Knotek: Measurements matter

In the world of mechanical industrial maintenance, one of the most vital concepts that is applicable to every operating machine is that measurements are extremely important. Measurements have a significant effect on the quality of the finished product, the quantity produced and the uptime of the machine. Everything that is manufactured is sized or has a dimensional tolerance target whether it is hot dogs or spaceship parts. When a maintenance technician is assembling, disassembling, maintaining or replacing components, taking measurements is imperative. Any machine Root Cause Failure Analysis (RCFA) process must include measurements, before, during and after the “failure.”

Dr. W. Edwards Deming, who is credited with starting the modern quality process, is legendarily quoted as saying “What gets measured gets done.” I think he may have borrowed on Lord Kelvin’s declaration, establishing absolute zero – “If you cannot measure it, you cannot improve it.” Or, you can go even further back to source the quote, to mathematician Rheticus. I generally agree with those statements, regardless of who said it first. But, learned people who understand business quality, management, marketing and productivity, such as Brian Strachman and the late Peter Drucker, caution us on the overreliance of measuring what doesn’t matter most. Multiple metrics taken at various times gives a balanced view of what is happening. These are the “key performance indicators,” or KPIs that allow management to change course, implement corrective measures and tweak the process. This strategy is most applicable to “A” criticality machines; if they stop turning, no widgets get made. In other words, don’t waste time on measurements that do not have a significant effect on the goal of safety, product quality, reduced operating costs or increased run time.

So how is taking measurements relevant to the average maintenance mechanic who is tasked with repairing broken machines? The mechanical world of industrial maintenance is full of opportunities to take measurements that matter: shaft coupling alignment, lubrication, V-belt tension and alignment, and the setting of gearbox bearings to name a few. Let’s use the installation, maintenance, lubrication and RCFA of rolling element bearings as an example. Three important issues relating to radial bearing performance are dimensional tolerance, fit and clearance, all of which involve measurements.

Rolling element bearings are manufactured to extremely close tolerances. What is a tolerance? It is a plus or minus allowable deviation from a set standard. Most quality manufactured bearings that comply with RBEC/ABEC/ISO published standards are made to within tens of thousands of an inch. The allowable amount is determined in part by the class of precision (1, 3, 5, 7, 9) and the overall size. The inner ring inside diameter, width, face axial run-out and bore radial run-out are measured and controlled. The same applies to the outer ring. The raceway geometry is tightly controlled and the rolling elements can approach near perfection. The only thing more precise in today’s world is nanotechnology. Every bearing and every part of the bearing is measured redundantly, using different means. A 25mm bore class 9 ABEC ball bearing inner ring has a plus zero, minus .0001-inch tolerance. That is the definition of precision.

This is an example of where and when you do not have to waste time measuring. If an exact part number replacement made by one of the reputable bearing companies is used, you can trust these manufacturers to produce a precise bearing. They employ redundant measuring processes, using a variety of methods to gauge every component.

The process of fitting a bearing onto a shaft and into a housing is a procedure where taking measurements are relevant to the maintenance technician. When a bearing fails and is removed from the shaft and housing supporting it, a common practice is to clean up both. Typically, some type of crocus cloth is used to remove the obvious corrosion or smooth out the galled surfaces, which results in material is being removed. I’ve witnessed the worst-case scenario where files and wire wheels were used. The original dimensions of both the shaft and housing were altered as a result, thus the fit has been changed. When this is done multiple times over the lifespan of the machine, the bearing inner ring may loosen on the shaft and/or the outer ring may creep in the housing. The result is less desired bearing life and more downtime. Bad tribal knowledge such as punch pricking or knurling the surfaces only hasten the destruction.

So how do we prevent this from happening? By taking accurate measurements. Use the Machinist’s Handbook or a bearing manufacturer’s published recommendations for the proper size of the shaft and housing based on the application. This is designated by an alphanumeric shaft or housing fit designation such as K5, which is probably the most common recommended shaft fit in the world. The suggested amount of press/tight or loose/slide differs with the size of the bearing, load, the application and which ring is rotating. Take multiple measurements at a minimum of four locations on the shaft and in the housing. Have another technician check the measurements for accuracy. If the measured components do not fall within the acceptable recommendations, replace them. Failure to do so only results in the repetitive madness of run-to-failure and ancillary damage to other machine components.