14 replies to this topic

[mariner]

Doing up a steel bolt into an alloy block the other day I started thinking about the forces when a bolt, or nut, is tightened ( no I didn't strip the thread while I thought!). This was apparently a Keith Duckworth interview question to graduate applicants

 

 As the bolt tightens it develops tensile load as the threads wind down and the head doesn't follow. That load creates friction at the threads which creates torsion loads in the shank of the bolt. The objective is to generate enough tension load in the bolt to exceed any likely load reversal in service so there is always  a residual tension to create enough friction to prevent the bolt vibrating loose. For things like flywheel bolts that residual tension has to give enough clamping force to allow high power transmission by metal to metal contact.

 

The text books explain that this means the highest load a bolt will ever see is during tightening as the torsional load vanishes when you remove the spanner. So why hasn’t anybody developed a way of pre heating bolts just before installation so they naturally clamp as they cool?

 

That exactly how hot rivets were use in ship and skyscraper building. The on-site heating is problem but it didn't stop the Empire State Building going up at one floor per day with hot rivets.

 

There is a gadget to heat bolts to remove them on the market

 

 

 

Could it be used to pre heat bolts and avoid over tightening them or let you generate even higher clamping loads as less torsional load would be needed?

Edited by mariner, 08 October 2016 - 11:29.

[rdyn]

The text books explain that this means the highest load a bolt will ever see is during tightening as the torsional load vanishes when you remove the spanner. So why hasn’t anybody developed a way of pre heating bolts just before installation so they naturally clamp as they cool?

Bolts are designed for this kind of installation. Tiny improvements are usually not relevant - if higher clamping loads are required it is easier to change to a larger diameter (simpler, cheaper and has less risks)

 

For very large bolts different tightening methods are common (hydraulic, thermal) because it is not practical to apply such large tightening torques with a giant wrench.

Edited by rdyn, 08 October 2016 - 20:22.

[Greg Locock]

It would be harder to control than torque, and would burn off the lube maybe

[imaginesix]

What's the advantage? Ability to use smaller bolts? Certainly wouldn't aid serviceability.

[Canuck]

I torque up fasteners on a daily basis to 1250 ft-lbs using a torque wrench and a torque multiplier. As you might imagine, this is done in steps. With the studs installed in the body, you'd need one hell of an input to overcome the thermal mass of 650+ lbs of steel. And having all those hot studs would create one more safety concern in a whole new area.

[Charlieman]

The text books explain that this means the highest load a bolt will ever see is during tightening as the torsional load vanishes when you remove the spanner. So why hasn’t anybody developed a way of pre heating bolts just before installation so they naturally clamp as they cool?

How would you get the bolts out again?

[gruntguru]

There is a small time window for fastening a heated bolt before heat transfer to the parts under compression eliminates the differential expansion. This creates difficulties when multiple bolts on the one flange need to be tensioned in a sequence and impossible if the tensioning needs to be multi-stage.

[bigleagueslider]

I think the best method for preloading threaded fasteners, that does not require expensive equipment, is using an angle-type torque wrench. With some basic information about the bolt/stud dimensions and threads, it is fairly easy to calculate the total wrench turn angle required to produce a given axial strain in the bolt/stud.

[gruntguru]

I think the best method for preloading threaded fasteners, that does not require expensive equipment, is using an angle-type torque wrench. With some basic information about the bolt/stud dimensions and threads, it is fairly easy to calculate the total wrench turn angle required to produce a given axial strain in the bolt/stud.

You also need to know the compliance of the joint/surfaces being bolted.

[munks]

Maybe I'm being daft here (partly because I have zero formal training in anything mechanical), but a heated bolt isn't going to expand in just one direction. It seems to me that in order to use this method, you have to increase the radial tolerances. In other words, the radius of your bolt needs to be smaller just so you can screw it in while it is hot and expanded. After cooling, this would result in decreased contact area needed for friction.

[Lee Nicolle]

Heating most bolts will weaken them. Unless it is a very low temp. Automotive that means about 150C max.

Actually heat still does cause breakages, many cast iron exhaust manifolds onto alloy heads using studs. The studs, usually on the end will break as the different expansion happens. Worse the studs seem to get more brittle. I have had to get experts in a few times to remove broken studs in the head.

most engines it seem use 8mm studs, I feel 10 mm [or 3/8] are better. On older stuff that was common and seldom caused grief though they often came loose. On many models part of the service is tightening manifold bolts

Edited by Lee Nicolle, 12 October 2016 - 02:46.

[bigleagueslider]

The primary concern is achieving a consistent preload strain in the bolt at every installation. Even if there is some way to ensure each bolt body is at a specific temperature immediately after being installed, you would still need some method to ensure each bolt is installed with the same amount of initial contact force. Can't think of any easy way to do this.

[gruntguru]

Torque wrench?

[blkirk]

The primary concern is achieving a consistent preload strain in the bolt at every installation. Even if there is some way to ensure each bolt body is at a specific temperature immediately after being installed, you would still need some method to ensure each bolt is installed with the same amount of initial contact force. Can't think of any easy way to do this.

Getting consistent preload using a torque wrench is also a problem.  The torque calculation assumes a friction value between the threads and under the head.  Actual friction is always different.  If you REALLY need to know the preload on the bolt (aerospace, nuclear, etc), use ultrasonics to measure the length of the bolt before and after installation.

[bigleagueslider]

Torque wrench?

A torque wrench would be affected by variation in friction due to heat transfer as the bolt and hole threads progressively engage. Ideally the heated bolt would be tightened just to where the bolt head face contacts the mating surface. Must be some clever way to accurately indicate this condition that would be practical for most applications.