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Knowing Your Teeth: Gear Terminology

July 17, 2014

Knowing Your Teeth: Gear Terminology - Groschopp BlogHave you ever looked at a gear drawing and been overwhelmed by all the different specifications, wondering what it all means? This blog will list and explain several common terms for gears.

Number of Teeth – the quantity of teeth on the gear. This term becomes slightly more complicated if sector gears are specified, in which case the manufactured quantity of teeth will be called out along with the total number of teeth of the full gear.

Diametral Pitch – the quantity of teeth per inch.

Module – the metric version of diametral pitch with units of millimeters (mm). It is instead distance (mm) per tooth, the reciprocal of diametral pitch.

Pressure Angle – the contact angle between gear pair’s line of action and direction of velocity at the pitch point.

Helix Angle – the tooth angle relative to the central axis. Helix angles typically range from 10° to 45° depending on the design requirements. Gears with  0° helix angles are called spur gears. Helical gears (over 0°) increase contact ratio and improve gear noise, but also produce an axial load that has to be accounted for in the bearing design.

Helix Hand – the direction of the helix angle expressed as either right or left. Looking down the central axis of the gear, a right hand helix angles from left to right down the tooth face away from you. The opposite is true for a left hand helix.

Lead – the axial distance required for a helical gear tooth to make one complete revolution around the gear.

Pitch Diameter – a theoretical diameter with no tolerance established by dividing the number of teeth of the part gear by the diametral pitch. The pitch diameter typically passes through the mid-portion of the gear tooth.

Outside Diameter – the diameter of the gear teeth tips.

Root Diameter – the diameter of the bottom-most portion, or root, of the gear.

Addendum – the distance from the pitch diameter to the tooth tip or outside diameter.

Dedendum – the distance from the pitch diameter to the root diameter of the gear.

Whole Depth – the gear tooth depth from the tip to the root.

Knowing Your Teeth: Gear Terminology - Groschopp Blog

Tooth Thickness – the thickness of the tooth measured at the pitch diameter.

Normal Tooth Thickness – the tooth thickness measured normal or perpendicular to the central axis of the gear—also at the pitch diameter.

Transverse Tooth Thickness – the tooth thickness measured normal or perpendicular to the helix angle—also at the pitch diameter.

Effective Tooth Thickness – the theoretical tooth thickness that accounts for manufacturing process errors.

Measured or Actual Tooth Thickness – the tooth thickness specified that does not account for manufacturing process errors and is used for measurement and inspection of the gear.

Measurement Over Pins – this pre-calculated tolerance is used to determine the tooth thickness. Typically two small cylindrical pins are placed between two gear teeth opposite each other. The distance across the two pins is then measured and compared to the theoretical calculated tolerance.

Master Gear – a very high quality gear that is rolled in tight mesh against the part gear to inspect test center distance, tooth-to-tooth error and total composite error.

Test Center Distance – a predefined tolerance based on the center distance in tight mesh (no backlash) between the master gear and part gear to inspect tooth thickness of the part gear.

Tooth-To-Tooth Error – measured error between adjacent teeth of the part gear using a master gear. Nicks or dings in the teeth will show up as tooth-to-tooth error.

Total Composite Error – error in the part gear comprised of both run-out and tooth-to-tooth error, checked using a master gear.

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