In a previous article, I discussed ways to reduce the recurring cost of hydraulic cylinder repairs. And one of the topics discussed was cylinder-rod buckling loads. In response to this article, many or our readers wanted to know how to deal with bent rods in a repair situation.
Checking rod straightness
Rod straightness should always be checked when a hydraulic cylinder is being re-sealed or repaired. This is done by placing the rod on rollers and measuring the run-out with a dial gauge (Figure 1). Position the rod so that the distance between the rollers (L) is as large as possible and measure the run-out at the mid-point between the rollers (L/2).
Figure 1. Checking rod straightness.
The rod should be as straight as possible, but a run-out of 0.5 millimeters per linear meter of rod is generally considered acceptable. To calculate maximum, permissible run-out (measured at L/2) use the formula:
Dealing with bent rods
In most cases, bent rods can be straightened in a press. It is sometimes possible to straighten hydraulic cylinder rods without damaging the hard-chrome plating, however if the chrome is damaged, the rod must be either re-chromed or replaced.
If a rod is bent, then it is wise to check actual rod loading against permissible rod loading based on the cylinder's mounting arrangement and the tensile strength of the rod material. The formulas and procedure for doing this are explained in detail on page 58 and 59 of Industrial Hydraulic Control. If actual road load exceeds permissible load then a new rod should be manufactured from higher tensile material and/or the rod diameter increased to prevent the rod from bending in service.
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