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In a previous article, I suggested that assigning serial numbers to components and tracking them to identify “rogue” parts is not a good way to ensure reliability. Rogue components are defined as components or assemblies that have a shorter service life than original equipment manufacturer (OEM) components.
For example, bearing failures in pump rotating assemblies often result in damage to the housing bore that contained the failed bearing. Because pump bearing housings are expensive spare parts, this damage is often repaired. The repair process is usually to bore out the damaged surface, insert a sleeve and then machine this sleeve to the bore diameter recommended by the bearing manufacturer using a standard shop lathe. This type of repair usually results in one dimension (the bore diameter of the new sleeve) being within the required tolerances for diameter and cylindricity, but there are two other critical tolerances that may be one or two orders of magnitude less accurate than in the OEM part. These are the “parallelism” and “concentricity” tolerances between the centerline of the repaired bearing bore and the centerline of the other bearing bore, with other functional machined diameters (Figure 1).
Figure 1. Important tolerances when repairing housings with two bearing locations
It is virtually impossible to match the bore alignment achieved in the typical manufacturer’s line-boring machine by machining just one bore in a standard lathe. Such errors, while small, may impose stresses on the bearing’s running surfaces that will shorten their service life and may cause the repaired housing to be regarded as “rogue.”
As materials, lubricants and machining technologies have advanced, it has become possible to achieve high reliability in much smaller components. However, to maintain this reliability during shop repairs, the same standards that are followed in the OEM manufacturing process must be duplicated. These standards do not apply only to machining accuracy. Parts must be stored and handled with the same high standards.
It is not uncommon to see bearings stored with a damaged protective wrapping or be unwrapped and exposed to a dirty shop environment prior to installation. I once observed an experienced mechanic removing a 3-inch-bore, single-row radial ball bearing from its box by turning the box on its end about 8 inches above a steel workbench and letting the bearing fall onto the benchtop. The impact on the running surfaces probably caused more damage than many years of normal service. I’m sure that such handling was the result of a lack of training and probably also a lack of supervision. Mechanics, stores staff and any others involved should make a practice of handling bearings “like eggs” both before and after installation. Repaired equipment should be packaged, stored and handled as the OEM would.
The same principles apply to manufacturing spare parts, either in-house or by local shops. To safely replace OEM parts, it is necessary to understand why they were designed the way they were and exactly what the material is. OEMs normally do not provide this information. When a part is copied, critical dimensions can be duplicated, but it may be impossible to tell where these dimensions lie within the allowable tolerance, or if the dimensions of the copy will be within those tolerances.
In one classic case, a reducer shaft and pinion were copied by a reputable gear shop. It was assumed that the pinion should have an interference fit on the shaft. This incorrect assumption resulted in three breakdowns over an 18-month period with a production loss of more than $1.5 million. The pinion should have had a close sliding fit on the shaft so the large retaining nut would preload the pinion against the shaft shoulder, greatly increasing the assembly’s fatigue strength.
Not only is it necessary to understand the design when substitutions are made, it also is important to be familiar with the operating context of all the equipment in which the component may be used. For example, manufacturers do not use Viton seals just to increase their costs. They use them to ensure that their customers have reliable equipment. Replacing Viton seals with standard nitrile rubber seals can save money, but this should only be done with the knowledge that they can operate reliably in the environment and conditions to which they will be exposed.
Substituting OEM components with copied parts, parts with a different design or those constructed of different materials is a decision that should be made by someone with a technical understanding of the equipment and its operation. It should never be decided by a buyer simply because of a lower price.