CRB-A step further!

[Rob Board]

Gents, as promised an update on the CRB saga. After obtaining the information stamped in the bearing itself, (Many thanx Lawrence!!!), I tracked down the bearing manufacturer in Germany, and subsequently their UK division. Spoke today to thier technical and applications department. Gent I talked to extremely helpful, and I gleened a lot of info, as follows;(Sorry if some of this is too technical or boring but gleen from it what you need)

Firstly the technical spec for the bearing is more than up to the job of dealing with the stresses of the Caterham, and should easily deal with the temperatures generated in the bellhousing.(Design spec is to work upto 120 c).

Secondly, and this is what blows all previous theories about failure out of the water, (I did say I intend to get to the cause of the snag), is that the bearing is designed to run CONSTANTLY against the fingers on the pressure plate. To get technical, the bearing should have a preload of 80-120Nm or 59-88lbft.Simply, the bearing must have a lateral force equal to the pre-load figure pushing it against the pressure plate before you even touch the pedal. If the pressure is insufficent, the bearing will not spin as fast as the pressure plate. This means that the plastic casing on the back of the bearing will wear away because of the speed differential between the two items.(basic physics!). However if the bearing pre-load is too great the preesure plate fingers will eat into the plastic and score it. These two factors lead to overheating and so failure. So people who have removed or tampered with the pedal spring are unfortunatley doing more harm than good.

Given that the spring only just removes the slack in the cable, I sumise that the pre load is not enough, but at present this is just a theory which I will discuss with Simon Lambert when my CRB is done in a few months (At 6K svc).

Another point to note is that the Clutch release/actuating arm, particularly the pivot point, is not too substantial, as far as I can gather from the build manual (Your views greatly received). This may allow the CRB to run eccentricly on the shaft (ie not true) so giving misalingment and undue stress=heat=breakdown. Again I intend to have a good look at mine when Caterham split the bellhousing/engine.

To summarise the CRB is as Caterham state not at fault, however there IS an underlying problem, that hopefully can be resolved. I am in Hants (Andover), if anyone within an hour(ish) has their K engine out at present or near future I'd love to have a look to see the relase arm 1st hand. Also if anyone has a knackered CRB please send it to me, so I can investigate further and send them to the manufacturer. The chap I spoke to says from looking at the bearing he can tell me the cause of failure. Post a reply and I'll e-mail my address. Thank you for reading this. It has taken a lot of hard work so far, but as an a proffessional engineering tech I do find this a great challenge.Your thoughts on this article, and if you would like to know more as things progress?? would be great. Cheers Rob (Sorry if there's too much techno stuff)

[Dave_H]

Hi Rob

 

That would certainly explain why there have been failures. Maybe the bearing is unsuitable for the application then. Is there another bearing available that doesn’t require this pre-load? I was planning on changing my CRB over the new year, but I don’t want to replace it with another that will most likely fail again.

 

Dave

 

[mic]

I'm sure the bearing does not have that amount of preload when used in Ford tin-tops.

[Nick Green]

Hi Rob.

 

Well done, on getting further down the information trail than I did when I tried! I can certainly claim no special expertise in the field of bearings, but in the absence of concrete information to guide me, I have been applying what I felt to be "common sense".

 

I've had a couple of reads of your post, and I have a couple of things I'd like to clarify though.

 

1) You say if pressure is too little, then

the bearing will not spin as fast as the pressure plate. This means that the plastic casing on the back of the bearing will wear away because of the speed differential between the two items.

 

I'm afraid I don't see why the plastic should wear at all in this case. The back part of the bearing, that fits in to the plastic holder, that clips into the clutch fork barely moves when the bearing is new, but the bearing that came out of my car last year, moved freely on its plastic back plate. (Incidentally, this is why I've drawn no attention to needing to centre the bearing. IME, after a while the bearing seems perfectly capable of doing this for itself.)

 

If anything is going to wear, given the above scenario, I would say the front face of the bearing which contacts the pressure plate fingers, is more likely to suffer.

 

2) The next bit:

However if the bearing pre-load is too great the pressure plate fingers will eat into the plastic and score it.

I also don't understand, as the pressure plate fingers don't touch any plastic.

 

Perhaps it's me, but I don't understand why a bearing which is not moving should wear faster than a bearing rotating at engine speed.

 

Cheers,

 

Nick.

 

PS I think I've got quotes sorted now...

 

Edited by - nick green on 13 Dec 2002 12:07:13

 

Edited by - nick green on 13 Dec 2002 12:09:43

 

Edited by - nick green on 13 Dec 2002 12:16:00

[Old Septimus]

This may be irrelevant - if so my apologies. I have just re-assembled the engine/gearbox for my 68 S3 which has a standard Ford 1600 North-South setup. The CRB was all metal and had a curved annular pressure ring. This was polished on the foremost part of the surface as were the ends of the clutch fingers, indicating some rubbing contact.

 

The only pre-loading is a compression spring inside the slave cylinder that keeps the pushrod in place by acting on the inner face of the hydraulic piston. This is very unlikely to give any significant force. After at least 70000 miles the CRB seemed fine and I cleaned it up and put it back.

