LT5 Assembly Project

[LGAFF]

I know I am flooding info right now, but just reading up on Cam failures in case I decide to go with regrinds.


From: Dave BakerNewsgroups: rec.crafts.metalworkingSubject: Re: Help with cam lobe destructionDate: 31 May 1999 16:24:57 GMTFor some reason I can't see the original post to this thread. However, thetheory and practice behind cam follower design and break in procedures is asfollows.Pushrod engine type followers have a radius of about 1 metre ground on therunning face. They are also offset from the centreline of the cam lobe bybetween 1 and 2 mm. The cam lobe is ground at an angle of around 3 thou perinch to ensure initial contact towards the side of the follower. Thecombination of angle, radius and offset ensures that the follower rotates toeven up wear over the entire running surface. If a follower stops rotating itwears out in very short order.The break in procedure is not just to "match" the follower to the cam lobe asis commonly thought. It is also to work harden the surfaces for which cast ironis an excellent material. The high point loadings rapidly compress the surfaceskin of the lobe and follower and create a hardened skin which then resistswear. All cam systems wear at a given rate during their service life and therate of wear depends on lubrication, material hardeness, regularity of surfacefinish and other factors. The aim is to reach a rate of wear at which thefreshly exposed metal work hardens quick enough to resist further wear. If thefollower stops rotating or the lubrication boundary fails then wear takes placefaster than hardening takes place leading to catastrophic failure.Modern overhead cam engines running on bucket followers use hardened steel asthe bucket material and these are ground flat not radiused although there isstill an offset to ensure rotation.Break in procedure is to coat the running surfaces with molybdenum disulphidelube and then run at 1500 to 2000 rpm for 15 to 20 minutes. Low speeds createhigh point loadings and high speeds create rapid wear rates. Intermediateengine speeds allow work hardening to take place without excessive point stresson the parts.99% of cam failures are due to inadequate break in procedures.Dave Baker at Puma Race Engines (London - England) - specialist cylinder headwork, flow development and engine blueprinting. Web page athttp://members.aol.com/pumaracing/index.htmFrom: Dave BakerSubject: Re: cam follower grindingDate: 28 Nov 1998Newsgroups: rec.autos.makers.vw.aircooledApart from overhead cam engines with bucket lifters under the cam, all camfollowers are ground with a convex curve on a radius of approx 1 metre. The camlobes also have a slight taper across their face, usually 2 to 3 thou acrossthe lobe. This ensures that the contact point is on one side of the followerwhich causes it to spin. Obviously the taper must slope the correct waydepending on which side of the follower the cam sits. (cam lobes run towardsone side of a follower, not right across the centre line)This evens out the wear and enables the cam lobe and follower to bed intogether. Spin rates are critical for long life and there is plenty of researchin old ICMA papers from the 50s and 60s on design parameters. Too slow a spinrate and the follower scuffs - too fast and it wears in the follower bore.Sometimes there is more to engines (and engineers) than meets the naked eye !!Some years ago a large firm of engine component manufacturers over here madesome special race cams for a well known racing driver. They kept wearing outand taking the engine with them. It took them ages to realise that a taperacross the cam lobe was needed !!Just because someone makes and sells something doesn't mean they know damn allabout it.Glossy adverts sell more parts than proper R&D.One day perhaps I'll drag myself out the local technical library and get thebalance a bit more right !!Dave Baker at Puma Race Engines (London - England) - specialist flowdevelopment and engine work. .