21 Finally, although not a design parameter, the subject of bolt installation clamp load must be addressed. It is a fundamental engineering concept that the force in a bolt in an ideal preloaded joint will remain equal to the clamp load until the externally applied force exceeds the clamp load. Then the force in the bolt will be equal to the external force. This means that fluctuating external forces will not cause fluctuating forces in a preloaded bolt as long as the clamp load exceeds the external force. The result is that fatigue failure will not occur. In a non-ideal joint, such as in a connecting rod, the bolt will feel fluctuating stresses due to fluctuating rod distortions. These are additive to the clamp load, so that fatigue could result. In connecting rods, precise clamp loads are required because if they are too low, the external forces (the reciprocating weights) will exceed the clamp load, thus causing fatigue. If they are too high, they provide a high mean stress that combines with the fluctuating stresses due to rod distortion. Again, fatigue is promoted. The objective, then, is to preload a bolt so that it just exceeds the external load, and no higher. To sum up: both insufficient and excessive clamp loads can lead to fatigue failures. Appropriate clamp loads are specified for each ARP bolt. These clamp loads can be attained in a connecting rod by applying proper torque using a torque wrench or by measuring the amount of stretch in the bolt using a stretch gauge (it is known that a bolt stretches in proportion to the tension in it). The torque method is sometimes inaccurate because of the uncertainty in the coefficient of friction at the interface between the bolt and the rod. This inaccuracy can be minimized by using the lubricant supplied by ARP. Other factors, equally as important as design, include material selection, verification testing, processing, and quality control. These aspects of bolt manufacturing are discussed elsewhere in this document. The foregoing discussion concentrated on the design of bolts. The same considerations apply in the design of studs. Motorsports Fastener Engineering for the Non-engineer FASTENER TECH
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