Bolt stretch

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SteveM
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Bolt stretch

Post by SteveM » Tue Aug 07, 2018 9:05 pm

The lug nuts on a Toyota Sienna are M12 x 1.5.

The manufacturer spec is 80 ft-lbs, which produces a predetermined (but unknown to me) bolt stretch.

The dealer put them on with 130 ft-lbs.

Is there any way with just that information to calculate if the bolts have been excessively stretched?

Steve

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warmstrong1955
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Re: Bolt stretch

Post by warmstrong1955 » Tue Aug 07, 2018 9:42 pm

Don't know the grade, but a 10.9 is 100-115 Mn. (74-85 ft/lbs)
Torque values are rated at 75% x proof x stress area, so at 85 ft/lbs, that'll give you 98 ft/lbs.
Does not take into account friction. Is it a flanged nut or a tapered nut? Regular old tapered lugnuts take a lot more torque to get to stretch than a flanged nut, or a nut with a flatwasher.

Also, how are you measuring the installation torque? That can be quite deceiving if you are using a beam or dial wrench to test by removing.
Plating makes a big difference too....especially over time.

Bill
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warmstrong1955
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Re: Bolt stretch

Post by warmstrong1955 » Tue Aug 07, 2018 10:00 pm

Just to add.....

I'm a heavy equipment guy. So.....
If you compare wheel mounting nut torque, hexnuts & flatwashers vs Budd nuts, the tightening torque on the Budds is 30% to 40% higher. Compensation for friction.

Bill
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SteveM
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Re: Bolt stretch

Post by SteveM » Tue Aug 07, 2018 10:22 pm

The lug nuts look like this:
21260489_dag_6112121_pri_larg.jpg
The taper is only there for use with the spare tire - the flat washer rests against the wheel.

I don't have a dial or beam wrench, so I used a click wrench and kept lowering the setting until the nut moved before the click. I realize that's not going to be a laboratory-grade measurement, but it was sure a lot more than the 80 ft-lbs I was installing them with. I tried several nuts to verify.

I used a wrench about 20" long to get them off. If someone, particularly a small woman, had to remove that with the 10-12" wrench supplied with the car, they would have had a tough time.

Steve

Chipper5783
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Re: Bolt stretch

Post by Chipper5783 » Tue Aug 07, 2018 11:51 pm

Tha break away torque on a bolt that is torqued to 80 ft-lbs will be significantly higher than 80 ft-lbs. the reason is that the static friction that has to be over come to get it moving will be a good bit higher than the sliding friction when it was being tightened.

How much higher? Tough to guess. Perhaps 80 x the static coefficient of friction divided by the sliding coefficient of friction. 130 seems a bit high.
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Magicniner
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Re: Bolt stretch

Post by Magicniner » Wed Aug 08, 2018 4:04 am

The best way for re-usable fasteners is to mark the orientation of the fastener, loosen it and then torque it back up to a test figure and see if it reaches the marked point of rotation, loosen again and repeat with increasing torque until you get the fastener back in the same orientation. You won't get a perfect result but it will be fairly close.

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liveaboard
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Re: Bolt stretch

Post by liveaboard » Wed Aug 08, 2018 4:13 am

Just take the wheel off and eyeball the threads of the bolts. If there's stretch, it will be near the base, under the nut position.
I would expect damage to the contact areas of the aluminum wheels first.

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warmstrong1955
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Re: Bolt stretch

Post by warmstrong1955 » Wed Aug 08, 2018 6:47 am

A non-rotating flange nut like you have, will have similar torque characteristics as a Budd nut. (spherical seat)
And, what Chipper5783 said. Friction is at it's highest when it is static. It will take a higher torque to remove them, than the torque they were installed.

I think what you are experiencing is pretty much normal.
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Mr Ron
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Re: Bolt stretch

Post by Mr Ron » Wed Aug 08, 2018 11:17 am

I'm assuming the threads are not lubricated as lubed threads can be torqued higher because there is less friction. If the elastic limit has been exceeded, the bolt has stretched beyond where it will return to it's original length and is no longer useable. I would go by the recommended torque figure from the factory.
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warmstrong1955
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Re: Bolt stretch

Post by warmstrong1955 » Wed Aug 08, 2018 12:33 pm

Mr Ron wrote:
Wed Aug 08, 2018 11:17 am
I'm assuming the threads are not lubricated as lubed threads can be torqued higher because there is less friction. If the elastic limit has been exceeded, the bolt has stretched beyond where it will return to it's original length and is no longer useable. I would go by the recommended torque figure from the factory.

Lubed threads should be torqued lower. The higher torque is not required to achieve the desired bolt stretch due to a lower coF. A lubed torque spec is generally 25% less than dry.
Also, plating qualifies as 'lubed'.

Bill
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Russ Hanscom
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Re: Bolt stretch

Post by Russ Hanscom » Wed Aug 08, 2018 1:31 pm

The nuts probably got installed with an impact wrench with little or no thought to torque. That is a frequent occurrence. I typically loosen and retorque the nuts after the tire guys finish. Made a cheater bar for the daughter so she could get the nuts loose.

John Hasler
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Re: Bolt stretch

Post by John Hasler » Wed Aug 08, 2018 1:49 pm

warmstrong1955 wrote:
Wed Aug 08, 2018 12:33 pm
Mr Ron wrote:
Wed Aug 08, 2018 11:17 am
I'm assuming the threads are not lubricated as lubed threads can be torqued higher because there is less friction. If the elastic limit has been exceeded, the bolt has stretched beyond where it will return to it's original length and is no longer useable. I would go by the recommended torque figure from the factory.

Lubed threads should be torqued lower. The higher torque is not required to achieve the desired bolt stretch due to a lower coF. A lubed torque spec is generally 25% less than dry.
Also, plating qualifies as 'lubed'.

Bill
This needs to be emphasized. The goal when tightening a bolt is to achieve a specific amount of tension in the shank, not to reach a specific torque. The torque is just a convenient but indirect (and inaccurate) indicator of tension. Part of the torque goes to fighting friction, part to stretching the bolt. Lubrication reduces friction, leaving a larger share of applied torque for tension. Torquing a lubricated bolt to a dry spec over tensions it. Torquing a dry bolt to an oiled spec under tensions it.

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