Tooth part Bevel & Miter Gears
TOOTH PARTS
Tooth parts and dimensions are important because they provide valuable information when quoting customer gearing. Let’s review the parts of a miter or bevel gear’s tooth by learning their definitions and referring to Figure below. (See Figure 5.5 )
• Addendum – the distance the tooth projects above, or outside of, the pitch line or circle
• Dedendum – the depth of a tooth space below, or inside of, the pitch line or circle. (
Note: In order to provide clearance, the dedendum is usually greater than the addendum of the mating gear tooth.)
• Clearance – the amount of space by which the dedendum of a gear tooth exceeds the addendum of a mating gear tooth.
• Whole Depth – the total height of a tooth, including the total depth of the tooth space.
• Working Depth – the depth of the teeth of two mating gears at the point at which the teeth mesh. Working depth is also equal to the sum of the teeth’s addenda.
• Tooth Thickness – the distance (along the pitch line or circle) from one side of the gear tooth to the other. It is nominally equal to one-half the circular pitch.
(Note: The difference between circular pitch and tooth thickness is the width of the space between the teeth that is necessary to accommodate a tooth of the mating gear.
• Face – the length of the tooth.
PRESSURE ANGLE
The pressure angle of a gear is the angle between the line of action (the direction of push from tooth A to tooth B) and the line tangent to the pitch circles of mating gears. A gear’s pressure angle also corresponds to the pressure angle of the cutting tool used to create the teeth. (See Figure 5.3)
Important: In a gear set, both gears must have the same pressure angle.
PITCH DIAMETER
The pitch diameter is the diameter of the pitch circle. On both miter and bevel gears, the pitch diameter is measured on the pitch circle – at the large end of the teeth. The formula for determining the pitch diameter follows:
• Pitch Diameter (P.D) = Number of Teeth ÷ Pitch (D.P) (See Figure 5.4)
