Bearing radial stiffness in precision applications

wehnelt · Aug 22, 2021

I’ve been looking at spindle designs and am having a hard time understanding what parts are press fit and which are slip fits. Let’s suppose I have a bearing arrangement where there’s a pair of angular contact bearings pressed onto the end of a shaft with their associated spacers. This is a setup meant to take both radial and axial stiffness, and once tightened, there’s axial preload. Then I want to drop this into a housing that has a precision bore. Should both bearings be press fit on both the shaft and the housing? Should one? Should neither? Where does the bearing setup’s radial stiffness come from if both bearings have radial clearance?

jim rozen · Aug 22, 2021

sp1nm0nkey said:
I’ve been looking at spindle designs and am having a hard time understanding what parts are press fit and which are slip fits. Let’s suppose I have a bearing arrangement where there’s a pair of angular contact bearings pressed onto the end of a shaft with their associated spacers. This is a setup meant to take both radial and axial stiffness, and once tightened, there’s axial preload. Then I want to drop this into a housing that has a precision bore. Should both bearings be press fit on both the shaft and the housing? Should one? Should neither? Where does the bearing setup’s radial stiffness come from if both bearings have radial clearance?

The specific design matters - in some cases the outer races of an angular contact pair are held in hard contact with each other via a recess in the bore and a clamp ring to apply the clamp pressure. This means that althought there's clearance around the outer races in principle, they're not free to move radially.

A light press for a pair like that works because one outer pushes on the next outer and the balls dont see force as the inners are not yet clamped by the spindle nut. This is one reason why you get one shot at installing bearing pairs like this, pressing them back out typically means pressing though the balls.

wehnelt · Aug 22, 2021

Ah… so designing such that the bearings are held stiff by the static friction resultant from pressure on their faces is an acceptable design decision?

I’m quickly seeing why cylindrical bonding adhesive has gained so much popularity.

gbent · Aug 22, 2021

Generally, the race where the pressure always loads the race in the same spot is the slip fit. This allows the race to slowly slip to load a different spot.

wehnelt · Aug 22, 2021

So if there’s a slip fit allowed, then what stops this showing up as additional effective “backlash” in a mill?

jim rozen · Aug 23, 2021

sp1nm0nkey said:
Ah… so designing such that the bearings are held stiff by the static friction resultant from pressure on their faces is an acceptable design decision?
...

Combined with shrink or light press-up.

In the case of an hlvh, for example, the cast iron housing is warmed to permit the spindle with bearings to be installed.

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Bearing radial stiffness in precision applications

wehnelt

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jim rozen

Diamond

wehnelt

Aluminum

gbent

Diamond

wehnelt

Aluminum

jim rozen

Diamond

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