Services

BENT ROD REPAIR

Bending of cylinder rods (bent rod) may be caused by various direct and indirect factors. They include, but not limited to:

insufficient rod diameter
material strength
improper cylinder mounting fitting

In certain cases it may be a combination of all of the above.

Once the rod has ‘identified’ bends or imperfections ( i.e. 'bent rod'), excessive loads placed on the rod seal may result in premature failure of the seal. At the very least the unit may be to be unreliable in the future.

Rod straightness is always checked by Hydraulic RAM Repairs UK Ltd when a hydraulic cylinder is being repaired at our workshops. To test for straightness, the rod is positioned on calibrated rollers and we measure the run-out using a dial gauge. The rod is positioned so the distance between the rollers (L) is as large as practical, then we measure the run-out at the midpoint between the rollers (L/2).

The rod should be as straight as possible. In practical terms, a run-out of 0.5mm per linear meter of rod length is generally considered acceptable. To calculate maximum, permissible run-out (measured at L/2) we use the following formula:

Run-out max. (mm) = 0.5 x L ÷ 1,000. L equals the distance between rollers in millimeters. An example. If the distance between the rollers is say 1.3 meters, then the maximum, allowable run-out measured at L/2 would be given by 0.5 x 1,300 ÷ 1,000 = 0.65mm.

In many cases, bent rods can be straightened in a press. It is sometimes possible to straighten 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 determined ‘bent rod’ i.e. outside of acceptable tolerance, the actual rod loading should be examined against permissible rod loading based on the cylinder’s mounting arrangement and the tensile strength of the rod material.

Following testing, if rod load exceeds permissible load, and repair is not possible or economically viable then we recommend a new replacement rod is fitted using higher tensile material. We also recommend consideration is given to the use of higher rod diameter to prevent the rod from bending whilst in service. Information will be provided on request.

Degradation of Cylinder Tubes

Degradation or ‘ballooning’ of the cylinder tube is typically caused by insufficient wall thickness and/or material strength for the cylinder’s operating pressure. Once the tube degrades, the correct tolerance between the piston seal and tube wall will be lost and high-pressure fluid may by pass the seal. This high velocity fluid may well erode the seal, and localized heating caused by the pressure drop across the piston may significantly reduce seal life.

Tube Integrity Testing

The normal way of testing the integrity of the piston seal in a double-acting cylinder is by pressurizing the cylinder at the end of stroke and measuring any resulting leakage that passes the seal. This is commonly referred to as an end-of-stroke bypass test.

Insufficient Bearing Area

If the internal bearing areas in the gland and at the piston are insufficient to carry the torsional load transferred to the cylinder, excessive load is placed on the rod and piston seals. This results in deformation and ultimately premature failure of the seals.

Rod Finish

The surface finish of the cylinder rod can have a significant effect on the life and efficiency of the rod seal.

For example, if the surface roughness is too low, seal life can be reduced through inadequate lubrication. If the surface roughness is too high, contaminant ingression of ‘foreign objects’ is increased and an unacceptable level of leakage can occur.

With regard to of extending the all important cylinder service life, consideration should be given to the surface of the cylinder rod as a ‘lubricated wear surface’ and treat it accordingly. Hydraulic RAM Repairs UK Ltd will be pleased to offer advice on bent rod problems.

Click here for information on alternative Rod Finishes