This arrangement sounds like an attempt by Renault to provide a fine degree of valve timing adjustment without using a hub with slotted holes to the wheel - which is the usual arrangement.
If I'm right, then it's cheapskate engineering in about the worst possible place.
659.
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Lots of cars now running wih keyless cranks.
What a poor (best word I could use without upsetting the swear filter) idea !
Im sure some engineer will come along and say how sound the idea is! But to someone who looks at and works on things such as this it can only be described as "a disaster waiting to happen!"
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woodruff would be spinning in his grave ( unless he's keyed up)
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Firstly,thankyou all for showing your interest.According to the relevant Haynes manual on the car (no.3916)page 2B3 quote 'The design of the camshaft and crankshaft sprockets are slightly unusual in that no method of positive location of the sprockets(such as that afforded by a Woodruff key) is employed.Instead,the sprockets are retained purely by the clamping action of the sprocket retaining bolts/nuts'.
So the way I see it,should anything like a piece of shredding auxiliary belt find its way through the small hole in the front edge of the timing cover as in the first instance,or say a water pump/air con pump/alternator for instance seize momentairily thus putting strain on the auxiliary belt which then strains the crankshaft sprocket,then its possible for the sprocket to slip,leading to valves and pistons going head to head as they've done.
The reason for not fitting a key according to the manual is to allow for fine adjustment when tensioning the cambelt(the crankshaft sprocket is left untightened for this to happen)But on all other engines I've worked on,you always had a key and the camshaft sprockets were finely adjusted to suit.My personal feeling is that if as is quite possible,there is something seizing up,if there was a key in the crankshaft sprocket atleast if it was one of the auxiliary items,the auxiliary belt would take the damage rather than the engine internal parts.
Having delved into the subject a bit,I've realised having read other articles by other Scenic owners on this site(especially one by Lizzie,which I believe has now gone,which had nearly 40 responses)it's a reasonably common problem that the cambelt slips in this way,and this is backed up by the rescue driver who helped my wife out when the car first broke down.The difficulty seems to be in finding what is seizing up,especially when it might only happen for a brief moment,whether it is an auxiliary part or even connected with the camshafts as someone suggested in one of the replies.I would'n't mind betting Renault might know the answer.......
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Ford have been using this design on their cam sprockets for years. Works fine as long as the correct torque and tightening procedure is used.
I remember changing Ford cambelts where you could take the bolt out completely and the sprocket would stay attached to the shaft. Took a smart blow with a screwdriver behind the cover to break the taper.
Cheers
DP
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Yes Dp but that doesnt make it a good design!
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I had an old 1950's motorbike with a Villiers 8E engine. The flywheel/magneto was attached to the crankshaft like this. It had the points cam built into it, so it had to be adjustable. Damn thing was always slipping. It's good to see how far technology has advanced in the last fifty years!
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I had an old 1950's motorbike with a Villiers 8E engine. The flywheel/magneto was attached to the crankshaft like this. It had the points cam built into it so it had to be adjustable. Damn thing was always slipping. >>
The Villiers of my old Webb mower is like this - got to get it just right to gap the points...and then tighten up REALLY hard.....but my almost-as-ancient Briggs and Stratton flywheel on another mower is Woodruffed - much easier.
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Tapers can be an excellent method for connecting rotating parts - there's nothing cheapskate about it. The cost of maching two tapers to fit well enough to grip is at least comparable with a parallel hole and shaft, and the machining of the key slots.
I think Renault should have put the taper fit on the cams, where there's more room to design the parts with adequate stiffness, and the loads are shared, and hence lower.
In these applications, is the crank pulley bolt torqued up, or is a torque plus angle called up in the spec?
There are some details of the joint which need to be exactly right to avoid fretting fatigue failures (but, that's not an issue here). For a bad example, consider the taper fit of the flywheel to the crank on the original Mini.
For a good example, there's a very highly stressed gear and shaft in the traction gearboxes on TGVs which rely entirley on a taper fit.
I'm inclined to think that only Screwloose is on the right track, the effort should be directed towards finding out which part of the engine is siezing.
If you think about it, if keyed connections actually relied upon the key to transmit torque to the drivebelt, the keys would have to be much larger, and the pulley and shaft would have to be a really tight fit, or else torque reversals would hammer the key to nothing in short order. In fact, the key is just an assembly aid, and the torque is actually transmitted from crank to wheel by friction, pre-loaded via the crank pulley bolt.
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Compare it to a typical vauxhall design crankshaft pulley then?
The pulley is bolted to the crank (4 bolts) and there is no need to touch the crank directly. Obviouslybeing bolted to the crank its much more secure than a taper!
Far better design! Far less chance of slip, and never heard of 1 of them causing a problem other than tensioners or excessive mileage etc!
Progress? advancing backwards imo !
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For a really tight fit, nothing beats a tapered spline joint.
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>>nothing beats a tapered spline joint.
Expensive though!
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Yes expensive but 110% sound.
Referring to taper fit hubs, sprockets etc some cars used to (and other applications in industry) had taper shaft to hub and woodruff key as well. I had one on a half shaft on a 1950's car, needed about 20 tons on a press and heat to shift it. Went with a bang like a six pound field gun and it was not rusty either.
If you do have one then certainly agree with the guy who said about lapping the sprocket to the shaft.
Keyless fits of course do generally work or their would not be so many of them under bonnets running around our roads but it can fail and with interference engines it is very bad news indeed when they do. give me a key every time.
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>>1 of them causing a problem other than tensioners
Yes, if there isn't adjustment at either the crank, or the cam, at leat 2 adjustable tensioners are necessary, to move the slack from the drive side to the slack side as required to obtain accurate timing. Not especially elegant, and not especially reliable either.
