Metric Spur Gears and Metric Spur Gear Sizes

The terms below are commonly used throughout the industry regarding metric and standard spur gears.

• Addendum: The height of the tooth was measured above the pitch circle.
• Backlash: The play (distance) between mating teeth at the pitch circle.
• Center Distance: The distance between the center of the mating gear.
• Circular Pitch: Arc length of the pitch circle between the center of other corresponding points of the adjacent teeth. Circular pitch = 3.14159/ diametral pitch.
• Clearance: The radial distance or separation between the top of one tooth and the bottom of the mating tooth space.
• Crown: The face of each gear tooth has a slight outward bulge and is thinner on each end. This feature helps accommodate slight misalignments of gear teeth or shafts on which they are mounted.
• Dedendum: the depth of the tooth measured below the pitch circle.
• Diametral Pitch: The ratio of the number of teeth to the number of inches of the pitch diameter. Diametral pitch = 3.14159/ circular pitch.
• External Gears: Gears with teeth that are cut on the outside.
• Face Width: The axial tooth length.
• Tooth Flank: the gear tooth's working, or contacting, side.
• Gear Center: The center of the pitch circle of the gear.
• Gear ratio: The ratio of the number of teeth in the driven gear sets. Typically, it is the number of teeth in the driven gear divided by the number of teeth in the driving gear.
• Line of Centers: Connects the centers of the pitch circles of two mating gears.
• Outside Diameter: This term applies to external gears and is the distance from the top of one tooth to the top of another tooth on the opposite side of the gear that is measured through the axis of the gear. Outside diameter = number of teeth +2 / diametral pitch.
• Pitch: The distance between similar, equally spaced tooth surfaces along a given line or curve.
• Pitch Circle: Circle where mating gear contact occurs. Pitch circles are tangent in mating gears.
• Pitch Diameter: The diameter of the pitch circle. Pitch diameter = number of teeth/ diametral pitch.
• Point of Contact: Any point where two tooth profiles on gears touch each other.
• Pressure Angle: The angle between a tangent to the tooth profile and a line perpendicular to the pitch surface. Standard metric spur gears are 14.5° or 20° pressure angled.
• Root Diameter: The distance from the bottom of one tooth to the bottom of a tooth is measured through the axis of the gear.
• Tooth Thickness: The thickness of a tooth at the pitch circle.
• Tooth Surface Aera: The contact sides or faces of a gear tooth or the area, including the tooth face and tooth flank.
• Shaft Angle: The angle between the axis of two non-parallel gear shafts. This is often referred to as misalignment.
• Spur Gears: Gears with straight teeth that are always parallel to the axis of rotation.

The information below is intended to guide gear selection. It's important to know that there can be many factors in selecting the right and exact gear to match expectations. For the basic loading guide, the rolling stress load on the tooth σ=589N/mm^2 and allowable bending stress σ=200N/mm^2.
Gear Strength: to choose the metric spur gear based on the strength of the gear, you need to get the permissible bending strength of the gear, which is referred to as the allowable tangential force at the pitch circle and relates to the allowable load on the tooth and resulting bending stress in the root of the tooth under load.
Surface Durability is the safe force on the tooth without any occurrence of surface failures. Lubrication will be an important consideration for the teeth that mesh to keep working surfaces lubricated. The speed of the gears in the mesh will also play a role as will the type of lubrication used.
Selection is the next step that needs to be taken; this should be made based on the torque load of the application. The allowable torque for each gear is listed on the charts below and on the product pages themselves. Different manufacturers may use different loading criteria for loading based on service factors and material stress levels; the published torque ratings are intended as a guide.
Gear Size is generally proportional to the load capacity; a hardened gear will have more load capacity but less flexibility for absorbing or allowing shock loads and other application requirements. Ground gears will also improve positioning accuracy.
Load or service factors for external dynamic loads will also influence the operation and life of the gear. The below service factors that can be applied to the load will help extend the service life.
Lubrication: Less than 0.5m/s grease will suffice for peripheral speeds, but speeds over 0.5m/s oil should be considered. Safety factors should also be considered for safe working loads.
Speeds: C45 (1045) metric milled spur gears (un-ground) can be applied at rim speeds up to 12 m/s; in comparison, ground gears can be applied to 25 m/s. Hardened milled gears usually have a maximum limit of 8 m/s due to tooth distortion from the tooth hardening process; consider multiple stages of gear sets for high reductions.
Noise Consideration: Milled gears are the quietest when held at five m/s rim speed, and ground gears are typically suggested for more continuous operation, primarily for minimizing noise. Also, manual drives and pinions with 8-12 teeth can be used for low-speed applications. For gears that require higher demands, 25 teeth or more should be used.
Service Factors:

Conversion Factors:

Selection Formulas (Metric & Inch Units)