Torque Tightening

Thanks for your prompt reply Sir,

1. STUD BOLT QUALITY / GRADE : ASTM A193 B7

2. COATING: PTFE COATED

3. SIZE: M27 x 270MM LONG

4. APPLICATION: MATERIAL WILL USED IN OFFSHORE WITH ENVIRONMENTAL EFFECT FPOR LIFTING OF HOSES.

Awaiting your reply Sir

You could have said that in the first place, Mildred, couldn't you? Now stop SHOUTING, and go away and use Google to find your answer. I'm not going to do that for you, because that might make you lazy, and we don't want that now, do we?

You don't mention thread pitch (there are at least 5 for M27) and that will make a difference to your torque specs. Generally, the finer the pitch, the higher the tightening torque.

Again, generally a PTFE bolt will have a tightening torque of approximately 60% of that of an un-coated bolt.

My ASTM A193 B7 chart does not show specs for an M27 PTFE bolt, but it does show for a 1" x 8 TPI PTFE bolt, which would be very close to an M27 x 3mm (1.063" x 8.47 TPI).

The figures on that chart for a PTFE coated 1" bolt are 280 ftlb or 380 Nm.

If you compare those figures to the 1" x 8 TPI specs on this chart, and multiply by 0.6, you will get pretty much the above numbers

In general the finer the thread the lower the torque for same preload. Of course this is valid in my world.

PTFE has a friction coefficient of about 0.03-0.04. Uncoated threads lubricated have coefficients around 0.08-0.10 and unlubricated 0.12-0.16. The ratio is not 60% in any case.

Now, a stud has NOT to be tightened to the same torque as a bolt with same thread since the friction surfaces are VERY different (a lot lower in the stud case). The nut on the stud has to be tightened at same torque as a bolt.

"In general the finer the thread the lower the torque for same preload. Of course this is valid in my world."

You're correct in that statement, but I made no mention of preload.

In almost all cases, tensile, yield and shear strength are considerably greater in a finer threaded bolt of the same size and material, this means that a greater tightening torque can be applied before the bolt's elastic limit is exceeded.

Excellent post.

You must have stayed up all night composing that.

Wish I could give you 2 GAs but I'm sure others will oblige.

Ditto!

GA....

On the grounds that you deserve two GAs for the earlier post: if someone else will join me.

Everything I always wanted to know about torque,bolts,nuts and thread pitch,but was afraid to ask.

Excellent information that I will archive for reference.

I can actually "see" the factors you are speaking of.

You missed your calling if you are not a teacher.

A GA from me also.

Thanks for taking the time to give such a thorough explanation.

I also give a GA.

A further consideration is the circular expansion of the nut under the hoop stresses induced by the sloping pitch. This reduces the shear area. The less high the nut, the less material, the higher the stress and so expansion of said nut.

There is also a contributory factor to how many threads on the bolt should stick out past the nut : the bolt end will dimple if for instance there is less than 1 whole thread sticking out. This again lowers the shear area. But this is not good practice and is generally avoided.

Do you mean the tensile, yield and shear strength are considerably greater in a finer threaded bolt because the stress area is greater?

The usual basic formula I'm aware of is T = 0.2*F*d/1000,

T = torque N*m, F = bolt load N, d = nominal dia mm.

Based on dry threads, and derived by assuming pitch = dia/8, and that only 10% of applied torque results in bolt tension, the rest lost in friction. Pitch = dia/8 is right or nearly so for most coarse series metric bolts, and for other pitches the formula (in my data) doesn't change, presumably because though finer pitch would produce higher load, this is offset by greater friction loss. So with finer thread hence greater stress area, for a given stress, load and torque are higher. Some people might think the above approach is a bit crude, but for general purposes it works OK in practice. I notice in the link in #2, 0.16 is used instead of 0.2.

I don't think ordinary grease makes much difference, my theory is the extreme contact pressure cuts through the grease layer, but the grease is still good to stop corrosion. Maybe it's different with special greases eg moly, or PTFE coating.

I wasn't talking about what torque could be applied by differing pitches but rather what the bolt could stand before exceeding its elastic limit.
The tensile, yield and shear strength are considerably greater in a finer threaded bolt of the same material simply because it has more meat on it, finer threads are shallower, so there is a greater minor diameter for the same major diameter.

I do not want to discuss the formula. I only want to make a point with respect to grease effect. Your theory would be valid if the flank surfaces would be mirror like. In fact surfaces are rough and grease enters as well the roughness of the male as the one of the female thread. Under pressure peaks are flattened and some grease becomes free to lubricate the surfaces in sliding. There are of course also metal to metal contacts and shear resistance of the peaks. Resulting friction is as called "an intermediate friction" between the dry friction with direct contact metal to metal all over the contact surfaces and the hydrodynamic friction when the film thickness is such that no contact can appear any more.

However the minute quantity of grease is sufficient to reduce considerably the friction. The effect is more important with dry lubrication as MoS2 grease for instance since the MoS2 particles (as graphite also) do have a high adherence to the surfaces and present a low shear resistance.

Only as information the mentioned formula in my comment is pretty well validated by measurements and is used in friction or tightening analysis.

OK, I wouldn't argue with that. There have been several threads on bolt tightening and as I said on earlier one - I've been greasing bolts (for ease of future dismantling) for a great many years, and never had any work loose, or broken any off on tightening.

