Camshaft Design- New
Camshaft Design- New
(OP)
I would like to start this thread over again. The last one had drifted too far off topic. My proposal for the topics would be the following:
1. Cam profile design and ramp design
2. Cam and lifter materials
3. Camshaft manufacturing
4. Valve spring design, materials, manufacturing
5. Valve train dynamics
I don't think we should get into applications (what cam should I put in my car?) or valves and ports. That should be the subject of another thread.
Most of the above subject matter is in the hands of a few specialty companies and consultants, but I doubt if a lot of it is proprietary. Engineers involved in engine design and testing could benefit from at least a working knowledge in these areas. If a problem comes up, it would be good to be able to take a first shot at it before turning to consultants, especially if your company does not have deep pockets.
Thanks,
John Woodward
1. Cam profile design and ramp design
2. Cam and lifter materials
3. Camshaft manufacturing
4. Valve spring design, materials, manufacturing
5. Valve train dynamics
I don't think we should get into applications (what cam should I put in my car?) or valves and ports. That should be the subject of another thread.
Most of the above subject matter is in the hands of a few specialty companies and consultants, but I doubt if a lot of it is proprietary. Engineers involved in engine design and testing could benefit from at least a working knowledge in these areas. If a problem comes up, it would be good to be able to take a first shot at it before turning to consultants, especially if your company does not have deep pockets.
Thanks,
John Woodward





RE: Camshaft Design- New
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RE: Camshaft Design- New
1. Cam profile design and ramp design
The ideal is to minimise return spring stiffness (friction), without allowing follower to cam contact loss. Most seem to offer good timing characteristics with elliptical shaped risers.
2. Cam and lifter materials
Tribology plays as much a part in this as any. If the oil film is thick enough, there will be no metal to metal contact.
3. Camshaft manufacturing
This should be interesting. I have seen hydroformed cams, which struck me as a good way to save weight. I imagine only limited machining was necessary.
4. Valve spring design, materials, manufacturing
To some extent, this will depend on point 1.
5. Valve train dynamics
Are you interested in vibration or fluid dynamics? I imagine the ideal motion to be a cosine pulse, with a dwell between events. The cam profile ideally reproduces this.The tricky part is optimising the timing.
Are you considering VVT mechanisms, desmodromic, and actuated cams?
Mart
RE: Camshaft Design- New
Each specific configuration of engine will require a certain design & is dependant on the bore, stroke, rod ratio & the flow characteristics of the induction & the exhaust system.
The valve timing events would be a good place to start & then again is application based.
Taking into consideration that an engine to produce good power needs pressure, flow & rpm, this would be a good talking point to start things off, what timing events are the most important & why.
RE: Camshaft Design- New
My interest has been in how to translate the required lift and duration into a profile that will not cause dynamics problems. At my place of work we have been having valve and seat wear problems and we have been experimenting with different profiles and spring rates. More on that another time.
Willeng- there was a good SAE paper one time that examined all the points. The intake closing and exhaust opening points were said to be the most important. More recently I have seen mention of the overlap period being used to control emissions. More overlap at low speeds acts like built-in egr. I always thought overlap was used for high speeds.
Till next time,
John Woodward
RE: Camshaft Design- New
Engineering is the art of not constructing...of doing that well with one dollar what any bungler can do well with two after a fashion.
RE: Camshaft Design- New
Well the design of a camshaft is a broad topic. But there are some essentials. First you have to have in mind in which engine you will fit the camshaft, if it requires torque in the low range or in the large one. With this in mind the experts use their experience in the design of the features of the cam, lobe separation, overlap, lift etc. In cam design there's a rule that says that the derivate of the acceleration must be finit, it doesn't matter if it's not continous. Another rule I've read is about the lobe separation, in an engine bigger tha 350 ci, add 5 degrees in lobe separation for every 25 ci up in displacement, in engines with less than 350 ci, take out 5 degrees in lobe separation for every 25 ci
For an engine cam, it was demonstrated that the best function for the displacement going up or down is a 3-4-5-6 polynomial function because it offers the minimal values of acceleration and in consecuence force.
The materials most commonly used are hard ones such as cast iron, gray cast iron or steels with mid content of charcoal.
The best way for machining cams is using CNC as far as I know.
There is more info in Norton's book "Design of Machinery" about cam design and cam dynamics
RE: Camshaft Design- New
In your last post you stated that,
At my place of work we have been having valve and seat wear problems and we have been experimenting with different profiles and spring rates.
What test engine is this in, pushrod, OHC etc?
RE: Camshaft Design- New
We have an industrial air-cooled V4, ohv. This engine has been in production for years. We started with cast iron seats and Stellite valves and wore out the seats, so we changed to nickel based seats with the Stellite valves, and now we wear out the valves. The weird thing is that we have this problem on only one cylinder, which suggests a heat problem, but other cylinders seem to run hotter.
