At 05:09 PM 2/10/98 +0000, you wrote:
...
>In structural applications, A325 and A490 bolts, by code, are limited to
>one[1] torque cycle and have to be destroyed when removed. I don't know if
>the A449 and A354 bolts have the same restriction. The SAE bolts apparently
>don't.
>Steve
Structural bolting applications (A325 & A490) are essentially one time
assemblies of steel buildings and they require several limiting conditions
in their application. One of these limitations is the use of hardened
washers on both the bolt head and the nut, without a lock washer. Torque
specs typically load the bolt to 85% of tensile yield under these
conditions. In the normal application of structural bolts, the preload and
number of bolts used in the design is set so that shear is controlled by
the friction between the structural members and not by the shear of the
bolt diameter.
Automotive applications are quite different in demand. From the wheel
studs to the head bolts, the proper bolt is strongly dependant on the
individual application and type of loading. While higher tensile
strength bolts are desireable in some applications, a lower tensile
strength bolt typically has a higher fatigue life. If you like and can use
the extra margin high strength bolts can give in certain applications, you
have to be prepaired to comensate by replacing these bolts on a more
frequent basis. All professional racers toss perfectly good looking bolts
in the dumpster because the cyclic fatigue is usually catastrophic, and its
better to toss 'em and replace 'em routinely rather than to suffer a
potential failure and build another race car.
The most typical example on a Tiger is the front fulcrum pin. While the
actual alloy and heat treatment has never been explored in detail, it is
obviously a high strength alloy, and a rather poor design. As such, it is
prone to fatigue failures, several of which have ocurred. It is unlikely
that it was ever a problem with the Alpine, or even low mileage stock
Tigers, but high horses, wide sticky tires, and autocrosses or open track
applications increase the duty cycles to the point where failures are not
that unusual.
Bottom line: If you use grade 8 bolts, you should consider serious
application of hardened washers, locktite, a torque wrench, and routine
replacements on any application where cyclic loading is part of the game
plan. I wouldn't worry nearly as much about the bolts holding the roll bar
to the body (as long as you have nut plates) as I would about suspension
attachments.
Tom Hall
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