Page 219 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 7 Bearings, Couplers, Gears, Screws, and Springs 197

• Shield (optional) Some thrust bearings come assembled with thrust
washers in a ready-to-use unit with a shielded cover. Shields help stop gunk
from getting into the rolling elements.
• Thrust load or axial load This load rating describes how much weight the
bearing can handle while spinning.

Thrust Washers and Bushings
Thrust bearings with no rolling elements are called thrust bushings or thrust washers.
They look just like your average washer, except that they’re made from slippery
material and have a higher quality flat surface to support rotating things. They often
come in sets with thrust ball and roller bearings to make sure the rolling elements
have nice smooth, hard surfaces to interact with. You can use a thrust washer by itself
as a thrust bushing to decrease friction if rolling elements aren’t necessary.

Linear Bearings and Slides

Linear bearings allow motion in a straight line, often along a shaft. There are a variety
of types with rolling elements in them. The most common are meant to ride on shafts,
as shown in Figure 7-8. The cylindrical sleeve has a kind of cage that holds steel balls,
as in other bearings, but these allow the bearing to roll along a shaft instead of spin
around it. Linear bearings are designed to carry heavy loads on precision, hardened
steel shafts, so the system components can get expensive pretty quickly.

Another type of linear bearing is a drawer slide or track roller. You’ve probably seen
these on the sides of dresser, kitchen, filing cabinet, or shop drawers. They allow you
to pull a drawer out while supporting the weight of the contents in a smooth,
relatively frictionless motion. These can be repurposed for many different projects that
need smooth, linear motion.

Linear bushings offer an economical alternative when you have light loads and a small
bit of friction is okay. Linear bushings, also called linear plain bearings, look a lot like
radial bushings. In their simplest form, they are just small, hollow cylinders of a
slippery material like plastic or bronze. These are the type used in MakerBot’s
CupCake CNC (see Figure 7-1). Higher-end linear bushings have Teflon or other
slippery linings on the inside surface. They perform better than linear ball bearings
when dirt, water, and vibrations are involved, but have slightly higher friction. Some
have grooves that allow dirt and debris to slide right through them.