Also in my booklet on ductile iron pipework, it gives flange bolting torques and says "The relationship between applied torque and the actual load imparted by the bolts is not precisely predictable, therefore the values given in the charts are of necessity an approximate guide." The torques are mostly well below what's needed to give say 80% proof stress. But it doesn't say what the bolt load should be or give the torque formula used so it could be back-calculated. So even if there is an accurate way to relate bolt load to torque it doesn't help much.

Merry Christmas!

In standards the friction coefficient used for the torque computation IS specified. I checked and found that they used same relationship as I presented. the problem is that in the tables the SAME value is used for both surfaces which is not true.

As an order of magnitude if the bolt is well lubricated friction can go as low as 0.08 !

If grease is sparse it goes up to 0.12...0.14 and if the thread an the other surface are dry it can grow up to 0.22. You see the difference.

As for the tightening of flanges the problem is more complex since the flange and gasket stiffness play a non neglectable role. Even if the recommended tightening order is respected the preload is NOT uniform. This is the reason the tightening is done in steps -small steps- to avoid big differences. But what ever is done the distribution is still not fully uniform.

As for the bolts, it is not usual to obtain available of the shelf fine pitch.

Merry Xmas to all

Ref tightening of flanges - I just meant that if you want to go through the full flange design procedure a la eg ASME VIII, using gasket pressure at bolting-up and operating conditions etc, to arrive at a bolting-up bolt load, AND you have a good bolt load vs torque formula, you're OK. But most people won't go to that trouble, and just using the data in my pipe booklet, knowing bolt load accurately from torque doesn't help much, as you don't know the bolt load.

Glad you agree only one pitch, whether it's called coarse or standard, is available off the shelf.

Merry Christmas again!

What are the 5 (or more) pitches for M27? I'm curious. According to my data metric bolts are available in 2 pitches, coarse and fine, and fine is only used in special cases. M27 is a non-preferred size, with pitch 3mm (same as M24). My data shows fine pitch going up to M24, which has 2mm pitch. Could be out of date, but if not M27 is only available in coarse.

I assume we're talking about nuts and bolts. For other types of thread could be different eg electrical thread, but for that there is no M27, just M24 and M32, both with pitch 1.5mm.

One search, two sites - Engineer's edge and MD Metric - that show 5 separate M27 pitches ranging from 0.75mm up to 3mm.

Another, Newman tools shows 3 fine M27 pitches of 1.0, 1.5 & 2 mm.

I didn't say that all were readily available or even commonly used, I simply stated that they are there and can be had if required.

The OP didn't specify what he was using the thread for, it may well have any of them, torque specs are considerably different between finest and coarsest pitches.

OK, thanks for the links, interesting.

I was aware metric threads are defined for loads of diameters, every mm for larger sizes, even smaller increments for smaller, but didn't know there were quite that many! But fortunately, AFAIK many of them aren't available as bolts, even as non-preferred sizes. Eg I don't think you can get M25, M28, M32, or M35 bolts. Similarly with the range of pitches.

To somebody my age who remembers dealing with BSF, BSW, UNF, UNC to name just the main ones, metric bolting was like a breath of fresh air. Maybe the Americans will eventually catch on ! One of the advantages was having just one pitch (coarse) for nearly all applications. I don't think most of the other pitches in the links are available as bolts, but I could be wrong. If they were the situation would be as confusing as in the bad old days.

You say torque specs are considerably different between finest and coarsest pitches. If you can be bothered to go through Nick Name's detailed analysis to verify that, will you post the results please?

Another gripe is the tendency to supply spanner sizes in every mm across flats (in the typical automotive range say M6 - M20), although not all are standard AFs. Though it can be useful as I have found 12mm and 15mm AF bolts on cars. I once had a case of a pipe flange with M27 bolts, and we went equipped with 41mm AF spanners, but they were 42mm, and kit that size isn't cheap. If all manufacturers stuck to the spec things would be simpler.

Merry Xmas!

But there's nothing to calculate when using a table.

May I add to your exhaustive comment that the friction coefficients are DIFFERENT on the 2 sliding surfaces (thread flank and nut/heat ).

The assumption they are equal leads to important errors an should be accepted ONLY in the case of not high loaded assemblies.

My reference does not mention that, but I can see the validity and importance of the difference; thank you!

So modified... T = (F_a / 12) [(P /2π) + (R_tf_t / Cos θ) + (R_sf_n)] ?

Length of lever times the force applied. Did I miss anything?

start in the middle and work to the edges

And what does the stud manufacture recommend? They should have all those specs on file. I would be calling them and not some anonymous forum. I would think they would be responsible for any failure within their torque specs and your responsible for anything outside their recommended torque.

Contact the manufacture, period.

The student asked the instructor "Why do we need a torque wrench?"

"Well, you really don't.Simply tighten the bolt until it almost breaks."

The student thought about the answer a minute,then asked:

"How do I know when the bolt will break?"

"That is why you need a torque wrench."

Seriously,there are plentiful tables and charts that list the maximum torque for bolts

and studs according to the size,material,and thread pitch.

Solar Eagle gives a very good link to start.

You must be a fledgling in the mechanical field.

I could give you the information outright,but then you would never learn to fly.

Stretch your wings and take the leap.

Genius is not so much about knowing something,as it is in knowing where to find it.

Learn how to use Google,it is the closest thing I know to a real "Know it all."

Flog it up until it breaks, then back it off a quarter of a turn.