Mark, The theory is that the co-efficient of static friction between two objects is usually slightly higher than the co-efficient of sliding friction. In general terms, what this means is that if an object is already moving, it requires less force to move than if the object is stationary. This means that if you loosen the nut, and then tighten it to 29 ft.lbs, it will take a little more than 29 ft.lbs to loosen it. In practice, if you back off the nut and then tighten it to 28 ft.lbs, then if you set your torque wrench to 29 ft.lbs and then tighten the nut without backing it off, chances are that the wrench will click before the nut moves. In this case, the nut has been tightened (moving) to a setting of 28 ft.lbs, even though you believe it is at 29 ft.lbs. So, if you don't back off the nut, you may not have enough torque on it because you have assumed that the co-efficient of static friction is the same as the co-efficient of sliding friction. Maybe that degree in Physics wasn't entirely wasted! Brian '74 Eldo '78 T3 '76 T3 basket
Tim Crump wrote:
Never could figure out why you would loosen heads to tighten them,what is the THEROY behind this.Mark
.....it's kinda like a rolling start......lets you build up a little speed.........
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