Fastener hex size and how it affects needed torque value

[parris001]

Okay, I'm not much of a poster on here. I mainly like to just stalk and I'll admit I learn a lot from some of the brilliant minds on here.

I'll preface my question by saying that I am an instructor at a trade school, one of the oldest of its kind in the country. And I had this discussion with a fellow instructor the other day and we came to an impasse. There was a lot of his argument I understood.

So the question is, does the size of the head of a bolt affect the needed torque to get the desired bolt stretch that ensures the maximum clamping force for the bolt. I'll go ahead and answer the question, it's yes. My case in point is back when I was building Detroit Diesels there were two possible main bearing bolts. One had a 12 point 3/4" head (early) and the other was a 15/16" head (later). I always hated dropping the oil pan and finding the 3/4" heads because my Ingersol Rand 244 wouldn't take them down, always ended up having to get out my big 3/4" ratchet and breaking them loose. If I dropped the pan and found the 15/16" head it was no problem. The 244 would rip them down after a little hammering.

The stated torque specs in the book were 250-260 lb/ft for the 3/4" head bolt and 230-240 for the 15/16" head. Which, is even kinda messed up right on its face because the underside on the head of the bolt on the 15/16" is a larger surface area and would have you believe there would need to be more torque applied to overcome that friction.

My question to the group is, why does head size dictate this? I tried to break it down to an analogy where a theoretical bolt, let's say it's 1/2"-13, has a head that's an absurd 2" hex. And the same bolt has a 3/4" head. With just your two fingers and hand strength you could get the 2" head bolt tighter than you could the one with the 3/4" head.

Let me know where I'm missing the obvious answer.

 

[PoorUB]

I would suspect it was the material the bolts were made off more that the type of head. The manufacturer used different heads to easily identify the two.
I am not certain the area under the head has much to do with it other than to spread the clamp load over a larger surface of the mating part.
do you remember if Detroit made any issue why the bolt change? Any main cap failures in the early engine?

 

[Leaflessshadetree]

Hex/head style or size does not affect the torque requirement to achieve proper bolt stretch.

The different spec more likely is due to thread size, shank diameter or some other design change was made as the result of performance experienced with the earlier design.

Thread pitch, platings/coatings, length of thread engagement are all things that could affect the force required to loosen a fastener and there could be many more (loctite, anti-sieze, heat).

Look at any engineering table for torque requirements. Head size/style is not one of the criteria.

 

[parris001]

I would suspect it was the material the bolts were made off more that the type of head. The manufacturer used different heads to easily identify the two.
I am not certain the area under the head has much to do with it other than to spread the clamp load over a larger surface of the mating part.
do you remember if Detroit made any issue why the bolt change? Any main cap failures in the early engine?

Same bolt material and hardness. When I'd find a engine being junked that had the 15/16" head bolts in it I'd always grab them up. And there was never an answer given as to why they made the change.

 

[parris001]

Hex/head style or size does not affect the torque requirement to achieve proper bolt stretch.

The different spec more likely is due to thread size, shank diameter or some other design change was made as the result of performance experienced with the earlier design.

Thread pitch, platings/coatings, length of thread engagement are all things that could affect the force required to loosen a fastener and there could be many more (loctite, anti-sieze, heat).

Look at any engineering table for torque requirements. Head size/style is not one of the criteria.

The bolts were identical and from the same manufacturer (F-C). All that was different was the size of the head.

 

[cmandp]

I think your answer is more to do with the rotational moment of inertia of the socket. The R squared term drives the equation. Whats the diameter difference of a 3/4 and 15/16? At least 3/16".

Its like how the Honda crank bolt sockets work. See here.

 

[cvairwerks]

Part of the answer is the area of the contact surface area under the head of the bolt. I’d be willing to bet that if you calculated that area and then multiplied it by the torque, you would come up the same numbers.

 

[parris001]

So the instructor I was having the conversation with was saying that the area of the underside of the bolt was the difference in the two torque values. And I had that lightbulb moment where I thought, dang. I bet that's it! Then I shot that down when I realized the bolt with the smaller contact surface area under the bolt required a higher torque value than the larger contact area. Which, that seems backwards.

