You Can Learn a Lot from a Filter

Don Searles, Machinery Maintenance for Extended Life
Tags: contamination control

From 1962 to 1970, I worked for two companies: for a Caterpillar dealer as a field service mechanic, and for a highway contractor as a master mechanic. In those positions, I was often called upon to troubleshoot a machine when a front-end loader or bulldozer was slow to raise. These devices are raised by hydraulic cylinders that are part of the machine’s hydraulic system. The problem could have various causes: a worn pump, broken springs that hold the vanes against the cam-ring in a vane pump, oil leaking internally past the cylinder packing, a stuck or leaking valve, a broken spring that held a valve closed, and many others.

One of the first things I did was to remove the hydraulic filter cartridge. I could immediately see if the cartridge was collapsed or damaged. This gave me a general idea as to how the machine was being serviced. I would then cut the element open with a hacksaw. I would lay out the pleats of the element on a table and look at the debris. As a rule of thumb, if the hydraulic pump was badly worn, the filter would have a lot of visible debris. A magnet would be used to help identify the metallurgy of the debris. This assisted in determining its source within the system. A magnifying glass was used to assist in identifying the remaining debris and in determining its source. Sometimes I found pieces of a broken spring or other parts of the system that could be identified. Other times I found pieces of cylinder packing, which would indicate the cylinders were the problem. In more than 80 percent of the cases I witnessed, the filter would provide clues that led to the problem.

"The next time you replace a filter, cut open the used one, look for debris and try to identify the debris. Also look closely at the construction of the filter."


Once I was called to listen to a knocking noise that would come and go in the engine of a Caterpillar D8-H bulldozer. The machine was still under factory warranty. Another Caterpillar mechanic listened to the noise and did not know what to do. When I arrived, the customer, the dealer and the Caterpillar mechanic were discussing the source of the noise. The opinions varied from a wrist pin knock, to a piston slap, to a rod bearing out, to “it’s nothing, don’t worry about it.” When I listened to the engine, the knock was hard to distinguish above the normal diesel engine noises. I could, however, hear a sporadic knocking noise. With the noise being the only concern, a decision to disassemble the engine could not be made. I then cut open the filter and found sufficient debris to justify disassembling the engine. When the engine was disassembled, one of the piston wrist pin bushings was found loose in its bore, and showed signs that it had been moving in and out of the bore as the engine ran. This explained why the knocking noise would come and go. After the engine inspection, and with the exception of the gaskets, the connecting rod, wrist pin and wrist pin bushing were the only parts replaced.

Since learning this lesson, I have continued to regularly cut open filters, even when a problem is not indicated. For years, I cut open filters with a hacksaw. I recently found that Caterpillar offers a nice tool for this. The tool is well-made, is similar to a pipe cutter and sells for $85. It will cut open filters from 3 to 6-1/2 inches in diameter. In my opinion, it is well worth the money.

I recently cut open a spin-on diesel engine oil filter. This filter was purchased at the largest auto parts dealer network in the United States and it was made by one of the leading filter manufacturers in the U.S. The filter consisted of a steel headpiece with threads, so it can be screwed on the engine; a steel can crimped to the head; a separate cartridge inside the can and a spring at the bottom of the can which holds the cartridge against the head. The cartridge has two metal ends affixed to a metal center spool. The cartridge was not bonded to the head and was held against the head only by the spring. This permitted the steel end of the cartridge to move against the steel headpiece during operation. Movement occurred and metal was worn away. The wear metal was on the clean side of the element and entered directly into the engine. With some filters, the filter must be added as a source of wear metal. I will not use this brand of filter in the future.

The next time you replace a filter, cut open the used one, look for debris and try to identify the debris. Also look closely at the construction of the filter. You may be surprised at what you have protecting your expensive machinery.

You can learn a lot from a filter.

About the Author