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Steve,
This looks like a good read.
Hope it helps a bit,
Jeff...
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Hello NC!
I was wondering if you have heard of this book
Cam Design Handbook: Dynamics and accuracy by Harold A. Rothbart
I am interested specifically in the formulas for calculating the minimum diameter of a solid, flat bottom lifter for a given lobe shape. I would like to also calculate the lift in 1 degree increments for a given shape and the acceleration/G force so that I can better select valve springs.
Keep in mind that this pertains to model engines. I'm sure that this is not the modern way of doing things but would be a big improvment over the flat flank designs being used in models today. I make the cams in a fixture so i can get close to a predetermined lift, duration, and base, flank, and nose radius.
This text is expensive (>$100) but worth the coin if it answers the above questions. If It is not so good, could you advise as to a better book or source for this math.
I'm not even sure that I will understand what i'm reading but I'm willing to try to figure it out. You probibly think i'm nuts to go thru all this for a model engine. I'm just wierd that way. Trying to "step up my game" in the camshaft area.
You may have seen my latest effort. This is for a V4. It is just under 3 inches long. Altering the depth of cut alters the lift and base circle. If i change the center to center (offset) i can change the flank radius. The number of cuts i make rotating the shaft in the fixture alters the duration.
After you stop laughing you can see that the level of quality will never be to yours but I believe if I can get a handle on the math, i can design model cams, followers, and springs that will work very well.
Steve
Steve,
This looks like a good read.
Hope it helps a bit,
Jeff...
I will look it over
Thank you!
Neat workmanship on the V4 cam!
Lifter diameter must be such that the point of contact never reaches the edge of the follower. This means the follower will certainly be smaller than the base circle diameter + twice the lift.
How much smaller can be determined by making cardboard scale models of the cam perhaps 5 times larger than life size. You need a reasonable margin for not dropping the cam off the edge of the follower - perhaps 0.04" radially for a round follower. The width of the cam will also affect this margin.
Apart from the limiting the peak acceleration, you should also aim for a smooth "jerk" profile - that is the rate of change of acceleration should not be too abrupt, as this can excite vibration at the components' natural frequencies, causing much higher than calculated loads, leading to noise, wear, peening, spalling, fatigue and breakage.
Jerk can be calculated by taking the table of acceleration values you have per degree of rotation and deriving the differential. This is simply the table of values obtained when you subtract the second acceleration value from the first, the third from the second, the fourth from the third and so forth.
Perhaps NC Cams can tell us what values for jerk would be considered acceptable. What I know is that you want the jerk profile to be as smooth and flat as possible...
Red to red and black to black, or it's ashes to ashes and dust to dust.
Just a thought but...
Wouldn't it be possible to make a cam copier?
i.e. a machine that could follow a full size cam profile hacked from a commercial cam shaft, and with a simple lever mechanism to reproduce the shape, suitabley scaled,
with a grind-wheel.
Check This out:
http://www.andrewsproducts.com/Cam_I...y_features.htm
Jeff...
Lots of cam grinding information on this site:
http://www.homemodelenginemachinist....action=search2
Cheers,
BW
Rothbart was one of the first books I bought to learn cams. IT is a good book, very good book actually. Helps to have college math in some areas.
Peak acceleration does not DIRECLY control tappet diameter - peak velocity does. You can determine peak vel of the cam by reading it with a BLADE/FLAT follower. and record lift in 1 deg increments. Subtract deg by deg CHANGE in lift and wah-lah, velocity. We do the same with a piece of drill rod as the "follower". this way, we can deal with ANY velocity that comes our way.
A quick and dirty formula for tappet dia is as follows
Max velocity = Tappet dia x .008726
move aroounnd the formaual a bit and if you have vel, you can calc for tappet dia.
This assumes NO od chamfer and NO part of the follower is overhanging the lobe.
Double any tappet chanfer and subtrackt from lobe dia if you do have any od chamfer on tappet.
WHen you start to push the limit, you do let the follower overhang the load carrying surface of the tappet. It is an empirical thing that is only achieved via trial and expensive error.
AN easier to understand book on cams is called the "camshaft reference manual" by DOn Hubbard. Last known contact was dhubbard at peganet.com. Not as intense as Torhbart. Book is pricey at #150 but what is a lifetime of knowledge worth.
Please don't call unless youre' TURLY interested. Don'd not ffeling will due to health problems and tire kickers and kibitzers are not needed.
Note acceleration (change in vel over change in time) determines how fast you reach max velocity - tappet dia with flat tappets determines peak velocity you can attain. Peak accel also determines peak inertia loads that the valvetrain will experience.
Rollers are a WHOLE different story.
Yes you can do that.Originally Posted by BillTodd
When I made the cam for the peewee, I made a cam as designed. Then i made another with 10 more degrees of duration on the exhaust. Then i made another with 20 degrees more duration and more lift.
Cant do that with a copier. There is no flexability with a copier.
I know. My engine is project of the month!
Is the hubbard book easier because the book is aimed at a dummy like me. Will it tell me what i want to know. I dont mind laying out the $150 as long as it answers my questions.
I dont want to get into rollers. Its flat tappit for me.
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