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  #1 (permalink)  
Old 05-14-2004, 10:39 AM
bullheimer's Avatar
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NO reason to know this but...

how come they dont put lock washers on main bearing caps nor rod caps??

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Last edited by bullheimer; 05-14-2004 at 04:20 PM.
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Old 05-14-2004, 12:19 PM
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I read that itīs the stretch in the stud/bolt that keeps it in tension and tight.If it was an issue wouldīnt they be wired to stop them loosening.
Sorry thatīs answering a question with a question.
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Old 05-14-2004, 01:15 PM
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well, it's obvious they dont need them, i'm wondering why. you just gave it also the most obvious, but it doesnt seem like that enough to keep them tight for 100,ooo miles to me when i tighten nuts and bolts on my body of my truck just as tight and they are always falling off when i go wheelin
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Old 05-14-2004, 02:16 PM
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To understand how those highly stressed joints stay together indefinitely while other seemingly lightly loaded bolts loosen with little stress you need to understand the mechanism for bolts and nuts falling apart.

Think of the thread on a nut/bolt as a 'ski slope'. Unwrap it from the bolt and stretch it out straight and it presents a fairly steep slope. Now envision the mating element (nut on a bolt or bolt hole in a block on a bolt) as a 'snow sled' sitting on the 'ski slope'. It may have enough friction to sit there and not move, (i.e., tension from bolt torque), but of you start jiggling the 'ski slope', the sled breaks loose and begins to slide down the slope. When the 'ski slope' is wrapped around the shaft of a bolt in the form of a thread, the 'sled' sliding down the slope is the nut and it inevitably loosens and the joint fails.

That may make sense but you ask, "what can cause the 'sled' to jiggle on the 'ski slope'? Isn't the fit between the bolt and nut so tight that nothing can move?" In a short answer, no that clearance isn't small enough to prevent movement. There must be a slight clearance for the treaded connection to screw easily together. Only a few 1/10,000ths is enough to do the deed. If the joint only moved in the same axis of the bolt (tension and compression only), the 'sled' wouldn't move. However, most automotive bolts see a lot of side loading vibration and the bolt moves back and forth inside the threaded joint. This back and forth movement 'walks' the bolt down the thread 'ski slope' and eventuates in a failed connection.

Once you understand the mechanism of failure you can design a preventative measure. Actually, mechanical Lock washers are a pretty bad solution. They not only dig into the two joint elements and ruin the finish, they also introduce a 'soft' element into a joint so after initial torque is set, they can relax or dig into the metal further and lose some of the tension preload that the torque setting is supposed to provide. LockTite works by filling the thread clearance with a tough, non-compressible solid that eliminates the side movement in the thread. NyLock nuts do the same thing. It is the elimination of the side to side movement that is most important - the adhesive effect of 'gluing ' the joint together is of secondary importance.

Still haven't answered your original question! With all of their bad features, in conjunction with the extreme side loads experienced in crank shaft joints, mechanical lock nuts would not survive reliably there. Thus engineers designed those joints with precision step locks that absolutely prevent sideways movement of the joint and the threaded joint internals. All main caps have a step joint that is a precision fit in the mating element. Rods can have a collar around the bolts, steps like main caps, v-grooves, some sort of lock. The bolted joint therefore need only resist tensile (push pull) loads and there is no mechanism to cause the 'sled' to slide down the 'ski slope'.

Last edited by willys36@aol.com; 05-14-2004 at 02:21 PM.
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Old 05-14-2004, 03:28 PM
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So would the angle of the thread help, a shallower angle?
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Old 05-14-2004, 04:05 PM
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Quote:
Originally posted by malc
So would the angle of the thread help, a shallower angle?
You betcha! That is the reason they make fine threaded - UNF bolts to complement coarse threaded - UNC bolts the same diameter. Coarse threads are more resistant to thread stripping so are great for under-grade-5 soft steel, steel in cast iron (used for main cap bolts), soft brass, aluminum, etc. materials but fine threads are more resistant to unthreading so are good for high strength, high vibration applications in high strenght steel (used in rod bolts). Fine threaded bolts are a little stronger in tension since the root diameter is a little larger than the deeper coarse thread.

Last edited by willys36@aol.com; 05-14-2004 at 04:32 PM.
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Old 05-14-2004, 04:06 PM
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thnx 36. once again btwn you and 4jaw, you have proven that ignorance is bliss.
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Old 05-14-2004, 04:31 PM
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Little known, less cared about fact - for nuts and bolts of the same grade, it only takes three full threads of engagement to develop full strength, i.e., less than 3 threads, the threads will strip, 3 or more threads of engagement, the bolt will pull in-two.

Last edited by willys36@aol.com; 05-14-2004 at 04:56 PM.
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Old 05-17-2004, 12:30 PM
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yeah, i read that somewhere as well. it seemed alittle strange to me because i always tried to make sure the bolt went all the way thru the nut, but as most nuts are like 4 threads, i guess that wasnt nec. ps, you're still my hero
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Old 05-23-2004, 10:38 AM
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thnx 36. once again btwn you and 4jaw, you have proven that ignorance is bliss.
Yep!! And I'm living proof of that!! Just ask 4 Jaw...

Chickie.
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Old 05-23-2004, 11:36 AM
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Originally posted by bullheimer
thnx 36. once again btwn you and 4jaw, you have proven that ignorance is bliss.
I hope bliss is a good thing! Of so, I am real good...
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Old 05-23-2004, 09:38 PM
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Quote:
Originally posted by adtkart
I hope bliss is a good thing! Of so, I am real good...
That reminds me of one of my favorite Curly Howard quotes, "It would be silly to be scared now. And I'm feeling really silly!"
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Old 05-23-2004, 09:54 PM
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Willys36 gave good answers but let me add one thing that will show how useless lock washers are. Grab a lock washer with a pair pliers that have jaws wide enough to span the washer's width. Squeeze the washer and see how it only takes a couple of hundred pounds or less to compress it. When a rod bolt is properly torqued the stretch of the bolt is putting thousands of pounds of clamping load on the bolted surfaces. The tension of a lock washer is just a drop in the bucket compared to a properly torqued bolt.
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Old 05-24-2004, 07:36 AM
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Unfortunately, the way lock washers work is that they dig into the bolt and part and prevent turning by damaging the surfaces.
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Old 05-26-2004, 04:32 PM
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well then tresi and 36 answer me this. if the head of the bolt is pushing down with thousands of pounds of force, and i'll take your word for it, let me direct you to my question (thread in engine forum) of wondering if i really need thread sealer on my head bolts and if i should worry about water leaking because i didnt use any sealer.
how could water leak out when my radiator cap blows at 15 psi? isn't that the max pressure my water inside the block will have? how could 15psi possibly work it's way past a bolt-head that is pushing down with thousands of pounds of force? (yet sometimes this does happen apparently) would there not have to be some kind of irregularity on the bottom of the head bolt or the heads hole that would account for some passegeway for steam or water to escape with that kid of pressure holding the bolt down? (btw head bolts are another bolt w/o a lock washer)

Last edited by bullheimer; 05-26-2004 at 08:09 PM.
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