Collapsible Column Impact Clamp

[bob_rich]

Hi Folks

Could some one advise on the torques setting and or type of bolts required for the impact clamp?
I have attached a note explaining in detail my question

thanks in advance for any help

and of course Seasons Greetings to all !!


Bob

[oldelanman]

Hi Bob,

I found this data for Triumph Spitfire which is the same column I believe - draw your own conclusions !

http://www.triumphspitfire.com/Torque.html#Steering

[ricarbo]

I've recently done this job and messed it up! I thought the workshop manual figure of 26 to 32 lb ft applied to bolts A and C in your attachment. The result was I stretched the two bolts A and had to replace them. I did up the replacements to the figures given under 'General nuts and bolts' in the manual, which I think was 5-7 lb ft, and reserved the higher figure for C. Since I have no method for measuring the torque applied to a normal L shaped Allen key, I bought a short straight key that is fitted into a 3/8 drive socket and put my torque wrench on that. It seemed much tighter than I would have been able to tighten it with an L shaped key which makes me wonder why the L shaped key is shown in your attachment. There's a similar drawing in the Lotus manual. Anyway, it didn't fail at that torque. I then just nipped up the locknut. Maybe most people just do it up tight with an L shaped key, that's what I've done previously, but you won't find out if it's wrong without a crash test. I'm still wondering if the locknut should also be torqued.
I see Brian Buckland gives an even higher torque of 40-45 lb ft, which I think must be in error. Very unusual for him.
regards
Richard

[bob_rich]

Hi Both

Thanks for the info. I have use the general data figures for the two 1/4" UNF bolts 7lbf-ft that I fitted new and then used the wrench attachment I made up to tighten the camp screw to around 25 to 30lbf-ft. It seemed to work OK --it sort of "felt right" If you know what I mean. I used my standard torque wrench on the 1/2" bolt hex head on the attachment and the small increase in overall length ( around 1 in 15), made certain it was inline, should not change the reading that much. Used a bit of loctite on both screw and the backing nut.

thanks for info

regards

Bob

[billwill]

Hey, you want this thing to slide down the shaft don't you, when you hit it with your head & chest in the event of a crash, so you really really don't want it done up too tight.

On the other hand you don't want the steering wheel to pull towards you when turning into a tight corner or pulling on it to pull yourself or the seat into a forward position, so you don't want it too loose either.

Some of those torque figures that you are bandying about seem too high to me and would defeat the object of having a collapsible column.

Interestingly, the earlier Service manual for the 2-seater Elan shows two pictures of the clamp. The first (page J11) does not have the Hex socket setscrew and locknut and the flat pat is described as spring steel plate. It is not threaded but has spring washers and nuts to secure the bolts from the U portion. The second picture on page L29 does show the more solid threaded 'flat' portion with the hex setscrew and its locknut; but the parts description on page L30 calls it a LOCATING SCREW, which implies that it doesn't really take part in determining the collapsing force. The earlier manual does not give any torque figures.

The later Service manual on page H6 shows the impact clamp with the threaded 'flat' portion and setscrew. In the assembly instructions no specific torque figures are mentioned, just "Ensure the Impact clamp is fully tightened". General bolt figures at the front of the manual gives 5-7 ft Lbs for 1/4 UNF

So without going down & undoing my own steering clamp, I think you first have to determine if the column clamping action occurs with just the two outer bolts with the setscrew out, In which case I suggest that you do not exceed 7 ft lbs, while using the actual bolt-force to determine the collapsing force.of the column. In this case the setscrew is merely used to be sure that the flat portion of the lower column is in the correct rotational position, as implied by its description as "locating screw".

