Gear Types

Part of Gear Making

An overview of the main gear forms — spur, helical, bevel, worm, rack — and when to use each.

Why This Matters

Choosing the right gear type for an application is as important as calculating the correct ratio. Each gear type has particular strengths: spur gears are easy to make and work well at low speeds; worm gears provide very high ratios in a compact package with self-locking capability; rack-and-pinion converts rotation to linear motion. Using the wrong gear type creates unnecessary complications or results in a drive that cannot achieve the needed performance.

In a rebuilding scenario, the practical constraint is always manufacturability: what can you make with available tools and skills? Spur gears are achievable with basic iron-working. Bevel gears require more skill. Worm gears require a reasonable lathe or turning setup. Knowing the characteristics of each type allows you to match the gear type to both the application requirements and your manufacturing capability.

Spur Gears

Spur gears have straight teeth cut parallel to the shaft axis. Both shafts are parallel. This is the most common gear type for industrial and mill machinery.

Advantages:

• Simplest tooth form to cut — straight chiseling, filing, or a straight pass of a cutter
• No axial thrust force generated (the tooth force is purely in the plane of the gears)
• Easy to inspect and maintain
• Works over a wide range of speeds and loads

Disadvantages:

• Teeth engage abruptly (one tooth pair leaves contact before the next fully engages), causing slight speed variation and noise at higher speeds
• Limited contact ratio compared to helical gears

Best applications: All-purpose gear drives at low to moderate speeds (below about 5 m/s pitch-line velocity), machinery where simplicity of manufacture is paramount.

Helical Gears

Helical gears have teeth cut at an angle to the shaft axis. The tooth is a helix rather than a straight line. Both shafts are still parallel (in standard helical gears), but the teeth are inclined at the helix angle (typically 15–30 degrees).

Advantages:

• Multiple teeth in contact simultaneously → smoother running, less noise
• Higher contact ratio → lower peak tooth stress
• Better load capacity per unit face width

Disadvantages:

• Generates axial thrust force (the oblique tooth force has a component along the shaft axis that must be restrained by thrust bearings)
• More complex tooth form to cut — requires a helical cut path rather than a straight one
• Opposing-hand helical gears (double helical) cancel axial thrust but are much more complex to make

Best applications: Higher-speed gear drives where noise and smoothness matter (power transmission in mills, machinery with human operators nearby), situations where load capacity must be maximized.

For hand manufacture, helical gears are more demanding but achievable with a lathe setup that simultaneously rotates and advances the cutter.

Bevel Gears

Bevel gears transmit power between shafts whose axes intersect at an angle — most commonly 90 degrees. See the dedicated bevel gears article for detailed construction.

Straight bevel gears: Teeth are radial lines on a cone surface. Simplest to make but generate noise and thrust similar to spur gears.

Spiral bevel gears: Teeth are curved (like helical but on a cone). Smoother, stronger, but much more complex to manufacture without specialized equipment.

Miter gears: Equal bevel gears at 90 degrees (both shafts at 45 degrees to the common apex). Ratio is 1:1; used only to change direction, not ratio.

Best applications: Mills (waterwheel to millstone), any machinery requiring a 90-degree shaft direction change.

Worm Gears

A worm gear consists of a worm (a threaded shaft, like a lead screw) meshing with a worm wheel (a large gear with curved teeth). The shafts are typically at 90 degrees but non-intersecting (offset in space).

Advantages:

• Very high ratios (10:1 to 100:1 or more) in a single stage
• Extremely compact — the worm and wheel take up much less space than an equivalent spur train
• Self-locking in many configurations — the driven load cannot back-drive the worm; the drive can only go one direction
• Smooth, quiet operation

Disadvantages:

• Lower efficiency than spur or bevel gears — typically 60–90% depending on lead angle and materials. High sliding contact generates heat.
• Worm wheel (bronze preferred) requires different material from worm (hardened steel or iron) to work well
• The sliding contact requires good lubrication more critically than rolling-contact gear types

Best applications: Hoists and winches (where self-locking prevents load from running back), indexing mechanisms (dividing heads), very high ratio drives in compact space.

Making worm gears: The worm is turned on a lathe with a form tool cutting the thread profile. The worm wheel is cut using the worm itself as a hob (hobbing): the worm is pressed against the rotating wheel blank and driven across it, cutting its own matching tooth form. This self-hobbing technique produces perfectly matched mesh geometry with basic tools.

Rack and Pinion

A rack is a flat or curved bar with straight (or helical) gear teeth. A pinion (small circular gear) meshes with the rack, converting rotation to linear motion (or vice versa).

Advantages:

• Converts rotation to translation — essential for mechanisms requiring linear motion from a rotary power source
• Simple geometry
• Easy to extend the rack for unlimited linear travel

Disadvantages:

• No speed ratio change (the relationship is linear: one full rotation of pinion of diameter d moves the rack by π×d)
• Limited to small contact angles; typically lower load capacity per tooth than equivalent spur gears

Best applications: Lifting mechanisms (rack-and-pinion hoists), door drives, traversing mechanisms, lathe saddle drives, hydraulic pump plungers.

Making racks: Lay out tooth spaces on a flat bar using the same pitch as the pinion’s circular pitch (= π × module). Cut teeth with saw and file. The teeth are identical to spur gear teeth but laid out in a line rather than on a circle.

Internal (Ring) Gears

An internal gear has teeth cut on the inside of an annular ring. A pinion meshes inside the ring, both rotating in the same direction (unlike an external pair, which reverses rotation direction).

Internal gears are compact and provide a large effective gear ratio in small space. They are used in planetary gear systems (epicyclic trains). More complex to cut than external gears — the concave tooth space requires formed tools or careful hand filing. Worth the effort when compactness or planetary operation is needed.