There is more to Torque

[kartracer55]

Well we all know what a torque wrench is, and how to use it (hopefully) but whoda thought it was this complicated. Im going to post this, and hope somebody wants to comment, provide more info, clarify, or discuss this. Personally I think all this sort of thing is really cool, unfortunatly, it opens up a whole new world of second guessing myself when I go to torque something...

"From Shigley's "Mechanical Engineering Design" 7th ed.

Bolt Condition K Factor

Nonplated black finish .30
Zinc-plated .20
Lubricated .18
Cadimum-plated .16
With Bowman Anit-Seize .12
With Bowman-Grip nuts .09

Torque = K*Fi*d

Fi = Sp*At*0.75 (or use 0.90 for permanent connections)

You can look up At (Tensil Stress Area) and Sp (Proof strength)



Example: I have a 7/8-14UNF-2A Grade 8 bolt that is phos & oil and spec'ed to 400-425 lb-ft.

Phos & Oil
K = .16 (from here)
Sp = 120 ksi (aka 120,000 psi) Min Proof Strength for Grade 8 from .25"-1.5"
At = .509 in^2
I'll assume non-permanent so 0.75
d = .875in

Torque = 0.16*120,000*0.509*0.75*0.875 = 6413.4 in*lb
Divide by 12 to get ft*lb
Torque = 534.45 or 535 ft*lb (for you huge nerds out there don't worry that the units don't cancle out...that is taken care of in the K factor )"




Pretty cool, but

Jim


Discuss!

 

[Fast Orange]

Jim-
I believe what you've posted is the correction factors for calculating maximum torque for various conditions.While this is interesting reading,very seldom will a mechanic ,twisting wrenches on a motor need to get this involved.These calculations are normally done by an engineer designing/spec'ing hardware and procedures for assembly /repair of machinery.
Other factors involved in determining proper torque values are the co-efficient of friction between materials used for the male/female threads,amount of crush for gasket materials. The formulae you posted appears to be for a simple bolt/nut
assembly,showing factors to compensate for lubrication, self locking and non locking nuts and ultimate yield strength.
Interesting info,but only needed if you're building fom scratch and engineering to a specific set of circumstances-for example cranes,bridges and aerospace.
Go by your service manuals-don't try to re-invent the wheel.

George

 

[kartracer55]

haha yeah, It was part of a big argument about wet torque vs dry torque and all that sort of stuff (which quickly got over my head) I just find this sort of stuff extremely intriguing(sp?) I was actually thinking about getting a copy of that book, but Its not cheap, and most of it is probably over my head, but there def. seems like there is a lack of good entry level books on this sort of stuff. So is this sort of stuff what M/E is?

Jim

 

[79rallysport]

Jim, I am 8 credits away from my BS in mechanical engineering from SUNY Binghamton (upstate NY). The book you referenced from is a text book from one of my classes, Machine Design. I took the class about 2.5 years ago. That book is a very useful one if you plan to get into the design of machines (involving linkages, gears, sprockets, cams).

This is one small part of what ME really is. Mechanical Engineering is a pretty broad field. Some other aspects of ME are: Thermodynamics, Fluid Dynamics, Heat Transfrer, Vibrations, Material Science. Of course there are fundamentals that every engineer must have: Math, Physics, Chemistry, Statics & Dymanics, Circuits

Sounds like you've been contemplating if you should get into ME after high school. If you think you'd be interested in any of these subjects, you should give it a shot.

Sorry I got off topic.

 

[kartracer55]

Yeah, Ive actually been thinking about it alot. I know I still have a ton to learn, maybe im behind where I should be, but Ive tried to arrange my courses around some sort of engineering field. I take the most advanced courses I can in the "important" subjects (math and science) and I have a few other honors courses thrown in to help out t he GPA. Ive also been taking CAD courses, that might help me out a bit.

I have a feeling that textbook is a bit over my head, Is there anything out there thats simpler? Kinda like a good intro/basic reference for things like using gearing pullies bearings etc? I dont think i can handle a full blow ME reference book just yet

Thanks

Jim

 

[79rallysport]

I can't think of anything off hand other than maybe the "bible", the Machinery's Handbook.

Edit: Or maybe Standard Handbook of Machine Design. I've never seen or read the latter, just FYI. I'm not sure how introductory it is.

 

[CraigFL]

As a mechanical enginner, I've been involved in the bolt torque problem -- lubricated or unlubricated, many times. When it really comes to critical structural or mechanical design, you want to use the "turn-of-the-nut" method. Most design is based on the clamping force between the two pieces generated by the bolts or studs. It is extremely accurate to use the turn-of-the-nut method because you specify exactly how many turns(or flats) you move the nut(or bolt head). and clamping force is directly related to the stretch of the bolt. Since the pitch of the thread is directly related to the stretch of the bolt, the clamping force can be accurately predicted. For example, if your desired clamping force means the bolt needs to stretch 2.5mm, and the thread pitch is 1.25mm, you need to turn the head two complete turns from "snug tight". Here's where it gets a bit vague... What is "snug tight"?

The problem with the turn-of-the-nut method is that the amount varies with the diameter of the bolt, length of the bolt and pitch. Someone had to invent a way to make it simple so they came up with a torque method. Even though it has all these problems if you look at it "microscopically", it still is amazingly close to what you need. Companies that make torque indication devices have tried to incorporate more of these variables into their products for improved accuracy.

Probably the greatest research on this subject was done by civil engineers in their efforts to provide efficient and safe bolted connections for things like bridges and other structures.

 

[MXtras]

Probably 98% of the engineers in the real world - myself included - never contemplate designing close enough to the edge to require such a level of detail when designing most equipment or machinery. Aircraft, aerospace and possibly automotive engineers are the guys that use this realm of engineering frequently - don't know about Civil (maybe they do, but I doubt it). I am pretty confident that almost all designs will typically utilize industrial standards that are accepted, known and safe - but I am into machinery and automation so I obviously cannot speak for all engineering branches .

I was involved with a leading edge technology where the limits of materials were put to the test and thread torque had to be right. This is one of the only times in my career that I actually had to calculate theoretical torque and stretch as part of the design and thankfully I was only out of school for maybe 2 years so my math was still fairly sharp. The part was for an ultrasonic cutting system and was oscillating at 20KHz while moving up to .1MM!!! That's a LOT of stress! It was also a lot of fun. I got to play with lots and lots of Titanium and breaking stuff was completely expected! Talk about fun!

Anyway - I think that most designers play it safe and take the safe approach to design by avoiding using anything outside the norm. Well - this is my method most of the time with only a few exceptions - like machine anchoring. There are always so many more issues to deal with than having to worry about your assembly falling apart due to fastener/joint failure because you designed things too close to the limits.

Sorry for the blabber - I agree that torque is critical and I agree that most mechanics probably don't comprehend how complex it can be. But then again - they aren't designing the engine - they are assembling and hopefully they know what torque is and where to find the numbers.

Scott

 

[YJTypeR]

Would that number you calculated be the applied torque to the bolt that causes failure? So you would not want to necessarily tighten it that much if its going to be seeing any load that is trying to stretch the bolt more. Seems that that makes more sense to me? (I'm also an engineering student) You have to REALLY like math if you're wanting to take this route of study. Through my classes I have heard mention of industry standard safety factors that have to be taken into consideration in any case like this. Say hypothetically you would want a 5% margin of error because as stated earlier, you don't want to operate right at the designs limits, you would be just asking for failure.

 

[CraigFL]

In general, you stretch bolts and studs to 90% of their yield strength to get the maximum clamping force. Calculated/measured torque tries to include the extra torque required for friction between the turned element, friction in the threads(using some lubricant for what's left from the manufacturing process and protectant), and starting(sticking) torque. So what torque is left, is what goes into stretching the bolt thru the ramp angle of the threads. Usually, bolts and studs that are torqued have a minimum yield strength which would give you at least a 10% safety factor and more than that on breaking.