Valve spring pressure?? how to calculate required pressure?

[Majik]

I'm trying to do the dificult job of calculating what the required valve spring pressure would be for my engine. I'm going over my books and trying to find some equations that would help me out but I can't find or my brain is to dusty to find the correct equations that will help me calculate this... this is what I have so far... The mass I'm moving is the weight of the valve, retainer , and keeper... the engine is a SOHC so there's no pushrods or lifters.. it does have rockers and I'm sure they add some to the weight... I just can't figure out how to calculate them into the equation... they have mass and acceleration but at the same time the spring is always pushing back during valve lift so it's gotta at a minimum cancel out that... anyone have a program that can calculate this?? I think I'm going to have to mathematically take the force from start of the lift over the time... and constantly see what force would be needed to counter act the force/time of the valve opening... as the valve closing should take care of itself. I know it seems like I'm wondering back and forth but it can't be as simple as F=MA... or if it is.. it's going to be a convoluted version of it... needless I'll toss the question to you guys... how does one get a rough but semi accurate calculation of spring rate required for valve lift? I know it woudl be incridibly complex to take into account the acceleration ramps of the cam design... or the variable rocker arm ratio present in my bmw.. 1.1:1 is close enough... cam lift is 10.7mm duration is 304 degrees... anyone got a spreadsheet they could slide me or a kick in the right direction?? Thanks

Kris

 

[EngJW]

Kris,

It actually is just F=MA in principle, but the application is difficult. You need to know the acceleration profile of the cam, and that with the effective weights as seen by the valve, would give you the inertia force throughout the cam rotation.

For the spring, you would assume an installed load and a spring rate. Given the cam lift profile, you would then calculate the spring force at each point.

If you plot the curves of inertia force and spring force throughout the cam rotation, the spring force should be greater than the inertia force at every point (say 10% greater). Too much will increase stresses and deflections.

One has to decide on what the effective weight is that would be seen on the spring side of the valve train, depending on if you have pushrods, overhead cams, etc. Lift and acceleration also have to be multiplied by the rocker arm ratio. In addition to spring force, the natural frequency of the spring should be several times greater than the engine speed.

There were a number of SAE papers back in the 1960s that gave more detail, but I don't know of a single paper that would explain it all. I have my own program, but it requires a lift table and the way it is compiled it may not work on every computer. It is in Visual Basic.

John Woodward

 

[patprimmer]

The weight of the rocker arm will be seen as polar moment.

The wight of the spring itself will have some effect, as will internal friction in the spring, and friction between the spring and damper.

Also, the safety factor of 10% should be after the losses from initial relaxation of the spring.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[desertfox]

Hi Majik

Would it be possible to take out one of the other valve springs and measure it ie:- wire dia, number of turns, outside diameter of spring etc. If you can do this then you can calculate the spring to find out what its properties are
or take the spring to a spring maker and get a new one made.

Regards desertfox

 

[WilliamH]

Majik,

If you really want to get into valve spring/cam dynamics, contact Steve Grunwald at Integral Cams:

Ph 507-645-9308
steve@integralcams.com

Integral Cams
212 East County Road 1
Dundas, MN 55019

Steve developed a very comprehensive software program for valve spring design.

Valve spring dynamics are extremely complex and therefore interesting.

WilliamH

 

[victorymotorsports]

Take rocker arm mass and multiply by 0.33 and add this to the mass of the valvetrain. Then and 1/2 the weight of the valvespring. This is your total effective mass at the valve for a finger follower valvetrain. Then just use F=ma. It is that simple.

Preload is driven by worst case pressure differential across the valve when it's on its seat. Open valvespring load is driven by the effective mass of the valvetrain and acceleration of the cam. Set-up a force balance of the two and your set. Find the contact force between the cam and rocker and do not let it become less than zero throughout valve event.

 

[victorymotorsports]

One more thing. It's valve spring FORCE, not pressure... Pet peeve of mine.

 

[ivymike]

A kinematic separation cover factor of 1.0 (no zero-force events over the nose, but very close) is poor assurance of maintaining contact throughout the valvetrain. Modeling the valvetrain dynamics would be much more effective. Spring surge and component vibration can easily result in separation where kinematics alone say there should be none.