Torque to Yield???

Topic: Torque to Yield??? (Read 2094 times) previous topic - next topic

Torque to Yield???

March 14, 2013, 08:00:46 PM

Torque to Yield???

Reply #1 – March 14, 2013, 08:19:29 PM

I think we talked about this concept in school. I can't say I'll be of any use in this discussion as I don't remember how that conversation went. If I had to make a guess, I'd say that that would probably be how you'd do it if dealing with head studs or with non-tty bolts. I'll be watching this thread, because it's relevant to my interests.

Torque to Yield???

Reply #2 – March 14, 2013, 08:55:31 PM

I do it the old fashion way as you did .....that bolt doesn't know how it's being tightened as long as it ends up @ the finished torque. The only thing is I don't do is re-use the old bolts.
....just me. :D

Torque to Yield???

Reply #3 – March 14, 2013, 09:31:13 PM

this is something I have wondered as well. How can you be sure that you get them all torqued properly if you are adding that extra 90* turn? the 90* is just a guess since there is no torque wrench that I've seen that has a degree setting.

Torque to Yield???

Reply #4 – March 14, 2013, 10:27:53 PM

Quote from: 88turbo;411214

this is something I have wondered as well. How can you be sure that you get them all torqued properly if you are adding that extra 90* turn? the 90* is just a guess since there is no torque wrench that I've seen that has a degree setting.

Actually, my Snap-On torque wrench (atech2fr100a) has a torque angle setting. The LCD displays the angle until I hit the set point, then it beeps and buzzes.

http://image.snapon.com/international/pdf/atech1fr240_atech2fr100a_atech3fr250a_npa.pdf

Torque to Yield???

Reply #5 – March 14, 2013, 11:33:04 PM

Quote from: TOM Renzo;411203

Ok just about once a month i have to use my angle torque wrench. So here is my question??? I just did those heads on the 3.4 and the GENERAL wants a 44 FT/LB initial torque followed up by 95* in sequence. So i did that and then i tested the actual torque. By twisting the bolts to 90* in stead of 95* Then used a beam torque wrench to add the final 5*. This way i could get a torque reading. So if i know the actual torque reading of the final bolt torque was 85 FT/LBS. So here is the question can i torque the head down in 3 steps to 85 Ft/Lbs and be dun with it. Comments on this any one!!

Fasteners are 'tightened' to achieve a certain preload (longitudinal force on the bolt). When you tighten a fastener by torque, you are assuming you achieve the desired preload. Normally this assumption is correct, or close enough not to matter. For the 3.8L heads (and many other applications), the engineers decided the variables are too great to get a sufficiently repeatable "nominal" preload. Thus, they chose TTY fasteners. [I was actually skimming through the SAE paper detailing the development of the 1982 3.8L Tuesday. Ford said they had issues with fastener preload due to the alum heads/iron block, fastener length difference between the upper/lower bols.]

On a TTY bolt, once you begin yielding the bolt there is no direct relation between torque and preload. Rather the relation is between degrees of rotation and preload. You can tighten a TTY bolt to a certain torque, but you won't know the actual preload (or more accurately, the preload variation of tightening by torque is greater than the variation by angle of rotation for a TTY bolt).

In other words, tightening to 44 ft-lbs, then going to 95 degrees will give you a different fastener preload than tightening to 44 ft-lbs and going to 85 ft-lbs.

In reality, will you have issues by tightening by torque rather than by angle? I don't know.

Torque to Yield???

Reply #6 – March 15, 2013, 12:47:58 AM

I can't imagine TTY bolts supplied by different manufacturers are exact replications of one another so therefore I can't find torque angle to be relevant anymore

Torque to Yield???

Reply #7 – March 15, 2013, 05:45:48 AM

This

Torque to Yield???

Reply #8 – March 15, 2013, 12:19:50 PM

Yeah, I'm not sure why TTY is needed. I didn't like having to do TTY on the Subaru that I replaced the head gaskets on some months ago. The entire process of working on that car, and all the things that completely fail on that vehicle at around 100k miles, make fox body repairs seem like nothing.

I'd think a properly torqued stud would still be way better than a TTY bolt. Perhaps TTY works better for aluminum parts due to the completely different rate of expansion? I could never make sense of the need for TTY other than what you found - it MAY prevent the bolt's torque from being off so much at the final torque value. Why should this even matter though? The manufacturing line and repair shops should already be using the right tools and parts, and have the education/experience to properly torque a bolt.

I just do what the manufacturer requires. I don't work on many vehicles though, so I can deal with the occasional TTY requirements.

Torque to Yield???

