Just pondering the 9 ft/lbs specified for the rear seal housing on my Cressida per the factory manual. How are bolt torque numbers determined?
- posted
16 years ago
Just pondering the 9 ft/lbs specified for the rear seal housing on my Cressida per the factory manual. How are bolt torque numbers determined?
Basically it is a specification that has been determined by past testing and documentation of the given fastener type plus engineering consideration of the assembly parts in question. The two most important things regarding the fastener are 1) it must not break/strip threads at this torque value and 2) it must not back out from lack of tension. The makeup of the parts and their intended duty modify this equation as well.
In the case of a final assembly, lots of data is reviewed before assigning a value to a fastener. Machining and material spec data publications are used by design engineers as a baseline towards engineering their specific requirements.
Toyota MDT in MO
They are determined by "tensile strength"
Here's an interesting link for ya:
dahpater wrote in news:1189567888.580758.94690 @o80g2000hse.googlegroups.com:
Another good one:
In a nutshell.... Bolted connections work because tightening the bolt generates a clamping force between the two items. The trick is to generated as much clamping force as possible without damaging the bolt by permanently stretching it. In a range, a bolt can be tightened(stretched) and loosened without permanent change in length- it will return to its original length. If you stretch it too far, it "yields" and will not return to its original length. The best amount of tightening would be to about 90% of the point where it wouldn't return to its original length-- a little safety factor for the unknown. Now the trick is to try to relate "torque" to the stretch in the bolt, including all kinds of other factors like type of metal, strength, thread pitch, lubrication, etc. Testing and usage has verified common engineering formulas for this and people have set up tables for common bolt size torque values. If you think about it, "threads per inch" tell you that with one turn of the bolt, exactly how far it will stretch. Say... if you have 16 threads per inch, then one turn will stretch the bolt 1/16" of an inch.
The most important thing is that the fastener is preloaded more than the load it will see.
This is a very complicated issue. In general the engineers would determine a target clamping force and work backwards from there to determine a tightening torque. For a gasketed joint, they would determine a bolt pattern and pick a bolt size to provide the required clamping force (and consider the mechanical strength for a load carrying member). The bolts would be sized so that when properly torqued they would be at something like 80% of their proof load. The torque needed to achieve this clamping force is a function of the materials involved (bolt material and base material for female threads), plating (these affect the frictional characteristics of the joint as the bolt is torqued), lubrication (another contributor to frictional characteristics), head style, etc. The type of loads the bolt would be subjected to would also be important (axial, shear, combination, static, varying, etc.) For a joints subjected to varying axial loads, they would go for a higher percentage of the proof load (90 to 100%). Most modern head, rod, and main bearing cap bolts are torqued to the yield point. This is done using specialized machines that sense a change in the stress strain relationship as the bolt reaches the yield point. This is a very good way to torque bolts for variable loads, but difficult to duplicate in a repair environment.
I have some information of torque settings at
Ed
I have seen bolt elongation gauges advertised and recommended for performance engine building. They are not so expensive.
I have never used one of them, and wonder if anyone here has any comments on them.
If you know the elastic modulus of the bolt that you are tightening, it is theoretically MUCH more accurate than torque and completely takes thread and nut/surface interface friction out of the picture. However, I have not used one either.
nate
We use them on airplanes. They are much more accurate than torque gauges because they don't rely on thread friction.
They would seem overkill for auto applications, but more importantly they would seem useless because the numbers in the engine manuals are all torque specs rather than elongation specs. So you'd have to develop your own nominal values for reference, which is probably fine for a one-off engine that is being constantly rebuilt but otherwise impractical.
--scott
I think that is where they are most often used, in race or race/street engines where the stock specs don't even apply anymore. I think that ARP will give you stretch specs for their fasteners if you ask.
nate
I've use them on sbc rod bolts. Arp gives a couple of torque values (depending on the lube used) and a stretch value.
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