 

[wssix99]

Here's a decent article that explains some of the key considerations here: https://www.mromagazine.com/features/importance-of-proper-bolt-torque-in-power-transmission/

The short answer is that standardized bolts of a common specification will have uniform torque values that change by size. Those torque values match head sizes not because of the head size, but because the head size is standardized to the bolt size and type. (Per all the aspects that @Leaflessshadetree mentioned above.)

The surface area on the bottom of the head is insignificant compared to the surface area of the bolt threads. So much so, that I recall engineers typically ignore it from the friction calculations that lead to figuring out these torques. As you can see in the picture below, the head plays its part in transferring the clamping forces to the pieces.

As the bolt tightens, the threads engage and the bolt threads start to touch the threads in the tapped piece or nut. When this happens, the bolt stretches, the threads come in closer contact and clamping forces start to develop along the threads.



As the bolt stretches and the threads come into closer contact, friction also develops along the thread and resistance to the bolt "unscrewing" out also develops. I recall also that customized torque values may also control for this and have lower values for some applications vs. the maximum torque values listed for bolts to develop their maximum clamping forces. (before failure)

As the article points out, there is a difference between wet and dry torques and all the aspects of a bolts specification play a part in what the maximum torque will be. (Material, bolt size, bolt length thread lubrication, thread pitch, thread angle, bolt coatings, number of uses/cycles on the bolt, enviornmental conditions, ... and then all the same things for the tapped material or nut.)

 

[wssix99]

The stated torque specs in the book were 250-260 lb/ft for the 3/4" head bolt and 230-240 for the 15/16" head. Which, is even kinda messed up right on its face because the underside on the head of the bolt on the 15/16" is a larger surface area and would have you believe there would need to be more torque applied to overcome that friction.

These are not standardized bolts and I would expect the torque values are specially calcualted for the application.

In this case, I would guess it's about clamping force and both applications would give the same "lbs" of clamping. The 3/4" bolt would develop more tension in the bolt and have a higher PSI of pressure at the head. The 15/16" bolt would have less tension in the bolt and lower PSI of pressure at the head. Even with lower pressure exerted on the piece, the higher surface area of the 15/16" bolt could equate to the same total lbs of force imparted on the part.

My guess is the company had issues with the original bolts and changed them to create a kinder and gentler environment. (With the diameter of the actual bolt shaft, one could do this calculation and check the proportion to the torque.)

 

[parris001]

These are not standardized bolts and I would expect the torque values are specially calcualted for the application.

In this case, I would guess it's about clamping force and both applications would give the same "lbs" of clamping. The 3/4" bolt would develop more tension in the bolt and have a higher PSI of pressure at the head. The 15/16" bolt would have less tension in the bolt and lower PSI of pressure at the head. Even with lower pressure exerted on the piece, the higher surface area of the 15/16" bolt could equate to the same total lbs of force imparted on the part.

My guess is the company had issues with the original bolts and changed them to create a kinder and gentler environment. (With the diameter of the actual bolt shaft, one could do this calculation and check the proportion to the torque.)

Note that it's not actually a 3/4" bolt and a 15/16" bolt we're talking about here. The bolt is kinda an oddball. It's an 11/16"-11. Won't find that in the bin at Home Depot. I guess that old thread had been used way back since the inception of the V-71 Series.

 

[Walkers]

The torque values may be the same, but picture this. You have two jars of tomato sauce, both equally torqued. Both jars have the same size threaded opening. One lid has an od of 1” and the other has an od of 3”. which one is going to be easier to remove? The 3”, because you have better leverage. I would think the same principal is applied to your bolt heads. Your impact wrench is marginal at that torque and the bigger head has a little better leverage.

 

[parris001]

The torque values may be the same, but picture this. You have two jars of tomato sauce, both equally torqued. Both jars have the same size threaded opening. One lid has an od of 1” and the other has an od of 3”. which one is going to be easier to remove? The 3”, because you have better leverage. I would think the same principal is applied to your bolt heads. Your impact wrench is marginal at that torque and the bigger head has a little better leverage.

That's exactly the same argument I made with the other instructor. Evidently great minds think alike. I gave the jar analogy of with just your bare hand and no wrench to measure the torque needed to remove the lid, the lid of a larger outside diameter will take less effort to remove than a lid that would be smaller.

Take it to an extreme and let's say my main bearing bolt head has one that's the early 3/4" head and it takes me with a torque wrench straining my guts out to torque it to 260 lb/ft, and the same bolt but the head of it is the size of a dinner table. And my 7 year old grandson can torque it to spec with almost zero effort.

 

[jack stand]

I might be taking this the wrong way, but not considering the +/- surface friction under the fasteners head wouldn't the measurement of the imagined centerline of the fastener to the actual wrench or socket surface give a larger bolt a slight advantage?
Now it's probably an irrelevant difference, but "on paper", taken to the extreme- what would 100ft/pounds do with say a 5/8 thread when comparing a 3/8 wrench on a cap (allen head) vs the same 5/8 thread that had a weird head that took a 4" monster socket?
Torque is rotating force around (I guess) the exact center of rotation. (??)
Wow I'm rambling but this post is interesting and inquiring minds.....
Maybe this expresses MY added question better, foot pounds are the effort at a 12" c/l radius
Would it change if you're "grip" was with a 3/8 allen wrench on a cap bolt vs a hex head that itself had a 2" radius to the wrench/socket grip point.
(Edit) You guys hit on my point/question while I was trying to spit it out

 

[cvairwerks]

A quick and dirty run on the values of the head contact area, including removing the shank area and neglecting hole clearance, the numbers are:
3/4 head has an area of about 1.08 square inches with a value of about 229 when you divide the low torque by the area.
15/6 head is about 1.92 square inches and has a torque/area value of about 120...
So, the 15/16th's has about twice the contact area as the 3/4 and only requires about half as much torque to provide the same clamping force, based solely on head contact area. Remember with this dirty calculation, we are only looking at the torque required to meet the clamping force and not taking friction or bolt stretch into account.

 

[parris001]

A quick and dirty run on the values of the head contact area, including removing the shank area and neglecting hole clearance, the numbers are:
3/4 head has an area of about 1.08 square inches with a value of about 229 when you divide the low torque by the area.
15/6 head is about 1.92 square inches and has a torque/area value of about 120...
So, the 15/16th's has about twice the contact area as the 3/4 and only requires about half as much torque to provide the same clamping force, based solely on head contact area. Remember with this dirty calculation, we are only looking at the torque required to meet the clamping force and not taking friction or bolt stretch into account.

I think you've nailed it. I reached out to a friend I consider to be one of the greatest mechanical minds out there. He said he thought the clamping force under the head of the larger bolt took less than the smaller. Basically, it's a pounds per square inch thing. Smaller surface area is going to need more to do the work, larger surface needs less.

 

[cvairwerks]

Yep...that's why you will see washer head bolts or large area washers or clamping plates where the required clamping force is great enough to damage the materials being clamped. In the case of the Detroit heads, the 3/4 bolts were probably found to be inflicting long term damages on the heads around the bolt holes and the easy solution was to move to a bigger head bolt and adjust the torque value to maintain the same clamping pressure.

 

[king nero]

So the question is, does the size of the head of a bolt affect the needed torque to get the desired bolt stretch that ensures the maximum clamping force for the bolt.

No. the stretch (and subsequently the clamping force ) is basically/predominantly dictated by the friction (underhead + threads) , the bolt diameter and the torque applied to the bolt.
A bigger hex head evidently gives you an advantage to apply the torque, as mentioned in different examples above.

No idea why different bolts in the same application would require a different torque, though. Your example is counter-intuitive, I would assume the same as you did. However: are the two bolts identical (material strength, delivery conditions)? The smaller head one might have required a lower initial torque value because the threads were prelubed, a washer was provided, the threads were plated, anything that might have lowered the required torque...

 

[PoorUB]

The torque values may be the same, but picture this. You have two jars of tomato sauce, both equally torqued. Both jars have the same size threaded opening. One lid has an od of 1” and the other has an od of 3”. which one is going to be easier to remove? The 3”, because you have better leverage. I would think the same principal is applied to your bolt heads. Your impact wrench is marginal at that torque and the bigger head has a little better leverage.

