Typically, the slicker the lubricant, the greater the "preload scatter" or preload error there will be during installation. Slicker lubricants may reduce the required torque by as much as 20-30% to achieve the desired preload, but compromise in areas of major importance such as preload repeatability, and may yield the fastener prematurely. In fact, ARP's in-house Research and Development department has proven that new fasteners using motor oil and other commonly used lubricants such as Moly and EPL typically require 5-7 cycles before final torquing to level out the initial friction and achieve the required preload. This "friction" inhibits the fasteners ability to achieve the required preload on the first several cycles. As discussed earlier in this section, friction is at its highest point when a new fastener is first tightened. One of the most overlooked aspects of choosing a fastener assembly lubricant is…the lubricant's ability to "control" the normal function of friction inherent in all high performance engine fasteners. The main factor in determining friction in a threaded fastener is the lubricant used, and therefore influences the torque required for a particular installation. Since most fasteners are installed blind and can't be accessed from both ends to monitor stretch, one will most likely use a torque wrench or other torque angle monitoring device for the majority of assembly work. However, since stretch can only be measured with the use of specialty type gauges or expensive ultra sonic measuring equipment, it is only practical for measuring the stretch on connecting rod bolts and other fasteners, where it is possible to monitor the overall length of a fastener, as it is being tightened. Of these methods, measuring the amount of stretch of a fastener has been proven to be the most accurate.
The importance of tightening fasteners to their required preload cannot be emphasized enough. To sum up: both insufficient preloads and excessive preloads can lead to fatigue failures. The objective, then, is to preload a bolt so that it just exceeds the external load, and no higher. If they are too high, they provide a high mean stress that combines with the fluctuating stresses due to rod distortion. In connecting rods, precise preloads are required because if they are too low, the external forces (the reciprocating weights) will exceed the preloads, thus causing fatigue. These are additive to the preload, so that fatigue could result. In a non-ideal joint, such as in a connecting rod, the bolt will feel fluctuating stresses due to fluctuating rod distortions. The result is that fatigue failure will not occur. This means that fluctuating external forces will not cause fluctuating forces in a preloaded bolt as long as the preload exceeds the external force.
Then the force in the bolt will be equal to the external force. It is a fundamental engineering concept that the force in a bolt in an ideal preloaded joint will remain equal to the preload until the externally applied force exceeds the preload. A typical bolt configuration is shown below.įinally, although not a design parameter, the subject of bolt installation preload must be addressed. This further increases the fatigue resistance of the bolt. Therefore, when the bolt bends due to deformation of the connecting rod, the bending stresses are reduced below what they would otherwise be. It reduces the bending stiffness of the bolt. The reduced diameter shank is helpful in another sense. Thus this configuration represents the optimum with respect to fatigue strength. Providing a local fillet radius at the location of the maximum stress further reduces the local notch effect.
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This leads to the standard configuration used in most ARP rod bolts: a reduced diameter shank and full engagement for the remaining threads. By removing the remaining, non-engaged threads, the local notch effect can be reduced. It is well known that the maximum stress in an engaged bolt occurs in the last engaged thread. In bolts, this would correspond to the notch effect associated with the thread form.
Fatigue failure is frequently caused by localized stress risers, such as sharp corners.
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