High angles and very low torques

[adi1]

Joint is a 6 bolt joint and soft one with a gasket in between. We are using torque control angle monitor shut off strategy.

Torque – 240-260 in-lbs
Angle shut off monitor limit – 750 deg
Snug – 25 in-lbs (random guess)

We are using 750 deg monitor because the very first bolt in that soft joint always shows high angles to achieve the target torque of 250 in-lbs and the rest of them something below 750 as the joint gets stiffer and stiffer after each bolt is tightened.

But for some reason we are getting high angles of more than 750 deg on some bolts and the tool shuts off and the torque achieved is very low, some thing in the range of 40-100 in-lbs. I know this generally occurs when something in the joint is deformed. I check for bolt and joint deformation but nothing occurred.

Any ideas?

 

[CoryPad]

How are you tightening them? Simultaneously is best, but not possible for you?

Are you using a "star" pattern? Are you using multiple passes?

"Crosstalk" is always a problem for multi-bolted joints. It may take some detailed investigation to learn the source of the problem.

Regards,

Cory

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

 

[desertfox]

Hi preload/adi1

Is this relating to your earlier fastener threads?


Regards

desertfox

 

[adi1]

simultanoeous is not possible. we are using single spindle dc tool. yes we are doing cross pattern to minimise the effect of cross talk.

clamp load is good. we dont have any failures in that aspect. the only problem is for some of the assembly rundowns we are hitting our max angle of 750 deg even before we reach our target torque.

for example here is the rundown sample

250 in-lbs reached - angle is 400 deg - good pass
220 in-lbs reached - but angle reached is 780 deg - tool shut off due to high angle

today we increased the snug torque as we thought the low snug torque is causing all this high angle problems. we are doing good with an exception of 1 or 2 high angle fall outs which is acceptable

Desert fox, no this is not the previous issue.

 

[CoryPad]

If you don't use multiple tools, or if you don't do multiple passes, or you don't use a complex algorithm like those developed by George Bibel (

http://www.und.nodak.edu/experts/view.jsp?dsas_id=bibelgeorge92616),

then you will have this as a recurring problem.

Regards,

Cory

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

 

[adi1]

Cory,

how does multiple passes help my situation?

 

[NickE]

Multiple passes will help by drawing the two components toghether evenly (and evenly compressing the gasket) at all bolts until on your last pass all you are doing is getting the final tension (torque) into the fastener.

I would use multiple passes at some angle with a rotating star pattern. (IE: start and finish each pass on a different bolt.) Then on the last pass I would concentrate on getting my torque.

This makes lots of sense to me.

I could see at the extreme using n+1 passes total, where passes #1 to n are only to draw up and compress the gasket, and the +1 pass is a torque pass. (And in this case I would set n as the number of fasteners.)


Nick
I love materials science!

 

[CoryPad]

adi1,

NickE has explained the essentials. For a more detailed explanation:

1) Obtain the book Handbook of Bolts and Bolted Joints and read chapter 24.

2) Obtain and read United States Patent 5,278,775.

Regards,

Cory

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

 

[adi1]

Nicke,

Thanks for the explanation.Sounds good to me.But is this process feasible on production line?????? dosent this kill extra time,labor charges and more cost???

Cory,

Thanks for the refrences. Will do that

 

[NickE]

Sure it takes tons of time to perform, and it also takes a good operator.

However How much time do you spend filling out 8-D's and CAR's.....

I go through this all the time where I work. Someone says how do we stop this <insert defect> from happening. After a bit of analysis I come back with a workable solution. The ususal answer is that we cant afford to do that in production.

My usual reply:

How long can we afford to lose business because of low quality?


Nick
I love materials science!

 

[adi1]

Nicke,

Instead of doing multiple pass tightening, why cant we have a clicker wrench set at target torque and make sure all of them reached the torque. This would save more time than doing multiple pass right?

correct me if I am wrong

 

[CoryPad]

we are doing cross pattern to minimise the effect of cross talk

You already have stated why you should use multiple pass tightening (in the absence of simultaneous tightening).

Regards,

Cory

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

 

[NickE]

You could, however you would then have to remove the requirement of angle. Also I dont think you will get even compression across the gasket.

Maybe thats not important, I'm not sure, I do know that multiple passes worked very well when I assembled the primary case (with a thin RTV "gasket") of my 1970 Triumph.

I guess the answer in thsi case is (as it often is) "it depends".

You would get your torque, obviously, what that is going to do to the joint would have to be investigated. If you run the first bolt to torque in one operation the next (assuming a star pattern) would then pull the other area down, then proceding around the assembly.

(Im not really a bolt/joint engineer, just a metallurgist with a whole lot of experience putting things together and taking them apart.)

 

[adi1]

Nicke Thanks for the explanation.

Cory Thanks for the support.