I disagree. The head diameter has nothing to do with the torque as far as tightening or loosening goes. It will make a difference with the jar lid because you are dealing with your grip, where with a bolt head it doesn't come into play. It may have a bearing on the amount of friction under the bolt head, but it was already discussed that the thread are by far the highest pint of friction.
If you have a couple washer head bolts and everything is the same other than the size of the bolt hex it will take the same amount of torque to turn either. If you notice, in general, bolt torque charts do not consider the style of bolt head, just strength, diameter, coatings and pitch.

 

[parris001]

Yep...that's why you will see washer head bolts or large area washers or clamping plates where the required clamping force is great enough to damage the materials being clamped. In the case of the Detroit heads, the 3/4 bolts were probably found to be inflicting long term damages on the heads around the bolt holes and the easy solution was to move to a bigger head bolt and adjust the torque value to maintain the same clamping pressure.

Well, it was main bearing bolts that we were talking about. The reason this was always an issue is that lazier mechanics would always torque their main cap bolts to the lesser value because it was, obviously, easier to do. They would say, "don't matter the head size" and take the easy route. I'm equally lazy but at least I was salvaging the larger head bolt when I could find them and I'd throw the smaller headed bolts away.

 

[jeffg]

I dont have any theory or math to support this, but I know of a few counterpoints to this idea. On my Subaru, there are many 12mm bolts (head size), and many different torque specs. Now obviously this could have been based on the usage, but I imagine you would want the maximum clamping force in most cases, or at least enough that they wouldnt come loose. The torque specs range from a few inch pounds to several ft-lbs. I assume this was because the actual bolts were different materials, threaded into different materials, etc. I assume a bolt threaded into an aluminum part would have a lower torque spec than a bolt threaded into steel or a nut, but they would have the same size bolt head.

 

[parris001]

No. the stretch (and subsequently the clamping force ) is basically/predominantly dictated by the friction (underhead + threads) , the bolt diameter and the torque applied to the bolt.
A bigger hex head evidently gives you an advantage to apply the torque, as mentioned in different examples above.

No idea why different bolts in the same application would require a different torque, though. Your example is counter-intuitive, I would assume the same as you did. However: are the two bolts identical (material strength, delivery conditions)? The smaller head one might have required a lower initial torque value because the threads were prelubed, a washer was provided, the threads were plated, anything that might have lowered the required torque...

I do agree that bolt stretch is ultimately what we are trying to achieve. We had an instructor here come up with an exercise where he threaded a bolt into a steel billet, where just a couple threads of the bolt protruded out the back side of the billet. And he machined flat the bit of bolt sticking out to make it flush with the piece of billet. He would loosen the bolt in this exercise and tighten to just hand tight and you could see where the bolt was recessed. Apply the correct torque and every time the bolt would become flush from the stretch. Now eventually wear came into play and the bolt quit returning to its original length. But his assessment was that he could tell you exactly how much any given bolt would yield to a stretch and be at it's proper torqued value. This is done with rod bolts all the time in racecars. They don't even bother putting a torque wrench on a rod bolt. They just tighten it until the micrometer tells them it's at the correct stretch.

We tend to know that if there's a certain value of pounds per square inch being exerted by the underside of the head of a bolt that if everything else in the equation is right, the bolt will be at the correct amount of stretch.

As far as these two different bolts go, they both used what Detroit Diesel calls International Compound on the threads and under the head of the bolt and the washer. There is no initial torque. But they came from the same manufacturer and were made the same. Just different sized 12 point heads.

I'm losing faith in my loosening the top on the jar analogy. That's going to start heading down the path that the length of the torque wrench affects the torque delivered if you carry that reasoning out to its conclusion.

 

[parris001]

I dont have any theory or math to support this, but I know of a few counterpoints to this idea. On my Subaru, there are many 12mm bolts (head size), and many different torque specs. Now obviously this could have been based on the usage, but I imagine you would want the maximum clamping force in most cases, or at least enough that they wouldnt come loose. The torque specs range from a few inch pounds to several ft-lbs. I assume this was because the actual bolts were different materials, threaded into different materials, etc. I assume a bolt threaded into an aluminum part would have a lower torque spec than a bolt threaded into steel or a nut, but they would have the same size bolt head.

