How Are Torqueing Specs Determined

I watch every episode of How It's Made: Dream Cars, most of them more than once. One thing they always mention is how certain parts are torqued to certain specifications. Tonight I was just watching the episode on the California T. I think more than any other show, they mentioned how many different sections were torqued. In fact for the timing chain cover they actually use a numbered template to let the tech know the order for the 40 bolts that need to be tightened.

I was wondering how do they determine just what is the right torque for a particular bolt. Is it derived from a formula based on the stress of the part, is it done through trial and error, or is there some testing to derive the correct torque, or is it done some other way?

Anyone know how a torque spec is determined?

 

Torque is specified for two main reasons;

1. To prevent loosening (friction)
2. Clamping load (strain)

The clamping load is impossible to monitor since it relates to bolt stretching, thread pitch and elasticity or bolt strain therefore we resort to torque which is easy to monitor and is influenced mainly by friction (thread and head). In manufacturing, we often resort to angle as our final setting i.e. reach preset torque -> set at angle -> monitor final torque.

To answer your question, engineers would specify the torque based on requirements as mentioned above. A series of tests are then performed which include plotting stress vs. strain curves and determining the Ultimate Tensile Strength (UTS). Since a bolt should always be used within its elastic region, then the yield point should also be known. This now completed the initial design where we have;

-Bolt OD
-Clamping load requirement (strain)
-Bolt grade. 10.9 is common in automotive for high clamping load applications. The 10 referred to the Ultimate Tensile Stress (1000 MPa) and the 9 means Yield point is 90% of UTS.

A thorough validation plan (thermal + vibration) would confirm the torque specification is either correct, incorrect or over designed. We typically look for any fracture, bolt loosening, shear etc. before design release.

Since we typically use torque to set fasteners, one major factor to keep in mind is debris or lubricants within the thread. These contaminants may not effect torque but will vastly effect the clamping load resulting in either under stressing or over stressing of the joint. Hope that answers your question.

 

Sam has posted great info above. It's often very difficult to find torque charts - wonder why?

The reason is, design clamp load. 2 fasteners used for different purposes often have different torque requirements and the reason is desired clamp load. Using charts is by far less than ideal. When you have nothing else it's valuable but it's always best to get torque specifications from the manufacture of the assembly.

Remember: every action has an equal but opposite reaction.

A bolt provides a clamp force because it is being stretched and the stretch always wants to return to the un stretched state. That force is the clamp load.

Another example: if you weigh say 180 lbs. when you stand on the ground, the ground reacts your weight by pushing up 180. (normal force)

 

Yes, yes, and yes -- plus a lot of prior experience

One point I'd add is that on many (if not most) of the fasteners on an F engine, the torque is much lower than for a similarly-sized steel bolt + steel nut (or steel bolt into steel female thread) because the limiting load factor is pulling/stripping the aluminum female threads out.

 

Steve - good point but from my experince it seems Ferrari uses studs for those purposes while other connections in aluminum (with bolts) have metal inserts.

 

No, you miss my point -- the stud pulls out of the aluminum before the threads in the (steel) nut or the threads on the (steel) stud strip. (For example, the M6 cam cap nuts spec being the lowish 6 ft-lbs; whereas, the typical value for a decent quality M6 steel bolt into a steel nut might be 8~11 ft-lbs.)

 

Ah, thanks for those explanations.

So if you replaced a bolt or bolts that were either stronger or weaker than the original bolts, your torque spec would actually be a lower or higher value than the original. Would that be correct?

 

No, I didn't totally miss you point but thanks for the clarification. Regarding studs, however, I would imagine they are often used over bolts so the stud can stretch and also so removal (cycles) of the nuts is not within the aluminum.

So hear is a question for you Steve as I have never installed studs in a Ferrari head - are these bonded in or do they use threaded inserts? I ask because the cylinder head nut torque is substantial.

 

In theory, yes but not necessarily as one would need to consider the design clamp load and if the fastener was near yield. Obviously you should use the same fastener grades as OEM unless you have some valid reasons to not or originals are NLA.

 

Neither - they use a larger thread size on the stud end going into block:
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So although the torque on the M10x1 cylinder head nut is sort of "normal-ish" for that size, the load it generates on the threads in the block is much, much lower than would be typical for an M14 thread size. I think you will find that, on any stud going into aluminum where the thread is the same size on both ends, the torque specified by F for the nut is lower than would be used on a steel bolt engaging steel threads (even though the stud and nut are steel).

 

Let's start by using the correct terminology. Bolt strengths are classified by grades. The higher the grade (number before the period), the higher the Ultimate Tensile Strength. This is a property of the material and its hardness and irrespective of the bolt size so you can have 5mm and 20mm bolts with the same grade (same strength).

