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The Oil Pickup Story

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During tightening any threaded fastener is subjected to two very different stresses:

 (1) The tension stress set up by the actual stretching of the bolt as it is tightened.

 (2) The torsional stress due to friction between the ­male and female threads and between the undersurface of the bolt head and the washer. This stress varies from virtually zero at the time thread engagement begins to a very high value indeed at the end of the tightening operation.

The tensile stress in the bolt is what we are looking for. It is the force that will clamp the parts and lock the male and female threads together so that the assembly will not loosen in service. The male threads elongate as the bolt ­stretches while the female threads compress. Resulting in an increasing interference condition which resists loosening ­due to vibration and so on. Unfortunately a large percentage of the actual torque required to tighten any threaded fastener is used up in applying the torsional stress necessary to overcome friction. This means that, while the calculation ­of the proper amount of pre-load is not all that difficult, its accurate measurement IS.

The torque required to produce a given tensile varies with plating, lubrication (or lack of it) length of engaged thread and class of thread fit. As an example of the magnitude of what we are talking about. FIGURE 1 shows the effect of different levels of lubrication on the tightening torque required to achieve various levels of installed stress and corresponding pre-load.

What we are really looking for is a level of installed ten­sile stress that is somewhere below the yield strength of the bolt material. Standard torque tables are usually compiled for plated fasteners, without lubrication. Critical tension assemblies subjected to high levels of cyclic stress such as cylinder heads, connecting rods, fly­wheels and the like require specialized fasteners. Each such fastener usually has its very own recommended torque value. These values are arrived at experimentally either by torquing a series of joints to bolt failure and setting the rec­ommended torque value at about 60% of the level at failure or, preferably, by actually measuring the elongation of the bolts and specifying the torque required to elongate the bolt sufficiently to develop the optimum amount of pre-load required for the assembly. The optimum pre-load is normally just below the yield strength of the bolt, or about 60% of the ultimate tensile strength.

In critical aerospace applications, stress-sensitive washers and various types of stress-indicating bolts are used to ensure proper bolt pre-load. Most clued-in engine builders measure the actual stretch of the connecting rod bolts rather than using an indicated torque value. Only in this way is it possible to avoid fastener failures by taking full advantage of the high stress levels available in the current generation of high-strength bolts.

As a point of interest, we can now readily obtain bolts with an ultimate tensile strength of 220,000 psi, a yield strength of 185,000 psi a shear strength of 132,000 psi and a tension endurance limit of 80,000 psi for 8 million cycles. Bolts are available (and nuts to match) up to 300,000 psi, but they are difficult for us mortals to obtain. There IS a difference.

The only good point about the torsional stress produced by the friction of tightening is that it goes away shortly after tightening is completed (without relative movement there is no friction). Everything relaxes just a little bit and the bolt is left under tension only. This means that, assuming a rigid joint and no cyclic stress, the maximum stress level that the bolt will ever see occurs during tightening. Strange as it may seem at first, it is actually better to overtighten a bolt than to undertighten it! As an example, in a laboratory test a 180,000 psi NAS bolt with a shank diameter of 0.374", when tightened to a residual stress level of 72,000 psi and subjected to a cyclic tension load of 12.000 lbs failed after 4,900 cycles. An identical bolt, pre­-stressed to a level of 108.000 psi and subjected to the same cyclic load, went more than 6 MILLION cycles before failure.

 

Interestingly enough, a residual stress (pre-load) of 108.000 psi works out to 60% of the Ultimate Tensile Strength (UTS) of the bolt and a cyclic load of 12,000 lb works out to about 60% of the ultimate strength of the bolt. The cross-sectional area of a 3/8” bolt is (0.375/2)^2 X 3.1416 = 0.11045 in² so the ultimate strength of a 180,000 psi bolt will be 180,000 x 0.11045 = 19,880 lb. 60% of 19,880 lb is 11,928 lb. In our earlier discussion of strength of materials we discovered that a rule of thumb states that the yield strength of most steels is about 60% of the UTS. None of this is coincidental. It DOES pay to properly tighten bolts!

 

Theory be damned! It is easy to overdue this tightening bit. We have seen that, due to the torsional resistance to tightening caused by thread friction, the highest total stress that a bolt will ever be subjected to occurs during the act of tightening, so that, if the bolt doesn't fail while being tightened, then, within the parameters of its assembled en­durance limit, it never should. BUT, if Super Mechanic with his eighteen-inch wrench exceeds the elastic limit of a bolt while tightening it, then the bolt MUST undergo plas­tic deformation. It will do so locally, beginning at the root of the starting thread and progressing through the bolt sec­tion with the highest unit stress, the unengaged threads, and the bolt will never return to its original length.

You read a lot of this sort of thing in fastener manuals. It is perfectly true-FOR RIGID ASSEMBLIES. There are no such assemblies in the racing car and WE should never pre-load a bolt (or a stud) quite to its yield strength. When you feel a bolt yield while being tightened, take it out and throw it away-don't even look at it. Every time that you remove ANY bolt, look at the threads for signs of elonga­tion. If there is any sign at all (as in FIGURE [145]) or if the threads are damaged in any way, give the bolt a floata­tion test (a popular test from my U.S. Navy days: Throw the metallic item under question into the nearest large body of water: if it floats. save it).