Torque as applied to fasteners

[hedjehog]

Hi all,

I have seen a few threads discussing the pros and cons of having a torque wrench. I'll probably get one one day, but at the moment I don't have one, and Toby R has reminded me of a question I have about torque.

I know it has a lot to do with 'feel' but is there an amount of torque that if you saw in the manual it needed applying to a screw fastener on the bike you'd think 'Oh, I've got to do that nut up very tight', or 'OK, that nut is supposed to be relatively loose'. I would like to be able to judge in that manner so that when the Haynes says 17 ftIbs or whatever I can know at the very least if it is important to do it up mega tight, or not too tight. At the moment, I just don't know what the numbers mean. IYSWIM

I had a bit of trouble expressing myself on this one

Thanks,
Jack (hedjehog)
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1988 CG125 BRE; 2000 GPZ500S D7; VFR 800 FI-X; 1980 CB 250 RSA; 1998 FZS 600 Fazer; 1986 Honda C70 E

[veeeffarr]

Basically you want to know how torque settings relate to how tight you should tighten a bolt by feel?

For example 4 ft/lbs = just over finger tight (not a real example) .

I just do finger tight + quarter turn, but whatever you like :0

[hedjehog]

[mr.z]

Without allot of experiance its not possible, you'll get replys to threds "oh you dont need a torque wrench, just guess" if you know how hard it sould be done up good for you, but you won't start off knowing how much or how little force is needed, a monumental fuckup is only an overtightening away, undertightening is just as bad, bits vibrateing loose and falling off isn't so great either...

Screws e.t.c on none important parts are easy enough, just do them up till they stop turning, but larger bolts like axel bolts or especially engine bolts is asking for trouble...

Let me put it this way, if you were to overtighten say a brake bleednipple fre example, and it snaps off, you'll have to take the brakes off, pay a mechanic £20 to remove it (if you take it to them allready off, that would be cheap), if the thred is stripped out then thats a new caliper body, so your looking at new brakes.. before you know it you've spent £90 and been bikeless for a week or more...

my two pee
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>RidingSkills<->Tech Tips<->MyBikes<

[mchaggis]

Torque applied to a nut or bolt is pretty much meaningless anyway, other than a rough and ready approximation. When you fasten two items together using a bolt/stud + nut, the main thing you are endeavouring to do is clamp the two pieces together. The interface between the two items being clamped provides the friction force which stops them falling apart.

This clamping force applied by the bolt is primarily related to the length of the bolt. That is to say, the extension of the bolt when it is tightened. Knowing the stiffness of the bolt and the extension, you can work out the force it is applying. When the clamping force is really critical, an ultrasonic probe is used to measure the untightened and tightened length of the bolt, so that the force can be calculated.

The torque you put on a bolt to tighten it does not relate very well to the clamping force, as the tightening effort/ torque is primarily related to how much friction there is between the threads. That is sky high if either is rusty or clarted up with crap and corruption. Torque values are only given for clean threads, preferably oiled so that you can tighten them with your fingers all the way in, and then stretch the bolt with the wrench. A brass brush and handy oil are essential when stripping and rebuilding, as is a healthy dollop of grease/ copper slip on the thread. Threadlocking compounds are bad news and should not be necessary if the pieces can be clamped properly.

Washers are used to distribute the force from the bolt head/ nut further into the material of the clamped piece so as to minimise the localised stress.

Use a torque wrench by all means, but don't trust it implicitly. Ensure that both internal and external threads are clean, free from corrosion and greased before you do anything.
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I must not be a troll...
Mmmm, Guinness
Discovering the delights of Hammerite and a 3/4" brush.

[kruzerkev]

the torquing of certain bolts is essential,especially if a gasket is used between the 2 items to be tightened or 1 or both items are aluminium e.g cylinder head bolts.torquing down ensures both pieces meet at a uniform pressure throughout the mass of the object & that too much pressure is not applied to the gasket.
not torquing is asking for trouble

[hedjehog]

Another day, and my heads clearer! Yesterday is what you get sitting in front of the computer at work not doing the paperwork you're supposed to

I'm interested in the technical side of bike mechanics but just don't have the experience. Thanks for the info everyone

hedjehog
____________________
1988 CG125 BRE; 2000 GPZ500S D7; VFR 800 FI-X; 1980 CB 250 RSA; 1998 FZS 600 Fazer; 1986 Honda C70 E

[Guest]

0 - 15 lb/ft wrist tight

16 - 50 lb/ft elbow tight

51 - 100 lb/ft shoulder tight

over 101 lb/ft stand on the spanner