Reply #9 – March 15, 2013, 01:18:08 PM

Quote from: jcassity;411230

I can't imagine TTY bolts supplied by different manufacturers are exact replications of one another so therefore I can't find torque angle to be relevant anymore

I'm sure there are specifications the fasteners have to meet (material strength, shank diameter, thread class, etc) that makes them interchangeable. Much like grade 5/8 fasteners are interchangeable.

Quote from: TOM Renzo;411236

This is my problem at hand. I have 2 TTY torque wrenches and they work great. BUT!!!! When i use an aftermarket head bolt arrangement or studs from ARP for example they do not recommend TTY!!!. So as i am finishing up the new engine for the Midnighter i used studs. Now studs as mentioned above never use TTY????. So as i know the engineers want the bolt on or near it's yield point how critical is it??

EXAMPLE. Some ford 3.8 V6 engines require to torque the heads down then back them off then torque them back down. Now Fel Pro says that is not necessary??????????????? So another scenario. Just something that is bothering me is it really necessary. My research on this seems to stem from inaccurate torque wrenches or not properly prepared bolt. LUBE ETC!! When turning a bolt a certain number of degreese torque values are not in the equation. So another thing that is rattling around in my BRAIN. Is it really necessary??????

The main reason TTY fasteners are used is to achieve more constant preload. If an OEM is having problems with preload (clamping load) inconsistency, they may use TTYs fasteners to overcome this. Ford did this with the 3.8L engine. I'll post up portions of the paper from Ford concerning the 3.8L development describing their problems on Monday.

You generally want a fastener to xx% of yield to increase fatigue life and decrease the chances of the fastener loosening up. This only concerns non TTY fasteners. In a TTY fastener, you go beyond the elastic region and permanently deform the fastener during installation.

Quote from: Seek;411259

Yeah, I'm not sure why TTY is needed. I didn't like having to do TTY on the Subaru that I replaced the head gaskets on some months ago. The entire process of working on that car, and all the things that completely fail on that vehicle at around 100k miles, make fox body repairs seem like nothing.

I'd think a properly torqued stud would still be way better than a TTY bolt. Perhaps TTY works better for aluminum parts due to the completely different rate of expansion? I could never make sense of the need for TTY other than what you found - it MAY prevent the bolt's torque from being off so much at the final torque value. Why should this even matter though? The manufacturing line and repair shops should already be using the right tools and parts, and have the education/experience to properly torque a bolt.

I just do what the manufacturer requires. I don't work on many vehicles though, so I can deal with the occasional TTY requirements.

Once again, TTY fasteners are used to increase clamping consistency.

You can do all the right things wrt fastener assembly, but a TTY fastener will simply give you better consistency then a standard torqued screw. This is due to the shank of the fastener yielding to achieve the desired preload rather than using the thread interface friction and mating surface friction to get desired preload from a torque.

Are studs better than screws? Yes. Are studs better than TTY? I don't actually know, could be!

Torque to Yield???

Reply #10 – March 15, 2013, 08:35:47 PM

Put a drop of moly assembly lube on that bolt and I'll guarantee that TTY bolt will go beyond it designed limit.

You could always do it the shady way.......tighten till it strips.....back off 1/2 turn........BINGO!

Torque to Yield???

Reply #11 – March 16, 2013, 08:11:47 AM

@JeremiB

Exactly

Torque to Yield???

Reply #12 – March 16, 2013, 12:37:43 PM

Why are these 70-100ft/lb bolts so sensitive in the first place? With thick enough surfaces, it shouldn't matter at all if it's +/- 10ft/lbs. I don't think the bolts on our heavy, thick-enough fox body motors are too sensitive to the torque values compared to newer vehicles. Get it in the general vicinity and they last beyond the point where freshening up is needed. Is it due to thinner castings and aluminum being used instead of iron? That should be much more sensitive to torque values.

Pieces clamping down bearings I can see being a little more sensitive to torque. I just don't understand TTY for heads, especially when it appears to do nothing for extending the life of head gaskets or head straightness when poor choices were made elsewhere by the engineers (Subaru notably).

Torque to Yield???

Reply #13 – March 16, 2013, 05:56:08 PM

True

Torque to Yield???

Reply #14 – March 17, 2013, 10:48:28 AM

Just for S+Gs , torque any fastener (wheel nut etc) with a click wrench to its desired setting. Wait a minute, and re-torque VERY softly again until it clicks and see if it moves any more. I have come across this in the past when re-torquing fasteners and found additional movement.
This is side-lining this thread (sorry Tom) but I`m curious if anyone else has come across this as this may be beneficial in critical parts as rods, caps etc.