Torque Education Needed

[Oka]

Hello folks, I would need some clarification on the science/physics of torque specifications.

A. 2006 Toyota Sienna, AWD has a lug nut torque of 75 ft.lbs

B. 2001 Volvo XC70 owner's manual specifies 100 ft.lbs for lug nut.
On line specifies 105 ft.lbs, which is 5 ft.lbs more than the OEM manual.
1. Logically, 5 ft.lbs is not much of a big deal, right?
2. Why would a 2001 Volvo XC70 have a higher torque than a 2006 Toyota Sienna?

C. 2001 Volvo XC70 Vibration Damper (4 screws)
Stage 1 = 18.5 ft.lbs
Stage 2 = 30 Degrees
3. First stage means to torque to that set amount, then turn 30 Degrees.
4. If so in (#2), why isn't the "30 Degrees" calculated and included to the Stage 1 torque. In such case, there would be only one torque to say, example, instead of the specified "Stage 1 18.5 ft.lbs" it would be just one 20 ft.lbs torque?

Thanks, for I am still work in progress!

[chris11211]

1. Lug nuts vs lug bolts. Big difference.

2. The idea is to stretch the bolts to secure better, but many mechanics just ugga dugga with their fancy impact wrench.

[jonesg]

5 ft lb isn't a lot but 5 more ft lbs is exponentially greater when its already at 100.
I hit one with the impact, check with the torque wrench and ugga dugga them all to the same amount.

[RickHaleParker]

C. 2001 Volvo XC70 Vibration Damper (4 screws)
Stage 1 = 18.5 ft.lbs
Stage 2 = 30 Degrees
3. First stage means to torque to that set amount, then turn 30 Degrees.
4. If so in (#2), why isn't the "30 Degrees" calculated and included to the Stage 1 torque. In such case, there would be only one torque to say, example, instead of the specified "Stage 1 18.5 ft.lbs" it would be just one 20 ft.lbs torque?

Thanks, for I am still work in progress!

It is done this way because the point of Maximum Tensile strength is in the Plastic range.

The 18.5 ft.lbs is the end of the elastic range ... The Yeild point.
30° stretches the bolt a specific length in to the plastic range to get Maximum Tensile strength.

Elastic Phase: The bolt will stretch while under torque, but returns to its original condition.
Yield Point: The point that separates the elastic phase from the plastic phase is known as the yield point.
Plastic Phase: Once a typical bolt reaches the yield point and you continue to apply torque, it enters the plastic phase. Bolt will not won’t return to its original length and condition when torque is removed.

Don't over stretch the bolt. If you do you will over shoot the Maximum Tensile strength and can stretch it to the point of failure. This is why one does not reuse the bolts. Go slow on the stretch.

You know when you entered the plastic range. The bolt will suddenly start turning easier as if the bolt turned into plastic. You have all done it at one time or another and wondered what the hell!

STYbolt.png (29.64 KiB) Viewed 980 times

[volvolugnut]

Thanks Rick Parker. This explanation of torque and plastic strain is from college level engineering course in Strength of Materials. Careers are built on these ideas. The concepts of bolting torque expands on elastic/plastic strain and is complicated by friction between the bolt head and clamped surface, nut and bolt thread, and the nut and clamped surface. Dirt, lubricants, imperfect machining of threads, previous damage, and any washers in the stack add other factors to the torque to obtain the desired bolt stretch. The tools to measure torque are also subject to errors. Thick books have been written about all these details.

The joint design difference (real or imagined by the designing engineer) are the likely source of the Volvo and Toyota torque specification differences.

volvolugnut

[kallekula]

I " ugga dugga" with my Ingersoll Rand till it stops when I change wheels. Never had one come off yet .

[volvolugnut]

I " ugga dugga" with my Ingersoll Rand till it stops when I change wheels. Never had one come off yet .

That is the thing with torque specs, there is a very wide range of what will work for most cases. What will often cause a failure is the small fastener under the hand of the large installer.

volvolugnut

[kallekula]

I used to work for a shop where my colleague forgot to tighten the lugnuts on a customer car. Twice. He was out smoking so much so he forgot.
Thank god the customers survived and that they didn't sue. Since then I tight lugnuts probably more than they need to.

[TisMe]

I used to work for a shop where my colleague forgot to tighten the lugnuts on a customer car. Twice. He was out smoking so much so he forgot.
Thank god the customers survived and that they didn't sue. Since then I tight lugnuts probably more than they need to.

If you were on the east coast I'd almost assume that was my dad's one-time use tire guy, but looks like this is a countrywide misstep from too much daytime devil's lettuce.

This is logically why a lot of mechanics overtighten your lugs when you cruise in for a tire swap, as you point out. Can't blame them.

[kallekula]

Thats right, rather too much torque