Spur Gear Preview

Pitch Diameter—

Outside Diameter—

Root Diameter—

Base Circle Dia.—

Circular Pitch—

Addendum—

Dedendum—

Tooth Thickness—

What is a Spur Gear?

A spur gear is the simplest and most common type of gear. It consists of a cylinder with teeth projecting radially, and the edge of each tooth is straight and aligned parallel to the axis of rotation. Spur gears mesh together to transmit motion and power between parallel shafts.

Key Parameters

Number of Teeth (N): The count of teeth around the gear. More teeth = larger gear at the same module. Minimum practical value is typically 12-18 teeth.
Module (m): The ratio of pitch diameter to number of teeth (in mm). Standardized sizes include 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10 mm.
Pressure Angle: The angle between the tooth face and the gear wheel tangent. Common values are 14.5° (legacy), 20° (standard), and 25° (heavy-duty).
Profile Shift (x): A correction factor to avoid undercutting in gears with few teeth or to adjust center distance.

Formulas

Pitch Diameter: d = m × N
Outside Diameter: d_a = m × (N + 2)
Root Diameter: d_f = m × (N − 2.5)
Base Circle Diameter: d_b = d × cos(α)
Circular Pitch: p = π × m

Spur Gear vs. Other Gear Types

See how spur gears compare to other common gear types across key engineering criteria.

Feature	Spur Gear	Helical Gear	Bevel Gear
Noise Level	Moderate — sudden tooth engagement creates impact noise	Low — gradual engagement reduces noise	Moderate — depends on tooth geometry
Load Capacity	Moderate — single tooth contact limits capacity	High — multiple teeth share load simultaneously	High — conical geometry distributes forces
Efficiency	95–99% — no axial thrust losses	94–98% — slight loss from axial thrust	93–97% — complex tooth contact
Cost	Low — simplest to manufacture	Medium — angled cutting requires more setup	High — specialized machining required
Complexity	Low — straight teeth, no thrust bearings needed	Medium — requires thrust bearings for axial loads	High — precise cone angles and mounting
Typical Applications	Clocks, conveyors, pumps, machine tools, printers	Automotive transmissions, industrial gearboxes, turbines	Differentials, hand drills, marine drives