So let's examine both scenarios with the assumption that the original design was correct:

1. Suppose you use a stronger bolt (higher grade) where a weaker bolt (lower grade) used to be. Assuming you apply the same torque, then the joint should have sufficient clamping load but you will likely reach torque with less angular travel. This is because higher grade bolts are harder and less ductile. Problems however may arise under cold conditions where the joint may come loose.

So replicating the original design intent by adjusting the torque alone will be difficult for reasons already mentioned above (it's all about the clamping load).

2. Now suppose you use a weaker bolt (lower grade) where a stronger bolt (higher grade) used to be. Again, assuming you apply the same torque, then the joint may be over stressed since the bolt could exceed its yield point. This is the point where the bolt goes from elastic deformation into plastic deformation. Under high temperature, the bolt will yield even further resulting in either a loose joint when it returns to room temperature or even total failure.

If you simply lower the torque for this scenario, then you will likely not have the required clamping load over temperature as originally intended.
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Can we also get a short lecture on single use TTY (torque to yield) fasteners vs re-useable?
Oh and how bolt shank dia plays into this? Distribution of friction tq etc...

Bolt science is fascinating in a boring and tedious way

J/K

Sam, Steve, great job keeping it simple and precise!

 

Great thread
Very informative

A possibly tangential question: how do safety wiring and thread locking adhesives factor into design planning and maintenance?

 

Awesome - thank you Steve - learn something new everyday

When I worked for GM, I don't recall them doing it this way but that was a long time ago and I don't recall all of the details

 

Great, informative post, Sam

 

Adding to this. Torque is applied/set at room temp. At operating temps, with a steel bolt in aluminum, the pre-load applied by the bolt will increase as the thermal expansion of alum is greater than steel. Thus the initial pre-load may be lower in this instance to achieve the desired value when hot.

As to why a bolt is torqued it does not increase the ultimate strength of the part. Pre-load, from torque, is applied to prevent parts from separating under load, e.g. to keep the head seated on the block.

 

As a TR owner with the single wheel nut (actually a bolt), I would have loved the extra assurance a safety wire provides. It's there to limit the nut from spinning completely off which is why Ferrari revised the design as with the F40's. This joint is a good example of where retention is more critical to function than load.....not to understate the importance of loading up the wheel against the hub taper (vibration, instability etc).

As for adhesives such as Loctite, I would guess it's used for low torque applications that require retention over preload but I know that's not the case. I used it on my brake calipers and flywheel which require high tensile bolts and high torque. I'll just resort to saying its for extra retention but others may have a better explanation.

There are other locking techniques such as Nyloc fasteners (nylon strip through the thread longitudinally) popular with grub screws due to the lack of a head to provide clamping or even locking nuts with a nylon insert. I'd say these are predominantly used for retaining cosmetic features without the need for a mechanical load.

 

Thanks guys. SMG, you've got me thinking. When I find time, I want to find the relationship between diameter-tensile load and diameter-friction (torque). Should be interesting to plot over thread number engagement

 

I'd lean towards the use of loctite for fasteners that experience harmonic frequencies that would loosen the bolt/nut assy. Suspension, engine etc.. go thru various frequency ranges it's quite helpful.

 

Here's a site for some light reading...
Bolt Science Web Site

 

Ditto, very informative.

I also wondered about Loctite/Threadlocker. It is specified for some fasteners on my Harley. And along those lines I have also used anti seize lubricant on spark plug threads that are torqued. I would assume if there is any impact in using these, Harley already factored it in to their specs.

I also recently assembled a stationary bike with nyloc nuts. They are convenient, and I assumed they supply those rather than lock washers because the skill level range of the end assembler may vary widely!

 

Nyloc nuts are used because they work, not the skill level of the end assembler. Used widely in the aerospace industry.

 

I agree. As an example, the targa latches in my GTS keep loosening. This is due to vibration and also give the size of the fastener (m5 or 6), if I torques them to proper values for bolts that size, I would strip the threads without question.

Same goes for very fasteners....

Blue Loctite is very good for vibration holding and keeping fasteners from coming loose. I just keep forgetting to put it on my targa latch bolts

 

And Auto. I think the difference is lock washers add to the clamp load and stretch of the fastener. I don't think Nyloc's do that (do they?) I've never looked up the engineering data behind them....

 

The use of washers is an interesting one, aside from the obvious spreading out the load, there is the surface friction to consider and that inner dia of the washer should be as close to the shank dia as possible to be most effective. Material science plays a role too as the material in question being bolted/clamped will also determine the use of a washer or not and what kind or type.