on pistons).