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194 Making Things Move

is how much the bearing can handle while not moving, like the bearings in your
inline skates if you’re just standing still. This radial load acts perpendicular to the
axis on which the bearing rotates. Dynamic load is how much the bearing can
handle while moving. For example, you wouldn’t use bearings on your inline
skates with a dynamic load rating of 10 lbs if you weigh 200 lbs. Dynamic load
ratings are usually more than twice the static load ratings.

NOTE Bearings can handle more load when they’re spinning because more
of the rolling elements are sharing the load. When a bearing is not moving,
all the load is concentrated on just a few rolling elements, so is more likely to
cause wear and dimples in the bearing material.

Ball bearings are the best choice when you have high speeds and light to moderate
loads, as in skateboards and inline skates. Each ball only contacts each race (inner and
outer) at one point, so there is very little rolling friction. Roller bearings can handle
heavier loads, since the weight spreads out over a line along a cylinder and not just a
point on a ball, but friction is slightly higher than in ball bearings because of this extra
contact. Needle roller bearings have rolling elements that are longer and thinner than
cylindrical bearings. They are useful when radial space is limited. 1
To use a bearing properly, you want one race of the bearing to stay still while the
other one moves. Generally, you install bearings on smooth shafts, but it’s possible to
install a bearing on a snug-fitting threaded rod as well. Although unconventional, this
does secure the inner race to the rod so they rotate as one. Figure 7-5 shows an

FIGURE 7-5 Installing a bearing on a threaded rod (images used with permission from
MakerBot Industries)