Washers, Bushings, Bearings

[jwatte]

I'm looking for fact checking on my mechanics basics here. Any advise (other than "go back to school and take an undergrad mechanics curriculum") welcome!

From what I understand:

Ball bearings are used on axles to fix the axle against movement in the plane perpendicular to the axis of the axle, when the axle needs to transfer substantial load in this plane.

Needle bearings are like ball bearings, except they are longer/slimmer, and can counteract some off-axis twist of the axle.

Thrust bearings are used when something that turns is forced against something it turns relative to along the axis of turning/the axle it turns around.

Ball bearings can provide some degree of axis-aligned force management as well (like a thrust bearing) if the force acts only on the inner part of the bearing, not the outer, mounting part.

Bushings are like ball bearings (or, long bushings like needle bearings,) but for lighter loads.

Washers are like thrust bearings but for lighter forces.

Bushings can be made to provide a washer / flange end that can provide some washer-like functionality on one end as well.


How do you know when you need a ball bearing, versus when a bushing is good enough?

How do you know when you need a thrust bearing, versus when a washer is good enough?

Is two washers with grease between them a reasonable substitute for a thrust bearing? Or a single washer with grease between it and the thrusting surfaces?

[Th232]

Mostly good, although I'd just like to say that thrust bearings can use either balls or needles, so the description of just a ball or needle bearing can sometimes be misleading.  Not your fault though, I think it's quite common to assume that it's a regular bearing, if it's a thrust bearing (or other kind) then it'll be explicitly stated.  I personally think of it as a function of the contact surfaces (ball, needle or sliding) and the construction of the casing (regular, thrust, tapered or spherical).  That said, your descriptions are fairly good, although if I may offer a terminology suggestion, the terms axial and radial force may help make things a bit clearer.

On a continuum, ball bearings can run faster than needles, which can run faster than bushings/washers, then just go back up the chain for the amount of force that each of them can withstand (greater contact area and all that).

Regular bearings can withstand radial forces acting on the shaft, but only relatively low axial forces, thrust bearings are the opposite.  Tapered bearings can withstand axial forces in one direction as well as radial forces, but not as well as their regular/thrust bearing counterparts.  Spherical bearings are like back to back tapered bearings, but are more complex and expensive.

Personally, given the greater losses due to friction (and resulting wear) from using use bushings/washers, I only use them where:
* The axle is expected to remain stationary for the majority of its life, and if it does move, that it'll do so slowly.
* I simply don't have space for a bearing.

Those're my personal rules though, and I have seen cases where washers have been used in dynamic situations.  Still not my first choice though.

Can't really say whether two washers + grease is a reasonable substitute without knowing your application, but if it's for your suspension then I'd just go with thrust bearings.  If you do have to do it though, definitely two washers unless the surfaces pushing against said washers are both harder and smoother than the washers.

[jwatte]

the terms axial and radial force may help make things a bit clearer.

I agree! I'll try to memorize them for the kinds of things I'm describing.

Your other answers are very helpful, too. Thanks a bunch!

[Gertlex]

In pondering what TH232 said, it seems to me that if we were talking non-wheeled bots, bushings are generally fine, as the amount of rotation from a servo in a join is far less than for a wheel.

(That said, Dynamixel MX servos and beyond use bearings for the idler support... but I suspect metal bushings would be sufficient)