A lot of "unimportant" bolts in an engine aren't torque to yield (or to the maximum the bolt can physically take) because they maybe hold on the water pump. And too much would squeeze the gasket out. Or a bolt may thread into aluminum and you'd pull the threads out. Or the bolted on part might be plastic. So you see, there's lots of bolts that have what got called in the Detroit Diesel torque spec section "exceptions to common torque". These usually all went less that the given or commonly accepted torque value for these fasteners.

 

[cvairwerks]

Ultimately, the torque on a fastener is a simple, repeatable way to apply a compressive force on a material joint. Whether it is done with TTY, torque to stretch dimension, hydraulic nuts, eddy nuts, you want the force the designer calls for, and torque on the fasteners is one of the easier ways to accomplish it.

 

[wssix99]

Note that it's not actually a 3/4" bolt and a 15/16" bolt we're talking about here. The bolt is kinda an oddball. It's an 11/16"-11. Won't find that in the bin at Home Depot. I guess that old thread had been used way back since the inception of the V-71 Series.

Yes, then both are a 11/16" bolt. One with a 3/4" head and the other with a 15/16" head. That's exactly what I was illustrating, although - did they have different washers or a flange head? That would also change things, too.

 

[FMB4]

Like others above have said; fastener torque specifications are dependent on application, the grade of the faster (bolt or nut/stud), and the length of the said fasteners. The 'size of the head of a bolt or nut' has little to do with torque specs.

Edit: bolts and other fasteners do have a maximum torque specification per diameter and length that should not be exceeded. But again, the head size for said specs is not always dependent on just the head size (external or internal hex, spline or torx, etc).

 

[wssix99]

The torque values may be the same, but picture this. You have two jars of tomato sauce, both equally torqued. Both jars have the same size threaded opening. One lid has an od of 1” and the other has an od of 3”. which one is going to be easier to remove? The 3”, because you have better leverage. I would think the same principal is applied to your bolt heads. Your impact wrench is marginal at that torque and the bigger head has a little better leverage.

That's exactly the same argument I made with the other instructor. Evidently great minds think alike. I gave the jar analogy of with just your bare hand and no wrench to measure the torque needed to remove the lid, the lid of a larger outside diameter will take less effort to remove than a lid that would be smaller.

Take it to an extreme and let's say my main bearing bolt head has one that's the early 3/4" head and it takes me with a torque wrench straining my guts out to torque it to 260 lb/ft, and the same bolt but the head of it is the size of a dinner table. And my 7 year old grandson can torque it to spec with almost zero effort.

Both jars take the same amount of effort to unscrew. If a human percieves a difference, it's because the grip of their hand is different.

If you put a jar wrench (for arthritis patients) or a good strap wrench on the two jars, they would "feel" the same.

Head has nothing to do with it. The moment arm extends from the center of the bolt to the end of the tool.

 

[wssix99]

Now eventually wear came into play and the bolt quit returning to its original length.

This is because the coefficient of friction was increacing with wear. This changes all the torque equations.

The solution to this is to oil the threads before doing the experiment and demonstrate a wet torque. If one does this, everything should last a lot longer and be more repeatable.

 

[parris001]

Yes, then both are a 11/16" bolt. One with a 3/4" head and the other with a 15/16" head. That's exactly what I was illustrating, although - did they have different washers or a flange head? That would also change things, too.

Same type washer but a smaller diameter because it was proportional in size to the 12 point head.

 

[wssix99]

Same type washer but a smaller diameter because it was proportional in size to the 12 point head.

Then the forces are being transferred through the head and to the washer. As @cvairwerks pointed out, the change was probaly made to address finer issues related to metal fatique around the head of the bolt and the surrounding metal. The larger bolt head (with its 2X surface area) is MUCH kinder to the washer underneath.

Small changes in the washer diameter can adjust to keep the clamping loads the same with the different torques. (Or maybe Detroit Deisel also intended to lower those loads with the change they made.)

