Thin Section Bearings for Tight Spaces and Hollow Shafts

Picture a joint housing that is already modeled. The gearbox wants a slice of the height, the cable run wants another, and the bearing gets what is left. When the leftover is small, you are in the thin ring aisle.

Two shapes spend a small height well. One is a deep groove ball bearing with a low cross section, the other a crossed roller ring that carries every load direction in one part. Which one fits comes down to four lines.

1Start With the Space, Not the Bearing

Most designers pick a bearing first, then find the housing will not close. Flip the order. Measure the height you have, take out the wall you need, and see what is left for the ring.

A bigger bearing is not the stiffer answer when the housing is thin. A ring stays round only if enough metal around it holds it round. When the wall flexes, the ring follows.

2The Budget Has Four Lines

Write these four lines on the drawing and the choice narrows fast. Each one can be read off the model.

3Thin Ball Series and What They Give Up

Here is the ladder at one bore size, 20 mm, from our data sheets. Same shaft, five heights, five ratings. The 6804 bearing sits at the thin end, and the 6904 bearing one step above it.

Read the table as a trade, not a ranking. The thin end buys height and gives up load. A 6804 lives in a 7 mm slice, while a 6304 asks more than double that height.

Two rules cover most picks. If the load is radial and the speed is high, stay in the thin ball family as long as the rating clears your load. If the joint also has to hold a tilt, no ball bearing does that.

4A Crossed Roller Ring in a 5 mm Section

A crossed roller bearing does the same job with a different shape. Rollers sit at 90 degree alternating angles in a V raceway and make line contact across both rings, so one ring takes radial load, axial load both ways and tilting moment.

The RAU series covers a 20 mm bore up to 350 mm in 5 mm, 8 mm and 13 mm wide sections. RAU 2005 is a 20 mm bore inside a 31 mm outside diameter, and RAU 10005 is a 100 mm bore inside 111 mm.

The rings are one piece inner and outer with no split ring to bolt together, and preload is set at the factory. Accuracy grades P5, P4 and P2 are available, and moment stiffness runs three to five times an equivalent ball bearing.

5When the Shaft Has to Stay Hollow

Plenty of joints are built around a hole. Cables, air lines, a drive shaft or a slip ring pass through the middle, and nothing may block that passage. A solid shaft carrying two ball bearings cannot open a path through its own center.

A thin ring with a large bore is the only shape that leaves the passage open, because the ring is built around the hole instead of beside it. The rollers sit in the narrow annulus and the middle stays clear.

6The Seat Is Half the Job

A thin ring is flexible next to a standard one, so it does not correct a rough seat. It copies it, and what you do to the housing reaches the joint.

A Thin Ring Copies Its Seat

Flatness and roundness errors in the seat show up as runout in the ring. Runout is the wobble a dial gauge sees on a turning surface. A dip in the housing face tilts the ring, and an out of round housing squeezes it.

Check the seat with a gauge before the ring goes in, not after. If the seat wanders, no accuracy grade will rescue it.

Wall Thickness Around the Ring

The wall around the outer ring holds the ring round under load. A thin wall flexes and the ring follows, so the pair moves together instead of acting as one stiff unit.

Support the ring with the seat, not with a cover plate that touches it only near the middle.

Bolting a Split Ring or Clamping an Integrated One

A split ring is bolted together around a shaft, which helps when the shaft cannot be threaded through a bore. The joints are also where stiffness leaks away. Our rings are one piece on both sides and clamp between a shoulder and a cover. Our notes on integrated and separated ring designs cover the difference.

Send the clamp force into the ring face, never through the seals or the rolling elements.

7Three Envelope Situations and What Fits

Three shapes cover most of the questions we get about tight envelopes. Each ends with the series that fits.

Compact Actuator Joint

A small servo actuator with a short stack leaves only a few millimeters per bearing position. A thin ball ring matched to the shaft bore keeps the stack closed and carries the light axial load. Start with a 6804 or a 6904.

Hollow Wrist

A hollow wrist runs cable and hose through the joint center while the payload hangs off the end. The ring has to hold a tilt and take axial load both ways, and a ball pair cannot do that without eating height. A small RAU ring in a 5 mm section fits that annulus, with tool center point deflection under 0.01 mm at full payload.

Small Rotary Positioner

A positioner that indexes a part and stops holds repeat only if the ring keeps the table from rocking. A crossed roller ring with factory preload holds the moment without a separate thrust pair, and the accuracy grade can suit the index.

8Frequently Asked Questions