Correct procedure for wet and dry torque specifications 2

[Ocean76er]

Hello,
This is my first post, and I hope I posted in the correct forum.
My question pertains to the Automotive field.
My understanding is that when torque is applied to a dry bolt, that the majority of the torque is applied to friction at the threads and the nut or bolt face areas. The information I see is varying, but it seems like the amount of friction between the bolt face and the threads is almost equal, lets say roughly 45 percent to the threads, 45 percent to the bolt face, and only 10 percent of the torque applied stretches the fastener.

Now my questions, and for an example I will refer to a simple fastener like a shock bolt, or a caliper slide pin bolt:

A) Most automotive service information does NOT specify whether a fastener torque specification is classified as wet or dry. Is it safe to assume that if not stated otherwise, they are referring to DRY torque specifications?

B) When using a WET torque specification, is it proper procedure to lubricate only the threads of the bolt, or both the threads AND under the face of the bolt or nut? If there is a washer, lube both sides of the washer?

C) When using a DRY torque specification, is it proper procedure to be sure that the entire fastener is completely clean and dry, with absolutely no lubrication whatsoever, at any of the friction points?

Sorry if my questions seem remedial, but I am crossing over from years of experience of tightening fasteners by feel, to tightening to manufacturers specifications with a proper torque wrench. However service information can be vague when it comes to details, thus my questions.

Thanks in advance for information provided, as the internet is loaded with misinformation, and opinions stating (what I usually do, or how I interpret it). I would rather have factual information, and it seems to me when half of the friction is at the bolt or nut face, the proper lubrication procedure would be relative.

 

[drawoh]

Ocean76er,

Friction torque on bolts is highly variable. If you need precise control over bolt tension, you need something other than bolt torque to control it. Bolt installation procedure also is wildly variable outside a shop of highly disciplined mechanics and technicians. If you are the designer, be careful making assumptions.

When I see bolt torque specifications, I pull out my old machine design textbook, and I redo the torque calculations. I want to know what assumptions were made. Did they assume lubrication? What percentage of yield/proof stress did they use?

If you are using the torque equation T=CDF, your torque coefficient C is very crude. If you need to do this accurately, you probably need to work out an installation process, and do practical tests with it. Then, you need to get your assembly people to accept your procedure.

--
JHG

 

[DAVIDSTECKER]

There is a "welding, bounding and fastener engineering" form here that you should probably look for.

This subject has been covered in the past with great detail.
Maybe try a search.
Best regards, David

 

[mfgenggear]

as a hands on guy, it's easier with a torque wrench, dry, clean and not damaged, how ever it's my under standing bolt (elongation) tension is way more precise and accurate.
I like using Loctite when permissible, and low strength Loctite as not to vibrate loose.

 

[TugboatEng]

If it's an important fastener, the fastener will have a specific procedure that should be adhered to.

Otherwise, experience says it's really not important for general bolting. Most fasteners come with a coating on them anyways that does effect friction and renders the dry torque meaningless.

For us working with heavy duty engines, important bits such as heads, rod and main bearing caps are all done torque angle now. A quick torque to 150 ftlbs and then crank it another 180 degrees is an example head bolt procedure vs the older 420 ft-lb final torque. Torque angle takes advantage of the flat stress strain curve during yield so you don't have to be terribly precise about where you end up.

 

[drawoh]

TugboatEng,

Half a thread pitch (180[°]) is quite a lot of strain to put on a bolt. Are you torquing down ductile bolts one time only, or are you compressing gaskets?

--
JHG

 

[dvd]

You need to correlate torque with tension -

https://www.skidmore-wilhelm.com/

make testers that will perform this function. Fastenal has some reasonably good literature on the topic of bolt tension in joints -

https://www.fastenal.com/content/feds/pdf/2017/02/Bolted Joint Design rev 2017-02-21.pdf

 

[mfgenggear]

I mean it's very simple far as the procedure it self. millions of automotive vehicles are repaired every day. hard part is what torque values to apply from the designer.
aluminum is the tough one company's don't like even giving values because of stripping out so easily. but Loctite and anti seize are your friends.

