The piston rod (Figure 300-18) is a principal component in the running gear. The primary design factor regarding piston rods is a phenomenon loosely called “rod loading” described in detail in Section 351. As the rod is subjected to high alternating stresses (compressive and tensile), its surface finish must be extremely smooth to avoid stress intensification, which could lead to fatigue failure. The threads on the rod where it attaches to the crosshead are critical for the same reason.
For hardness and surface finish specifications for commonly used rod materials, refer to API 618 and/or Section 800. For clean non-corrosive gas service, AISI-4140 carbon steel is a typical rod material. For this material, guidelines for the allowable stress at the root area of the threads at the crosshead are:
Thread Type Stress, psi
Cut threads have a higher stress intensification factor.
These guidelines may be used by dividing the vendor’s maximum allowable continuous rod load rating (in pounds) by the thread root area (square inches). Obviously, if the rod material is not 4140, the guideline stress values should be adjusted by the ratio of tensile strengths of the two materials. Rolled threads are preferred for all applications, but may not be available on some of the smaller machines.
Some new piston rods are provided with metal coatings for certain services. Metal coatings are also used to recondition rods. A summary of Company experience with piston-rod reconditioning is included in Section 830.