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RICKZ28
User Rank
Platinum
Re: Torque?
RICKZ28   2/24/2012 6:47:00 PM
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When I worked on cars, for 30 years on my Hot Rod cars (all daily drivers),  I always thoroughly cleaned all fasteners and all threads before re-assembly.  All fastener threads were properly lubricated, "Loctited", anti-seize used (exhaust fasteners), or silicon sealant used (head bolts (screws) threading into the engine block).  Every fastener (screw) that went into a tapped hole was sequentially tightened using a torque wrench.  The nuts were always tightened on the bolts using a torque wrench for all critical applications such as suspension and brakes.  Such thoroughness meant I rarely had a problem on the road, and had much better success than other home mechanics I knew.

William K.
User Rank
Platinum
How much thread engagement is enough???
William K.   2/22/2012 9:24:55 PM
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Palmer is correct about Helicoils and similar products. But they only are beneficial when used on material thick enough to allow them to be installed correctly, which usually does not include sheetmetal. although it is common to put self threading screws into pierced holes in thin sheetmetal, on manyoccasions the result is an inadequate joint, only strong enough to hold correctly until the product is first used. When the hole for the screw is punched instead of being pierced the joint integrity is usually even worse.

Thread engagement in tapped holes in material much thicker than sheetmetal is a totally different case. Many folks assert that most of the load is carried by the first two threads, but they don't realize is that as soon as those threads deform just a bit that the load is then shared by quite a few more threads. Also, that conceptis most true for "avaerage quality" threads. It is not that much extra effort to produce threads that have greater engagement and can carry a larger load than standard threads. So it is a better approach to select threaded connections to be at least 2 screw diameters deep.

Dave Palmer
User Rank
Platinum
Re: Torque?
Dave Palmer   2/22/2012 9:49:04 AM
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@ed_bltn: I respectfully disagree.  There is nothing fundamentally wrong with using high-strength fasteners in aluminum, provided that the joint is properly designed.  There is a long history of thousands and thousands of successful designs which use high-strength fasteners in aluminum.  Heli-coil inserts are a wonderful thing, but there is no reason to include them in every single design, unless you don't care about cost.  As you mention, they are a good fix in certain situations, such as when a very high clamp load is needed, or when the threads need to stand up to lots of repeated assembly and disassembly.

ed_bltn
User Rank
Iron
Re: Torque?
ed_bltn   2/21/2012 4:32:28 PM
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I don't like the idea of threading a high strength fastener directly into aluminum in the first place...especially fasteners that will be removed and reinstalled a few times over the life of the product, and definitely ones that will see big and/or live loads. I've found it better to use helicoils or something similar in aluminum.

The good insert manufacturers can provide strength data for design.

When using helicoils (or similar non-name brand wire inserts) you should not use thread locking compounds on the fasteners. Best case, they can adhere the screw to the wire insert so it backs out when the screw is removed. Worst case, it reduces the strength of the completed joint because the locker prevents the load from distributing properly over the length of the insert. Better to use locking inserts, which have one convolution deformed.

proper
User Rank
Iron
Grease
proper   2/21/2012 1:25:18 PM
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benmlee2 – I agree with fredsay.... 

Using LH studs on the LH side of the vehicle was due to the idea of self-tightening of the threaded fasteners from rotation of the clamped (but almost unclamped) components.  Raced pre-war and post war MG's had a little habit of loosening the left rear axle nut (RHT) allowing the drum to come off.

It's not a current practice and we have no serious issues with wheels falling off, so at best it could be seen as an extra expense and complication to solve a non-existent problem while creating a new problem for non-suspecting owners when the need to change a wheel.

It still is common practice to use a LH thread for mounting a bicycle pedal to the left crank arm.  Just 2 months ago I had to hit the trail after only putting my left pedal on finger tight, but after the ride it needed a wrench to get it off.

fredsay
User Rank
Gold
Re: Grease
fredsay   2/21/2012 11:26:05 AM
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I believe he is referring to left hand threads used on one side of the car and right hand threads used on the other side. While this is probably a good thing for axle nuts where the spinning torque can loosen one side if both sides use a standard RH thread, it's probably not necessary on wheel studs because of their arrangement off of the center of rotation.

