A. Straight Axles:
If lubrication is through the hub, a plain axle is used. However, if greased through the axle, then specify a zerk type axle. Straight Axles Quote

B. Spanner Bushings:
Spanners are standard on all casters with roller bearing wheels and with most straight axles. The spanner is drilled if the axle is drilled. Spanner Bushings Quote

C. Shouldered Axles:
The wheel rides directly on the shouldered part of the axle, except with roller bearing wheels, in which spanners are necessary. When used with an antifriction bearing wheel, the axle is drilled. Shouldered Axles Quote