John. Thanks for elaborating further on the additional forces that take place at the lobe/tappet. Good job on making that explanation as short as it was. Frank and I had pondered over whether it was a subject that should be touched upon but felt like it would be an overly complicated answer that would also be too lengthy to post. You managed to get the point across in a reasonably short explanation.

Lorena, Texas (South of Waco)

No, it's no good because I now have more questions

Wouldn't the lifter be tangent (or whichever appropriate engineering term) to the lobe where they make contact, in which case the area cannot be calculated and the pressure should always be approximately the same? The only way the pressure could change with a larger base circle is if the cam or lifter surface deflect or perhaps the oil film transfers some of the load which can be represented as a surface area and that could conceivably increase with a larger base circle.


Lawrenceville, GA

Charlie, The larger base circle will allow a larger contact area with the lifter, and you can use less lobe lift resulting in a stronger camshaft.

Frank/Rebop

Bristol, In ( by Elkhart) 

I understand how it is being represented.
By nature, I'm trying to understand the logic behind the math. If you take two curved surfaces and touch them together, the point of contact cannot have an area because it will be either a point or a line depending on the complexity of the curves. So where does the area come from?


Lawrenceville, GA

Charlie a larger radius has more contact area than a smaller radius I can't think of a simpler way to put it. There is a difference between static or applied pressure and pressure in pounds per square inch. A sharp knife does more work than a dull knife with the same applied pressure. Simply because psi goes up on a sharp knife ,the cutting area is reduced and the static pressure the same.

http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

I guess I'll have to accept that "a larger radius has a larger contact area" is how this problem is modeled for engineering purposes, but in the simplest terms, any radius will contact only at a point or a line and those by definition have no area.
Take for instance a circle and a line that is tangent to that circle, where they make contact will be a single point, it doesn't matter how big the radius is, until it reaches infinity, it will always be a single point. Make that circle into a cylinder and the line into a plane. Now where they make contact can be represented by a line which has a length, but no width, and therefore, no area. The only way to have a contact area is if the edge of the circle or cylinder deflects to make more contact with the line or the plane. Do you see what I mean?
In terms of the cam and lifter, the larger radius will be flatter so perhaps the oil film is less inclined to disperse and effectively creates what can be considered a larger surface area. This also makes sense with the knife analogy, the cam will be less likely to cut through the film.

Since y'all didn't want to make it too complex, I'll just drop it from here.


Lawrenceville, GA

Charlie, you're right, but the flatter or larger radius does carry oil better. And the less severe angles deflect less and create lower forces at the intersection. Think about the clearance ramps on a flat tappet cam. Also, a lesson I still have trouble with, even metal deflects slightly under force. NOTHING is completely rigid. So in actuality there can be more area. Good luck trying to measure it though.
The reduction in friction must be large with rollers, many street Hydraulic cams have .600 or so lift and don't wear out quickly.

Frank/Rebop

Bristol, In ( by Elkhart) 

Charlie mechanical engineers use Hertzian Stress calculation for this type of useage everyday. This instance would use contact mechanics for an elastic sphere and an elastic cylinder. Where the lifter is the sphere and the cam lobe is the cylinder. Since all things made of metal have a density value this will be figured in along with the radius. The the 60" to 80" radius of the lifter will be figured along with a density value. The value of indent can be figured with weight of the lifter itself or under the load sping x rockers ratio thus an "area" can be determined for both weights as well as if the radius of the nose is changed. Metal parts do not have infinite hardness nor are we on the moon. I understand your theory of point or line contact but your theory says that there are no area differences in a cylindrical roller bearing or a ball bearing of the same diameter or ball bearing to ball bearing of any diameter. Yet all of these bearings have different load ratings and usages.


http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

Thank you, that is awesome. I realize what the simplicity of my example implies about the load rating of bearings and such but I also know that more area doesn't just come out of nowhere and something else has to be going on. That is really cool.


Lawrenceville, GA

I always assumed that the reason lifters had a concave face, was to give more contact area with the cam, thus eliminating the straight line theory.

Glen Henderson



Freedom is not Free

Letohatchee, AL