Spur Gears Information
Spur gears are simple, easily manufactured gears and are usually the first choice when exploring gear options. Transmitting power between parallel axes, the teeth project radially on the disc. The sole variance in their identity remains the rack, which is covered more in-depth in Engineering360's Rack and Pinion Specification Guide.
Spur gears are regularly used for speed reduction or increase, torque multiplication, resolution and accuracy enhancement for positioning systems. The teeth run parallel to the gear axis and can only transfer motion between parallel-axis gear sets. Spur gears mate only one tooth at a time, resulting in high stress on the mating teeth and noisy operation.
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Gears mate via teeth with very specific geometry. Pitch is a measure of tooth spacing and is expressed in several ways.
Diametral pitch (DP) is the ratio of the number of teeth to the pitch diameter of a gear; a higher DP therefore indicates finer tooth spacing. It is easily calculated by the formula DP= (N+2) ÷ OD, where N is the number of teeth, and OD represents the circumferential measurement.
Circular pitch (CP) is a direct measurement of the distance from one tooth center to the adjacent tooth center. It can be measured by the formula CP= Π ÷ DP.
Module (M) is a typical gear discipline and is a measurement of the size and teeth number of the gear. Gears measured in inches earn 'English module' distinction to prevent confusion. M = OD ÷ N
Pressure angle is the angle of tooth drive action, or the angle between the line of force between meshing teeth and the tangent to the pitch circle at the point of mesh. Typical pressure angles are 14.5° or 20°.
Gear vernacular identified. Image credit: Boiler Invasion
Selection tip: Gears must have the same pitch and pressure angle in order to mesh. Spur gears do not have helix angles.
Consider the gear center, bore diameter and shaft diameter. The gear center can be a bored hole or an integral shaft. The bore diameter is the diameter of the center hole. The shaft diameter is the diameter of the shaft for gears with an integral shaft. Helical gears can be mounted on a hub or shaft. A hub is a cylindrical projection on one or both sides of a helical gear, often for the provision of a screw or other shaft attachment mechanism. Hubless gears are typically attached via press fit, adhesive or internal keyway.
Shaft mounting choices include the following:
Keyway: One or more square cutouts exist in the gear bore for exact mounting on the shaft.
Image credit: Direct Industry
Set Screw: The gear is attached to the shaft by screws through the hub.
Image credit: WM Berg
Hub Clamping Screws: The gear is attached with a screw that squeezes the inner diameter of the hub to a tight fit around the shaft.
Image credit: SDP/SI
Split: The hub is split into several pieces that are tightened down by a separate clamp to grip the shaft.
Image credit: SDP/SI
Simple bore: A straight bore designed for adhesive attachment
Image credit: QTC
The following application requirements should be considered with the workload and environment of the gear set in mind.
Power, velocity and torque consistency and output peaks of the gear drive so the gear meets mechanical requirements. Spur gears' design contraints limit their ability to transfer energy at high speeds and torques.
Inertia of the gear through acceleration and deceleration. Heavier gears can be harder to stop or reverse.
Precision requirement of gear, including gear pitch, shaft diameter, pressure angle and tooth layout.
Gear lubrication requirements. Some gears require lubrication for smooth, temperate operation.
Mounting requirements. Application may limit the gear's shaft positioning.
Noise limitation. Commercial applications may value a smooth, quietly meshing gear. Spur gears are particularly noisy in operation.
Corrosive environments. Gears exposed to weather or chemicals should be especially hardened or protected.
Temperature exposure. Some gears may warp or become brittle in the face of extreme temperatures.
Vibration and shock resistance. Heavy machine loads or backlash, the deliberate surplus space in the circular pitch, may jostle gearing.
Operation disruption resistance. It may be necessary for some gear sets to function despite missing teeth or misalignment.
Spur Gear Materials
Gear composition is determined by application, including the gear's service, rotation speed, accuracy and more.
- Cast iron provides durability and ease of manufacture.
- Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
- Cast steel provides easier fabrication, strong working loads and vibration resistance.
- Carbon steels are inexpensive and strong, but are susceptible to corrosion.
- Aluminum is used when low gear inertia with some resiliency is required.
- Brass is inexpensive, easy to mold and corrosion resistant.
- Copper is easily shaped, conductive and corrosion resistant. The gear's strength would increase if bronzed.
- Plastic is inexpensive, corrosion resistant, quiet operationally and can overcome missing teeth or misalignment. Plastic is less robust than metal and is vulnerable to temperature changes and chemical corrosion. Acetal, delrin, nylon, and polycarbonate plastics are common.
- Other material types like wood may be suitable for individual applications.
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