boring ænewbieÆ torque-to-yield questions 1

[jbqwik]

Hello all,
Please overlook my laymen‘s ignorance on this subject.
I’m curious to know more about so-called “torque-to-yield” fasteners (in this case bolts) I have used.
I don’t know why this method is preferred over more ‘usual’ torquing methods.
For example: one TTY bolt precedure I have done requires a certain preload (30 lb.ft.), then a further tightening of 180 degrees, followed by another 90 degrees. Often, while following this procedure, you can feel the bolts stretching.
In another, automotive instance, Buick used TTY bolts on their (1980’s) turbocharged engine head bolt. Today, many engine builders just use standard Grade-8 bolts with apparent success.

So, then, I don’t understand what the ‘big deal’ is with these TTY bolts.
Here are my questions:
Is this process more accurate than torquing to foot-pound spec?
Does this procedure and bolt offer more clamping force?
Are these bolts of a different material than other Grade-8?

I really hope you all will help educate me as to why this method is sometimes preferred over more usual torquing methods (i.e. torque wrench).
Thanks! very much...
Steve Hill

 

[CoryPad]

Torque-To-Yield is more accurate than using a torque only specification.

TTY does offer more clamping force than torque only.

No automotive manufacturers use inch-pound units (e.g. Grade 8) for fasteners. Metric fasteners are used, with Property Class 10.9 being roughly equivalent to Grade 8. Any fastener can be tightened using TTY.

TTY relies on the fact that the variation of 1) joint member elastic properties, 2) joint member dimensions, and 3) screw yield strength is much smaller than the variation in friction during tightening. Thus, a higher preload with less scatter can be achieved for the entire population of fasteners to be tightened.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jbqwik]

alright, so, you suggest TTY is a method only, and, not tied to any special fastener material? Interesting.
I have been trying to quantify the properties of TTY vis-à-vis' torque wrench method. I certainly appreciate your comments, and, this helps. Thanks!
Anyone else?

 

[jbqwik]

Also, I have read the TTY ‘plastic’ or ‘yield’ stage affords special properties that allow a certain give under tension and compression loads, all the while maintaining proper clamp load. Can anyone add to, or, expand upon this?

 

[CoryPad]

Try this:

http://www.boltscience.com/pages/tighten.htm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jbqwik]

thanks corypad!