Torque to yield bolts?

[dlundblad]

Why is this method used over a simple torque spec with thread locker?

Her Volvo has many of these.. Say 90 ft lbs plus an additional 60 degrees of rotation. As a result of this, they are technically 1 time use fasteners and need replaced at every job.

The example I go back to is a photo I saw of a subframe bolt off a Volvo that was the same vintage as the wife's. The guy thought it looked rusted because it was thinner in the center. I say no; it was plastic deformation like you'd see in a tensile test and was on the verge of failure. Should have never been on a car IMO.

 

[Kestas]

It's all about clamp load. With TTY bolts you get a guaranteed clamp load. With simple torque specs you are at the mercy of the condition of the threads. That's the pro side of things. There are plenty of cons to this as well.

 

[dlundblad]

Originally Posted by Kestas
It's all about clamp load. With TTY bolts you get a guaranteed clamp load. With simple torque specs you are at the mercy of the condition of the threads. That's the pro side of things. There are plenty of cons to this as well.


Aren't you at the mercy of the threads either way?

 

[RayCJ]

A bolt operating below it's yield strength is more likely to snap in the event of certain types of impulse loads and frequencies. Keep in mind, these fasteners are taken "to" the yield point -not "beyond" the yield point. Also, the material and construction of the bolt has to be fairly precise and the actual max load on the bolt has to be well known. There are special test bolts that help determine how much strain (impulsive and static) the bolt actually experiences. https://www.strainsert.com/products/force-sensing-bolts-studs-general-product-information/ The graph below gives a rough idea of what's going on for static/steady loads but, it does not show how the bolt reacts to impulsive forces. The X-axis of this graph shows static strain. If it showed strain resulting from impulsive forces, you would see the bolt actually performs better with high preload near (but not beyond) the yield point.

 

[doitmyself]

Originally Posted by dlundblad
Originally Posted by Kestas
It's all about clamp load. With TTY bolts you get a guaranteed clamp load. With simple torque specs you are at the mercy of the condition of the threads. That's the pro side of things. There are plenty of cons to this as well.

Aren't you at the mercy of the threads either way?

Somewhat, but not as much, IMO. Achieving final bolt stretch (clamping load) via measuring turning force (torque) is a very crude measurement. Engineering forums often state that the possible error of torque measurements is around +/- 25 to 30% due to so many variables: thread quality, cleanliness, lube, surface friction of the surfaces, etc.. My theory is that these error sources are less pronounced at the preliminary low torque setting. You then turn the fastener a precise amount of degrees to get a more accurate higher clamp force, in effect bypassing these accumulation of errors. Just a hypothesis and an explanation to what Kestas already stated.

 

[RayCJ]

Originally Posted by dlundblad
Originally Posted by Kestas
It's all about clamp load. With TTY bolts you get a guaranteed clamp load. With simple torque specs you are at the mercy of the condition of the threads. That's the pro side of things. There are plenty of cons to this as well.


Aren't you at the mercy of the threads either way?


Not really. As other have mentioned, there are dozens of complicating factors. There are many industry-approved guidelines assuming certain rules are followed. In specialized cases (such as headbolts, bearing cap studs etc) the guidelines should be closely scrutinized and tested. That said, when the guidelines are followed, it's fairly rare to see threads ripped-out. Head and shank snapping are the most common modes of failure.

BTW: For normal stuff, thread depth vs bolt diameter is the guiding light. Each grade of bolt in a given class has a specified ratio of depth to bolt diameter whereby, it's a safe bet the shaft will snap before the threads pull out. As I recall: Grade 2 is 2x. Grade 5 is 3x and Grade 8 is 4x. E.g: For grade 2, if a 1/4" diameter bolt is threaded 1/2" deep (2x the diameter), the shaft or bolt head will fail before the threads pull out.

 

[Job]

It's more accurate as distance is measured not torque. Fel pro has some good info. My Toyota truck head bolts are TTY and have no neck.Some are made with a neck, different mfg. It's not that much stretch. Mine is about 3/4 mm stretch after torque number, many are much less. Mine is two 90 deg rotations after 29 ft lbs and the thread is 1.5mm. The bolts should be a quality make.

https://www.felpro.com/technical/field-test-garage.html

https://www.felpro.com/technical/field-test-garage/testing-head-bolt-stretch.html

 

[Cujet]

Some aircraft piston engines like the common Lycoming IO360 series, use connecting rod bolts (with nuts) that are stretched to a dimension. Starting torque is 35 foot pounds. if the required length is not reached, then up to 55 foot pounds is allowed.

