Camshaft design

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Gordon

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Is there any place where I can find information on basic cam design? Base diameter, working angle and lift are somewhat determined by the basic engine design. What I am trying to determine is how is the flank radius calculated. On the cam drawings that I have looked at the radius and the center seems to be a rather precise number so I assume that there is some way to establish this. One article states that it is easily established using a CAD program but I don't understand how.
 
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Gordon

The profile of a (an?) harmonic cam is determined by: the cam angle, the lift, the heel radius, the flank radius, and the tip radius. If you define any four of these parameters that automatically determines the fifth. I have found (ie worked out for myself) geometrical constructions for each case except unknown tip radius, and for that I came up with a simple trigonometrical formula. As you say, a good starting point is the angle and lift. Given a flat-faced tappet, the flank radius determines how aggressive the cam is in terms of acceleration and forces. The larger the radius, the hotter the cam.

I have been meaning to write all this up, sometime, but there is quite a lot to it with complicated diagrams to produce and I don't want to do it right now, but I can take you through the flank radius case.

First, as we are going to need it, do you know how to construct a perpendicular bisector of a line, using two circles?
 

Gordon

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Gordon


First, as we are going to need it, do you know how to construct a perpendicular bisector of a line, using two circles?
Not really sure what you mean by that. I know what a perpendicular bisector is on a straight line but not sure how circles are involved. It has been 65 years since I took trigonometry. I used it frequently in my work life but mostly triangulation stuff.
 

Gordon

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This is what I am playing around with at this point.

I just looked up perpendicular bisector so I know what that means.
 

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Brian Rupnow

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Gordon---I have a book that explains all that. In fact, I think I did a post about it. I will hunt for it. Basically, the small end radius can be thought of as a circle. The large end radius can be a circle. The flank radii have to be tangent to these two circles.
 

Brian Rupnow

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Gordon--I can't find the book I had in mind right now, but in Malcolm Stride's book "Miniature Internal Combustion Engines" on page 95, he states that " The flank curve affects the rate of acceleration of the valve, and a good compromise is to use a figure of around twice the base circle." (radius). On page 36 and 37 he also says that " The valve lift should be equal to one quarter of the valve diameter." The base diameter of the cam should be at least 0.150" greater than the camshaft itself, and the nose diameter is generally around 0.100".
POST EDITED BY BRIAN 14-AUG-2022
 
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Brian, read it again. "... if the the valve is lifted by an amount equal to one-quarter of the valve diameter, then the open area round the valve head is equal to the area of the valve port ...".

Mathematically, for valve seat radius R and lift H , port area is Pi R squared, and opening annulus area is 2 Pi R H. therefore H = R /2.
 
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Jasonb

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Rod who wrote the "CamCalc" program did an article on cam design in SIC, would be worth a read.
 

Gordon

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I did a little bit of looking at my book collection. Brian mentioned the Malcolm Stride book and I found that I also had that book. I also have a book by L C Mason "Model Four Cycle Gasoline Engines". The explanation in the Mason book looks to be pretty good so this afternoon is going to be study time. It looks like my original thought about just making a radiused flank is not the the correct route. I will go back to the original basic design. I have mostly just been blindly following cams designed by others for other engines and I think that I have followed a poor design. Back to the study and then back to the computer and CAD.

Thanks for the input from others.
 

gbritnell

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Wow! What a complex question! If you start with full sized camshaft applications the variations are endless based on carburetion, intake manifold design, head port flow, exhaust configurations which include the many types of headers available. Then you get into torque and rpm requirements like if the engine is going to be used in a motor home you want a lot of low rpm torque but if you need the high horsepower and rpm then totally different numbers come into play. The thing about building miniature engines is that we just want them to run, and run well. In all the years I've been doing this I've never spoken to anyone who said "I started out with a camshaft that had these numbers and it didn't do what I wanted so I made another whole camshaft". We don't have the availability of flow benches and dynos to prove what our little engines are doing. I have never made a camshaft with an acceleration ramp on it. My lifter comes of the base circle of the cam then starts to open. I have never had any trouble with my cams. Everyone asks about camshafts but no one that I know of has ever asked what pressure valve springs to use.
Here's the basics. The fuel charge is burned up, or should be burned up, at about half to five-eights stroke so the exhaust valve can open up anywhere between 30 to 50 degrees BBDC. Now the piston reaches BDC and starts upward toward the combustion chamber pushing the spent exhaust gases out of the cylinder. It usually stays open anywhere from TDC to 25 degrees ATDC. The reason for this is the gases have a flow out of the exhaust so by leaving the valve open a little past TDC the flow of the gases helps scavenge as much of the burned gases out of the cylinder. Now the number of degrees is based on exhaust design and flow and at what rpm the engine is running at. Now were done with the exhaust. The intake usually opens a number of degrees BTDC. You have to realize that when the crankshaft is almost at the top, or bottom of it's stroke the piston isn't moving up down very much so by opening the intake a few degrees BTDC you utilize the scavenging effect of the moving exhaust gasses to help pull some of the fresh air/fuel charge into the cylinder. Now the piston starts it's downward travel and starts pulling more of the fresh charge into the cylinder. Much like the exhaust staying open past TDC the intake stays open past BDC for the same reason. The incoming air/fuel charge has built up velocity and being as the piston isn't moving up very much (dimensionally) by leaving the intake open a number of degrees after BDC it helps put a greater charge into the cylinder. If the intake valve is left open to late then it will start to push the charge backwards out of the manifold and carburetor.
Going back to what was stated earlier the cam numbers are all based on how the engine is used and how it moves these gasses in and out.
For a low rpm model engine the numbers can be very conservative and the cam symmetrical. Intake Open 10 degrees BTDC. Intake Close 30 degrees ABDC. Exhaust open 30 degrees BBDC and close 10 degrees ATDC.
gbritnell
 

Brian Rupnow

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Okay Charles---I read it again. The cam lift should be equal to 1/4 of the valve diameter. I have amended my post.---Brian
George---Thank you for the information you posted.---Brian.
 
