Deep groove bearing axial preloading

[sp1nm0nkey]

Hey everyone,

I have a quarter inch shaft that's 6 inches long that must be free to rotate around its axle but needs to be completely spatially constrained otherwise. It can not have more than 2 tenths play in any dimension. That is to say, the ends can displace 2 tenths radially or the entire shaft may displace 2 tenths axially. It will not see loads greater than a few Newtons of force, though that force will be applied in such a way to axially and radially load the bearings.

What I want to know is whether one can axially preload conventional high-tolerance small (half inch and smaller diameter) bearings to take out the play, or if this is only appropriate for angular bearings. Should I instead switch to a combination of thrust and axial bearings with parts this size? Should I instead switch to an entirely different form of bearing? I am open to suggestions. It seems to me that jewel bearings might be appropriate.

Any thoughts?
Evan

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[generaldisarray]

I have shimmed radial bearings this way. To replace a rusted out set of angular contact ball bearings. But the speed they saw was essentially zero and the risk in its application was nil.

[ger21]

Yes, as long as the forces aren't too high, or the bearings may bind.
I use preloaded deep groove bearings on router to support the acme drive screws.

[Mike Everman]

Hi Evan,
Axial preload of radial bearings is an interesting topic. One overriding concept you won't find readily is what axial load capacity do radial bearings have? Interestingly, it is the same number as the radial capacity! Why is for another topic.

I have done this quite a bit. A couple of tips:
For best stiffness, shimming is best, but preload wave washers will be fine if the preload exceeds the axial force you will see. Tolerances are extremely tight, or off axis slop due to assembly clearances will crop up. It's tough to press fit to eliminate that radial slop, and still hit a sweet spot on axial preload.

Shimming comes in two forms, hard and liquid. My go-to is Loctite 638 cylindrical bonding "green goo". Basically, bonding the bearings inside and out, while enforcing the preload. You still need one shim, or some tricky two phase bonding.

Preload of duplex bearing pairs will take two forms:
Back to Back (where the contact angles look like a diamond; this one is best for moment stiffness. This one requires a shim between bearings at the inner race, and a push together at the outer races.
... and Face to Face where the contact angles make an "X"; which has the same axial stiffness, but allows a little moment flex for a "kingpost" radial bearing added some distance away. It will need a shim at the outer races, and a push at the inner. The latter would be best for a spindle, the first is appropriate for cam followers and other short baseline, high moment things.

The tighter (least clearance, least sloppy) radial bearings are actually less what you want if you have a choice, otherwise no biggie. If you have a handful of bearings, pick the sloppiest feeling ones, if you have a choice of course. The reason is that the radial bearing will have zero angle to the contact (the angle from the plane of the bearing to the line through the ball's contact points). The sloppier you start with, the more contact angle and higher axial stiffness you will have.