Wear of Equipment

1Q. What is wear?

1A. In its simplest definition it is the unwanted removal of material. Unwanted is used because many times metal removal is desirable such as with metal removal machining or self adjusting abrading seals. Wear is a process which can take on many forms, some of the more common being adhesion, abrasion, and corrosion. These can be broken down further. For example fretting is a form of corrosion and galling is a form of adhesion.

2Q. What are some important things to know about wear?

2A. Unwanted wear takes place in most equipment and it can be reduced or prevented in the design stage by the correct selection of hard surfacing materials. The in-service life can many times be determined and extended by analytical modeling and the subsequent selection of materials, load reduction, or other factors included in the modeling process. Modeling the wear process in the design phase is the most cost effective procedure. For example, extruders used to form plastic pellets have been analyzed for the screw separating loads. Wear calculations were then performed and the high wear regions determined. These selective surfaces could then be protected with a wear resistant coating. When wear history on a given extruder is known this information can be used to make the analytical model even more valuable. Wear history simply means how much the barrel or the screw has worn in such a time. An example would be to express the screw wear as 0.125 inches wear on the diameter in 2 years. This might be due entirely to start ups and shut downs or to continuous operation or to both. Which of these produces the wear could also be determined. This same process can be used for examining equipment such as automotive cylinders, excavating equipment tools, sleeve bearings, clutches and most any other equipment with rubbing surfaces that wear. One potential savings is of not having to buy new parts, however eliminating or extending repairs to a planned downtime can save much more by reducing lost production costs.

3Q. What is the harm with wear of a gear type coupling and what causes it?

3A. Gear type couplings are used to allow some misalignment between say a pump and a motor or a motor and a gearbox. As long as they remain lubricated and within their alignment capability of angular and off set they work fine. The problem usually occurs when the lubrication breaks down. When this occurs rubbing contact can occur and the teeth can wear away or "lock-up". When this occurs high moments and forces occur on the connecting shafts. The result is usually fatigue failure of the shafts or failure of the bearings. Non lubricated couplings, such as diaphragm, disk pack or elastomer are usually used in very dirty environments or when there is no opportunity to lubricate the couplings, as in large process centrifugal compressors and turbines.