Achieving High Torque Settings for Nuts & Bolts

The results of not going to that have been fixed by me more than once.

My torque wrench, which I thought was pretty big, 'only' goes

Hire a bigger torque wernch. I ended up borrowing one and putting some scaffold over the end to get more leverage as I jumped off te car bumper.

There should be an equation where a longer extension on that existing wrench would make it torque more. I have this misty memory of something like this, but it was a long while ago now Brian

Chuck the old VW flat 4 on the nearest scrapheap and buy something modern ?.

Or use a torque multiplier.

Or do what garage 'mechanics' do, just use an air gun with an impact socket and blast away at it, then finish with a four foot long bar.

NT

...or just use loads of Locktite.

There are ways of doing that sum, but it not straight forward. Basically you can calculate the clamping pressure exerted by the faster if you know a bunch of things like the bolt head diameter, the pitch of the thread, the modulus of elasticity of the fastener, and the coefficient of friction in the interface between fastener and what you are fastening. Then you could work out the relationship between rotation angle and stretch, and between stretch in the fastener and applied clamping force.

Alternative approaches would include using an extension on the *output* side of the torque wrench. i.e. use the wrench's indicator to measure the torque applied to the extension, and then calculate the torque applied to the fastening, using the ratio of the lengths of the wrench and the extension. An example here:

Or if you have a way of measuring the pull applied to the end of the bar, extend that, and just use force applied multiplied by the bar length to get a direct measure of the torque you are applying. e.g. 50 lbs of pull on a 5' bar would give 250 lbs.ft

bathroom scales

NT

Or, many people have luggage scales for when they don't want check-in problems.

Well there *are* some calcs which could be done, but in this case I would take a much simpler approach. You know how heavy you are. Stick a suitable scaffold pole over a suitable spanner, and apply your weight at the appropriate distance. You just need to place one foot there, and lift yourself off the ground. Try not to bounce too much.

Yeah, but I only want to apply 220ft/lbs., not shear the head off in a microsecond. ;-)

That was my immediate thought too. Bathroom scales and a long bar of a known length on a wrench.

If this was a once only tighten, I might consider say 45degs of additional turn to put the bolt into the plastic range after torquing to

Alternatively, perhaps noting the amount of turn, from 50 to 100, and

Can't remember the numbers, but last time I needed something out of range of my biggest torque wrench (18 inch long, 1/2 square drive) it was a couple of feet of scaffold pole. Long time ago.

:-)

I resemble that remark ;-)

Spring scales are what I used many moons ago. Something like what fishermen use might do

I like your thinking. This would be unlikely to be a linear relationship, so the results would best be graphed. It should be an excellent predictor of the required full torque in terms of degrees of twist needed. Thoroughly greasing the threads up first should also improve accuracy. Probably the best answer so far IMO, although Tabby's was pretty neat, too.

Thanks all!

which is done by practical testing with known torques, thus doesn't solve the problem. And IME bolt tightening v torque is far from linear.

I don't know of a reason to complicate what is a simple task.

NT

For the real pedants, this isn't a bad analysis of the problem:

Another reasonable treatment is given in a Handbook published by Molykote in 1991, but unfortunately they don't seem to have ported it to the web.

Basically, you have friction in two places: within the threads, and underneath the head of the bolt. If you are designing something complicated with multiple fasteners like a cylinder head or a helicopter gearbox, it's worth going into the detail.

For a single bolt like a crankshaft pulley where you have been given a target torque, just use a simple calculation. But the theory helps to explain why you want clean and well lubricated surfaces for the threads, and underneath the head of the bolt.

OK, your suggestion wins. A ten shilling postal order is on its way. ;-)