[Nick Green]

I've been thinking about this a little more...

 

Perhaps the pre-load requirements are necessary for where constant contact *does* take place, to ensure that, for instance, drag inherent in the bearing is overcome, so that the correct part of the bearing mechanism is doing the spinning.

 

Does it mean that constant contact *must* take place though ?

[Rob Board]

For Mic- I do not know about the amount of pre-load on ford tin tops, however the pre load figures were what were quoted to me by the manufacturers "Automotive Applications Expert" and I have no reason to doubt him. The figures are basically what the design pre load is. Further than that I am afraid I am a bit stuck. All I can confirm is that this bearing in it's current application ie CRB, is designed to work with that pre-load. I shall try and find out more!

 

For Nick- The reason that the plastic wears away when pressure plate and bearing (that is to say the complete CRB assy plastic and all) do not have the same speed of rotation, is because there is friction between the two. If they both spin at the same speed there is effectivley NO SPEED, therefore no movement BETWEEN the two pieces, although they ARE both spinning. Remember it is not the bearing that wears away, but the inner and outer plastic casing. Hope this explanation is ok?

Secondly I was not clear when I said the back of the plastic casing wears away,(sorry).I meant the back or part that is contact with the pressure plate fingers.

Regards your question of does it really need constant contact, again I am relaying what was told to me by the manufacturer. If you beleive this is not happening in the case with the Caterham set up, then the set up requires modification or the wrong bearing is being used, and one which requires no preload is required for this application.Either way, something has to change.

Lastly their "Expert" stated that as you say, when new the plastic casing should be difficult to rotate. However, if on removal it "spins freely"- his words, there is a preload problem.

I am DEFINITLEY not a bearing expert (thats why my queries have been directed at one). However, I am a professional Egineer,(Mechanical/Hydraulic/Electrical), which enables me to converse with other engineers. Hopefully my background can help me sort this problem out, or direct enough info at someone who can.

At the end of the day there is no substitute for the real thing, and I repeat my request to view 1. your knackered CRBs 2. Your bellhousing and CRB in place with the engine out. This will improve my understanding of CATS set up. I will travel!!!! Thanks once again for your replys Rob.

[Johnty Lyons]

Excellent work Keep it coming I'm on mt second CRB in 4000 miles unfortunatly I don't have the old one but do remember it span quite freely on the plastic housing which the new one didn't

 

jj

N.I. L7C AO.

Membership No.3927.

[Nick Green]

Rob,

 

There is no "inner and outer plastic casing"... There is a metal bearing, mounted on a plastic backplate which clips into the clutch fork. The plastic backplate has a sleeve which fits snugly over the nose of the gearbox.

 

The metal bearing is in a number of parts: The casing that is attached to the plastic backplate, the race itself, and the...other bit that spins on the race and actually makes contact with the pressure plate fingers.

 

I'm pretty sure I have some analog (ie not digital) photos from when I did the bearing swap last year. I can try scanning these and sending them to you if you like, but I would need an email address. Dave J has/had my old bearing off me, to further his own investigations

 

I am still unclear on why the bearing *has* to be in constant contact. I would be surprised if my foot pressure on the pedal does not cause the necessary preloads to be attained pretty darn swiftly when doing a gearchange.

 

The preloads you state for the bearing are (from a "doing up a bolt" point of view) very high! I don't see the spring Caterham fit on the clutch pedal being able to generate that sort of force... And the bearing can't wear anyway, if it isn't making contact.

 

I'll have a go at scanning these pics and getting them to you.

 

Cheers,

 

Nick.

[Rob Board]

Nick, thanx v much for your offer. As you are only in Malmesbury I can pop up and see them (if ok). Anyway I do have someone sending me a duff CRB, so I can veiw it. As you can tell from my descriptions it is very hard to get a good idea of the CRB make up without seeing one. I do agree why *has* the CRB to be in constant contact? Again when I have one in my grubby mit, then I can re phone the manufacturer and when he explains what to look for things will be clearer. Also I will ensure that their "Expert" has one at hand so we are both viewing the same item.The only CRB I have at hand at present is an old Renault 11 floating around in the garage so I do need a K series one. I have noted your concerns and issues, which are pretty much same as mine, and when I have CRB will hopefully get a little further. I am aiming to have solid facts, and some answers in writing from the manufacturers within a few weeks, which I can then present to Caterham. Hopefully then Simon Lambert and his team will be able to progress things futher if they perceive there is a problem. But unfortunateley it is a bit of a long process to get all the info and some CRB examples. But, I will continue. Cheers Rob (Nick have e-mailed you)

 

 

Edited by - ROB BOARD on 13 Dec 2002 17:12:36

[sjwb]

If I may make some observations.

The preload should read 80 > 120 N (not Nm which is torque). This is an appropriate load.

The release bearing is designed to run constanly, as are just about every other manufacturers applications. Wear to the diaphragm fingers and bearing face is eliminated by NOT having to constantly accelerate the race to match crank speed. Hence constant contact.

There is of course the consideration with hydraulic applications which self adjust by means of said preload.

