Thanks for your message.
One of the big problems on the design is the space constraint. The cam has to be so small that I need to use a small diameter cam follower roller in order to keep from undercutting the cam. The points on the cam near the dwells get too sharp with a larger diameter roller. It seemed even going to a 1/4" OD roller produced interference, but perhaps I could cheat a little. It does not look possible to use a larger roller than 1/4" OD and I would rather stay with a 3/16" OD roller so I can use the same cam and not redesign the curve for a 1/4" OD roller.
Presently, I'm making the rollers from 3/16" OD A2 drill rod. I cut the roller to length, drill a 2 mm hole in it, and then press a 2 mm OD hardened dowel through the on-center hole. The dowel extends from each end of the roller & the dowel "rotates with" the roller as it oscillates. The ends of the dowel are supported by plastic busings made by
As the roller oscillates, the dowel pin / shaft oscillates in the plastic bushings. It's a basic yoke type of roller mount.
Perhaps the plastic bushings are giving a little & I would be better off just letting the ends of the 2 mm OD dowel oscillate in the 4140 yoke, & lube it with some grease. It may be possible there is some deflection of the shaft causing problems, or perhaps heat treating the roller made it a little egg shaped. I did not think there would be any significant friction at these rollers, but I think the load is so high for such a small roller that this is causing the problem. The two rollers are pinching the cam rib, and causing drag. Springs return the cam to it's home position & drag is preventing this. Stronger springs mean more physical effort from the user. I need some way to reduce friction at the rollers as much as possible. Reducing the friction should also reduce the noise problem.
Other than this friction problem, the design works fine, which makes it frustrating the get everything together and run into an unforeseen problem like this.
Timken makes a 1/4" OD drawn cup needle roller. Perhaps I could install this over a 1/8" dowel and give that a try. The possible problem I see here is that drawn cup needle rollers rely on a press fit to properly size them, and make them truly round. It may also fit loose on the shaft unless I make a custom shaft.
makes a miniature stud type cam follower (Part # CFS 2.5 V) with a 5 mm diameter ( 0.197" OD) which should be close enough. It can handle a 220 pound static load, the only problem is that the last time I checked, they cost about $50.00 a piece and the product requires two rollers per unit. I think it will be cost prohibitive.
It seems a rolling element bearing is the best way to reduce friction, drag, & noise as much as possible, along with applying lube to the roller OD & making sure the surfaces are smooth. The problem is finding a bearing that has a diameter of 3/16" and that can carry a static load of 90 to 130 pounds.
A non-metallic self lubricating roller also seems appealing, but it's a matter of finding something that small that can handle the load.
Any other feedback would be appreciated.
Thanks again, John