Great conversation, guys. We might be electronics nerds, but the subject of practical mechanics seems to be as detailed as the many parameters of an op-amp.

Right now, I'm thinking, for 5,000 pounds of stretching force, that means 5000 pounds is the friction force on the threads and the interface of the bolt head to the outer surface. F=-KX doesn't say anything about the area of contact. That would seem to indicate that the number of threads is irrelevant in the friction department. Then again, K must be derived from something. Does a ton sitting on a square foot move easier than a ton sitting on a square yard?

 

The simple model of friction is that contact area doesn't matter because the frictional force per unit area is directly proportional to the pressure but the pressure is inversely proportional to the area of contact, hence it's only the total load that counts.

Like pretty much all simple models, that one is good to first order but not too good beyond that. If it were, then the traction obtainable by a tire would be independent of the size of the tire, yet one look at racing tires, particularly dragsters, indicates that that is not the case. Similarly, the behavior of tires in low-friction situations shows how important contact area is.

 

Ok, let me try this explanation. The torque value on a bolt is the amount of rotational force it takes to get the bolt to elongate enough to put the correct clamp tension on the joint. Until the head of the bolt contacts the part being clamped, there is only the small amount of friction in the threads, very negligible. Once the head of the bolt contacts the part surface, that is when the torque starts to build in value. Then one surface of the thread shape becomes involved in the friction causing the torque to rise. The whole time the elongation of the bolt shank being put in elongation/tension due to the pull between the one side of the thread and the underside of the bolt head. The amount of rotation during this "friction" is minimal and is why the pitch doesn't play that big a deal in it.

Torque in a bolt/nut situation is the easiest way of making sure that clamping force is "near" correct. If all of the procedures are followed, the correct lube on the threads, the part and bolt head lubed, the bolt and part threads correctly sized it works.

The torque to yield on the engine head bolts was to overcome the need for all this correctness. The automatic wrenches tighten to a smaller torque value, back off then go to that value a second time then turn "X" degrees(that was figured out from testing). To put the bolt in the materials yield range, insuring sufficient clamp.

 

Irrelevant question: If the bolt is already in its yield range, what happens when the explosions start in the engine?
I assume it must require more force to stretch the bolt farther, but you're already in its yield range. Another counter intuitive!

 

How are you getting that the slope is 1:13?

The circumference of the thread is nominally pi/2 inches, or 1.57", so the slope of the incline of the thread is (1/13)"1.57)" or ~1:20.

Ah yes, thank you for the correction. The concept of circumference completely escaped me. As I'm sure you exercised great restraint in not pointing out, "had I only carried the units," I would have realized that the unit "threads" simply disappeared, and that would have been a clue.

 

On the elongation point, I have seen a large dia. bolt with a small dia. bolt running down the middle of it. The small bolt had a washer under its head and was set with a feeler gauge so that the gap between the washer and the head of the big bolt was known. The big bolt could then be tightened up until the washer was no longer loose and so the elongation was known.

Mu understanding is as shortbus says, that pitch for all "normal" bolts makes very little difference because the frictional forces dominate.

 

Great conversation, guys. We might be electronics nerds, but the subject of practical mechanics seems to be as detailed as the many parameters of an op-amp.

Nerds will be nerds. We could take just about any universally understood simple concept, mechanical, electrical, societal, whatever, and dissect it and Rube Goldberg the shit out of it until only 5 of us can claim to still understand it, and even then, 3 of the 5 would probably be lying. I'm sure the mechanic nerds are the same way. They probably have a forum somewhere, where they have a strantor; someone who asks inappropriate electronic questions on the mechanical forum because he feels comfortable with the members and doesn't like making new friends. It probably sends them into never-never land on a regular basis.

Does a ton sitting on a square foot move easier than a ton sitting on a square yard?

I'm guessing that would have to depend on the type of surface. Is the surface soft, and likely to deform when a concentrated load is put on it? like sand; I imagine trying to drag a 1-ton cube down the beach would be harder than dragging a 1-ton plate, since the cube would dig in more, and you'd be moving more sand than cube. But if you were dragging a cube across plate glass, it would be easier than dragging a plate, since there would be less surface contact.

Notice in the bolt torque formulas and tables there are different constants for lubricated, dry, zinc plated, etc.

 

First, LOL
Second, I was assuming a hard surface to push the ton across.

 

Ok, let me try this explanation...

Thank you for your patience shortbus. I get it now. I try to think in extremes to better grasp concepts, but this concept precludes my process. I cannot think in extremes here, because the concept applies only to the real world where there are no extremes. Typical bolts are typical bolts, and the variance between their pitches is not extreme. In the range that their pitch does vary, the variance is not enough to offset the bigger issue, friction.

 

So all the online references - published by manufacturers working in the industry - that include a thread pitch factor are wrong? Sorry, I'm not buying it. I'll stick to the expert opinion until someone ponies up some credible data.

 

Irrelevant question: If the bolt is already in its yield range, what happens when the explosions start in the engine?
I assume it must require more force to stretch the bolt farther, but you're already in its yield range. Another counter intuitive!

