Question

Friction is extremely difficult to control. If excessively regulated, leads to exhaustive wear, while if loosely regulated, leads to ineffective maneuvers. As a mechanical engineer, you are to design a web handling device meant to transport the flexible films for monitor screens. Elaborate with the support of suitable diagrams the phenomena of wear that you should keep in consideration when designing the web handling device, and the sub- components associated with it. In your elaboration, at least provide three types of available wear in that web handling device to justify their differences

Answer 1

Engineers who build components to last are naturally concerned with wear. There are different types of wear, and some engineers may induce wear purposely for varying reasons. Three types of wear are abrasion, adhesion, and corrosion. Each type has inherent problems and benefits that can be affected by materials, lubrication, and surface finish.

Abrasive

There are two common types: two-body and three-body abrasion. Two-body abrasion refers to surfaces that slide across each other where the one (hard) material will dig in and remove some of the other (soft) material. An example of two-body abrasion is using a file to shape a workpiece. Three-body abrasion is where particles between the two surfaces remove material from one or both surfaces. The tumbling process is an example of this.

Tumbling involves using particles to sand and polish the surface of a part. The particles that cause abrasion are often called contaminants. Contaminants are anything that enters a system that creates abrasion. While lubrication is imperative, an active lubrication system can introduce contaminants that cause abrasion. Filters remove contaminants and are one of the reasons proper maintenance and replacement of filters is important. However, the lubrication, or the additives in it, can react with the metal, creating a thin monolayer of contaminants that also make proper lubrication selection important to reduce wear to your equipment.

Abrasive wear can have benefits, such as water jetting. Water jetting has the ability to cut through metal with relative ease. This can reduce property changes that can occur with other processes that generate excessive heat while cutting.

Surface roughness is another important variable for wear. Two-body abrasion is reduced by having smoother surface roughness. For example, a journal or sleeve bearing made out of a softer material will slide against a harder drive shaft with little to no abrasion due to the surface finish. Using materials with similar hardness is generally not advised. The reason for the softer bearing material is to further reduce wear. Contaminants can become embedded into the softer materials and stop three-body abrasion from occurring. This technique might damage the bearing, but is preferred as it is designed to be relatively easy and more cost-effective to replace than a drive shaft. The rougher surfaces can increase the coefficient of friction and micro-peaks can break off, contributing to contaminants that are related to abrasion.

Adhesion

Surface roughness also contributes to adhesion. For this type of wear a material’s compatibility will be important. Compatibility does not mean materials that work well together; rather, that the materials “like” each other, causing them to stick together. This compatibility forms a bond causing parts to seize and even become cold-welded together. There are a few general rules to follow for material selection to make sure unwanted adhesive wear doesn’t occur. Materials that make contact with one another, in general, should:
.Not dissolve in the other

• Not, in given environment and other conditions, form into an alloy

• Not be identical (e.g., an aluminum shaft with an aluminum bearing)

• Have at least one metal from the B-subgroup (e.g., elements to the right of Nickel, Palladium, and Platinum on the periodic table).

Adhesion is possible to calculate. The adhesion and abrasive wear calculations share the same formula; however, it can vary by as much as +/-20%. This inaccuracy is due to constant changing surface conditions and lubrication during operation. It may be better than no data, but designers need to be aware of the limitations and accuracy of the formula. Trying to calculate or predict wear is made more difficult if components have non-conforming geometries, such as when gear teeth and cams are involved. These components can have difficulty staying properly lubricated. To reduce adhesive wear, sometimes corrosive wear is purposely induced.

Corrosive

Chlorides, phosphates, or sulfides can be added to induce corrosion and reduce a more destructive adhesive wear. Corrosive wear is more often thought of as something you want to prevent. Rust, or oxidation, is the No. 1 form of corrosive wear. Lubrication, material selection, surface finish, including coatings—like in abrasive and adhesive wear—are the main factors to consider.

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