This question was posted on the cams page of the mechanism pages. It's interesting so I thought I'd re-post it to the blog where more people would see it.

Hi Rob, found the site this week and really like it, nicely done!

I was trying to envision a cam that could be used to make the cam follower appear to pause at both and upper end and lower end (in your animation it pauses at the lower end). 

To do that, would you just draw two circles of different dimensions, connect their outer edges, and spin along the center of the bigger circle?  I added to picture to show what I was talking about:http://www.tooz.us/cam.jpg

I realize that the pause and the top and bottom will be of different lengths of time, but that seems necessary to get the cam follower to roll along nicely.

Thanks and keep up the great drawings!

A cam is, at its simplest, a surface describing a movement, all wrapped round in a circle. The highs and lows of the surface create the highs and lows of the cam follower's path.

To create a flat area in a cam, an area where the cam follower doesn't move up or down, you need an area that follows the circumference of the circle centred on the centre of the cam. 

In this cam there are two quarter turn flat areas. In a mechanism this would give you a cam follower that stayed high for a quarter turn, drops down, stays still again for another quarter turn then rises again. It should do what you are after.