Gear structure

The composition and structure of gears are generally gear teeth, alveolus, end face, normal face, top circle, root circle, base circle and indexing circle.

Tooth number of gear

The total number of teeth of a gear is called the number of teeth, expressed in Z. The number of pinion teeth can be taken as z1=20~40. Open (semi-open) gear transmission, because the gear teeth are mainly wear failure, in order to make the gear not too small, so pinion does not also choose too many teeth, generally can take Z1 = 17 ~ 20.

Modulus of gear

Because the dividing circle diameter D of gear can be determined by its circumference zp, that is d=zp/pi. In order to facilitate design, calculation, manufacture and inspection, p/pi=m, m is called the modulus of gears, and has been standardized. It is the main parameter to determine the size of gear. The diameter of the indexing circle d = mZ, so m = D / Z.

Pressure angle of gear

That is to say, dividing the pressure angle of the circle and stipulating its standard value is a=20 degrees. It is the main parameter to determine the shape of gear profile.

Dividing circle diameter d=mZ/cos beta

Pressure angle RB = RCOs alpha = 1/2 mzcos alpha

At the tangent point P of the two pitch circles, the sharp angle between the common normal of the two pitch circles (i.e. the direction of force acting on the tooth profile) and the common tangent of the two pitch circles (i.e. the instantaneous direction of motion at the P point) is called the pressure angle, also known as the meshing angle. For a single gear, it is the tooth angle. The pressure angle of standard gear is generally 20%. In some cases, alpha=14.5 degree, 15 degree, 22.50 degree and 25 degree are also used.

Top Height Coefficient and Top Clearance Coefficient: h*a, C*

When two gears mesh, the top of one gear always enters the root of the other gear. In order to prevent thermal expansion and the space for storing lubricant, the height of the root of the gear is required to be higher than that of the top of the tooth. The top height coefficient and the top clearance coefficient are introduced.

Normal teeth: h*a=1; C*=0.25 short teeth: h*a=0.8; C*=0.3

A pair of meshing gears must have the same modulus and pressure angle. Pressure angle of standard gear (tooth shape angle for single gear) is 20 degrees