Collapsible Column Impact Clamp

[billwill]

Technical Data, no section letter, right at the front, Torque Loading figures, Front suspension, Steering Column Clamp. I believe the earlier manual only refers to the earlier style of clamp.

That line is simply not there in my copy of the later Service manual (36/T 327), the other detail for torque for the front suspension steering etc is there. It must have been added in a later printing.

Anyway what torque figure does it give? Does it differ from that in the Plus 2 manual.


I've been looking up stuff on collapsible steering columns, but nowhere have I actually found any figure of what initial and sliding friction forces are desirable in a collapsible column; there may be figures buried in research papers but they cost money to read.

What I did discover was that Lotus seemed to be ahead of most manufacturers in installing collapsible columns quite a few years before big name ones. There seems to have been a big fuss about the issue in the late 60's (Raph Nader etc) and I suspect it was then that Lotus included that torque figure in the Manual. But when you talk of 'Expert Advice' Where did Lotus get their advice for the setting?

Later systems seem to rely on pre-known strength of materials rather than obscure torque figures setting friction, for example plastic shear pins joinin the two portions of the column, or the zig-zag lattice crumple zone (made of steel ?) on the upper portion of the steering column.

Incidentally from the info I found on the net, it is pretty clear to me that the outer column is supposed to slide, with friction in the clamp to the dashboard, absorbing energy while it does so if the column is being pushed towards you due to frontal chassis collapse; The dashboard clamp should probably be tight enough to initiate the shortening of the column at the clamp being discussed from the Original Posting.





The ideal of course is that the steering wheel jumps towards you in a crash and then relatively slowly collapses forward so that the force on your body is never excessive... (they cal these Air-Bags ).

Bill, That clearly isn't what I wrote. But it was intended to illustrate that we have no idea of the mechanics involved - What heavy weight were you going to drop?! Why?

Aye, I know but what you wrote "In a crash at 30 mph your body weighs about 1 1/2 tons ." doesn't actually make any sense in Physics.

Since I reckon my body & head would start to crumple at around 100-150 lbs I would like to think that the intial collapse friction force was about 125 lbs, dropping to 100 lbs sliding friction for the few inches of the flat part of the steering column. Personally I would prefer if Lotus had made that flat portion longer because I am a little fella so my seat is fairly far forwards, so my steering wheel is far forward so there is only about 2 to 3 inches of flat part left for the column to collapse.

[Chancer]

Lotus neither designed nor made this state of the art safety feature, it was Standard Triumph and it was probably designed in to allow the column to be removed or even maybe to cope with manufacturing tolerances, when the Triumph Herald was made safety wasnt really an issue, no seat belts for example.

Caterham cars will have the definitive torque figure, you might find a PDF of a build manual somewhere on the net.

[oldelanman]

In the 2-seater, I think, (after checking the pictures Not by looking at the actual car) the outer tube is secured only to the dash with a U clamp, not also to the body and that part of the dash would be fairly weak in the event of a crash because of the two big holes for the speedo & tacho.

[Quart Meg Miles]

The numbers don't add up. As described, the two bolts at 7 lb.ft simply secure the block to the outer column but the pinch bolt (which has a flat on the bottom) done up to 26/32 lb.ft is acting against those torqued up bolts which must be exciting for them. Then the lock nut adds a bit more stress.

I tighten up the pinch bolt by hand with an Allen key (15 lb.ft ?) and not too much on the lock nut and there has never been any insecurity in the steering wheel's position or rotation.

There isn't much securing the outer shaft to the dash but it is not supposed to move in the event of an impact and even if it did it wouldn't move much, see below, and a seat belt should keep you clear of it (though there's another pseudo safety device if it is of enertia reel type, as most are). The steering rack and hence inner column can only move a few inches before the vacuum cross-member gets in the way (of the impacting object) and if that bends significantly you are in real trouble from the stresses on your body.

As it happens I was about to start a thread asking if anyone had experienced a frontal impact sufficiently heavy to move the steering rack and column and what happened to that assembly. Any survivors out there?

[AHM]

The numbers don't add up. As described, the two bolts at 7 lb.ft simply secure the block to the outer column but the pinch bolt (which has a flat on the bottom) done up to 26/32 lb.ft is acting against those torqued up bolts which must be exciting for them. Then the lock nut adds a bit more stress.

Putting some rough numbers into a 'Bolt' calculator - I couldn't find one that went to Grade S (8.8 ) so these are based on 8.2
proof stress of 2 1/4" unf is about 9000 lb
Preload at 7 lb ft is about 3500 lb

I don't know enough to calculate the force of the grub screw, but lets assume it is a Bolt in tension! then at 32 lb ft it is around 5000 lb - in compression it should be less, and the flat part of the clamp will also deflect

The only effect that the locknut has, as far as I can see, is to unload the grubscrew ie reduce the stress.

I've just noticed that of the of the 2 x 1/4 unf in my clamp, one is a bolt, and one a setscrew so probably both should be bolts.

I'll stick with the numbers in the manual.

[oldelanman]

Do the clamp bolts actually directly oppose all of the load applied by the grub screw ?
As the clamp is semicircular will there not be components of the force acting at right angles to the bolts putting them in shear as well as tension ? This might explain why they appear to be too small for the job.

[Quart Meg Miles]

Do the clamp bolts actually directly oppose all of the load applied by the grub screw ?
As the clamp is semicircular will there not be components of the force acting at right angles to the bolts putting them in shear as well as tension ? This might explain why they appear to be too small for the job.

Yes. Initially all they do is tighten the block against the cut-out area of the outer column and then the grub screw presses the inner column against the inside of the outer column and loosens the pressure of the block on the outer column. If 32 lb.ft exerts 5000 lb then it will lift the block off slightly and stress the bolts more (by 1500 lb)though not over their ratings.

There shouldn't be any or much side load and it would increase the stress rather than reduce it.

AHM Why should there be less stress in compression than in stretching the bolt, the thread angle is the same?

The lock nut, in this application, pulls the top of the grub screw thread in the same direction it is already stressed in so basically stresses the protruding bit of thread and the top of the already engaged bit. When a bolt is normally lock nutted you are correct, it unloads the nut, but not in this case where the grub screw is pushing.

[AHM]

Meg,

AHM Why should there be less stress in compression than in stretching the bolt, the thread angle is the same?

There was a good reason why when I wrote that, but It was late and I can't think of one now and haven't found one!

Re the lock nut, agreed it won't unload the grubscrew in the same way that a lock nut unloads the full nut (not the bolt) on a bolt in tension. Assuming that the threads deform linearly, will the force between the grubscrew and the column not reduce? the applied forces have the same sense, and the compressive force will load the threads evenly.

[bob_rich]

Hi Folks and a happy new year 2 all ! !

Thanks for tall the comments and on interesting discussion. Will be finishing off the whole steering assembly shortly and feel confident with the 7lbs-ft on the two 1/4" bolts and around 25lbs-ft on the grub screw with a bit of loktite and a similar pull on the lock nut with a loktite on that as well

cheers

Bob