Flat Face Cam Follower Spin

[strokersix]

I understand that flat tappets have a slight spherical crown that mates with tapered cam lobes to induce spin and spread wear evenly. Question is: How much spin is typical? It seems to me that it really doesn't matter how much spin as long as it's greater than zero. Any thoughts or experience here?

 

[evelrod]

All I can add is that I have had a couple flat tappets go 'flat' (no pun) and presumably stop spinning, one in a 292 Y block Ford and one in a 292 Chevy 6....hmmmm. I hadn't noticed, perhaps the 292 is not a good number. Anyway, they quickly wiped out the camshaft in each case. One add on...we had five 350 SBC in the 83 model year...the year GM 'saved money' by reducing the diameter of the tappets...none of the five made it beyound 72K miles before the cams died!

Rod

 

[EngJW]

I recall a number of about 25 rpm somewhere in the literature but I don't know the specifics. Should give you an order of magnitude anyway. It would not be a high value since the cam turns at 1/2 engine rpm and each time the lobe comes around it would only rotate the lifter a small amount. The tangential force on the crown has to increase to an amount that would overcome friction of the lifter against the bore, but a the same time an increase in tangential force will increase the side loading of the lifter, which also increases friction.

 

[franzh]

As EngJW stated, the tappet rotates slightly with each lift, about 1/3 turn. This is also dependant on the engine rpm.
Rod, I too remember the GM camshaft fiasco. The shop I was affiliated with had at one time, 32 5.0 and 5.7 camshaft jobs waiting in line, and almost all were the #6 exhaust lobe and all were in the 35K to 50K mile range. We had to analyze the failures for any possible warranty issues and GM stated that they had not had any complaints from other locations that the problem was localized, probably due to owner neglect (HAH!) One thing of interest, once we changed the cams, they did not fail again. I don’t recall any change in lifter diameter though.
Franz

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[evelrod]

Franz, that was the "official" reason given the owner of the company. Four needed camshaft and cylinder head work and one was a total needing a complete new engine...I think that one died around 35,000 miles. My truck made it to 60 something. I really disliked those trucks, I am not a Chevy man, but after the engine work they ran well (excepting a few flex plates, ps pulleys, alternators, ign. coils and, the occasional center bearing in the drive shafts) to around 150,000. Don't know what happend to them...the company sold them and I moved on. Interestingly enough, the old guy that owned that outfit converted to Dodge/Cummins in the '90s...only took him twenty years to give up on GM.

Back to the original post...I do recall watching my pushrods rotate...pretty slowly at idle...maybe 2, 3 turns a minute...a guess. Actually I think the taper on the cam lobe is responsible for the tappet rotation and the convex finish to the tappet is more for break in purposes.

Rod

 

[franzh]

Rod
Chev had more than their fair share of engine problems in the late 70's and early to mid 80's. Too many exhaust valves tuliped into the heads, flat camshafts, and #1 and 2 rod bearings spinning. Toss in the aforementioned flexplates, AC compressors and evaporator coils, PS pulleys, cracked fan blades, diode trios, and lousey engine and transmission designs, its a real miracle they survived even that. More than one company switched to Fords, and many to Toyota PU's too.
As for the OT, the lifter convex surface mates with the cam taper for the lifter rotation. Only a portion of the lifter face actually contacted the cam lobe. When the taper is interfered with by a small piece of debris and the lifter quits rotating, its only a matter of time until the bottom is wiped out.
Franz

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[WilliamH]

From an engineering point of view, a more reliable method of lifter rotation is the offset method. This method does away with the tapered cam lobe. The lifter centerline is offset from the lobe centerline by approximately 5mm. The lifter face has a small convex radius and rotates nicely for hundreds of thousand miles.

Block and camshaft manufacturing machining is slightly more critical with this setup.

Will

 

[strokersix]

WilliamH,

I don't understand the configuration you state. I'm visualizing a convex tappet running on a straight (not tapered) lobe will result in edge loading of the lobe and not tappet spin. What am I missing?

 

[EngJW]

When a crowned lifter is put against a taper on the lobe, the actual point of contact will be off center on the lifter. Any force applied by the lifter will create a rotating moment.

If the lifter is offset and run against a flat lobe, contact will be in the center of the lifter regardless of the offset. A flat surface has no idea where the offset is.

Another purpose of the crown and taper is to allow for misalignment. A crowned lifter on a flat surface would not have edge loading, but without the taper there is nothing to make it rotate.

 

[franzh]

The taper of the cam lobe against the convex tappet bottom is enough to force the cam rearwards. On most applications, this is no problem as the rear of the cam gear contacts the block surface, but on other applications, there is a purpose built thrust plate or bearing to retain the cam movement.

Franz

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[Tmoose]

Somehow OHC engines with lobes running directly on tappet shims seem to work flawlessly. When I peek in the oil filler hole the lobes appear to bear on the tappet shim full width, but the shime removed to have a faint circular wear pattern. Maybe it helps that they live in oil.

 

[evelrod]

Having some experience with DOHC engines where the cam lobe runs directly on the tappet (Lotus, Alpha, Jaguar, Kawasaki 650 and a couple of Kawasaki 900's that used a huge floating shim on top of the tappet) I can say that, improbable as it might seem, the tappet does rotate.

Rod

 

[GregLocock]

I have known of a problem with a production engine when the valves stopped rotating. I can't remember the fix, but it was a big deal.



Cheers

Greg Locock

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[bobqzzi04]

Tmoose (Mechanical) 30 Mar 06 18:57
Somehow OHC engines with lobes running directly on tappet shims seem to work flawlessly. When I peek in the oil filler hole the lobes appear to bear on the tappet shim full width, but the shime removed to have a faint circular wear pattern. Maybe it helps that they live in oil.


I think you'd find that even though they look centered, they are slightly offset (not anywhere near 5mm though)

 

[strokersix]

I'm interested in arguments regarding whether 1 degree per lift is better or worse than 1/3 turn per lift and why?

There must be some difference in hydrodynamic conditions, sliding velocity profile across the lobe, wear life, and I'm sure others. Any thoughts? Is the tappet rotation effect linear with engine speed? With applied force? Cam ramp slope?