A Keyway is a slot in the bore of a gear corresponding to the slot milled in the diameter of a shaft that fits (usually tightly) together with a square (or rectangular) key aligned with the two slots, eliminating slippage between the two members. There is usually on keyway, but often there is two at 90, 120, or 180 degrees apart. Sizes as large as 4” wide can be manufactured. Odd sizes and metric sizes can also be produced.
Splines are sometimes used instead of key and keyway drive in gears. Splines are either straight (as in multiple keyways equally spaced) or involute (as in gear teeth).
Straight splines are multiple keyways in the bore (usually six out of ten is also fairly common) driving multiple keys equally spaced on the shaft. The major and minor diameters are curved (instead of straight as in actual keyways). There is often minimal clearance between major diameters and minor diameters of the mating parts.
Involute splines are gear teeth with the same number of teeth in both the male and the female parts. Involute splines will usually have teeth with 20 or 30 degrees’ pressure angle. Involute splines can have any number of teeth (most commonly any number between 9 and 32).