Torqueing Bolts Up to Yield

[DrWazzock]

Fitting a new clutch to my tractor flywheel using M8 grade 10.9 bolts that came with the clutch.
Manual recommends 70 nm which is about twice what I can find on the internet as a tightening torque. Seems a hell of a torque to me.
11 of the bolts went up to that torque and seem OK, just.
1 of them just kept on stretching and wouldn't get up to torque. Backed it out before it snapped and its developed a very thin waist.
Backed one of the others out and it hasn't developed a thin waist.
So I am presuming a duff bolt rather than a misprint in the manual.
Ordered a new bolt.
Does this sound right?

 

[Courier]

STANDARD TORQUE VALUES - METRIC SYSTEM

Sounds way over standard acceptable torque - Much nearer what you would expect for a M10, 10.9

I think a query to the manufacturer with a written reply would be in order...

 

[DrWazzock]

STANDARD TORQUE VALUES - METRIC SYSTEM

Sounds way over standard acceptable torque - Much nearer what you would expect for a M10, 10.9

I think a query to the manufacturer with a written reply would be in order...

It felt and seems way over the top but maybe there is method in their madness. As suggested will check with manufacturer. Thanks.

 

[dave mountain]

seems to me that they've muddled their m8 and m10 torques, it is quite rare to get a duff graded bolt nowadays. be interested to know what the manufacturers reply is.

 

[DrWazzock]

I have taken all of the bolts out and even though they have been up to 70 nm , only the one bolt is visibly stretched and has a serious waist in it. The rest look OK. The generally accepted max standard torque of around 33 nm doesn’t feel that great TBH. Feels like 50 would a put a bit of stretch in them without going OTT. Anyway, fresh set of bolts ordered and will see what the manufacturer has to say otherwise I’m giving them 50 nm and leaving it at that. In fact 33 nm then a fixed further angle would be better way of stretching them a bit as with head bolts.

 

[TechWise]

Recommended torques have always seemed a bit of a mystery to me. Especially since that table indicates that a fine or course thread is irrelevant. A finer thread will produce a greater force for a given torque.

 

[dave mountain]

Recommended torques have always seemed a bit of a mystery to me. Especially since that table indicates that a fine or course thread is irrelevant. A finer thread will produce a greater force for a given torque.

afraid not. the tensile load for a given torque is the same for fine and coarse as the angle of the threads to the direction of the tensile load is the same on both. however that table isnt correct either. the recommended torque for fine thread bolts is very slightly higher due to the slightly larger cross sectional area of bolt carrying the load.

 

[Mur Huwcun]

I’ll have to confess to have never torqued a pressure plate bolt ever, just use spanner on the m8 ones, 3/8 ratchet or small swivel bar on m10s or 12s!!

 

[TechWise]

afraid not. the tensile load for a given torque is the same for fine and coarse as the angle of the threads to the direction of the tensile load is the same on both. however that table isnt correct either. the recommended torque for fine thread bolts is very slightly higher due to the slightly larger cross sectional area of bolt carrying the load.

Interesting. I was thinking of it a bit like a lever, but one that rotates. If you were to turn each bolt one revolution, the coarser thread would advance further into the hole, therefore the coarser thread would sacrifice torque and gain distance, whilst the finer thread wouldn't travel very far but would do it with a lot of force. However, that's if we consider revolutions as the input rather than torque, in which case your point about the thread angles and cross sections makes more sense.

I suppose that's why I'm an electronics engineer and not a mechanical one

 

[ACEngineering]

Interesting. I was thinking of it a bit like a lever, but one that rotates. If you were to turn each bolt one revolution, the coarser thread would advance further into the hole, therefore the coarser thread would sacrifice torque and gain distance, whilst the finer thread wouldn't travel very far but would do it with a lot of force. However, that's if we consider revolutions as the input rather than torque, in which case your point about the thread angles and cross sections makes more sense.

I suppose that's why I'm an electronics engineer and not a mechanical one

i knew exactly what you were on about ( i think! ) and i say your right. you will get a higher clamping force with a finer thread than a course thread using the same torque to tight the bolt.

got 2 vices in the workshop, a cheap piece of crap with a course thread and a record with a much finer thread, the record clamps far tighter.

 

[DrWazzock]

Well I wish I’d just used my judgement and a small spanner not looked in the workshop manual. Complete new set of bolts arriving Tuesday as I don’t trust the overtorqued set. A couple of quid but 4 days delay.
It doesn’t add up though. 12 M8 bolts holding the clutch on a 140 hp engine. Tightened to the standard torque of about 33nm doesn’t seem enough for the size and weight of the thing with added heat, vibration and cyclic loading. But the 70nm specified by the manual seems OTT.
I’ll give them 50 ish. They seemed happy up that point.
The manual actually says 7 daNm. Am I right in thinking this is decaNm, so 70 nm

 

[DrWazzock]

seems to me that they've muddled their m8 and m10 torques, it is quite rare to get a duff graded bolt nowadays. be interested to know what the manufacturers reply is.

Manufacturer says 35 nm. My manual is out of date. Design has been modified.

 

[jd6820]

i knew exactly what you were on about ( i think! ) and i say your right. you will get a higher clamping force with a finer thread than a course thread using the same torque to tight the bolt.

got 2 vices in the workshop, a cheap piece of crap with a course thread and a record with a much finer thread, the record clamps far tighter.

Not strictly true... If both fasteners were torqued to the same the course thread may actually clamp tighter. This is due to the finer thread have more surface area and as a result more friction during tightening hence why course acme type threads are used on scaffold/screw jacks.

 

[jd6820]

Well I wish I’d just used my judgement and a small spanner not looked in the workshop manual. Complete new set of bolts arriving Tuesday as I don’t trust the overtorqued set. A couple of quid but 4 days delay.
It doesn’t add up though. 12 M8 bolts holding the clutch on a 140 hp engine. Tightened to the standard torque of about 33nm doesn’t seem enough for the size and weight of the thing with added heat, vibration and cyclic loading. But the 70nm specified by the manual seems OTT.
I’ll give them 50 ish. They seemed happy up that point.
The manual actually says 7 daNm. Am I right in thinking this is decaNm, so 70 nm

You'd be surprised! An M8 10.9 grade has a 2.1 tonne shear strength, so 12 bolts.... Gonna be some 140Hp to break em! No chance those bolts are suppose to be 70Nm I'd say a typo for sure.

 

[Munkul]

12 M8 bolts holding the clutch on a 140 hp engine.

...Holding the clutch pressure plate, and the pitch circle diameter of the holes is the lever action of the torque on them. I imagine it's a decent diameter of clutch all those bolts really have to do is hold the clamping force on the clutch, which isn't tremendous. The twisting force on the clutch is halved between the flywheel and the pressure plate.