Moparts Tech Archive
New roller tappet concept?
I’ve been looking at the posts concerning serious motors (SB & BB, Indy, W5, etc.), with roller cams. I assume that these are high-dollar projects (which are never really finished) where anything that would provide an advantage is of interest.
The cam lobe base circle center is directly under the tappet centerline with any cam. With a flat tappet (solid or hydraulic, only referring to the contact surface) cam turning CW, the point of contact begins (very) near the left edge of the tappet, reaches the center of the tappet at full lift, and passes across to (nearly) the right edge at closing. This works fairly well, and no improvement immediately recommends itself. The dynamics of a roller cam are very different than a flat tappet cam. A major difference is that the point of contact between the lobe and a flat tappet is always the highest point of the lobe acting on the lowest point of the tappet.
A roller tappet makes contact between the closest point of the roller with the closest point of the lobe (the point of tangency), which is only the highest point at full lift. With a very radical roller cam, the lobe makes contact with the side of the roller, and as much as 30% of the force vector is side-thrust rather than lift.
However, the geometry of a roller cam was “scienced out” for use in automatic looms and weaving machines 200 years ago. Devices purpose-built for roller tappet use do not place the tappet centerline on the lobe center. Harley-Davidson was not breaking new ground in 1920 when their V-twin motor had the tappet centerlines offset against the direction of cam rotation by 1/8”, it was common industrial practice.
What does this do? It places the point of contact above the rising lobe, so that the thrust vector is directly (mostly) upward aligned with the tappet axis. This greatly reduces the amount of effort required to rotate the cam, and adds some life expectancy to the tappet body (I’m afraid I don’t know enough about effects on the roller to comment). It also gives a small mechanical advantage to the lobe against spring pressure (harmless, since spring pressure here is typically not critical).
It has been suggested that the optimum amount of offset is ½ the lobe height. H-D motors with lobes between .240” and .400” all used 1/8” offset.
I think it might be possible in 1 of 2 ways:
1. a motor still in the machining stage could have it’s tappet holes bored oversize and bushed down eccentrically to move the tappet centerline over. How much is possible? I’m sure 1/16” can be done; how much more depends partially on the block and who’s doing the work.
In both instances, the cam will not time in on the original marks. What I can’t get straight in my tired old mind is whether a new cam will be required. I don’t think so – all events (left and right bank) will be advanced by the same amount, so retarding the cam should “clock” it back to where it was. Will it change the effect of the lobe shape on what the valve does? Yes, but I don’t think it’s critical. Wish Harold were here.
Mushroom tappets install from the bottom as well.
I was just thinking a little about this and was comparing a centered lifter to what you propose but on more of an extreme for comparison purposes.
First I'm thinking of using a standard roller cam with a symetrical lobe.
By moving the roller lifter against the direction of cam rotation you would be reducing side load but I think the farther you go from cam centerline you get less lift per degree of rotation than compared to a centered arrangement. Now on the flip side as the cam rotates past peak lift you would get much more lifter "drop" per degree than a centered cam.
This makes me believe to achieve similar valve action you would need to alter the lobe profile to be steeper on open and alot more gradual on close. But wouldn't this negate the reduction in lifter side load? Basically what I am saying, is lifter side load directly related to valve action (sine wave) no matter where the lifter is positioned to the cam?
Also what would a non symetrical lobe profile do to duration.
I think that with a sewing or assembly machine the designers were alot more concerned with one particular motion but in an engine I think the opening and closing are just as critical.
It’s a concept that has good theory but I would have to believe any horsepower gain just would not be worth the effort and if it was, then I would have to believe a WJ, or a Bob Glidden type would have done it by now. The pushrod motor design is inherently flawed by the placement of the cam. Put the cam or cams over the valve and now you have a real possibility. In drag racing Ford tried it, McGee refined it for the Hemi and Batten perfected it on a big block Chevy. But for some reason we all still like our pushrod V8 and the overhead designs never went anywhere in drag racing. Somewhat strange as just about every “new” production motor design is an overhead valve configuration. And if you look at the high horsepower to cubic inch ratio engines (excluding Nitro Hemi’s cause that is just a 6 second time bomb) you find an overhead valve design. With today’s CAD and CNC manufacturing it is not that far out there for someone to make a production drag overhead valve design at a reasonable price. Just thinking out of the box!
I agree with CJK440 that moving the lifter in the manner described will make the lobe lazier on the opening and quicker once it gets over the nose. Of course you could grind the cam to compensate for this but I think the real advantage might be in reduced frictional losses and wear from the reduction in side-loading on the lifter when the load on it is the greatest, on the opening flank. Good question for UDHarold.
I'm afraid you're right, the opening rate would slow down; not good for most motors, since the trend is to get the valve to follow the piston speed, which peaks around 72-78 ATDC, 30 degrees before intake CL. The closing rate increases (again, not good, since this is what floats valves).
I can't figure out whether total curve area remains constant, I have a feeling it's reduced, but offset allows faster acceleration than is used now, which means (bad) new profile needed to get it back. The net result (mechanicall) is definitely positive for loads, side-thrust, power consumed, etc. - requires less power to rotate than std., and if you tried to turn it backwards, effort is much higher. Unfortunately, this doesn't mean much if you need to put development time into a new profile to get the curve area back. A friend is testing some Buick GN profiles on a fixture he made for load vs. offset, should be interesting. I'm going to make a guess: it might be worth 20-30 hp on a 8K RPM motor with 600 lb. springs.
There's nothing wrong with pushrod OHV except compliance in the parts. Big V8 motors just don't turn fast enough to make OHC a big advantage, since the valve gear tracks very well at any speed the motor can make power - just takes expensive parts and big springs.
I wonder if the looms and weaving machines used minimal lubrication due to the state of sealing technology in that era. A leak in a pressure lubrication system may have resulted in staining of the fabric. Therefore the machines may have used minimal lubricant and so the wear on sliding friction elements was high. Especially since materials like oilite bushing had not yet been invented. I had a professer in a friction studies class read a section of a owners manual from a Franklin car, that described the lubrication required at regular intervals. It was quite an extensive list. The Professor's point was that the state of materials has advanced so much in the last 80 years that many problems of the past are not germane to today. Now a friend of mine has a circa 1952 Harley Panhead with near 100,000 miles on it. I don't know if it has the offset lifters or not. He reground the cam once during an earlier engine rebuild and has now had to replace the cam the last time he had the engine apart. The cam lobes were spalled probably from surface fatigue from contact with the roller lifters. However the aluminum tappet block was in fine shape even though it was apparently the original part. I would have thought that the aluminum bores that are subjected to sliding friction would have a longer life than a rolling contact cam. Now if the lifters are offset maybe this contributed to the low wearon the tappet block. Another thing to note about this motor is that the aftermarket cylinders that were installed after the orginals could no longer be bored anymore seem to be quite a bit more durable than the orginals. Is this because the modern cast iron is better, this seems doubtful, but it may be possible. Now back to your earlier point, I am not sure the offcenter thrust on the lifter causes much wear. I really wonder if the shock loading of the valve train is that major contributer. I would think that flat lifters with their sliding contact and offcenter loading might also have fairly high side loading. Since I don't understand the friction and dynamics well enough I will not enven venture a guess as to whether the roller or the flat tappet have higher side loading.
Before I read your post I thought they might be on the "New" Schubeck roller lifter design. It just happens to be the same design that Land Rover started using in the early 50's and they have a lot more wear problems than the current roller lifters.