Secondly if two outer bolts merely clamp the clamp together without affecting the collapsing force, do them up to 7 ftlbs (with the setscrew loose), then do up the setscrew to achieve the desired collapsing force e.g. so that it doesn't move in normal use but the setscrew is not so tight that you think it will never move even during an impact. Finally, in my opinion, the locknut should be done up to the normal torque for a nut of its size; its only function is to stop the setscrew from coming undone.

~~~~~~~

Personally, rather than trying to do i with a torque wrench, I would do up which ever screw(s) determine the collapsing force a small bit at a time and after each tightening I would give the steering wheel hub a strong pull; If I can move it I would loosen the screw a little, push the steering wheel back into place and do it up a little tighter. When I reach the setting where I could no longer move the steering wheel, I would probably do up the screw a further 1/8th turn and then apply any locknut.

[bob_rich]

Hi Bill

Thanks for the comments. Firstly my car +2S has an arrangement exactly like the picture in my note above which was taken from the Section H of the Lotus +2 manual section 8 page 9 and secondly the torque figures were taken from the Technical Data section pages 22 and 23 that details all the torque figures.

I can also confirm that if I did the clamp 1/4" bolts up to full ( 7lbs-ft) with the hex head screw removed the lower column was in no way clamped or restricted. In fact the upper column would revolve around, and slide up and down, the lower one if the hex bolt was removed. I then did up the hex head bolt to a figure near the lower end of the recommend torque.

By using the term "felt right" was to imply that if I held the torque wrench there was no yielding felt

To be sure I had not misunderstood I tested a brand new 1/4" bolt with a torque of 26 lbs-ft. It did have a "yield" feel to it and eventually I was able to shear it. This is not surprising as the maximum torques for a 1/4" bolt is a round 7lbs-ft.

With the safety implications of faulty steering I did want to get this right and have made every effort to ensure that the whole assembly is not under too much strain.

The other posts provided help but clearly there are some contradictory ideas and values given so it will be interesting to see if anyone else has comments

thanks for your comments

Bob

[AHM]

Bob,

My understanding is as you and others have written. The 2 1/4 unf set screws are to hold the 2 parts of the clamp together - with just those 2 screws it does nothing!

You then apply the torque defined in the manual to the grub screw. This puts the whole assembly in a pre defined tension and results in the end of the grub screw exerting a predefined frictional force on the inner shaft and the clamp a predefined frictional force on the outer shaft.

The bit I am stuck on is what torque on the locknut, since this torque will unload the grubscrew. Edit: I just did a quick envelope sketch - I don't think lock nut has much effect re. tension, and I don't think it unloads as a locknut onto another nut does on a bolt or stud (explaining may confuse the current topic). So I am going to use the standard torque for the locknut. as with all such discussions - your car your settings!




Bill,
I wouldn't do it like that! The torque on the grubscrew is loading up the 2 1/4 setscrews - I've no Idea what tension that torque is creating .. do you? without the knowledge of the forces involved and the characteristics on impact you are just guessing. You might find that your column collapses too easily and you bang your head on it even harder when it hits the stop.

[billwill]

Given that there are no torque figures given in the 2-seater Elan service manuals for the setscrew and that the car existed for 9 years (?) before the Plus 2, there must be an awful lot of cars on which the setscrew was done up by a method similar to mine.

I think you are thinking that it would create loads GREATER than those discussed above, but I very much doubt that, those figures seem far too much for the threads of the setscrew.

Given that Bob has found that the clamp alone with its two assembly bolts tight has no significant effect on the collapse force, it follows that the prime function of the setscrew is to provide a single point of defined friction for determining the collapse force. (the force on the curved surfaces will be much spread out compared to the inner end of the setscrew) I'm not sure how hard is the metal of that setscrew nor whether it has a point or a rounded surface at the contact end. If it is a standard hex grubscrew it will be hard metal and have a hard point I hope not, because doing that up too tight even once will make a pit in the inner shaft which will possibly give totally the wrong collapse effects.

If I had a spare steering column, I'd be happy to do tests of the collapse behavior by dropping heavy weights on the steering wheel end with the bottom end secured vertically in a vice, but failing that I'm happier using my method than relying on a torque figure which seems wrong and might be a misprint and differs drastically from book to book.

