July 2012 Electrical Apparatus

July 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus July 2012 featured technical article,  by Richard L. Nailen, P.E.  

Assemblies of terminal connectors or busbars commonly require bolted joints, which must be tightly compressed to minimize contact resistance and the associated heating and voltage drop. Initial bolt tightness can be achieved using a calibrated torque wrench.

However, such connections may loosen in service for several reasons. One is the expansion and contraction of the joined members during heating and cooling cycles. Vibration, impact, and corrosion can also cause the clamping force to diminish. Joints involving aluminum conductors are especially troublesome because compressive forces cause the metal to exhibit shrinkage (cold flow) regardless of temperature.

Periodic re-tightening of bolted joints can be impractical or even unwise. Fastener threads may be damaged in the process. Including conical spring Belleville washers in the bolt assemblies offers two benefits. First, because the spring can be compressed flat under a known tightening force, measurement of bolt torque becomes unnecessary. Second, the spring action of the washer will maintain clamping pressure on the joint despite dimensional variations caused by thermal cycling or cold flow.

Opinions differ as to whether bolts should be initially tightened to flatten the washers, then left in that condition, or that initial tightening torque should be slightly reduced to allow for joint expansion. The manufacturer’s recommendation should be followed. Experience can dictate future practice.

To facilitate coordination between washer flattening force and bolt tension limits, multiple washers can be used in various combinations, provided that only a washer’s concave side is in contact with the joint members. Many Bellevilles have been mistakenly installed upside down, resulting in loss of much of the expected clamping action.

Washer material (usually spring steel) should suit the environment. A protective surface coating may be advisable. If hydrogen is present, electroplating should be avoided, because that can result in embrittlement causing the washer to break apart.

To order a back issue with the full article, “When, and how, to use Belleville washers” call 312-321-9440 or visit our online webstore.

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