Choosing a Gear Type: Spur, Helical, Bevel, Worm or Rack

**Parallel shafts.** *Spur* — teeth straight along the axis. The cheapest to make and to inspect, no axial thrust, and noisy at speed because each tooth pair takes up the load all at once. The default for anything slow, and for anything that must slide in and out of mesh. *Helical* — teeth on a helix. Quieter, stronger for the same size because more teeth share the load, and it generates an axial thrust the bearings must carry. *Herringbone or double helical* — two opposite hands on one blank. The thrusts cancel, so a large helix angle becomes usable. Expensive to cut. *Internal* — teeth on the inside of a ring. Compact, does not reverse direction, and is the outer member of every epicyclic train. **Intersecting shafts.** *Straight bevel* — the spur gear of the bevel family, for shafts that cross, usually at 90°. Both members are thrust loaded and both must be located axially, because the mesh position sets the tooth contact. *Spiral bevel* — the helical equivalent: quieter, stronger, more thrust. **Shafts that neither intersect nor are parallel.** *Worm and wheel* — a large ratio in one stage, in a right angle, quietly, with a sliding contact that costs efficiency and needs the right oil. *Hypoid* — a spiral bevel with the axes offset. The rear axle of a car. *Crossed helical* — two ordinary helical gears on skew shafts. Point contact, so it carries very little; useful for light drives. **Rotation to straight line.** *Rack and pinion* — a rack is a gear of infinite diameter. Same module, same pressure angle, and the tooth flanks are straight.

**Spur.** Cut with a hob or a shaper cutter, one setup, inspected with a caliper and a pin. Held by any pair of bearings; no axial location needed beyond keeping the gear on the shaft. Cheapest by a wide margin. **Helical.** Same hob, the machine's differential set to generate the helix. A little more setup, the same cutter. The cost is downstream: a thrust bearing, or a pair of angular contact bearings, and a housing stiff enough to take the thrust without opening up. **Bevel.** Cut on a dedicated machine, and made and lapped **in pairs** — bevels are not interchangeable, and replacing one means replacing both. Both members need axial adjustment, usually shims, and the mounting distance is a dimension on the drawing that must be held. **Worm.** The worm is ground like a thread; the wheel is hobbed with a hob that is a copy of the worm, so again they come as a pair. Bronze wheel against a hardened steel worm, because the contact slides rather than rolls. The worm takes a large axial thrust equal to the wheel's tangential force, which in a reduction is the big one. **Rack.** Cut on a bar and supplied in lengths that are butted end to end — the joint has to be set with a spare tooth pitch gauge or the pitch error at the joint is permanent. The pinion must be held against the rack at a fixed centre distance along its whole travel, which usually means a stiff rail, not the rack itself.

**Slow, cheap, parallel, and noise does not matter.** Spur. Do not pay for anything else. **Parallel and it will be heard.** Helical, 8° to 15°, and budget the thrust bearing at the same time. Pitch line velocities above roughly 5 m/s are where spur noise becomes the complaint. **A right angle, and efficiency matters.** Bevel. A bevel stage runs at 97–98%; a worm of the same ratio will not. **A right angle and a ratio above about 10:1 in one stage.** Worm — that is the thing it does that nothing else does. Accept 50–80% efficiency, provide for the heat, and use an oil made for worm drives. **It must hold when the power is off.** Fit a brake. Choose the worm for ratio and efficiency, and treat any self-locking as a bonus, not a mechanism — the friction coefficient that provides it is not a design value you control. **Straight line motion.** Rack and pinion for long travel and moderate accuracy; a ball screw when the accuracy matters more than the length. **In every case, check what the ratio does to the bearings, not just to the speed.** A reduction multiplies torque, and torque on the output shaft is radial load on the output bearings. The gear is rarely the part that fails first.