[AT] Statistics In Tractor Manufacturing

[James Peck]

One co-worker described some of his experiences as an Army tank mechanic. One was driving an Abrams through a woods seeing how they could break it.

[szabelski at wildblue.net] My guess is that when the engine overheats the head expands and gets thicker (standard thermo dynamics). When it expands it stretches the head bolts, all of them. As a result it would appear that the head got thinner.

Finding all the head bolts loose, one might guess that’s the head shrunk uniformally from the overheating. If the head shrinks every time it overheats, one should eventually see a head that is getting noticeably thinner and thinner.

Note that when the bolts are stretched the bolt shanks get a little thinner. If they get too thin they will not hold the required torque, and will loosen up easier. This is due to the fact that a bolt that is too thin will twist easier without applying the expected clamping force.

When we serviced military equipment that required a high torque on certain bolts, we always replaced those bolts with new ones before applying any torque. Torquing a bolt does stretch it and you can only stretch a bolt so far before it no longer will clamp properly. Not knowing how many times a specific bolt was torqued, or if it was ever over torqued, made it necessary to not take a chance on reusing a bolt that was stretched to far. 

We also never reused lock washers or self locking nuts since, there was no way of determining how many times they may have been taken off and put back on. They were simply replaced and we knew they were 100% good.  When the bolts, lock washers, or self locking nuts were removed, they immediately went into a scrap bucket to make sure they didn’t make it back into the assembly.

A five ton main gun slamming rearward as it comes out of battery (it’s mounting) due to degraded bolts will easily slice a tank commander or the loader, sitting basically behind it, in two, maybe both of them. It can also do some serious injury to the gunner sitting along side of it. There are also dozens of pieces of equipment mounted to the interior walls. If they break away and become secondary missiles, they can do a lot of secondary damage and injury. We and the army never wanted to risk that or anything else like that happening, and always put crew safety as our first priority. It did drive up the cost of the field installation kits because we had to included a replacement bolt, lock. washer, and nut for every one that was touched during maintenance, retrofit, or upgrade.

[ Jim Becker] I agree the whole head isn’t going to shrink.  However, I can imagine the pressure from the head bolts acting on the overheated head to cause some displacement of the head material.  When cooled back to a normal temperature, the dimension from where the head bolts contact the head to the gasket surface could be reduced.  This could be confirmed by measurement before and after.

Having not seen the actual wording of the bulletin, I could imagine it stating something to that effect.  I would presume someone at Satoh made such measurements.