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I support the "Wow" comment:
I have never needed to do Cam Design, but am very interested to learn from you all. I have only ever "swapped cams and followers" in Motorcycles, to note the variation of tractability and driveability.
e.g. I used Triumph 500cc engine in the 70s and swapped from "hot" cams (Something like110mph "top whack" on a 1955 Factory racing engine, 10.5:1 compression) to "standard" cams, as for commuting the "Hot" engine was just too uncomfortable in traffic below 30mph. But great on the open road. The Standard cams were OK, but a bit sluggish off-the mark from stationary at traffic lights. So I tried "trials" cams, which were really tractable around town, good from traffic lights, but just failed to keep up with highway traffic - even when I used the racing twin carb head instead of the single carb head, and 9:1 compression instead of the trials 7.5:1 compression. So eventually I settled on the "regular" cams, as a compromise, with 9:1 compression and twin carb head, unless I was going to have a weekend of fun riding, when I could change to the 10.5 comp pistons. The "Hot" cams were really only comfortable when ridden really above half throttle and >3500rpm.
I also had a pair of ex-racing cams that were seriously lumpy, but found out they blew-up the racer's engines too quickly, so he sold them second hand... which is how I ended up with them. Only good for drag racing at max revs for a very short time! One thing I learned, the cams only operated the valves for "factory" timing documentation with the correct cam follower radius. SO, I hope to learn how this can be modelled/determined?
K2
 

Bluechip

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I support the "Wow" comment:
I have never needed to do Cam Design, but am very interested to learn from you all. I have only ever "swapped cams and followers" in Motorcycles, to note the variation of tractability and driveability.
e.g. I used Triumph 500cc engine in the 70s and swapped from "hot" cams (Something like110mph "top whack" on a 1955 Factory racing engine, 10.5:1 compression) to "standard" cams, as for commuting the "Hot" engine was just too uncomfortable in traffic below 30mph. But great on the open road. The Standard cams were OK, but a bit sluggish off-the mark from stationary at traffic lights. So I tried "trials" cams, which were really tractable around town, good from traffic lights, but just failed to keep up with highway traffic - even when I used the racing twin carb head instead of the single carb head, and 9:1 compression instead of the trials 7.5:1 compression. So eventually I settled on the "regular" cams, as a compromise, with 9:1 compression and twin carb head, unless I was going to have a weekend of fun riding, when I could change to the 10.5 comp pistons. The "Hot" cams were really only comfortable when ridden really above half throttle and >3500rpm.
I also had a pair of ex-racing cams that were seriously lumpy, but found out they blew-up the racer's engines too quickly, so he sold them second hand... which is how I ended up with them. Only good for drag racing at max revs for a very short time! One thing I learned, the cams only operated the valves for "factory" timing documentation with the correct cam follower radius. SO, I hope to learn how this can be modelled/determined?
K2
This book ( UK Link ):


Has a section ( T16 ) on cams and their geometry. I've had a quick look and it does seem to mention such considerations as 'constant velocity' , 'constant acceleration' etc. so it may be relevant.
I have NOT looked very closely the T16 section, but it is a fairly cheap buy so your call.

D.
 

GreenTwin

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My dad owned a 112 foot towboat, and it had two large diesel engines in it that were directly connected to propellers via shafts.

To reverse the boat, the engines were stopped, the cams shifted, and then the engine started in reverse, where they ran backwards.

I always use to scratch my head about how they were able to shift those cams, and I still scratch my head about that.

I did see a tugboat with the same type of engine, up on the Great Lakes, and it was a museum.
They installed a plexiglass cover over part of the camshaft, and it looked like the cam lobes had ramps on the side, which allowed the shifting.

Rather clever stuff, and no gearbox problems.

.
 
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Sounds like the sort of thing that may have been on early diesel locos? - As they would need to run equally in reverse as forwards, as steam locos do by reversing the valve gear. Easily done if the valves are driven by eccentrics - like a steam gear - with similar reverser..? Or maybe somehow the drive from the cam followers did the reversing?

I.E. If you call cam A the "exhaust" for "forward" and Cam B the "inlet" for forward, the 4 strokes would be B - blank - blank - A, repeat. => Inlet - compression - firing - Exhaust.
Then "reverse function, so the valves get their drives from the opposite cam follower, the drive becomes: cam B the "exhaust" for "reverse" and Cam A the "inlet" for reverse, the 4 strokes would be A - blank - blank - B, repeat. => Inlet - compression - firing - Exhaust.
Very complex for 6 or 8 cylinders though...
But Interesting to find out how it was done?
K2
 

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