Best Contact Shape for cup that contacts a single rolling ball

[luthster]

Hello,
I'm forming a cam with a single ball riding in a V-shaped groove in a shaft. The groove runs parallel to the axis of the shaft and then wraps around the circumference causing 90 degrees of rotation. Opposite the groove, the ball is installed in a blind reamed hole in the housing. Contacting the ball is a cup that transfers spring force to the ball from the bottom of the blind hole. Integrated into the ball cup is a positive stop that limits the travel of the ball cup so that the ball stays in the groove (and the spring is not over compressed). The ball needs to roll in the groove and cup as the shaft oscillates back and forth.

I started with a cup having a radius .005" larger than the ball. The ball contacted the bottom of the cup. I'm having two issues:
1) The ball sometimes doesn't want to roll in the cup
2) The ball is wearing into the cup

The ball is 4mm diameter tungsten carbide and the cup is prehard 4140 steel with a hardness of Rc 30. A guy here at our shop says I should be contacting the ball with a drill point not a radius. He also recommends using annealed 303 SS for the cup material.

What shape should the contact feature in the cup be for best rolling and wear with the ball?

Thanks!

 

[MikeHalloran]

The ball can't roll on both surfaces at once.

You are trying to make a two-way cam follower. I think the classical way to do that is to use a spindle normal to the shaft, free to rotate in needle bearings. The spindle would be preloaded into the shaft, possibly by a ball and spring at its distal end. The end contacting the shaft would be half-spherical, or somewhat better, conical if the shaft grooves are V-shaped.

Some kinds of 'ballscrews' use a helical array of such rotating pin followers inside their nut.



Mike Halloran
Pembroke Pines, FL, USA

 

[luthster]

Thanks Mike, You are correct, the ball cannot roll with "no slippage" contact on both surfaces.

Assuming the ball maintains "no slippage" contact with the groove (rolls along it) and slips inside the ball cup (ball rolls within the cup surface) what is best ball cup contact geometry?

 

[MikeHalloran]

All I can think of is a hemispherical cup, lined with a large number of smaller balls on which the bigger ball can roll. The scheme assumes the smaller balls will recirculate, which is not guaranteed, but is allowed.


Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

unlubricated metal-on-metal sliding accompanied by high friction and wear? I don't think I'd expect any thing different.

I'd be thinking I need lube, or one of those teflon bronze matrix bearings that claim to transfer the teflon to the other part. Depending on your speed and load and required service life one of the leaded bronzes might at least reduce the socket wear. I'd try to score some of the ceramic balls ot of a "hybrid ceramic" ball bearing to at least get the vicious steel-on-steel tribology out of the picture, spray the hemi socket with Molykote, or at least put a dab of moly grease in the socket. A grease pocket or reservoir in the socket might be helpful

 

[luthster]

Good catch on the lubricant! I forgot to say the entire assembly in submerged in hydraulic oil.

 

[dgallup]

I think the cup needs to be a lot harder, I would probably go with something like 52100 or 440C at Rockwell C 58-60. Try to keep the spring load down too.

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