For plants which have many hydraulic systems, hydraulic cylinder services and maintenance expenses take up a significant chunk of the plant maintenance budget. A good number of breakdowns in hydraulic cylinders are caused by improper selection of hydraulic cylinder designs.
In such cases, while the cylinder will perform the required job satisfactorily, it won't give you an acceptable service life compared with a cylinder that was designed for that specific application. The following are some design-related reasons behind frequent hydraulic cylinder breakdowns and how they can be addressed.
1. Bent rods
Cylinder rods may bend over time because of incorrect mounting arrangement, inadequate rod diameter and/or poor material strength. When the rod is bent, too much weight is placed on the rod seal, which culminates in the seal failing prematurely.
During hydraulic cylinder service, the rod should be checked for straightness by placing it on rollers and using a dial gauge to quantify the run-out. Place two rollers as wide apart as possible underneath the rod and then take the run-out measurement at the mid-point between the two rollers. It is acceptable to have a run-out of 0.5mm/metre in linear rods.
If the rod bends beyond the acceptable threshold, use manufacturer's recommendations to evaluate the real rod loading vis-à-vis permissible rod loading, considering the rod material strength and cylinder mounting arrangement. If the real load exceeds the allowable load, you'll need to manufacture a new rod from a material with higher tensile strength or increase the rod diameter to prevent bending in future.
2. Ballooning tubes
Cylinder tubes may balloon because of having inadequately thick walls and/or using materials with lower tensile strength than the cylinder's operational pressure requires. When cylinder tubes balloon, there is loss of tolerance between the tube wall and piston seal which means that high-pressure/high-velocity fluids will erode and eventually break the seal. The fluid causes localized heating around the seal attributable to the drop in pressure across the tube, which in turn reduces the usable life of the seal.
3. Inadequate bearing area
This happens where the inner bearing areas within the gland and on the piston are not enough to bear the torsional load weight transmitted to the cylinder. As a result, this places excessive weight on the cylinder rod and piston seals causing gradual deformation that culminates in premature seal failure.
4. Rod surface finish
Surface finishing on the rod directly impacts the rod seal life: too smooth and the seal lifespan is reduced by insufficient lubrication; too rough and there will be an increase in contaminant ingression causing higher-than-acceptable leakage levels.
To extend hydraulic cylinder service life, think of your cylinder rod surface like it's a lubricated wear surface, and preserve it as such. In some cases, using surface finishes with superior mechanical properties will increase the usable life of both seal and rods. For instance, you can replace the traditional chrome plating with high-velocity oxygen fuel or black nitride.