If you don't know that a nut or bolt is LH, you might add a lever to your wrench and snap the bolt before it unthreads. RTFM, a good shop manual can be one of your best friends. The harmonic balancer (actually it's on the lower pully for my supercharger which is bolted to the actual harmonic balancer) on my Ford is a LH thread, but it is clearly marked on the pully. However I have run into LH bolts which are not marked. I guess that's the real trick.

benmlee2
User Rank
Gold
Re: Grease
benmlee2   2/21/2012 11:08:01 AM
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What is the LH thread trick????

benmlee2
User Rank
Gold
Friction Affects Clamping Force
benmlee2   2/21/2012 11:05:32 AM
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I stripped out several studs on the exhaust manifold while torqueing it down. Made the mistake of using anti-seize. So happen at work, I was investigating clamping force and thread lubricant. At work, we also broke a stud because it was coated with moly sulfide lubricant. Turns out, lubricant have a significant effect on clamping force. Friction make up 80% of the clamping force on a screw. Did a webpage on it:

http://benmlee.com/4Runner/threads/threads.htm

Conclusion is it is ok to oil the threads, but don't use anti-sieze unless you account for it.

 

Dave Palmer
User Rank
Platinum
Re: Torque?
Dave Palmer   2/21/2012 10:52:42 AM
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Great questions and comments!

@TJ McDermott: Many liquid threadlockers serve as lubricants during installation, so they allow you to get more clamp load for the same amount of torque by reducing thread friction.  Dry threadlockers tend to increase thread friction, so you need to supply more torque to get the same clamp load.  In either case, though, the retained clamp load stays high, i.e. the joint is prevented from loosening over time.  Obviously, the point of using a threadlocker is the retained clamp load.

@fredsay: Generally, fasteners which are used in torque-to-yield applications are the highest strength grades (Grade 8 for English, Grade 10.9 for metric).  This makes sense, since to get the most bang for your buck, you want a fastener with a high yield strength.  But in terms of metallurgy and manufacturing processes, there is nothing different between TTY fasteners and other high-strength fasteners.  In fact, while we regularly use the term "TTY fastener," torque-to-yield is really just a fastening strategy, not a type of fastener.

Some fasteners which are specifically designed for torque-to-yield applications have a reduced shank, which ensures that they yield in the shank rather than in the threads.  In other cases, a "TTY fastener" is just a standard Grade 8 bolt.

@Brother John: Yes, torque-to-angle is also a good tightening strategy.  As you point out, tightening to a specified angle takes the effect of friction out of the equation.  We often install fasteners under angle control with torque monitoring (i.e. we tighten to a specified angle, but also monitor the torque to make sure it is within a specified range).

proper
User Rank
Iron
Grease
proper   2/21/2012 10:47:18 AM
NO RATINGS

My brothers ran into the debate about grease applied to automotive wheel studs, or not.  One recommendation was that you should never apply grease because this would cause bolt stretch and lead to failure.  But while growing up, in our driveway it was common practice to put a dab of grease on the threads.  So my engineer brain broke it down this way.  A little grease applied to just the threads is good to prevent the thread from freezing up and causing cursing trying to change a flat on the side of a highway.  But keep the grease off of the angled surface under the head of the bolt/nut.  Once the head comes into contact with the wheel, that diameter and angle area is a lot larger than the thread flank cumulative area, and it is the dry head contact area that will take the applied torque.  Avoid grease under the head to prevent too much of the torque transmitting into bolt tension/stretch.

There's another funny story about abusing a pry bar trying to get an old Alfa Romeo wheel off – luckily I figured out the LH-thread trick before we broke it.  When my Dad got home he explained why LH....

 

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