Project Manager's Weekly- Finish

Edition 4-

The finish of the fastenings and crane does matter also. I have researched on paint and galvanised finishes to even normal factory standard finish. From a strength and preload standpoint the ideal steel fastener would have a plain black finish. This finish would be unacceptable as it corrodes easily. The common solution is to apply a zinc or cadmium plating to prevent corrosion, and apply a conversion coating such as chromate to keep the finish looking nice. The fastener is usually polished and chrome plated.

Plating causes problems with high-alloy steels due to something called hydrogen embrittlement, if appropriate measures are not taken after plating to "bake out" the hydrogen. This is especially true of chrome plating which tends to lock in the hydrogen. Plating does not adversely effect the mild steel used for fasteners.

For an emergency crane this would be especially important as it will be used in many weather conditions where corrosive forces are bound to take effect. The torque-tension relationship is greatly affected by plating due to its effect on the friction coefficient. Cadmium plating reduces the friction by 25% and zinc plating increases the friction up to 40%. This requires a corresponding 25% reduction or 40% increase in required torque for the same tension. The fasteners used for the crane should therefore have this finish applied so that friction is reduced and to ensure the longevity of the parts.

As for the actual crane body, the type of finish would be less of a worry. The main base of the design would need to be plated with the same finish as the fastenings so that the level of frictions and longevity would be of equal terms. Places like the base of the legs, and mechanical winch needs to be especially looked at, as the winch would need a lubricant of some sort to ensure it maintained operation after several times use. Something along the lines of WD40 would be a good idea although with the right surface finish expressed above this would seldom be needed.

Because the emergency crane would be used in areas where there are higher disaster rates and specifically third-world countries where the climate tends to be extreme. The type of finish describe above offers protection in temperatures you are likely to find in these places.