# Basic parameters of involute standard spur gear

#### Names of gear parts

1.Cogging: the space between two adjacent cogs on a gear.

2.Addendum circle: the circle at the top of the gear is called addendum circle, and its diameter is represented by Da.

3. Root circle: the circle at the bottom of the tooth slot is called root circle, and its diameter is represented by DF.

4.Tooth thickness: the arc length between the tooth profiles on both sides of a tooth is called tooth thickness, which is represented by S.

5.Tooth slot width: the arc length between the tooth profiles on both sides of a tooth slot is called the tooth slot width, which is represented by E.

6.Dividing circle: the circle with standard modulus and standard pressure angle on the gear is called dividing circle, and its diameter is indicated by D.

7.Pitch: the arc length between two adjacent end profiles on the same side is called pitch, which is represented by P. Namely P = S + e

8.Tooth height: the radial distance between the tooth top circle and the tooth root circle is called tooth height, which is expressed by H.

10.Root height: the radial distance between the root circle and the graduation circle is called the root height, which is represented by HF.

11.Tooth width: the width of the gear measured along the axis of the gear, expressed in B.

#### Main parameters:

1.For a single gear, on the end plane, the acute angle between the radial line of any point on the tooth profile of the end plane and the tangent of the tooth profile at that point is the tooth profile angle of that point

Pressure angle of indexing circle the acute angle between the speed direction of the tooth profile curve at a certain point on the indexing circle and the normal direction of the curve at that point (i.e. the direction of the action line of the force), also expressed in α.

Tooth angle the acute angle between the radial line of any point K on the tooth profile of the end plane and the tangent line of the tooth profile at that point, expressed in α. The tooth angle of point K is α K.

The tooth angle of each point on the involute profile is not equal. The farther K point is from the base circle, the larger the tooth angle is. The tooth angle on the base circle is α = 0 °.

2.Teeth Z

The number of teeth in a gear is the number of teeth.

3.Modulus M

Because the length of dividing circle π d = ZP, the diameter of dividing circle is d = ZP / π

Because π is an irrational number, for the convenience of calculation and manufacture, P / π is artificially defined as a rational number, that is, the quotient of pitch P divided by pi is called modulus, which is expressed in M. That is, M = P / π (mm)

If the number of teeth is equal, the larger the module is, the larger the gear size is, the larger the teeth are, and the larger the bearing capacity is.

4.Tooth top height coefficient ha*

For standard gears, ha = ha * m is specified. Ha * is called the addendum coefficient. According to Chinese standard, ha * = 1 for normal teeth.

5.Top clearance coefficient C*

When a pair of gears are engaged, in order to make the top surface of one gear not collide with the bottom surface of the other gear, the root height of the gear should be higher than the top height of the tooth, that is to say, there should be a certain radial clearance, called the top clearance, which is expressed by C.

For standard gears, C = C * m is specified. C * is called the top clearance coefficient. According to Chinese standard, normal tooth c * = 0.25. 