 

[Mallen]

The under-head frictional force is solely determined by the tension on the bolt. The contact surface area is not relevant. That's the very first thing you learn about friction.


Consider this.

You have two identical bolts with 1" heads. You put an adapter on one of them that gives it a 2" head. Just a little metal cap. Now put a torque wrench that is 1' long on it and tighten it down by exerting 50lbs of force on the handle. What is the torque that the bolt sees? 50ftlbs of course.

Now take the second bolt, put the same torque wrench on it, but with a bigger socket. Apply 50lbs of force to the wrench. Does that bolt get tightened more? Of course not. Its STILL tightened to 50ftlbs of torque.

Now consider what's going on. There is a socket. Its attached to the torque wrench. The force is being applied at 3 sets of opposite corners. Let's simplify it.

Instead of a hex bolt it's a 2" slotted screw. Instead of a hex socket it's a 2" flat screwdriver bit. The screw slot is parallel to the torque wrench. Now apply the torque to the screw. Assume a normal right hand thread screw. On the near side the force is applied an inch closer. On the far side it's an inch farther away. The pivot point is the center of the bolt.

Its the same as two levers with their pivot in the center of the screw, one 11" and one 13 inches.


The force is split between the two levers but acts on the screw in opposite directions. But since they act at opposite points on the screw they create a net torque.

So let T be the torque due to the 11" lever and S be the torque due to the 13 inch lever and W be the force on the wrench handle. Let r be the radius of the screw head.

Total torque = T+S = (W/2)*(d-r) + (W/2)*(d+r) = W(d-r+d+r)/2 = W*2*d/2=d*W

So the radius of the bolt head falls out and the torque depends only on the force applied to the wrench handle and the length of the wrench.

Now, as was mentioned, what if the head was 5' wide? Shouldn't it be far easier to tighten? The answer is no. Look at the equation. There is no radius in the final answer. The ONLY thing that matters is d, the length of the torque wrench. If the bolt head is larger than the length of the wrench, it's irrelevant as you are still applying the force at distance d from the center.

Let's talk about clamping force now. The clamping force is solely determined by the amount the bolt stretches. The friction on the bolt is determined by the force normal to the surfaces of the bolt that contact the threads in the hole, plus the force normal to the surface under the head times the force normal to that surface. (again no area is used in these calculations) the friction on the threads is probably rather complex to calculate. I think you'd have to integrate over the radius to sum up the differential bits of friction due to the dot product of the differential bit of clamping force and the normal vector of the surface at that point. The friction due to the head is simple, it's just the friction coefficient times the clamping force. But the long and the short of it is, torque to tighten the bolt is some effective friction coefficient of the whole bolt, times the clamping force on the bolt. One uniquely defines the other. If you make the bolts the same, so they have similar friction coefficients, you can specify the torque, and the result is a predictable clamping force.


I agree that the different heads were probably made to reduce the pressure which was probably deforming the fastener or the part. I bet it's not even an oddball custom thing. Its probably a standard fastener for those sorts of applications.

 

[Walkers]

Both jars take the same amount of effort to unscrew. If a human percieves a difference, it's because the grip of their hand is different.

If you put a jar wrench (for arthritis patients) or a good strap wrench on the two jars, they would "feel" the same.

Head has nothing to do with it. The moment arm extends from the center of the bolt to the end of the tool.

I think it is really no different than using a long wrench vs a short wrench. Same amount of work, just different math to achieve it.

 

[finn]

The under-head frictional force is solely determined by the tension on the bolt. The contact surface area is not relevant. That's the very first thing you learn about friction.


Consider this.

You have two identical bolts with 1" heads. You put an adapter on one of them that gives it a 2" head. Just a little metal cap. Now put a torque wrench that is 1' long on it and tighten it down by exerting 50lbs of force on the handle. What is the torque that the bolt sees? 50ftlbs of course.

Now take the second bolt, put the same torque wrench on it, but with a bigger socket. Apply 50lbs of force to the wrench. Does that bolt get tightened more? Of course not. Its STILL tightened to 50ftlbs of torque.

Now consider what's going on. There is a socket. Its attached to the torque wrench. The force is being applied at 3 sets of opposite corners. Let's simplify it.

