# Thickness determines the size of the gear

The main function of the gear is transmission. Why are some gears large and some gears small? Some people may say it is because of the size of the mold, but not all gear moduli are the ratio of pitch to circumference. The larger the modulus, the stronger and thicker the teeth, and the larger the size of the gear. The tooth height of a standard gear is equal to 2.25 times the modulus, and the tooth thickness is equal to 0.5*3.14*modulus, that is, the larger the modulus, the larger the teeth. The modulus is also large, the higher the number of teeth, the thicker the gear teeth, and if the number of teeth is determined, the larger the radial size of the wheel. Gears with larger moduli also have larger numbers of teeth. The modulus determines the size of a single tooth profile, not the size of the gear, and has nothing to do with the pressure angle. The modulus is the largest parameter that affects the gear. This does not mean that the gear is large. This should also be determined according to the use environment. If a small gear should be used in a place, it is definitely not suitable for a large gear. So it is not the bigger the better, the right one is the best.
The diameter of the involute gear is d=zm, which is the modulus multiplied by the number of teeth. The modulus is related to the load-bearing capacity of the gear. When the gear is made of the same material and heat treated, the larger the modulus, the stronger the load-bearing capacity; the number of teeth is related to the transmission ratio. The speed of the gear is related to this number as n1*z1=n2*z2.

Therefore, there are two factors that determine the size of the gear, the load-bearing capacity and the transmission speed ratio.
Module m
(1) This module is equivalent to the model of the gear, which can determine the size of the gear. Other parameters are the same. The larger the module, the larger the size of the gear.
Number of teeth z
(2) Needless to say, the number of teeth is the number of teeth in the gear. This is related to the gear ratio. The two meshing gears are in the best possible state.
Pressure angle α
(3) The pressure angle usually refers to the pressure angle on the pitch circle. There are other pressure angles: the pressure angle on the base circle, etc. It is generally believed that the pressure angle is on the pitch circle, and the standard gear is 20 degrees.
Helix angle β
For cylindrical spur gears, there is no helix angle, or the helix angle is 0, but cylindrical helical gears have a helix angle. This helix angle also has a certain range of values. See the Mechanical Design Manual for details. Here it is mentioned that if it is a bevel gear, then the pressure angle we refer to is the positive pressure angle on the bevel gear partition ring.