Catastrophic camshaft and follower failure - any ideas?

timetraveller

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Well chaps, I have left this for some time just to see what calculations or guesses were put forwards. No responses so far so perhaps no one really cares. Let me go through the calculation and then some of you can check to see if my two remaining neurones are still working.

Start with a figure of one G is 32 feet/sec/sec (that is per second squared). Having measured many Vincent cams up over the years a typical figure for the maximum positive acceleration, which occurs at the start and end of the lift, is 0.0005 inches/camshaft degree/camshaft degree. Incidentally, the maximum negative acceleration which is when the valve spring is controlling the valve motion should be about -0.0002 inches/camshaft degree/camshaft degree but because of the nature of the flat based lever cam follower on Vins this figure is exceeded on all Vincent cams. Now at 6,000 engine rpm the cam is going at 3,000rpm so the question is how much time is taken for the cam to turn one degree. 3,000 rpm equals 50 revs/second, divided by 360 to give the time for one degree of camshaft rotation gives 1/50X360 of one second. However, note that we are here into degree/second square or per seconds squared so the above figure has to be multiplied by itself for us to work out the 'G's. So we now have a figure of 1/(50x360x50x360) which for those who care gives a figure of 1/32,4000,000. Multiply the value of 0.0005 inches given above for the maximum positive acceleration and we get 13,5000 ft/sec/sec which is about 422 G !! Go on, admit it, you knew that anyway didn't you? Calculating the reciprocating mass of a valve train is somewhat tricky as one has the valve, collar and spring cap and push rod reciprocating, the cam follower and rocker oscillating but a figure of about one pound weight will do for this purpose. Therefore at 6,000 engine rpm one has the cam and follower etc thinking that it is moving about 422 pounds weight. Note that there is a 'squared' in this so that at 1,000 rpm one has to divide this figure by 36 to give about 12 pounds weight.

Going back to the negative acceleration controlled by the valve spring and using a figure of -0.0002 inches/camshaft degree/camshaft degree we get a 'G' figure of about 169 pounds weight. Now you know why your valve springs are as strong as they are.

So now going back to the advice to rev the engine upon start up. Turn the engine over as slowly as you want and there are certain forces on everything. Rev the engine and you still have all those forces plus the inertial loading due to the 'G' forces caused by the acceleration of all the valve train components. Do you still think that it is a good idea to rev an engine from cold?

Now with regards to starting a new engine and anti scuff compounds. Everything above pertains to used engines being started after sitting for a day or a year or whatever. It is true that there are a variety of products made to aid the start up on new engines without too much wear on the highly stresses components like cams and follower. Some of these are intended to be 'stuck' to the components by baking or whatever. Putting on a grease ,unless it can give a molecular bond, is unlikely to be adequate. Using the above figure of about 400 pounds weight for the inertial equivalent of the valve gear consider that the whole of this movement it being produced by the movement of the cam follower around the cam. Let us take an area of half and inch wide by one thou thick as the pressure point between the cam and follower. That is a total area of 0.0005 square inch. This gives a resultant pressure of 2,000 times 400 i.e. 800,000 pounds weight/square inch. Eye watering isn't it.

Now let me know what I have done wrong. :)
 

bmetcalf

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Good thing that max G period is short. Did PEI do this calc when he decided on the diameter of the pushrods, to avoid buckling under the 422 pound force?

Maybe I should trade my Rapide for a Scott.
 

timetraveller

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The duration is not as short as you might think. I am going to try to attach a graph to the end of this but in might not work. A figure or more than half the maximum positive acceleration lasts for well over 20 - 30 degrees at the start and end of lift.

No, the attachment wont work but try to find the article I did for MPH years ago. :)
 

BlackLightning998

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Thought you might like an update....

Evening All,

Thought you might like an update.

Engine all re-built and 1500 fault free miles covered so far. Quiet (like it never was before) and oil tight, I'm delighted with the work that John Coates did for me in putting it all right.

What did we find, well:
Oversize valve springs
Valve springs not seating in upper valve guide groove as a result of over size
Excessive wear in upper valve guides - I suspect as a result of lateral pressure from poorly seating over-sized springs
Gearbox camplate not secured
Most components loose inside the kickstart cover
Exhaust valve lifter mechanism failed and jammed
Exhaust valve lifter mechanism "kissing" cam followers
Clutch fouling in clutch drum - some nasty bright polish marks
ESA "new" springs broken
etc etc etc

Numerous people have looked at the cams and followers, there is blueing on them but we haven't been able to identify if this is due to oil starvation, oil choice problems or mechanical tolerance problems driven by the valve spring issues described above.

I have just sent one follower away this week for detailed examination in a metalurgy lab - so watch this space.

Oh - and interestingly the carb. problems I was experiencing seem to have gone away now too - what a surprise!!!!!!!

Many thanks for people's support, encouragement and guidance. Appreciated.

Stuart Metcalfe
 

lindie

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Good thing that max G period is short. Did PEI do this calc when he decided on the diameter of the pushrods, to avoid buckling under the 422 pound force?

Maybe I should trade my Rapide for a Scott.


side topic: love my two strokes, question if cams are really needed.

none of my zweitakters ever require a tappet adjustment.

offhand, were the problems with your followers a lone case or have others had similair problems. what oil did you use so i can cross it off my list.
 

Tnecniv Edipar

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Ring dings are fine for dirt bikes but too compromised otherwise. Narrow power bands , burning their own lubricant , high fuel consumption and emissions , limited engine life and they sound rubbish !!
 

bmetcalf

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side topic: love my two strokes, question if cams are really needed.

none of my zweitakters ever require a tappet adjustment.

offhand, were the problems with your followers a lone case or have others had similair problems. what oil did you use so i can cross it off my list.

My semi-close cousin Stuart had the problem, not me. (Touch wood.) I have used a variety of 20w50 conventional and most recently 10w40 conventional.
 

roy the mechanic

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Ring-dings

I have to disagree with tecniv edipar, Stinkwheels are great. My RG500 Suzuki weighs about a third of my vin, makes more than double the horsepower, goes 25 mph faster than Rolle Free,and it's street legal. O.K, it uses fuel like an Irish navvy on pay day, but it shure has the feel good factor in spades!-Roy.
 

Tnecniv Edipar

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Ah , but if you could keep only 1 , which would you choose ? And would you care to ride the RG to say , the South of France ? Thats without mentioning the 30-40 years age difference between them !
Having said that , I do have a soft spot for my D7 !! LOL !!
 
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