One of our readers wrote to me recently about the following problem:
I am having problems with the boom on my crane creeping down. How do I
determine if it's the valve or cylinder that's leaking?"
A popular misconception about hydraulic cylinders is that if the piston seal is leaking,
the cylinder can creep down. Fact is, if the piston seal is completely removed from a
double-acting cylinder, the cylinder is completely filled with oil and the ports are
plugged, the cylinder will hold its load indefinitely - unless the rod-seal leaks. What
happens under these conditions - due to the unequal volume either side of the piston, is
fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs,
the only way the cylinder can move is if fluid escapes from the cylinder via the rod seal
or its ports.
In the above situation described by our crane owner, unless the cylinder is leaking
externally, the load or directional control valve is the cause of the problem - not the cylinder.
But as a product
group, hydraulic cylinders are almost as common
as pumps and motors combined. So if you operate a lot of hydraulic equipment, it's likely
that cylinder repair expenses are a significant portion of your operating costs.
It is often stated that up to 25% of mechanical equipment failures are failures of design. If we extrapolate this to hydraulic cylinders, as many as one in four hydraulic cylinders are not adequately designed for the application they are operating in. This doesn't mean that the cylinder won't do the job asked of it, it will - but not with an acceptable service life. If you have a particular cylinder that requires frequent repair, you may need to address one or more of these design-related problems:
Ballooning of the cylinder tube is usually caused by insufficient wall thickness and/or material strength for the cylinder's operating pressure. Once the tube balloons, the correct tolerance between the piston seal and tube wall is lost and high-pressure fluid bypasses the seal. This high velocity fluid can erode the seal and localized heating caused by the pressure drop across the piston reduces seal life.
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.
The surface finish of the cylinder rod can have a dramatic effect on the life of the rod seal. If the surface roughness is too low seal life can be reduced through inadequate lubrication. If the surface roughness is too high, contaminant ingression is increased and an unacceptable level of leakage can result.
In the context of extending cylinder service life, consider the surface of the cylinder rod as a lubricated wear surface and treat it accordingly. In some applications, the use of an alternative rod surface treatment with superior mechanical properties to conventional hard chrome plating, such as black nitride or High Velocity Oxygen Fuel (HVOF) metal spraying, can increase the service life of the rod and its seals. The installation of a shroud to protect the rod surface and seals from impact damage and contaminants can afford similar life extension benefits.
Bent rods are a common cause of rod seal failure. Bending of cylinder rods can be caused by insufficient rod diameter or material strength, improper cylinder mounting arrangement or a combination of all three. Once the rod bends, excessive load is placed on the rod seal resulting in premature failure of the seal. The permissible rod loading for a cylinder in an existing application can be checked using the Euler formula. A detailed explanation of how to do this is contained in Industrial Hydraulic Control.
Repair or Redesign?
Not all hydraulic cylinders are made equal. So if you have hydraulic cylinders that suffer recurring failure, it's likely that modifications to the cylinder are required to break the vicious circle of failure and repair.
"Thanks for the great work on the two publications, Insider Secrets to Hydraulics and Preventing Hydraulic Failures. I have been in the hydraulics business for the past 20 years and it is very difficult to find any decent material on hydraulic maintenance, troubleshooting and failure analysis. These two books cover it all in easy to understand language... I conduct hydraulic training courses and plan to purchase copies to distribute to my students to share your practical approach to understanding a not so understandable subject."
Paul W. Craven, Certified Fluid Power Specialist
Motion Industries, Inc.