21.1 Pin Breakage from corrosion fatigue
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When a load is applied to a chain the pitch hole clearance accumulates on the opposite side to the bearing surface of the pin and the inner wall of the chain bush. This area is thus exposed and is susceptible to corrosive attack if unprotected. The degree of corrosion will be dependent on the environment and the presence of water, acid based compounds or materials that produce acids when combined with water.
As with any structural member, chain pins will be subject to a bending moment under load. Under these conditions the surface of the opposite side of the pin is put under a degree of tension. If the pin has been pitted by corrosive attack each pit becomes a stress raiser. fig 21.1.2 illustrates how bending can eventually create fatigue cracks at the point of stress. These cracks can progress until the pin finally breaks during a load cycle.
Protective coatings such as water resistant lubricant or surface plating are often effective in preventing corrosive attack. Where conditions are severe ; corrosion resistant materials such as stainless steel offer the most effective solution.
Fig. 21.1.3 demonstrates an advanced stage of corrosive attack this depicts the entire process of corrosion fatigue, pitting, cracking and ultimately fracture. In this case the pin was operating in an environment where the product conveyed was of a corrosive nature. It is sometimes an option to reduce the corrosive impact of the material with neutralising additions. Lime is sometimes added to materials of an acidic nature to raise the PH value and reduce the corrosive impact.
To find the most effective deterrent for corrosion fatigue all aspects such as load, speed and material conveyed must be considered.
