Cams and Cam Design

[trumpy81]

GDay All,

I just finished designing a 3cc 4-Stroke single made from barstock, all except for the actual cam profile. My knowledge of cams is pretty sketchy so I'm looking for resources on the subject.

I've googled and found nothing that would assist me in designing a cam or for plotting the profile.

Does any one here have a good resource for design of cams, prefferably in laymans terms? ( Trigonometry baffles me ... :big: )

Or any ideas on plotting a decent cam profile?

Thanks to SteveHucks I have a fairly good method of machining the cam, now all I need is a profile to machine :big:

 

[Dave G]

Hello Mr Trump, Slow speed engines such as a hit-miss type can use a simple flat flank on the cam. If you want to run your engine at a higher speed a curved flank should be used. The method Steve uses to make a cam lobe is a good one.
When I design an engine I will take the speed into consideration. Full size engines, say a V-8 will use around .400" - .450" lift at the valve and around 260 degrees of duration. I will take the size of my engine ( bore and stroke ) and scale the size to the full size engines. If my engine is 1/4 scale the lift of the lobe will be 1/4 the full size engine. The duration of the lobe isn't scaled. If you use a rocker arm in your valve train and your rocker arm ratio is 1:1 the cam will have the full lift. If your rocker arm has a ratio other than 1:1 your cam lobe lift will have to be designed with this in mind. Say a 1.5:1 ratio, the lobe will only need 66% of total valve lift.

As the cam rotates 1/2 crankshaft speed the duration on the cam lobe will be 1/2 of total duration desired ( 260 degrees ).
So your opening and closing points on the cam will be 130 degrees apart. You will need to have some clearance in the valve train so the valve will be fully closed when running ( lash ). As you increase lash in the valvetrain you lose duration of the lift.

The intake and exhaust lobes for one cylinder will be around 110 degrees apart ( lobe separation angle ). The intake lobe will follow the exhaust lobe. The intake lobe will be installed 110 degrees after TDC of the piston. These are measured at the highest point of the lift of the cam (centerline ).

I like to layout a diagram of my timing events on paper showing 720 degrees of crank rotation. The intake valve will open just before TDC and the exhaust valve will close just after TDC. There should be about 20 - 30 degrees where the valves are both open ( overlap ). I hope this helps and if I got anything wrong I'm sure someone will correct me. There are quite a few valve timing diagrams available in engine books and these should be studied to give a much clearer view than a few paragraphs on the web. Dave

 

[gbritnell]

Hi Trumpy,
If you Google camshaft design you should get many hits on the subject from Hot Rod magazine, Circle track magazine etc.
I would suggest reading some of this information to learn some background on the why's and wherefores of cam design. This is not to make you a cam designer by any means but it will give you some insight into how they work.
That said, for ease of starting and operating a 4 cycle engine you would want to start out with a cam of mild specifications. By that I mean it would have moderate valve events, as Dave stated it would open the intake about 10-20 degrees BTDC and close the exhaust about the same amount ATDC. The exhaust could open and intake could close about the same amount before and after BDC. This is known as a symmetrically designed cam. We won't even get into asymmetrical.
As far as how to plot it out, once you know a little bit more about cam design it won't be so mysterious and laying out the profile will be quite easy actually.
Here again as Dave stated you will need to know certain things, actual valve opening, rocker arm ratio, lifter design, (roller or flat tappet). These things all come into play when laying out the shape of the lobe.
gbritnell

 

[mu38&Bg#]

Try this.

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

Beyond simple 4 arc cams, things get extremely complicated. Model airplane type engines have 260° to 300° duration and 40-80° overlap. Being conservative with the numbers results in docile handling, but less power. Another important design aspect is covered in the above link and that is the minimum cam follower diameter to prevent riding the edge of the follower. And as usual it's all a compromise. You have to juggle base circle diameter, lift, duration, to get a cam profile that works. That is to say if you change one it will in some way effect the others. Sometime not so much, other times significantly.

Greg

 

[trumpy81]

GDay All,

Thanks guys for the replies. I should have included a pic of the engine to give you a better idea of what I was about.

Engine Specs:
Bore 16mm
Stroke 15mm
Belt Driven Overhead Cam
Inverted Bucket Type Lifters

I was planning on using 270 degree duration, 105 degree lobe seperation and around 2mm lift. But I got stuck trying to calculate the flank radius and nose radius.

It's been a long long time since I was interested in cam design and I know I have/had an article that described the 4 arc layout method in detail, together with a lot of hand written notes that I made in my youth.

As luck would have it, I cannot locate that article. I have searched high and low but .....

Now I'm trying to refresh the old grey matter which is not so easy these days ... :big:

Thanks for the link Greg. I have been there before but had forgotten it was there. I'll be reading that again shortly.