The problem was accidently discovered while checking the valve clearance after several hours of hard running. There was none left. The standard procedure was to turn the engine over until one valve was open and the other closed, then set the lash. However, it was found that after rotating the engine around again to tdc compression, that the lash decreased by several thousandths. We suspected base circle runout or valve train deflection, but could not find any.
We are going to send the engine out to a local cam company and have them run it on their Spintron machine next.
John Woodward
RE: Camshaft Design- New
I have some questions regarding the manufacture of over-head cams.
Specifically does anyone have any recommendations as to:
1. Where to test the camshaft cores (I guess wear resistance, overall material quality, etc.)?
2. Where to grind the camshafts (United states)
3. And where to test the accuracy of the grinds?
4. Is there anything else I'm missing?
Thanks in advance!
RE: Camshaft Design- New
As far as material, grinding, and measurement, I don't think there is a difference between ohv and ohc cams so any cam grinder or manufacturer should be able to do them. Design is a different matter, however. The mechanical position of the lobes will be different from an ohv, depending on such things as type of drive, angle between valves, number of cams (sohc or dohc). The concepts of advance, retard, and lobe separation angle should apply equally. Another factor is the amount of lift that can be squeezed into the profile without getting excessive accelerations, since there may not be any rocker arms to multiply the lift.
John Woodward
RE: Camshaft Design- New
I was looking at an earlier thread and there was mention of a cam design program in VB.
I wrote a program in fortran about 20 years ago and have been trying to bring it to life by rewriting in qbasic. Am having some success but was wondering if an alternative program was available for the design of polynomial cams?.
The original program was used to design some cams, mainly for 4-valve engines including the weslake speedway engine.
It is many years since I was involved in I.C. engines so am a bit rusty.
Any help would be much appresiated.
Hughnew
hugh@newlyn.com
RE: Camshaft Design- New
The polynomial equations came from a number of older SAE papers. The methods of solving them came from a library book about matrix algebra (I don't know, must have cut class the day that was taught).
There are some programs that you can buy. Andrews is one of them. There are some posts in this thread about others.
These comments are just general. If you start on this project, post again and perhaps I can dig up some of my old information.
RE: Camshaft Design- New
Will come back when its running, shouldn't be long I hope.
Hughnew
RE: Camshaft Design- New
I agree VB is a horrible mess. For day to day work I now tend to use Scilab (www.scilab.org) which is an interpreted mathematical toolbox, rather like matlab, but free.
If you like Pascal you might like it's son, Oberon, also by Werth. It is a bit more graphically orientated than Pascal, but it still has a text based interface.
But qbasic is fine for typical engineering projects.
Cheers
Greg Locock
RE: Camshaft Design- New
Seems the best approach is to write out the equations for displacement, velocity and acceleration of everything from the geometry, input limits for the loads, lift, duration, spring, RPM, et cetera and start with a simple cosine shaped control curve. Then move the points on the curve until the opening limits are met, the stopping accelleration (reitterating the opening if the stopping requirements cannot be met), then work the closing curve the same way. The control curve would be interpreted with a spline for all the intermediate points required. A motion curve is calculated based upon the geometry (radius tappets, finger follower, inverted bucket). The control curve can be interpreted for whatever accuracy is required by the cam grinder.
RE: Camshaft Design- New
The emphasis in this book is that you build the profile from continuous jerk and acceleration functions in order to control the motion and stresses. The lift curve is the result, not the start.
RE: Camshaft Design- New
Either way it takes itteration. Just seems easier to start with the constants you know and work toward the maximum accelerations constrained by the limits. Also, that way you can change parameters (spring pack, masses, geometry) and come up with a new profile quickly.
I have not seen Norton's book but most were written before fast computers were available.
RE: Camshaft Design- New
RE: Camshaft Design- New
They also suggest constructing a straight line segment jerk curve and deriving acceleration, velocity and displacement from that.
From this you still have to backtrack to find the cam profile since the motion descibed is that of the follower and not the cam surface. The minimum cam radius and follower length have to be determined to find a useable cam profile without cusps or undercuts.
RE: Camshaft Design- New
The equation I am most familiar with is a 7th order polynomial with exponents 0,1,2,p,q,r,s. You have to know the boundary conditions, the maximum lift, and duration. You then fool with the exponents until you get the best compromise in terms of velocity, acceleration, jerk, and area under the lift curve. I don't know if the specialist cam companies use the same methods, but this one came from one of them.
Some good sources for this information are the SAE series Computer Applications in Valve Gear Design (title is something like that) or the Machine Design articles about Polydyne cams. You would have to search back to the 1950s and some of the names are Englemann, Dudley, and Stoddart.
RE: Camshaft Design- New
A spline curve would allow smooth transitions through the jerk curve.
RE: Camshaft Design- New
RE: Camshaft Design- New
RE: Camshaft Design- New