I'm sure this specific Renault design is horrid, but, the principle of using a taper for this type of joint isn't a bad one.
There is a pedantic issue with respect to lapping the taper fit, which is that in this case, the taper fit is also loaded by the crankshaft bolt. Obtaining a full contact without any preload does not necessarily mean that the parts will fit together well when tightened up.
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taper fits are sop for high stress/torque machine tools in engineering workshops,Mt1,mt2,etc.Lathes,horizontal/vertical mills,Normally tightned via a draw bolt.Minimal tourque applied.All locking/driving forces transmited by self locking taper.Overdoing it resulted in the morse taper refusing to seperate when required(often).Correct workshop practice was periodic lapping of componants to ensure maximum contact area.Failure to observe resulted in chuck/toolholder working loose with results that would put a £1000 engine repair bill into the shade.On a correctly lapped set up the drawbolt was almost uneccesary.avoiding being a pendant is not an option,It's how i was taught.
snip quoted post
Edited by Pugugly on 03/08/2008 at 13:43
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Taper on the spindles and work/toolholders of machine tools are a different kettle of fish - they are much stiffer, and the drawbolt, as you say, isn't particularly tight. The deformation from true of the tapers in a machine tool is minimal. The deformations of a small, highly pre-loaded taper are different.
What you were taught as correct practice for machine tools might not be entirely appropriate for car engines.
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Thanyou all again for your inputs.I would just like to add another point or two.When the car was towed home on the first occasion,before it went to the garage for the repair,I took the cambelt covers off and also removed the crankshaft pulley,whereupon I could see perhaps just how much the belt had slipped.It looked like it had slipped about 120 degrees.But having got this far I'd decided to see if I could re-align the belt.So having followed the manuals instructions I managed to do this,replacing everything to specified torques.I could get the car started after about 5 secs,but as expected it seemed to fire on only 2/3 cylinders,and I stopped after about 5 secs again.Now the main point I'd like to make is that the crankshaft sprocket as far as I could see does not have a tapered fit,just a tight parallel fit,which with a small effort I could push on with my hand and also rotate on the crankshaft end.As the manual said,the only thing which stops the sprocket spinning is the clamping action of the crankshaft bolt via the pulley against the sprocket(1st stage 15 lbf ft 2nd stage angle tighten 115 deg.)So hopefully this may clarify a few points mentioned above. Many thanks - and what a gem of a website!
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Cap'n
The VW LT's I work on have a similar tightening method for the front pulley/damper. (Just a small keyway for alignment)
A stretch bolt and a hefty 180nm and then 180 degrees. Using a three foot bar it takes two of us to tighten the bolt. And believe me, if it isn't tightened correctly, it will loosen !!
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A NEW stretch bolt, that is !
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The crucial point there is the use of a new bolt. The elasticity of the old bolt may have been compromised if it had been [over]tightened [in the usual way] with a hammer-gun.
There is even a bolt-stretch limit of 49.1mm. Regardless of that; if you ever see an elasticity spec for almost any bolt - you fit a new one. It's impossible to prove later, when accusations are flying, that you actually checked it - a new bolt shown on the dealer's belt-kit invoice is powerful proof of a job done to the letter.
There also appears to have been a great deal of variation in the torque setting. There have clearly been several revisions over time - a sure sign of in-service problems. Some give 40Nm +110 degrees - and specify a new bolt every time.
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Is everyone missing the point?, modern cars are disposable - not repairable. That is why they are so cheap relative to income and years ago. It is no good comparing a modern disposable car ( Renault) to say, a pre war Riley 12 with helical cut cam gears- I would like to, but a Riley Falcon in 1935 cost as much as a very good house?, how much was this Renault the other week?
A bit like Sherman tanks- make a lot some will succeed , some will fail.
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>>a new bolt shown on the dealer's belt-kit invoice is powerful proof of a job done to the letter.
>>There have clearly been several revisions over time - a sure sign of in-service problems.
Two excellent points there SL.
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BMWs diesels are like this too with their chain sprockets I to adjust static injection timing, it says in the manual to use a new bolt and it has a special green coating to stop it loosening. Did they use a new bolt on yours?
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Yes Dp but that doesnt make it a good design!
Actually with regard to the Ford cam pulleys, it is a good design, as it allows the cam sprockets to rotate freely as the belt is tensioned (the cams themselves are timed by slots in the tails). This pulley "freedom" allows the tension to be applied equally to both sides of the belt relative to the cam pulleys when the tensioner does its stuff.
Whether it's related, I wouldn't like to say, but the Zetec engines hardly ever suffer premature timing belt failure, despite a 100,000 mile change interval.
Of course this benefit doesn't apply to the crank pulley mentioned by the OP.
Cheers
DP
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I have to disagree then DP.
it may (or may not) be a good feature on the cam sprockets (there are better ways imo) but it cannot be described as good design when you look closely at the Renault system.
As mentioned, it all boils down to the integrity of the bolt and the surfaces of the 2 metal contact areas !
Much much safer to use the simpler (but dearer at build point) vauxhall (and other) design. Look at the typical Ford diesel desgin for example. Cam and injection pump adjustable but crankshaft fixed and secure. Quite basic, simple and effective.
The keyless crank is a typical piece of progress (advancing backwards by design !) imo !
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For everyone who has contributed to my story,may I thank you all for showing such interest.As a final note,the Scenic has now been sold for the princely sum of £500.By my reckoning,with the initial cost and subsequent repair,we have lost £2150 overall in the space of about 15 weeks,i.e. about £150 per week.Having explored this wonderful website,I feel now,as far as Renault Scenics are concerned,I've probably got away it reasonably lightly!,and having consulted the 'Which' magazine since,I clearly did'n't do my homework. All the best,and happy motoring.
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