Turbo-Jet

I have often read that roller cams are machined from steel, rather than made of cast iron because roller cams are required to be harder. In general, steels are harder and stronger than cast iron, and more widely heat-treatable. But nothing I’ve read would explain why the roller cam is required to be so much harder.

At first thought, one might think a flat-tappet cam would need to be harder, for two reasons. First, obviously they have much more friction resulting in wear, and second, they appear to have higher contact pressure. Let me explain. A flat tappet lifter has a 30” spherical radius ground on the foot. And a flat tappet cam has a slight rake angle of 1 deg on its face. This combination does two things; it rotates the lifter a few degrees with every cycle to produce even wear, and it generates some thrust to hold the cam in the block. The resulting contact of these faces is point-contact. A roller cam is subjected to line-contact, which appears to be a larger contact area. Because pressure=force/area, then if the same force is applied, and the area is smaller, than the pressure increases proportionally. If the contact pressure exceeds certain material properties, then the cam will suffer from excessive wear. So if the roller cam has a greater contact area, then the pressure should be less. This does not explain why a roller cam is made harder…

Possibly two reasons why:

-A flat tappet cam must be broken-in. In a short period, the cam and lifters wear together at an extreme rate, just as new gears or bearings do. This has the effect of increasing the area of the contact ‘point’, until the pressure is reduced to a non-wearing level. For this to be true, this new area must be much greater than that of a ‘non-wearing’ roller camshaft’s line contact.
???

-Due the elimination of a pressure angle, a roller cam is not limited by cam profile; rather loss of contact is of concern due to the much higher ramp accelerations available. This, combined with stiffer valve springs, produce forces which are large enough to result in higher contact pressures, despite the increased contact area.
???

Comments?



[Modified by Turbo-Jet, 1:41 AM 12/18/2002]

1979toy

My vote has to go to the increased spring pressures as the reason that roller cams have to be made from harder material. It takes a lot of spring to restrain and reverse the movement of a roller lifter. Rev kits, the springs that go between the lifter and the head, were developed so racers could run high RPM's with roller lifters. The excessive spring pressures required to run extreme RPM's with out a rev kit will cause the spring seat in the cylinder head to collapse.

norvalwilhelm

I don't know why roller cams have to be harder. In plain carbon steels the carbon is what causes a steel to be able to be hardened. In cast iron you have maybe 2 % carbon so cast iron is very readily able to be made very hard. It is brittle which means if you try and bend it in it's hardened state it will break. White cast iron is extremely hard. Normal steel is maybe .2-.4 % carbon and maybe in special alloy steels up to 1% and they can be harder to harden then cast iron. Usually normal carbon steels are hardened and then tempered or drawn back to a more useful range..
I know I haven't clarified anything only that cast iron can certainly be hardened to any specs that a normal alloy steel can.

Flareside

Not all roller cams are steel. The Comp "street rollers" use a cast core. If I had to guess, I would think that the race rollers use a steel core because of the extreme valve spring pressures involved. Maybe the springs press hard enough to deflect the cam and cause premature failure? There are lots of roller cams out there running 800 lbs. of pressure on each lobe. :eek:


[Modified by Flareside, 9:26 AM 12/18/2002]

gkull

I just don't see why you think that a roller cam had more surface area for contact. :confused:

A flat lifter is the whole width of the lifter bore. The cam lobe is the same width. My billet steel roller cam uses lifters with wheels that are not the width of the bore so the contact point is really a very thin line the width of the wheel.

Flat cams are made of soft material because it's cheaper to machine and they live in an environment that doesn't require much strength. If you installed much over 150 lbs closed pressure springs you would destroy the lifters and cam.

1979 Toy - Rev kits are installed for more a block and lifter saver in the event that you have upper valve train failure. I think that mine are rated at 25 lbs. which is nothing compared to 200# valve springs with 1.65 ratio rockers

MotorHead

I agree with gkull the roller cam seems to have a smaller contact area and added to the higher spring pressures makes it nessessary to have a stronger cam :D

Turbo-Jet

do you not see how contact between a spherical face and a flat face (or line) would result in point contact? in theory, the area of a point is zero, making for an infinite force, but because of wear patterns, material yielding, and oil film, the area is not zero, but still very very small.

Stingraycrazy

I'm going with the theory of higher spring pressure and faster ramps that require harder material. :crazy:

HDIronman

:iagree: Absolutely, point comtact. The bottom of a flat tappet lifter is not flat.

gkull

Turbo-Jet

I went out in the garage last night and grabbed my old XE274 cam and a Crane H-flat lifter. While rotating the cam in my hand and watching the lifter. I noticed that the lifter had whole width of the base across the cam and during the valve lift and closing phases that the flat base had most of it's surface area touching the cam lobe.

Then just think of a wheel rolling around a cam and it never touches more than a thin line the width of the roller wheel which is not as wide as the bore.

I also think that a roller cam has to have very good steel because of valve lash. That clicking sound you hear when they run is 16 little hammers beating the valves open to much higher lifts than is possible with a standard flat lifter.

Stingraycrazy

Don't forget gkull that they have solid flat tappet cams made from cast as well with the same little TaT TaT TaT... But your probably right, it is more severe with the higher spring pressure of a solid roller. As for contact the lifters only contact one side of the diameters face surface as the lobe revolves below. This causes the lifter to spin in the lifter bore when in operation. If a lifter is flat and not spinning it will cause uneven wear and will eventually wear down the lobe of the cam. If you look at a cam with low miles on it you can see the lobe surface is shiny on half of the contact surface. :crazy: