Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

valvetrain forces

valvetrain forces

valvetrain forces

can somebody point me to info on valvetrain forces in a pushrod engine, please?

im primarilly looking for valvetip force and lobe/lifter force on a v-8, (e.g. ls1), preferably real/measured.

RE: valvetrain forces

The range in spring pressure can be pretty large... for a stock LS, open pressure is like 210 lbs or something. For a fully built engine designed to run at high RPM, open pressure could be 500+ lbs.

RE: valvetrain forces

spring pressure is just one factor to consider. e.g. acceleration usually has a peak at about 30° after ivo.

RE: valvetrain forces

Correct- but in a properly designed system valve tip force will never exceed valve spring force; so if you know peak lift values and you know spring characteristics, you know what the upper limit for all the forces are. Whether or not you need a more detailed answer than that depends entirely on what you're trying to do, which so far you have not specified.

RE: valvetrain forces

if you look at a graph of measured force vs. degrees you will see that what your saying might not be correct. why should the acceleration of the valve be limited by the decceleration available from the spring? i will dig up the graphs for other engines and try to post one here as an example.

RE: valvetrain forces

Quote (roymychaj)

why should the acceleration of the valve be limited by the decceleration available from the spring

So that the valve tip doesn't bounce off of the lifter/bucket/shim/cam lobe/whatever and destroy the valvetrain. It's not all that complicated.

RE: valvetrain forces

Valvetrain dynamics is second only to combustion in terms of complexity. From the original post I'm going to wager you dont have a few years' experience specializing in the niche so would advise leaving whatever you're doing to someone who does. If you truly want to begin to learn the subject, SAE has several good texts.

RE: valvetrain forces

The valve spring is not the only "force" to over come, the exhaust valve see's cylinder pressure in its opening phase(cam timing dependent). Depending on rocker ratio the push rod side can see more force than the valve side. Your going to have a difficult time trying to measure it, not impossible but not easy.
Mention of not exceeding spring force. Is that true in a condition of spring harmonics, or valve float?

RE: valvetrain forces

yes, i dont have "a few years in specializing ..." but i can differentiate between decceleration and acceleration (the later easily being able to exeed max spring force).

so would anybody know where i can find a graph of valve tip force in a ls1 with stock cam (or similar engine)?

this is an example of a zetec engine (comparison of two cams):

RE: valvetrain forces

Quote (romychaj)

acceleration (the later easily being able to exeed max spring force).

Think about the physics of what would happen at the point on the cam lobe where the valve acceleration changes sign during valve opening if the force applied to accelerate the valve is greater than the force applied by the spring to keep the valve tip in contact with the cam lobe.

This is called valve float, and it's bad. Usually really bad.

RE: valvetrain forces

Back in the day we had manual methods to develop cam profiles which obviously included checks on valve acceleration. Sadly neither of my two remaining engine textbooks cover it. I'd have thought perusal of the SAE papers and the IMechE collections on small engines would be a good place to start.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: valvetrain forces

In simple terms the contact force is the sum of three components:
1. Spring force. This is very simple to determine. This force varies in direct proportion to valve lift. When you rotate the engine very slowly, this will be the contact force.
2. Inertial force. This is directly proportional to valve acceleration which can be determined from the valve lift profile (2nd derivative of lift). This force is +ve (adds to the spring force) at low valve lifts and -ve at higher lifts. This force increases with engine speed.
3. Gas force. Usually less significant than 1 and 2. Difficult to analyse precisely without actual data on cylinder and port pressure. Varies with engine speed and load and is obviously different for intake and exhaust valves.

je suis charlie

RE: valvetrain forces

The thing about gas force too is that it's highest when spring force and valve inertia are lowest... the relationship is almost purely inverse. If all you're looking to calculate is envelope forces it can be just about disregarded.

Of course we don't know what OP is really after so maybe it matters and maybe not.

RE: valvetrain forces

what op is after: "... where i can find a graph of valve tip force in a ls1 with stock cam (or similar engine)?" (citation from above.) exactly like the graph above. the same for lifter/lobe would be nice. other parameters like measured acceleration, stifness, friction, float analysis or other cams welcome.

grunt, add to it: friction, contact angles rocker/pushrod and rocker valve.

swinny, did you see the graph i posted?

these are the most popular u.s. engines. such measurements were surely done thousands of times by gm and others. i would have thought this was "common knowledge".

RE: valvetrain forces

174*2 = 348

348 > 315

RE: valvetrain forces

swinny, now you lost me.

looking at the graph, can you explain where the second 1400 peak comes from? i can see how the valve deccelerates when banging onto the seat, but how would that translate into contackt force? inertia would make the rocker tip lift, no?

there is no second peak here:

(doesnt upload, will try later.)



edit: uploaddoesn work.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close