Lifter Foot Load

[Taylor]

With a 1.6 rocker, P5249464 springs, and crane 753901 cam I calculate my lifter load to be 390.72# change that to 1.7 rocker and I get 415.14# HOWEVER I read a bit more and come up on this;

"The actual load on the lifter foot is the sum of the static spring pressure, the frictional loading in the valve train, and the inertial loading of the mass of the valve train."

Anyone mind helping deciphering this http://www.eng.wayne.edu/page.php?id=758

 

[BuickGN]

I think we all know about the effects of spring pressure, mass of the valvetrain, and different rocker ratios. I assume the "inertial loading of the mass of the valvetrain" is a reference to the three items mentioned above plus the ramp speeds of the cam.

When you look at a flat tappet cam and a roller cam together with the same duration, the roller cam still has more "area under the curve" and it's noticable to the eyeball with the top of the lobe being flatter. Even though it starts the valve opening process at the same time, it reaches fully open quicker, holds it open longer and closes it quicker, arriving at the closed point at the same time as a flat tappet with the same duration.

What you get is more power with no trade-off in drivability. I don't know why I went on this roller vs flat tappet tangent. Just trying to illustrate how a cam with faster ramp speeds can place a higher load on the valvetrain even with the same duration and lift.

 

[Taylor]

Yea I was wondering where you were going with that! But yes I understand the atvantages of a roller cam. I guess my question is, do you think the numbers I have come up with are, dare, I say "acurate?" Or am I leaving a ton of important factors out, and my numbers are waaayyy off?

Either way, thanx for the help! Did you see my comparison of the RL 5w30 to the Amsoil ACD on the HTHS thread?

 

[Taylor]

Anyone?

 

[mechtech2]

I am not sure about what you are asking.
Less pressure and less mass are best for wear.

 

[Taylor]

I'm trying to equate my lifter foot load

 

[Shannow]

Your calcs going from 1.6 to 1.7 are static values only "Over The Nose"...but still incorrect even in a static sense. The valve will be more open, so the spring pressure will rise, so you need to add that back through the rocker to get the static OTN load.

I'll add inertial load next (makes more sense here).

Imagine pushing the lifter up slowly, versus whacking it with a hammer (put a ruler over the edge of your desk, and slowly lift versus smacking it up...feel the change in force on your hand ?), the amount of clearance, plus the opening ramp determine how hard the cam pushes the lifter up, and the rest of the valvetrain weight "reacts" to this, giving inertial loading.

Finally friction..it opposes everything that you want to do, adding to the opening pressure.

 

[Gary Allan]

Give him a fudge factor that will more than account for the .1 higher rocker ratio.

I say 450lb+ (no-I won't explain it - I can't since I pulled it from my behind)

 

[Shannow]

Nah, when you throw in inertia and omega squared, all of the static stuff is meaningless.

You can get wear on the non pressure side by everything catching up and driving it in the back so to speak.

 

[Taylor]

The spring pressure I used was the open pressure. This is how I got there;

Spring specs;
Spring rate = 270#/in
120# Installed

Cam specs:
0.46" lift

Open pressure= installed # + (lift x spring rate)

Lifter load= open pressure x rocker arm ratio

Is all this correct??

What your saying makes sense, and I've also been doing a little more research. So we've got mass, velocity, rocker arm ratio, spring pressure, and friction. What else? Does push rod angularity play a role as well?

 

[Taylor]

Actually the lift for the intake is .456 and exhaust is .484. So my numbers a little off.

 

[mechtech2]

Also, new cams and lifters are not flat.
Th lifters should be convex, and the cam [at max lift] is not flat across the surface. This is for lifter rotation and wear purposes.
As they wwear, they get flatter.

This makes determining the actual load [pressure/area] very interesting and difficult.

 

[Taylor]

It would be the same pressure, just exerted on a smaller area. Or no?

I guess the load on the cam would be different...

AHHH

 

[Taylor]

Here is a decent read on the subject;

http://www.tildentechnologies.com/Technical/CamSprings.html

 

[BuickGN]

Originally Posted By: mechtech2
Also, new cams and lifters are not flat.
Th lifters should be convex, and the cam [at max lift] is not flat across the surface. This is for lifter rotation and wear purposes.
As they wwear, they get flatter.

This makes determining the actual load [pressure/area] very interesting and difficult.


That was one of the reasons some of the GNs will eat cams over and over. The lifter bore in the block for the #2 exhaust lifter is perfectly centered over the cam, no rotation of the lifter, and the second you put an aftermarket cam or valvsprings, this one always fails. The only choice is to convert to roller or replace the block.

 

[Taylor]

A lot of cam failures you see seem to be specific lobes withen each engine family, no? And it seems as if its predominatly exhaust lobes, maybe due to the higher lift rasing the pressure?

 

[OVERKILL]

Originally Posted By: Taylor
A lot of cam failures you see seem to be specific lobes withen each engine family, no? And it seems as if its predominatly exhaust lobes, maybe due to the higher lift rasing the pressure?


No, it's the added load of trying to force that exhaust valve open into a cylinder that is under pressure.

 

[Taylor]

Ahhh, I see! Thanks for pointing that out. But the higher lift isn't helping anything, correct?

So it's safe to say the greatest pressure is exerted on the nose of the exhaust lobes?

 

[BuickGN]

Originally Posted By: OVERK1LL
Originally Posted By: Taylor
A lot of cam failures you see seem to be specific lobes withen each engine family, no? And it seems as if its predominatly exhaust lobes, maybe due to the higher lift rasing the pressure?


No, it's the added load of trying to force that exhaust valve open into a cylinder that is under pressure.


You know, I've never really thought about it but on the "bad cylinder" on the GN, the intake lifter also looks centered. Maybe it's the lack of cylinder pressure trying to hold it closed and the boost trying to help it open anytime the rpms go up that keep it from failing too.

 

[ARCOgraphite]

You are talking thousands of PSI at the cam contact point due to the small area. Hopefully you get good oiling and a little oil wedge under there and the running PSI net will be managable. Shoot , we have problems with roller cams not rolling due to too much lube.