The yield range is 'just' entered on the individual bolt. Not to the point of 'total' yield. Then, for most all car engines you have four or some use five of these bolts taking the "explosive" load on each cylinder. Explosive is an often used but wrong idea in an internal combustion engine. It is a controlled burn, not an explosion. The only time there are explosions in the combustion process is when an engine goes into detonation. Detonation happens when something other than the spark starts the burn, before it is supposed to. Detonation is usually from the burn starting and then being compressed, resulting in excessive pressure in the cylinder.

Going back to the screw jack talk, a screw jack can't be compared to a clamping bolt situation. Look a any screw jack meant for a real load. You'll see a thrust washer of some type between the handle end of the screw and the 'anchor point on that end. For a light duty jack it may just be two hardened washers, on a heavier duty jack(like for a car) it will be a ball or roller bearing. The thrust washer prevents the 'friction' that starts the torque build up in a clamping situation. Then , in a jack, you only have the friction on one side of the thread.

 

So all the online references - published by manufacturers working in the industry - that include a thread pitch factor are wrong? Sorry, I'm not buying it. I'll stick to the expert opinion until someone ponies up some credible data.

The thread pitch factor only means that the threads can take more of a load. The threads in the bolt it's self and the threads in the part or nut. And that since the fine pitch threads need to have more movement put on them to get the elongation to give the clamping force. The only way to get that movement is to use more torque.

Torque is just a name for the easiest way to assure the amount of elongation. It is not torque as in the amount of force to move or lift something. How else would you propose to measure it? Something that is hidden, like the stretch/elongation of a bolt shank inside a part. You could now days put a strain gauge in every bolt made, but is that practical? Don't think so. The SAE and other groups decided long ago that tightening torque was close enough after doing testing.

 

And that since the fine pitch threads need to have more movement put on them to get the elongation to give the clamping force. The only way to get that movement is to use more torque.

Sorry if I'm being dense and not understanding you. The original question was how to relate measured torque to tensile force, and whether to include a factor for thread pitch when making this relationship. It appears to me the factor is small, on the order of ~10% more torque for fine threads to give the same bolt tension. Personally, I think this is large enough to include in an estimation although it barely exceeds measurement error.

 

Going back to the screw jack talk, a screw jack can't be compared to a clamping bolt situation. Look a any screw jack meant for a real load. You'll see a thrust washer of some type between the handle end of the screw and the 'anchor point on that end. For a light duty jack it may just be two hardened washers, on a heavier duty jack(like for a car) it will be a ball or roller bearing. The thrust washer prevents the 'friction' that starts the torque build up in a clamping situation. Then , in a jack, you only have the friction on one side of the thread.

That jives with what I've been pondering the last couple days. Since I don't have that car jack any more, I can't inspect it. But I'm pretty sure it did not have a frictionless bearing, however I think it did have a large thrust washer that we lubricated with a heave grease. Probably designed specifically to minimize friction.

I'm not absolutely convinced yet, but I am swayed that your position is very plausible. Given the mechanism involved and that it is really a question of the degree to which one factor is allowed to dominate the other factor, it is something that needs to be done experimentally. You've been involved with such experiments and so I'm at a point to grant the benefit of the doubt until (and unless) I do some experiments are see other experimental evidence to the contrary.

Things like this would make neat little projects for me to explore with my daughter at the right age, like perhaps when she is taking a high school physics class.

 

Sorry if I'm being dense and not understanding you. The original question was how to relate measured torque to tensile force, and whether to include a factor for thread pitch when making this relationship. It appears to me the factor is small, on the order of ~10% more torque for fine threads to give the same bolt tension. Personally, I think this is large enough to include in an estimation although it barely exceeds measurement error.

Ok. lets try this way, forgot it earlier, been a long time since I had to use this information. Along with what I said in post #32, the reason behind more torque for a fine thread is that torque and tensile strength of a bolt have NOTHING to do with its shank diameter or the "bolt size". It is the root diameter of the thread. This is the diameter that is the controlling dimension in strength of a bolt. That and material strength. Haven't looked at a screw thread size sheet, but I'll bet you if you do there will be a real close correlation between the difference in root diameter of the thread (coarse to fine) and the torque value for the bolt (coarse to fine).

Almost all bolts for 'critical' purposes are made with the shank between the end of the threaded portion and 'close' to the head, are made with that area at or close to root diameter of the thread form. I say "close to the head" because they have a short space that is bolt size diameter, to center the bolt in a hole or washer. This is done so the shank elongates properly at tightening. Remember I said critical purposes, go to your hardware store and they won't be made this way. Those are common bolts. But pull a headbolt out of your car engine and they will be 'necked down' under the head of the bolt.

Ever break a bolt when working on something? Did you think of why they always break at the thread at the parting line between the tapped part and the part being held to that tapped part? It's because the root diameter of the bolt thread is the smallest diameter of the bolt.

 

...the reason behind more torque for a fine thread...

So I think we agree that the proper tightening torque for a fine-threaded bolt is (minimally) higher than for a coarse thread? That's really the only point I was chasing. Strantor was asking about the effect of thread pitch on the relationship between torque and bolt tension, and I'm still struggling to understand where you stand on that.

 

I'm pretty sure there is no correlation between pitch and torque, other than the root/critical diameter of a fine pitch is larger than a coarse pitch. Which makes the fine thread marginally stronger. Theoretically if you could make one or the other with a flat topped thread form and the same root diameter, the torque would be the same, to give the same clamp force.