Lastly, clutch housing temperatures can, and do, get up to 150 Celsius so maybe 120 is a little conservative.

 

Steve B

[F355GTS]

Rob

 

My Engine/ Gearbox out at the moment (but only for a couple more days, and I've got two used CRB's that I don't think have failed but you're welcome to have them

 

I'm in Reading so if you're free over the weekend give me a call to arrange a visit

 

 

 

 

Mark

[Swiss_Tony]

Rob

 

Please can you describe the parts of the bearing that wear in a bit more detail?

Is it the same bearing as the one for a vx engine?

 

James

 

[Rob Walker]

Oh this has been done to death. IMO the bearing fails because it has been fitted with the plastic sleeve section not set concentic with the metal bearing section. When this happens the metal section scuffs against the clutch fingures prematurely wearing these and in the process generating heat, this unwanted heat then compromises the bonding between the plastic section and the metal bearing section and the two can separate. I am well aware that this bearing is designed to be self centering and in most cases this works as designed, its the ones that are too stiff to self center that fail and have been fitted without centering the bearing manually before installation.

 

 

IMHO if you were to apply 80-120 nm of torque to the release bearing against the clutch fingures you will partially declutch and clutch slippage will result.

 

The other factors that may influence the life of this bearing are the higher normal revs at which gear changes are made on the Caterham I cannot remember downshifting on my road Tin Top at 7500 and many are running with uprated AP clutch covers that has a significantly higher spring rate than a standard road spec cover.

[Bricol]

i think a simple analogy may be in order here:

 

think of the CRB as your rear wheels.

 

think of the clutch plate as the road.

 

 

Hoist car into air - rev hell out if engine with car in gear - wheels spin

 

( - CRB held by additional springs etc away from clutch plate - rev engine)

 

In each case, speed differential exists between two parts.

 

 

Now drop car onto road quickly, with wheels stil spinning - if nothing breaks, wheels spin, leaving black rubber marks on the road - tyres and road show signs of wear.

 

( - press clutch pedal down - CRB contacts high reving clutch plate - has to immediately spin up to engine speed - which will involve some slippage between CRB and clutch - shows signs of wear on both). the torque required for this wil also be transferred back to the plastic carrier - causing the wear there.

 

Now if the CRB is constantly in contact with clutch, it is constantly spinning at engine revs - no slip - no contact wear on either. No sudden torque load on the plastic carrier - maybe a slight increase as you depress the clutch pedal.

 

The preload ensures this constant contact and also no doubt takes up the tolerances in the bearing as designed.

 

80 - 120N is not great - I bet you can push with a force 80N (same force as a mass of 8kg - 12kg pressing down on the earth approx) - but I bet you cannot even slightly move a clutch overcentre to cause slippage or disengagement. And an uprated cover with be even harder.

 

And as to speed - 7500rpm is not a great speed for an engineered bearing - I bet design spec is a lot higher.

 

From what I remember of the clutch release arm, it allows slef centering of the bearing in all planes so eccentric contact shouldn't be a problem. Temp however could be a problem.

 

And while not a bearing specialist, I use them everyday in machine design so know a little.

 

Bri

[Foxy Smith]

Having had my hands on a couple of K series CRB's when trying to engineer a solution to John Howe's QED flywheel/Caterham dry sump mismatch I would offer the following observations.

 

1. The stiffness of the joint between the plastic carrier must be critical. Too stiff will result in failure to self centre and result in wear. Correct stiffness will self centre and hold centred by virtue of correct stiffness of joint or by the correctly applied preload. Not stiff enough and no preload will result in bearing constantly running in and out of centre and therefore wear.

 

2. I can't really see the need to have a bearing self centre in a situation like this, especially if it has a preload applied. On my own car, a Vx with Caterham dry sump which has a co-axial hydraulic slave the CRB is firmly held onto the slave actuator by a rubber annulus, there is very little if any radial movement and therefore is held centered by virtue of the concentric relationship of the slave actuator with the input shaft with the clutch cover etc. etc.

 

3. The K series CRB is a flimsy looking item and I'm not surprised to hear about these premature wear problems.

 

Alan

[Foxy Smith]

Just to add to the above posting, having sat and thought about it the self centering CRB must be a requirement of the lever arm clutch actuation. Which is another achilles heel. I'm told that even the uprated arms that come with the Caterham dry sump kit have been known to bend. (ref. Peter Carmichal) And what about clutch cables snapping?

 

There is a solution gentlemen. Fit a decent engine. Get a Vauxhall! 😬

[Old Septimus]

In my earlier post I overlooked the fact that the clutch arm is a first order lever with a mechanical advantage of 3 or 4 to 1. Thus my comparatively weak slave cylinder spring exerts a significant force on the CRB.

 

[John Vine]

The K series CRB is a flimsy looking item...

As I understand it, the CRB used in the K cars is in fact a Ford part, as used in Sierras etc. I've had four Sierras over the years, and never had a CRB failure with them. But I've had two in my 1999 7!

 

JV

[Rob Board]

Correct John, it was also original fit for the RS500 Sierras so well up to abuse!