Cam design please help
Cam design please help
(OP)
Hey all,
Is anyone good with cam calculations that might be able to give me a hand. I'm trying to find the force generated by a cam at different points on the cam. I've bin unsuccessful so far, I have a good idea where to start but I haven't done anything with cams before. Any help is greatly appreciated. (or any ideas as to where I might better find help)
Thanks,
Is anyone good with cam calculations that might be able to give me a hand. I'm trying to find the force generated by a cam at different points on the cam. I've bin unsuccessful so far, I have a good idea where to start but I haven't done anything with cams before. Any help is greatly appreciated. (or any ideas as to where I might better find help)
Thanks,





RE: Cam design please help
There are a few approaches you can take:
- kinematic: force depends only on the displacement, the geometry of the system, and whatever you're using to hold the system against the cam. You didn't mention whether you're dealing with an automotive valvetrain or something else.
- pseudo-dynamic: force depends on the above, plus instantaneous acceleration of the system (cam profile and rotational speed will influence this)
- dynamic: every component has inertia and stiffness, and will vibrate, so force at any interface will depend on a whole lot - usually this force is measured or derived by simulation rather than calculated by hand.
RE: Cam design please help
Thanks again
RE: Cam design please help
Then apply the energy conservaton equation, namely
F*dx=T*dtheta or T=F*dx/dtheta
where
F= piston force
dx/dtheta= ratio of piston to cam angle motions
T = torque on cam
Next draw the normal force vector actng on the cam at the surface. Finally,drop perpendicular from the center of rotation to the vector and call that distnce r.The torque can be written as
Fc*r where Fc is the normal force on the cam you seek.
Substituting into the eq above we get:
Fc*r=F*dx/dtheta and Fc=F*dx/dtheta/r
RE: Cam design please help
RE: Cam design please help
Repeating,torque on the cam is the normal force, Fc acting on the cam surface multiplied by the distance r from the centre of rotation to the line of action of that force( the fundamental definition of torque).You get the distance,r by first drawing a line perpendicular to the cam surface at the point of contact with the follower and then dropping another line from the center of rotation to that line; the distance from the Centre of rotation to the line is r.
You must do this for the entire motion in order to find the maximum value of Fc.
I might further add that for very high speeds you may have to add the effects of inertial forces including the potential separation problem if the piston force is insufficient to decelerate the piston on the downstroke of the motion.
RE: Cam design please help
once again great post! I love cams and have to spend more time learning about them.
RE: Cam design please help
CAMS Design ,Dynamic and Accuracy, Harold A. Rothbart 1956
RE: Cam design please help
RE: Cam design please help
Could you not just take the piston force x the length of the lever arm "r" to get the torque?
I just don't quite understand the value of the Fc ? Maybe I'm just not quite understanding right.
Thanks again
RE: Cam design please help
http://www.designofmachinery.com/Cam/index.html
Also if you are not sure about equations, do a FEM or kinematic modeling by "working Model".
RE: Cam design please help
The piston force in the direction of the piston motion is due to the fluid preessure in your case and as I have previously noted, the force normal to the cam surface is Fc.
There is an easier way of determining Fc for your special case of a translating follower obtained by a force analysis at the follower.F is the vertical component of the Fc force vector; i.e., if the angle between the F and Fc vectors is alpha, then:
Fc*cos(alpha)=F and therefore Fc=F/cos(alpha) which for your case is simpler than using the energy conservation equation I earlier proposed.Alpha is called the pressure angle and its maximum design value could be as low as 45 degrees if there is enough friction in the slide of the pistion.
For a clear discussion of these principles inluding the effect of friction, you should look into an elementary cam book. Rothbart's Cam book "Cams, design, accuracy", Wiley, 1956 is one, but is out of print.
RE: Cam design please help