[AHM]

Bill,

The earlier clamp is of a different design, for the later one there is a torque figure given on P.26 of the elan manual.

The +2 was 1967.

The 1/4 unf setscrews are holding a clamp together - the clamping force they provide is not that relevant as they act within the clamp itself. the preload is provided by the grubscrew.

The grubscrew is in compression and the torque does not seem abnormal for its size..

Newton's third law defines that the force applied to the inner and outer column is equal.

Friction is independent of surface area. So the frictional forces at each interface will be about the same assuming constant u.

The end of the grubscrew is flat.

In a crash at 30 mph your body weighs about 1 1/2 tons - you are going to need some big weights to drop.

Just my thoughts, when it comes to it we each have to do what we believe to be safe.

[Chancer]

I cant recall the figure but the torque on the centre pinch screw is quite low as the column has to yield at a certain force, one which is low enough to prevent breaking your ribs and puncturing your heart so this is a very important subject.

That design quickly was superceded by tubular lattice steering columns as most mechanics or owners would overtighten the pinch bolt with potentially fatal results.

This column does live on in one low volume production car, the Caterham 7, on the one hand its frightening o think that it has a potential defect that was recognised 40 years ago and superceded, on the other hand they do give owners builders precise instructions on what torque to tighten it to and IIRC the load figure that the columb should slide at.

I cant recall the figures but if you are concerned then ask a Caterham owner or service agent.

Me I just tighten them only enough to resist a good beating on the steering wheel, it can always be nipped up a bit more if needs be in service.

[oldelanman]

As I understand it the impact clamp is there to reduce the risk of the column intruding into the car in the event of a front end collision, not to collapse under the weight of the driver. The outer steering column is bolted to both the dash and the body and these mounts would have to be overcome before the inner column could move and the sliding clamp become effective. The steering wheel itself is likely to deform and break absorbing some of the impact from the driver but beyond that I suspect the impact clamp would remain intact unless impacted from below due to deformation of the chassis. Later innovations like the convoluted can under the steering wheel and the air bag are energy absorbing devices designed to cushion the driver against impact with the steering wheel and I don't think this crude device does that at all.

Just my thoughts. Fire away !

[Chancer]

You are absolutely correct, were I to have had either of the vehicles here I would have gone and looked before posting what might be rubbish as I have often done before.

Again this might be my memory playing tricks with me but I seem to recall using the clamp bolt to adjust the steering wheel fore and aft position on the Caterham but I might be dreaming.

[billwill]

You have a third version of the manual. Neither of my sections P go as high as page 26.

Section P of my later manual is Heating & Ventilation.

If your chest hits the steering wheel with a foce of 1 1/2 tons, all this discussion will be academic

[billwill]

As I understand it the impact clamp is there to reduce the risk of the column intruding into the car in the event of a front end collision, not to collapse under the weight of the driver. The outer steering column is bolted to both the dash and the body and these mounts would have to be overcome before the inner column could move and the sliding clamp become effective. The steering wheel itself is likely to deform and break absorbing some of the impact from the driver but beyond that I suspect the impact clamp would remain intact unless impacted from below due to deformation of the chassis. Later innovations like the convoluted can under the steering wheel and the air bag are energy absorbing devices designed to cushion the driver against impact with the steering wheel and I don't think this crude device does that at all.

Just my thoughts. Fire away !

What torque figures are given for the clamps to the dash and to the body. I think these should be not so tight as to prevent the outer tube from sliding forward in a crash.

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.

[AHM]

You have a third version of the manual. Neither of my sections P go as high as page 26.

Section P of my later manual is Heating & Ventilation.

P. for Page

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.

If your chest hits the steering wheel with a foce of 1 1/2 tons, all this discussion will be academic

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?

For me the logic of how the clamp works, its intended purpose, and the torque figures given in the manual are clear. It was designed by someone who knew what they were doing I'll stick with them rather than guessing, or assuming I know better!

If you have any doubt seek expert advice - Don't just guess!