Cam drive HP

[GT6Steve]

Could someone help me with a formula for calculating the HP required to turn a specific cam considering valve spring rates. College physics is too far behind me to ferret it out on my own.
I want to get a reasonably useful number to select a drive belt or chain for a six cylinder engine.
I can see accleration and impulse should be part of the equation but those components may be so small as to not be a factor? Any thoughts welcomed.
Steve

 

[EngJW]

I don't think you will find it as it is a complex problem that requires a simulation, and most of us mortals with shallow pockets can't afford the software. There are probably some older SAE papers that show it as a fraction of the total friction hp.

 

[GregLocock]

Perhaps more helpfully, work out the sliding velocity at each interface, and the normal force. If you know the coefficient of friction (which is likely to be non-linear) then just add them all up over 360 degrees and you'll have the total power lost, assuming no losses elsewhere - such as that lost when the valve hits the seat .

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[turbocohen]

Contact Crane cams and ask for Ralph Johnson or an engineer. Alternatively contact Jesel belt drives. A small cogged belt will do the job as long as oil is kept away.

 

[ivymike]

I don't think that calculating the friction losses to get average horsepower is the best way to go about sizing a belt drive- I expect that calculating peak instantaneous torque would be much more useful. If your rpm is relatively low compared to cam mode shapes and spring surge frequency, you might get close enough by:
1) doing a single-line kinematic calculation for one exhaust and one intake cam lobe at a particular rpm, then
2) calculating the (phased) sum of the forces due to all lobes.

 

[GT6Steve]

Thanks all for the inputs. So far I've simply selected a 1" HTD belt of the appropriate length reasoning that if it's adequate for the serious V-8's it's certainly adequate for my 200 HP six. I'm about ready to start machining.

This argument came up again when I asked why we use heavy duplex chains in our racing Triumphs as opposed to the lighter simplex chains and all the answers boiled down to insurance. All of the input was subjective and anecdotal (and SCARY) but very little genuine data. What forces do we actually see withn 150# seat pressures. Lord knows it's less than the average weekend drag racer is looking at.

And as an aside, the folks at Jesel were somewhat less than interested in discussing a one off application. Can't blame them for that...
Again, Thanx for the input, Steve

 

[facty]

Assuming you don't want to invest in softwares to do this, Greg's advice is how I'd tackle it too. There's a very useful book by Robert Norton called, "Cam Design and Manufacturing Handbook". I recall that there is information on general figures for the sliding friction produced by the follower on the cam. A link to the book is below if you're interested:

http://www.designofmachinery.com/CDH/index.html