Page 215 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 7 Bearings, Couplers, Gears, Screws, and Springs 193

• Outer diameter The outer dimension of the bearing.

• Outer race/ring The short cylindrical part outside the rolling elements.
• Inner race/ring The shaft you use should fit snugly into the inner diameter
of the bearing, so the shaft and inner race rotate together.
• Inner diameter Also called bore size, bore diameter, or just for shaft size in
reference to the size shaft it is designed to fit over.
• Ball/roller The spherical or cylindrical rolling elements, usually made of
hardened steel.
• Width The thickness of the bearing.

• Cage/separator/spacer/retainer (optional) This helps keep balls separate
so they don’t run into each other. Bearings without cages where the balls can
roll around without constraint are called full-complement bearings.

• Seal or shield (optional, not shown) Some bearings are open so you can
see all the rolling elements, and some have one or more seals or shields to
stop gunk from getting into the bearing.

Here are a few more useful bearing vocabulary terms:

• ABEC rating Sometimes bearings are rated with an ABEC number. ABEC
stands for Annular Bearing Engineers Committee. The ABEC rating ranges
from 1 to 9 (in odd numbers) and is a measure of precision. The higher the
ABEC number, the more precise the bearing, and of course, the more
expensive it is. More precision generally leads to longer life from less friction
and wear, faster spinning, and more reliable performance. For reference,
skateboard and Inline skate wheels are normally equivalent to ABEC-3.
• Revolutions per minute (rpm) This is how fast you expect your bearing to
be spinning. If you can estimate this, you can use the number to narrow down
your options on sites like McMaster that ask for an rpm range. Their ranges
are generally really high— maybe 15,000 rpm—so will rarely make or break
your design. You should always buy bearings that are rated for many more
rpms than you need.
• Static load and dynamic load You might see options for static load, dynamic
load, and dynamic radial load capacity ranges on sites like McMaster and Stock
Drive Products (www.sdp-si.com/estore) when you look for bearings. Static load