 

[TugboatEng]

3/4-16 bolts, ~6 inch grip length. There are two gaskets being compressed and some ferrules. It's a shim steel lower, aluminum spacer plate with rubber coated ferrules, and compressed fiber upper with a thin stainless steel fire ring. Other than compressing the fire ring I think everything else is already fully compressed at the initial torque.

 

[Ocean76er]

Thanks for the replies.
I am having a difficult time adjusting to using the torque wrenches, due to the sometimes extra leverage and bulkiness of the handles. They take away the sense of feel I have been accustomed to, so I am trying to duplicate exactly what the service information provides, but the information is vague, providing only a torque value. The torque amounts specified almost always feel a slight amount looser than I would have made the fastener by hand. I am using digital wrenches BTW. It seems like newer vehicles are being manufactured with inferior materials, compared to years ago, so I as I am getting older, I am trying to be a bit more meticulous in my work, and follow the service information, rather than relying on feel. I am seeing a bulk of the OEM parts coming from over seas now. I am seeing higher failure rates of critical fasteners, on occasion. A simple tire rotation takes me over an hour now, since I have to carefully inspect each wheel stud and nut for damage from improper installs. A majority of shops are still using heavy impact tools to tighten the fasteners. I think that was acceptable in the past, as I mounted thousands of wheels with impact tools with great success, but now I rarely use them. I won't even use the torque sticks.

 

[TugboatEng]

They take away the sense of feel I have been accustomed

That's the entire point of torque wrenches.

It seems like newer vehicles are being manufactured with inferior materials, compared to years ago

If you're working on newer vehicles, the fasteners come with lubricated coatings on them, you see it as the greenish color in Japanese vehicles.

since I have to carefully inspect each wheel stud and nut for damage from improper installs.

Yes, damage caused by not using a torque wrench or torque stick.

 

[Ocean76er]

From an engineering perspective, is there a general margin of error that is taken in to consideration when determining a published torque specification, that is not printed? + or - 15% for example? I see some torque specifications printed that way, 100ft lbs + or - 15 ftlbs for example.

 

[mfgenggear]

The margin of error is built into the torque wrench. in other words when it passes calibration. I have a beam style torque wrench because it will never lose calibration if taken care of. as soon as the pointer hits the spot eg ft/lbs in/lbs then your good. clickers are more precise but can lose calibration.

 

[TugboatEng]

Instead of giving a nominal value ± an error tolerance (mechanics don't like doing math) they give you a range. 100 ft-lb ± 5% is the same thing as 95-105 ft-lbs. Shoot for the middle and hope your torque wrench is in spec.

 

[mfgenggear]

it depends on the precision of the tools. cost $$ goes up

 

[DAVIDSTECKER]

I've had this book on fasteners for decades and if you can get a hold of a copy I highly recommend it.
Here are some pages I scanned on the topic.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1618429479/tips/21041414_27_20_c9twxu.pdf[/url]

 

[desertfox]

Hi Ocean76er

The answer to your first question is yes always assume the torque is a dry value unless a lubricant is specified. If any lubricant gets on the fastener and no lubricant is mentioned or specified then using the torque value laid down could over tighten the joint or the fastener itself.

The next question is a difficult one because even if you only apply lubricant to the threads I think it would be impossible to keep lubricant off mating surfaces of a clamped joint. For example imagine lubricating threads in a blind hole and then tightening a screw or bolt into it, the chances are that lubricant will ooze from the hole during the tightening.

The third question would be yes where at all possible.

At the end of the day it’s the designer who should specify what torque etc is required for the joint. Torque as others have said is not the most accurate way to achieve a bolt tension and the designer should select the method and lubrication if required whilst keeping in mind an acceptable tolerance on the bolt tension which we not impair the integrity of the joint.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Ocean76er]

@desertfox
Thank you for directly answering my questions!

@DAVIDSTECKER
Thank you for the reading material!

I specifically like the chart for typical targeted preloads, and the explanation of the 30 or 40 variables that can effect the friction seen in a threaded fastener. The chart helps me possibly understand now why I feel that some of the published torque specifications seem on the low side.