Instead of a hex bolt it's a 2" slotted screw. Instead of a hex socket it's a 2" flat screwdriver bit. The screw slot is parallel to the torque wrench. Now apply the torque to the screw. Assume a normal right hand thread screw. On the near side the force is applied an inch closer. On the far side it's an inch farther away. The pivot point is the center of the bolt.

Its the same as two levers with their pivot in the center of the screw, one 11" and one 13 inches.


The force is split between the two levers but acts on the screw in opposite directions. But since they act at opposite points on the screw they create a net torque.

So let T be the torque due to the 11" lever and S be the torque due to the 13 inch lever and W be the force on the wrench handle. Let r be the radius of the screw head.

Total torque = T+S = (W/2)*(d-r) + (W/2)*(d+r) = W(d-r+d+r)/2 = W*2*d/2=d*W

So the radius of the bolt head falls out and the torque depends only on the force applied to the wrench handle and the length of the wrench.

Now, as was mentioned, what if the head was 5' wide? Shouldn't it be far easier to tighten? The answer is no. Look at the equation. There is no radius in the final answer. The ONLY thing that matters is d, the length of the torque wrench. If the bolt head is larger than the length of the wrench, it's irrelevant as you are still applying the force at distance d from the center.

Let's talk about clamping force now. The clamping force is solely determined by the amount the bolt stretches. The friction on the bolt is determined by the force normal to the surfaces of the bolt that contact the threads in the hole, plus the force normal to the surface under the head times the force normal to that surface. (again no area is used in these calculations) the friction on the threads is probably rather complex to calculate. I think you'd have to integrate over the radius to sum up the differential bits of friction due to the dot product of the differential bit of clamping force and the normal vector of the surface at that point. The friction due to the head is simple, it's just the friction coefficient times the clamping force. But the long and the short of it is, torque to tighten the bolt is some effective friction coefficient of the whole bolt, times the clamping force on the bolt. One uniquely defines the other. If you make the bolts the same, so they have similar friction coefficients, you can specify the torque, and the result is a predictable clamping force.


I agree that the different heads were probably made to reduce the pressure which was probably deforming the fastener or the part. I bet it's not even an oddball custom thing. Its probably a standard fastener for those sorts of applications.

This!

 

[parris001]

I agree that the different heads were probably made to reduce the pressure which was probably deforming the fastener or the part. I bet it's not even an oddball custom thing. Its probably a standard fastener for those sorts of applications.

Explain this last part. I think I've got the whole of your argument except for this

 

[king nero]

Pressure - Wikipedia

en.wikipedia.org

The larger hex has a larger contact area under the bolt head. The clamping force is thus spread over (divided by) a larger area, reducing the pressure on the part (under the head).

high pressure can deform parts. The common solution when you cannot lower the force, is to enlarge the contact area.

One thing to consider is the use of washers (and sometimes the size of the hole, if there is a large chamfer (which is most often not the case for engines)! A large hex head with an undersized washer does not help the situation at all.

If you want an analogy: a lady in high heels stepping on you would hurt, if she's standing on you barefoot it's very much likeable. Her weight stays the same, the contact area is enlarged.

 

[metlmunchr]

A few Detroits used main studs with fine nuts and the torque spec for those 5/8-18 nuts was a bit lower than that for the 5/8-11 main cap bolts for the simple reason that a fine thread develops more tension than a coarse thread of the same size for the same torque.

All Detroit 71s with bolts on the main caps are 5/8-11 regardless of whether it's an inline or V type engine. The torque spec is 180-190 ft-lbs. IIRC, the latest version of the -71 service manual, published in the early to mid 80's, changed the spec to a lower torque plus so many degrees additional turn of the bolt.

I don't recall seeing any 12 point bolts used on main caps, but if they were used at some point, the flange diameter on a 12 point bolt is the same diameter as the washer face on the same size hex bolt. In the case of 5/8-11, that would be 15/16". So the bearing area under the head is a moot point

 

[larry_g]

Force

Above is a discussion on friction and how the area of contact is pretty much ignored. I remember having this discussion in physics class years ago and in friction equations, the area is not in the equation. There is also the difference between static and dynamic friction.

lg
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