Whats the point of these needle thrusts bearings?

[georgeburns]

http://www.cncrouterparts.com/parts.html

What is the point of the needle thrust bearings and washers (parts 3 and 4) in the linked design above?

Maybe I'm tired... but I can't figure out their purpose... seems to me they will just sit on the screw shaft without any function.

Thanks...
gb

[Geof]

They take the thrust.

Regular ball races are not designed for axial loads so the thrust from the screw has to be taken with thrust bearings.

This type of construction gives some backlash because things cannot be cinched up with zero clearance using thrust bearings like this. On a more expensive setup ball races designed for taking axial loads would be used, two of them back to back preloaded to take out an freeplay.

[jcc3inc]

As an added thought, one could use a belleville spring or wave washer to preload the needle brg setup. This takes out the endplay up to the spring load force. Likely a better solution as mentioned above would be either tapered rollers or angular contact bearings with a suitable preload.

JC

[rlrhett]

Sorry for being dense, but what axial thrust? Isn't this part simply holding the screw in place? The surfaces these thrust washers are up against don't move, right? Like the orignial poster, I don't see their function. Where is the axial load comming from?

PS, the non-exploded view of this particular assembly can be found here assembled view

[Geof]

Originally Posted by rlrhett Sorry for being dense, but what axial thrust? Isn't this part simply holding the screw in place? The surfaces these thrust washers are up against don't move, right? Like the orignial poster, I don't see their function. Where is the axial load comming from? PS, the non-exploded view of this particular assembly can be found here assembled view

The purpose of the screw is to move the nut which is attached to a gantry, or cross slide, or table; in other words to push on the nut and make it and whatever it is attached to move against a cutting force. So therefore there is a back force on the screw and the thrust bearings are to take this back force, and the force is aiong the screw, in line with its axis so it is called an axial force.

[rlrhett]

I guess I always assumed that the screw was free to slip axially on the skate bearing and so any back force would be transmitted directly to the coupler/motor/motor mounts. However, thinking about it what you said makes perfect sense. Thanks Geof.

[ger21]

Originally Posted by rlrhett I guess I always assumed that the screw was free to slip axially on the skate bearing and so any back force would be transmitted directly to the coupler/motor/motor mounts.

Motors are not designed to carry these loads. If you rely on the motor to do it, the motor bearings will fail quickly.

And the screw must be held very firmly in place. Any movement (backlash) will show up in your cuts.