I had one bolt that did not stretch at all, even at the maximum. Replaced it with another and it stretched properly.

https://www.lycoming.com/sites/default/files/SI1458G%20Connecting%20Rod%20Bolts%20%281%29.pdf

As you may have guessed by now, the stretch provides a spring-like clamping force that far exceeds any load the engine will ever place on the connecting rod cap. So the connecting rod caps never move around, and the unit behaves as if it were one piece.

The force the bolt feels NEVER CHANGES, so it's not subject to cyclic failure.

Short, fat, non stretch fasteners were tried. They broke after a known number of cycles due to "cyclic loads".

 

[Kestas]

The head bolts for my Mercedes E320 are TTY. They are also reusable... up to a point. The bolts are ground to a finished length. As long as the length doesn't exceed the limit, they can be reused. This means each time it stretches it incrementally nears the point of necking. Once it exceeds the max length, it's too close to necking and should be discarded. This is useful because some people can lose track of the bolts they've tightened on a job, and may have tightened them twice.

What RayCJ presented is a stress-strain curve illustrated for textbook learning purposes. I've seen stress-strain curves collected from tested material and it looks quite different. They don't necessarily have that bump at the yield point, and the part after the yield point is considerably flatter.

I've had many posters on the Mercedes web sites argue with me that TTY bolts are single use only. These people don't understand the stress-strain curve and how it applies to E320 head bolts. I'll bet the bolts I used only stretch one-fourth of the area between yield and necking. I've put 100K miles on the E320 head with reused bolts.

 

[Job]

So Meredes requires the staff to accurately measure the length of head bolts, and makes the bolts so accurately to each other? Must be costly bolts. They would need to be precision ground to size and on each end, not so easy to do. Better to listen to these guys, buy American made Fel pro bolts and change them every time. No need for all that other complication. The second link above I already posted also shows why bolts should be replaced, even if it's only recommended. They aren't that expensive.

https://fme-cat.com/livedocs/Head Bolt Sets.pdf

 

[dlundblad]

Originally Posted by Kestas
The head bolts for my Mercedes E320 are TTY. They are also reusable... up to a point. The bolts are ground to a finished length. As long as the length doesn't exceed the limit, they can be reused. This means each time it stretches it incrementally nears the point of necking. Once it exceeds the max length, it's too close to necking and should be discarded. This is useful because some people can lose track of the bolts they've tightened on a job, and may have tightened them twice.

What RayCJ presented is a stress-strain curve illustrated for textbook learning purposes. I've seen stress-strain curves collected from tested material and it looks quite different. They don't necessarily have that bump at the yield point, and the part after the yield point is considerably flatter.

I've had many posters on the Mercedes web sites argue with me that TTY bolts are single use only. These people don't understand the stress-strain curve and how it applies to E320 head bolts. I'll bet the bolts I used only stretch one-fourth of the area between yield and necking. I've put 100K miles on the E320 head with reused bolts.


As they've mentioned in the highly informative documentary, Jurassic Park, just because you can doesn't mean you should. (Reuse tty bolts.)

 

[Skippy722]

Every bolt is TTY if you're brave/strong enough.

 

[Kestas]

Originally Posted by Farnsworth
So Meredes requires the staff to accurately measure the length of head bolts, and makes the bolts so accurately to each other? Must be costly bolts. They would need to be precision ground to size and on each end, not so easy to do. Better to listen to these guys, buy American made Fel pro bolts and change them every time. No need for all that other complication. The second link above I already posted also shows why bolts should be replaced, even if it's only recommended. They aren't that expensive.

https://fme-cat.com/livedocs/Head Bolt Sets.pdf

If you read your link, you'd find it says that some bolts are reusable. It's not recommended to reuse them if you don't know the history of the bolts. Mercedes made each bolt precision ground to length. They also specify the maximum reuse length. So yes, I will listen to the Fel Pro guys.

On the second link in the previous post, I would say their opinion is biased, since they would like to sell you more product.

Explain why Mercedes would make such a reuse specification.

Those people who don't fully understand stress strain curves make blanket statements about not reusing TTY bolts. They only partially understand the stress strain curves.