Torque loading

[phil 01]

When you torque down a bolt with a nylock nut are the values given in say "the build manual" the actual loading or are they the loading after we have taken into account the preload of the nylock nut. If you dont add additional poundage would this lead to under torqued nuts and bolts, this thought came to me laying in bed thinking should get up its 0630 and it's your day off

 

Phil

[Paul Richards]

Phil

Nylock will make no difference. Just tighten to recommended torque settings. How is it progressing?

[Jonathan Kay]

Neither. The recommended value is the value at which you should stop twisting the head. What the stress is at various points in the finished assembly depends on all sorts of things in addition to that. You can't, for example, assume that two similar fastenings that only differ in one having a Nyloc nut and the other something else will end up with the same stress at a given location after they are each tightened to the same torque at the head.

 

There are lots of ways of setting bolts accurately other than the sort of torque wrench we mostly use. And the K series engine has an unusual fastening feature: the engine through bolts are designed to operate in plastic deformation rather than elastic.

 

Bolted joints, Wikipedia article

 

Jonathan

[phil 01]

Hello Paul, the car is just about fully stripped, found various problems as I've slowly dismantled the car but nothing major, hopefully it will be down to just the chassis and ready for Arch to do their magic in about a week or so. Probably the hardest part to remove was the carbon fibre dash I eventually bought what is a garrote with braided gold wire laced between the handles and worked this between the chassis frame and the dash,but I saved the dash. Another item is the wishbone bushes,I've a 6 inch record vice and jaws where bending as I extended the vice handle with the trolley jack handle I think these bushes will need pressing out

 

Phil

[Paul Richards]

Phil

I managed to remove my bushes using a large socket, a nut and bolt and a steel drift.

Hard to explain, but arranged so that the nut and bolt were pulling hard on the bush, then gave it a couple of whacks with a lump hammer which enabled the bolt to tighten a little further. Repeated a few times until bush fell out.

Good luck with it.

[garybee]

There will be a certain amount of torque required to turn a locking nut on the threaded portion, referred to as 'run-down torque'. On critical joints, you add run-down torque to the torque loading required to clamp the joint, giving you a torque figure to apply to the nut. I've no idea whether Caterham will have taken this into account when writing the build manual however, but would be surprised if they did. In fairness to them, I doubt that any of the joints that the builder will be making are that critical with respect to torque accuracy.

[phil 01]

Apparently the term is prevailing torque

[Roger King]

The whole torque thing is really rather misleading.

 

Assuming you are dealing with non-stretch type bolts, which rely on being deliberately taken beyond their elastic limit, the torque figure is simply the best approximation that can be made to ensure the correct amount of bolt stretch is obtained when tightening. This stretch is too little to exceed the elastic limit of the bolt which means that when undone it will spring back to its original length.

 

The problem is that when you tighten to the correct torque, numerous factors will likely contrive to ensure that you don't end up with the correct stretch. For example, if you lube the threads and put graphite paste under the head of the bolt, the same torque figure will stretch the bolt considerably more than if thread and head are left dry. Any torque figs should specify the lubrication conditions to be used. Even then, results can vary considerably, sometimes caused by variations in the surface texture of the threads themselves.

 

Respected bolt manufacturer ARP specify that big end bolts should ideally be tightened by stretch rather than torque. They also state that if you have to torque them you should tighten them and undo them several times before final assembly; the reason being to remove microscopic high spots that will potentially affect the final result. I used to tighten big end bolts by stretch but did try torquing and then measuring stretch to see how close they came; the results were VERY variable.

 

Stretch bolts are different; by turning through a specified angle you can ensure that the bolt suffers plastic deformation. With good design you can be sure that clamping force will then be correct; furthermore, with a bolt in plastic deformation you can be relatively inaccurate with your angles and it will make little difference to clamping force. This is ideal for both mass production situations and for maintenance in less than perfect conditions, although the downside is that such bolts should usually not be re-used (the K-series head bolt is an exception to this provided it falls within certain limits).

 

Edited by - Roger King on 17 Apr 2014 19:10:29