'sliding' ball bearing

[dsc]

Hi guys,

I'm designing a grinder which will use some parts normally seen in CNC machines and was wondering if anyone here can help me out with the red part on the following drawing:



It has to be a bearing as it supports the rotating shaft (green part on the drawing), but as the whole carrier/support (the 'box' with blue walls) moves up/down it has to slide along the shaft without any friction. Is there a bearing that can go both spin around and do linear movement? or perhaps I should use some sort of low friction slide bushing? The bearing is there simply to act as support for the shaft which is also supported in the upper part, with another bearing mounted on the upper plate.

Would appreciate any help.

Regards,
Tom

[HuFlungDung]

What kind of rpm does the shaft turn at? I cannot think of an antifriction (ball) bearing that will cope with both. I think you'd need to mount a ball bearing on/in a sliding bushing to accomplish this task. The ball bearing could cope with the higher rpm.

A slide fit is prone to locking up if the length/diameter ratio is near unity. Since most ball bearing inner races are not as long as their diameter, they are prone to binding. For a long lasting sliding fit, you'd be better off with a length to diameter ratio of 3:1 or greater. The lower the clearance between the slide and the shaft, the less it will want to bind, but then slight contamination issues will cause seizing.

[dsc]

Hi,

thanks for the reply, here's some more info:

The RPM is going to be around 300-400, pretty slow I guess. As this is a coffee grinder there will be beans coming from the top, do you think it will be a problem for the bearings (getting dirty and covered in oils and stuff)?

Perhaps using a nylon sliding bearing would work, something like this:



I was looking at metal sliding bearing but they tend to have some nasty stuff in them which isn't very food-safe.

I'm a bit worried about the sliding aspect as the whole thing might bind with slight contamination as you say. I can't really use good lubrication as there will be beans in the blue box area and the shaft has to be clean all the time. Because of this I'm thinking of moving the low shaft support from the carrier and sticking it under the carrier so the bearing doesn't have to slide on the shaft.

Regards,
Tom

[RICHARD ZASTROW]

Could you have the driving motor move up & down with the shaft? Ex. hollow shaft motor attached to the green shaft underneath the bottom plate. The bearing would then move up & down with the motor without the need for the shaft to slide thru the bearing.

Just another possibility.

Dick Z

[dsc]

Hi Dick,

not really as the motor is attached to the bottom burr which can't move up/down. The only thing that moves up/down is the upper burr and by changing the distance between the burrs you change the grind size.

Regards,
Tom