Secondary Technical Exam Fundamentals of Machinery What parts make up a machine? What are the characteristics that must be present in a standard gear?

Machines generally consist of a power part, a transmission part, an actuating part and a control part.

Standard Gears

Structural Classification

Generally there are wheel teeth, tooth grooves, end faces, normal faces, top circles, root circles, base circles, indexing circles.

The gear teeth

Teeth, for short, are the raised portions of a gear that are used for meshing. These raised portions are generally arranged in a radial pattern, and are in contact with each other on the paired gears, allowing the gears to continue to mesh.

Groove

The space between two neighboring teeth on a gear; the end face is the plane perpendicular to the axis of the gear or worm on a cylindrical gear or cylindrical worm.

End face

is the flat surface at each end of a gear.

Normal plane

The plane perpendicular to the tooth line of a gear tooth.

Top circle

is the circle where the top of the tooth is located.

The root circle

is the circle where the bottom of the groove is located.

Base circle

The circle in which the line of generation forming the involute makes a pure roll.

The indexing circle

It is the reference circle for calculating the geometric dimensions of the gear within the end face.

Main parameters

The number of teeth Z

Closed gear transmission is generally higher speed, in order to improve the smoothness of transmission, reduce the impact of vibration

Spiral bevel gear

Movement to the number of teeth more for the better, the number of teeth of the pinion gear can be taken for the z1 = 20 ~ 40. open (half-open) gear transmission, due to the teeth of the wheel teeth mainly for the abrasion loss efficiency, to The gear is not too small, so the pinion should not choose too many teeth, generally can be taken as z1=17~20.

Spiral angle

β > 0 for the left;

β < 0 for the right

Pitch

pn = ptcosβ (the subscripts n and t indicate the marking of the normal and the end face respectively)

Modulus

Modulus is the ratio (m=p/π) of the pitch p between two adjacent gear teeth on the same side of the tooth profile to the circumference π, measured in millimeters. Modulus is one of the most basic parameters of modular gears, the modulus of straight, helical and conical gears can be referred to the standard modulus series table (GB/T 1357-1987).

mn=mtcosβ

m=p/ π

The indexing circle of the gear is the benchmark for designing and calculating the dimensions of each part of the gear, and the perimeter of the indexing circle of the gear = πd=z p

Module m is a basic parameter for determining the dimensions of the gear. A gear with the same number of teeth has a large module, then its size is also large.

Pressure angle

α rb=rcosα=1/2mzcosα

At the point of tangency P of the two gear pitch circles, the acute angle between the common normal of the two tooth profile curves (i.e., the direction of the force on the tooth profile) and the common tangent to the two pitch circles (i.e., the direction of the instantaneous motion at the point P) is known as the pressure angle, or the angle of meshing. For a single gear that is the tooth angle. The pressure angle of standard gears is generally 20". In some occasions there are also used α = 14.5 °, 15 °, 22.50 ° and 25 ° and so on.

Diameter of indexing circle

d=m*z

Center distance

a=1/2*m(z1+z2)

Correct meshing conditions

m1=m2,α1=α2,β1=β2

In order to make gears free from root-cutting, for the standard straight cylindrical gear with α=20o, z1≥17. z2=u-u2

In order to make gears free from root-cutting, for the standard straight cylindrical gear with α=20o, z1≥17. z2=u-u2. z2=u-u2. z2=u-u2. z2=u-u2. z2=u-u2. z2=u-u2. z2=u-u2-u2-u2. 17. z2=u-z1.

Top height and backlash coefficients

-h*a , C*

When two gears mesh, the top of one gear always enters the root of the other gear, and in order to prevent the top of the gear from thermal expansion and to have a space for lubricating oil, the root of the gear is required to be larger than the top of the gear. In order to prevent thermal expansion and to have space for lubricant storage, the root height is required to be larger than the top height.

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