Re: Locknuts on the rod bolts

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Posted by TheOldHokie on January 16, 2019 at 10:30:57 [URL] [DELETE] :

In Reply to: Re: Locknuts on the rod bolts posted by Jim on January 16, 2019 at 08:46:10:

Short answer is yes. Long answer is more complicated.

Torque values are a crude measurement of the tension (clamping force) produced in the fastener. It is this tension and the friction it creates that keeps the fastener from working loose. This tension is typically chosen to be 60-75% of the fastener proof load. The torque needed to produce this specific tension is then calculated using an equation that contains a "K factor". The K factor represents the loss in tension attributable to friction between the threads and head of the fastener. This loss is typically 75-90% of the applied torque!!! So anything that chances that friction can substantially alter the torque needed to produce the desired tension.

Most automotive service manuals specify torque values calculated assuming oil lubricated threads. According to Henkel's technical data sheets Loctite 243 has the same K factor as engine oil so no adjustment would be needed. On the other hand an inserted or deformed thread lock nut will have a higher K factor and require a small (WAG 5%) increase in torque in order to obtain the same clamping force.

Keep in mind that torque wrench measurements as a means of measuring tension have an accuracy of +/- 25% to begin with so tiny variations in K factor become academic in most applications. Be more concerned when using torque values for dry versus lubricated threads where the difference in K factor becomes significant.


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