Critical alignment really is critical

Why designers, engineers and end-users should not accept rod seal failure as normal

By Bill Morse, TRD Manufacturing, a Bimba Company

A critical design feature of any mechanism where moving elements interact or work in tandem with each other is the alignment of those parts in relationship to one another. Design engineers commonly specify and make notation on design prints concerning the machining tolerances of those parts and mechanisms. Often, when a manufacturing company has a part or assembly made by an outside source, the vendor is required to adhere to the tolerances specified. If those tolerances are not adhered to, the overall machine usually will not perform or yield the life expectancy it was designed to offer.

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It is troubling that many design engineers do not concern themselves with tolerances associated within a purchased assembly of moving parts as long as that assembly interfaces dimensionally with the rest of the machine or mechanism being produced by their company. It is assumed that the manufacturer of that supplied assembly is producing a quality product with a life expectancy adequate to match the cost of ownership profile that the design engineer places on the machine or mechanism.

Such is the case with hydraulic and pneumatic NFPA tie rod cylinders. These mechanical assemblies all adhere to the same exterior dimensions and are dimensionally interchangeable from one manufacturer to another. Therefore, very little attention is given to the machining tolerances held by the cylinder manufacturer in implementing the cylinder into a machine. The NFPA cylinder is such a mature product in the marketplace that it is commonly selected by comparing price, while service or lead-times (delivery expectations from time of order) is the secondary issue. It is our experience that because the cylinder manufacturer is reputable and has been in business for many years, its cylinder design and the internal tolerances used are acceptable and the cylinder produced will function adequately within the designer's overall machine or mechanism.

Within an NFPA cylinder, all the machining tolerances associated with the alignment of the piston rod in relationship to the rod bearing (or bushing) are critical to not only the interface dimensions of the cylinder, but also the operation/performance of the cylinder and most importantly, the life expectancy. If you ask a hydraulic cylinder user, “what is the #1 point of cylinder failure that causes you to either repair or replace the cylinder?” the most common answer is “rod seal failure”.

In most cases, this failure can be greatly reduced by maintaining better tolerances when machining components associated with the centerline of the cylinder assembly. The reality is that tight tolerance components are more expensive to make, as one would expect. Therefore, some NFPA cylinder manufacturers have designed their way around the need for tight tolerance machining by utilizing a softer bearing material such as bronze, that in most cases can be seen as a good bearing, but is inherently known as a “wear material”. As a result of the piston rod moving back and forth within the rod bearing where the critical alignment is given the highest priority, the bronze material wears itself in to match the alignment of the rest of the cylinder parts. This wear is a predominant factor in the premature failure of the rod seal. A preferred design remedy is to use a harder, more precisely machined and located rod bearing which will hold the piston rod in proper compliance, providing greater protection for the rod seal.

As a result of hydraulic (and pneumatic) cylinders being designed and used that way for so many years, many designer engineers and end-users accept this reduced life (rod seal failure) as normal. Therefore, it is vital to resist the urge to apply the “commodity” tag to engineered devices or components such as NFPA cylinders; even though they may all have the same external dimensions, all NFPA cylinders are not created equal.

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