What is a gear

Closed gear transmission generally has a higher speed. In order to improve the stability of the transmission and reduce the impact and vibration, it is better to have more teeth. The number of teeth of the pinion can be z1=20-40. Open (half-open) gear transmission, because the gear teeth are mainly worn out, in order to prevent the gear from being too small, too many teeth should not be used for the pinion, generally z1=17~20.
Helix angle
β> 0 is left-handed;
β <0 is right-handed
Tooth pitch
pn=ptcosβ (subscripts n and t represent the marks of the normal direction and the end face respectively)
Modulus
Modulus refers to the ratio of the pitch p between two adjacent gear teeth on the same side tooth profile to the circumference ratio π (m=p/π), in millimeters. Modulus is one of the most basic parameters of modular gear teeth. For the modulus of straight, helical and conical gears, please refer to the standard modular series table (GB/T 1357-1987).
mn=mtcosβ
m=p/ π
The index circle of the gear is the basis for designing and calculating the size of each part of the gear, and the circumference of the index circle of the gear=πd=z p
The modulus m is a basic parameter that determines the size of the gear. A gear with the same number of teeth has a larger module, and its size is also larger.
pressure angle
α rb=rcosα=1/2mzcosα
At the tangent point P of the pitch circles of the two gears, the acute angle between the common normal line of the two tooth profile curves (ie the force direction of the tooth profile) and the common tangent line of the two pitch circles (ie the instantaneous direction of movement at point P) is called The pressure angle is also called the engagement angle. For a single gear, it is the tooth profile angle. The pressure angle of standard gears is generally 20″. In some occasions, α=14.5°, 15°, 22.50° and 25° are also used.
Diameter of index circle
d=m*z
Center distance
a=1/2*m(z1+z2)
Correct meshing conditions
m1=m2,α1=α2,β1=β2
In order to prevent the gear from undercutting, for the standard straight-edge cylindrical gear with α=20o, z1≥17 should be taken. Z2=u·z1.
Addendum height coefficient and head clearance coefficient
—H*a, C*
When two gears mesh, the tooth tip of one gear always enters the tooth root of the other gear. In order to prevent thermal expansion from topping out and have space for lubricating oil, the tooth root height is required to be greater than the tooth tip height. For this, the addendum height coefficient and the head clearance coefficient are introduced.
Normal tooth: h*a =1; C*=0.25 Short tooth: h*a =0.8; C*=0.3
model
According to specifications or size classification, gear models are divided into two types: standard and non-standard;
According to different measurement units at home and abroad, gear models are divided into metric and imperial systems.
Metric Gear Model
Domestically, the metric system/modulus (M/m) is mainly adopted. Gear modulus = index circle diameter ÷ number of teeth = gear outer diameter ÷ (number of teeth + 2).
The main models of metric gears are: M0.4 M0.5 M0.6 M0.7 M0.75 M0.8 M0.9 M1 M1.25 M1.5 M1.75 M2 M2.25 M2.5 M2.75 M3 M3. 5 M4 M4.5 M5 M5.5 M6 M7 M8 M9 M10 M12 M14 M15 M16 M18 M20 M22 M24 M25 M26 M28 M30
Inch gear model
DP gear is an inch gear (diameter gear) used in Europe, America and other countries. It refers to the number of teeth per inch of the index circle diameter. The larger the value, the smaller the tooth. Diameter DP=z/D (z—number of teeth, D—diameter of index circle, inch), the unit of diameter pitch DP is (1/in). Its conversion relationship with the metric system is m=25.4/DP, which means it is the same as our commonly used modulus.
The main models of inch gears are: DP1 DP1.25 DP1.5 DP1.75 DP2 DP2.25 DP2.5 DP2.75 DP3 DP4 DP4.5 DP5 DP6 DP7 DP8 DP9 DP10 DP12 DP14 DP16
classification
Gears can be classified according to tooth shape, gear shape, tooth line shape, surface on which the gear teeth are located, and manufacturing method.
The tooth profile of the gear includes tooth profile curve, pressure angle, tooth height and displacement. Involute gears are easier to manufacture, so in modern gears, involute gears account for an absolute majority, while cycloid gears and arc gears are less used.
In terms of pressure angle, gears with small pressure angles have a smaller load-bearing capacity; gears with large pressure angles have higher load-bearing capacity, but the load on the bearing increases under the same transmission torque, so it is only used in special cases. The tooth height of the gear has been standardized, and the standard tooth height is generally adopted. There are many advantages of displacement gears, which have been widely used in various mechanical equipment.
In addition, gears can also be divided into cylindrical gears, bevel gears, non-circular gears, racks, and worm gears according to their shape; according to the shape of the tooth line, they can be divided into spur gears, helical gears, herringbone gears, and curved gears; according to the gear teeth The surface is divided into external gears and internal gears; according to the manufacturing method, it can be divided into cast gears, cut gears, rolled gears, and sintered gears.
The manufacturing material and heat treatment process of the gear have a great influence on the load-bearing capacity and size and weight of the gear. Before the 1950s, carbon steel was mostly used for gears, alloy steel was used in the 1960s, and case hardened steel was used in the 1970s. According to hardness, tooth surface can be divided into two types: soft tooth surface and hard tooth surface.
Gears with soft tooth surfaces have low load-bearing capacity, but they are easier to manufacture and have good running-in performance. They are mostly used in general machinery with no strict restrictions on transmission size and weight and small-volume production. Because the small wheel has a heavier burden among the matched gears, in order to make the working life of the large and small gears roughly equal, the hardness of the tooth surface of the small wheel is generally higher than that of the large wheel.
Hardened gears have a high load-bearing capacity. After the gears are cut, they are then quenched, surface quenched or carburized and quenched to increase the hardness. But in the heat treatment, the gear will inevitably produce deformation, so after the heat treatment, grinding, grinding or fine cutting must be carried out to eliminate the error caused by the deformation and improve the accuracy of the gear.


Post time: Dec-18-2020