#### 1.Modulus

Module: the pitch circle of the gear is the basis for designing and calculating the dimensions of each part of the gear, while the circumference of the pitch circle of the gear = π d = Z P. Therefore, the diameter of the scoring circle d = Z P / π is not convenient for the location of the reference circle because π is an irrational number in the above formula. In order to facilitate calculation, manufacturing and inspection, the ratio p / π is defined as some simple values artificially. This ratio is called module > which is expressed in M, that is to say, its unit is mm. Therefore, the module M is a basic parameter to determine the gear size. If the module of the gear with the same number of teeth is large, its size is also large. In order to facilitate manufacturing, inspection and interchangeability, the module value of gear has been standardized.

#### 2.Diameter of graduation circle

The pitch diameter is the reference diameter of the gear. The two factors that determine the size of gear are modulus and number of teeth, and the diameter of graduation circle is equal to the product of number of teeth and modulus (end face). In the past, the pitch diameter was called the reference pitch diameter. Recently, according to ISO standard, it is called the diameter of graduation circle.

#### 3.Pressure angle.

The acute angle between the radial line of the intersection of the tooth profile and the graduation circle and the tangent of the tooth profile of the point is called the graduation circle pressure angle. Generally speaking, the pressure angle refers to the pressure angle of graduation circle. The most commonly used pressure angle is 20 °, but there are also gears with pressure angles of 14.5 °, 15 °, 17.5 °, 22.5 °.

#### 4.Helix angle

1) The acute angle between the tangent line of the cylindrical helix and the straight busbar of the cylindrical surface passing through the tangent point on the cylindrical surface is called the helix angle.

2) The acute angle between the tangent line of the conic helix and the straight generatrix of the conic plane passing through the tangent point is also called the helix angle.

3) . the angle formed by the edge of the helix and the central axis of the tool.

Increasing the helix angle β can increase the axial coincidence degree ε β = bsin β / π Mn, which is generally required to be ε β > 1 ~ 1.15, so as to improve the stability and reduce the noise of the transmission, so that the transmission is stable, but the axial force increases with it (referring to the helical gear). The helix angle direction of two gears on the same shaft shall be the same so that the axial forces cancel each other. In order to reduce the axial force of the low-speed stage, the high-speed stage spiral angle and the low-speed stage spiral angle should be taken as large. If β is selected properly, the center distance a can be rounded. If the helical angle is determined by offsetting the error of exchange gear of machine tool, the error of helical angle in gear hobbing can be effectively reduced.

#### 5.The difference between single head worm and double head worm.

The number of spiral teeth of worm is called “head number”, which is equal to the number of teeth of gear. The more heads, the greater the lead angle.

#### 6.How to distinguish R (right rotation) and l (left rotation).

When the wheel axle is placed vertically on the ground, the right-hand gear inclines to the right and the left-hand gear inclines to the left.

#### 7.The difference between M (modulus) and CP (cycle pitch).

CP (circular pitch) is the circular pitch on the index circle. The unit is the same as the module. CP divided by PI gives m (modulus). The relationship between M (modulus) and CP is as follows: m (modulus) = CP / π (PI). Both are units of tooth size.

#### 8.Backlash.

When a pair of gears are engaged, the clearance between tooth surfaces. Backlash is a necessary parameter for smooth operation of gear mesh.

#### 9.What is the difference between bending strength and tooth surface strength?

Generally, the strength of gear should be considered from two aspects of bending and tooth surface strength. Bending strength is the strength of the transmission power of the teeth to resist the breaking of the teeth at the root of the teeth due to the bending force. The strength of tooth surface is the friction strength of meshed teeth in repeated contact.

#### 10.Center distance.

Center distance refers to the distance between shafts of a pair of gears. The center distance has an effect on the backlash. The larger the center distance, the larger the backlash.