camshaft layout

[BERTOParticipant @berto]

Hi i was flicking through some of my books on I/C Engines and was reading up on camshafts .

These books seem to be mostly interested in describing how to make a camshaft by glueing the the lobes on or describing opening and closing of the valves and how many degrees of crank rotation they are open for etc.

Whilst this is all usefull info i am thinking about building a cam grinder that would use a template and a follower to to move the grinding wheel so it grinds the profile of the cam to what is set up on the template .

This is similar to the cam grinder featured in MEW a few years back but i would like to know if anyone knows of a book , website etc that shows how to draw these shapes ?

I am not worried about roller cams as they are two radii separated by flat flanks , what i’m after is cam lobes for flat based lifters which need a radius on the flanks .

What about asymetrical cams they have opening or closing ramps so how does one go about designing and laying out these ?

There are also things like what radius to use for the various sections of a cam lobe ?

Valvetrain velocity is another area that i would like to know about also.

 

 

Ian

 

[BERTOParticipant @berto]

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Berto ,

 

Don’t immediately start with design of the cam itself start with the valves :

 

(1) Decide what you want the valves to actually DO . When to open , when to close and whether you want snap or progressive opening and closing and how much lift .

 

(2) Layout the valve movements required as ordinates on a graph set on crankshaft angular position as a base line . Simple linear graph is ok but there are some math advantages in making it polar .

 

(3) By graphic methods or calculation do further graphs on the same baseline showing first , second and third differentials of the valve displacement .

 

(4) You now have graphs showing displacement , velocity , acceleration and jerk factor of the valve .

 

(4) Examine each graph and adjust the valve primary displacement and consequent differentials until the valve is doing what you want .

 

(5) Interpretation of results :

 

Practical – lift/opening must be adequate .

 

Practical – it must be physically possible to make a sutable cam . In most cases it works out ok anyway but watch for things like dips and rises in quick succession on the displacement graph because this would require a hollow in the cam which would be smaller than any practical follower and you wouldn’t get true following .

 

Displacement – timing must be right .

 

Velocity – can’t usually do much about it but avoid large variations and high local values .

 

Acceleration – should be smooth during the rise and fall phases .

 

Jerk – look out for jagged peaks since this denotes ‘rough’ working and possibility of valve bounce ..

 

(6) You have the radial ordinates for your cam from the final version of the displacement graph . It may be nescessary to scale these if you have asymmetric rockers or the like which alter the movement .

 

(7) Lay out the cam to a large scale and design a follower to suit . Usually straightforward but be aware that flat and roller followers behave in a different way when following a cam with tight hollows – as mentioned above .

 

Note : Take into account tappet clearances and any other sources of lost motion if they amount to more than a few percent of total motion .

 

Note : The displacement , velocity (etc) of valve , rocker , push rod and follower are all transforms of each other and generally on small engines if you get the cam right it is not nescessary to evaluate the other parts except as regards actual practical design .

 

All of the above can with advantage be done on computer – spreadsheet and CAD will usually suffice .

 

There’s a lot more to it so ask any questions you like .

 

Michael Williams .

Edited By MICHAEL WILLIAMS on 10/12/2011 10:18:57

[John Stevenson 1Participant @johnstevenson1]

Take a read of this.

 

http://modelenginenews.org/design/CamTable.html

 

And the associated links on the page.

 

John S.

[Author]

Posts