# Basic knowledge of gear (4)

1. What is the equivalent gear and equivalent number of teeth of helical gear? What is the purpose of equivalent number of teeth?

Answer: the straight cylindrical gear equivalent to the normal profile of the helical gear is called the equivalent gear of the helical gear. The number of teeth of an equivalent gear is called the equivalent number of teeth. The number of equivalent teeth is an important basis for selecting the tooth shape of the cutter in the process of gear copying, and the number of equivalent teeth is the main basis for the strength design of the gear.

1. What are the characteristics of worm gear mechanism?

Answer: (1) transfer the motion and power between the spatial staggered axes, that is, the spatial mechanism.

(2) In theory, the tooth profile contact is point contact when the worm gears are meshed, but the worm gear is added by the hob of the worm which is meshed with the worm gear, which is actually the space curve contact.

(3) The transmission ratio of worm and worm gear is calculated with the number of heads (lines) of worm.

(4) The pitch circle diameter of worm is not the number of heads multiplied by the modulus but the characteristic coefficient multiplied by the modulus, that is, D1 = QM

(5) The center distance of worm is also calculated by the characteristic coefficient.

a＝ m（q＋Z2）/2

(6) It can obtain large transmission ratio and self lock when worm gear is active.

1. Which is the standard parameter surface of worm gear? What are the conditions for correct engagement?

Answer: (1) it is the main section, that is, the section parallel to the end face of the worm gear passing through the axis of the worm is called the main section.

(2) Correct engagement conditions: m A1 = m T2 = m α A1 = α T2 = α β 1 + β 2 = 900 same rotation direction

1. Why is the characteristic coefficient Q of worm determined as the standard value?

Answer: (1) it is conducive to worm standardization and reduces the number of worms.

(2) The number of worm hobs for machining worm gears is reduced.

1. How to determine the steering direction of worm gearing?

Answer: first of all, determine the direction of rotation of worm or worm gear: erect the axis of worm or worm gear. The right side height of spiral line is right rotation, and the left side height is left rotation. Then determine the direction of turning: the right-hand rule is used for right-hand rotation. For the active worm, the right-hand four fingers hold the worm along the worm’s direction of turning, and the thumb point is opposite to the node speed direction of the worm gear, so as to determine the direction of turning of the worm gear.

1. What are the characteristics of the straight bevel gear mechanism?

Answer: (1) transfer the motion and power between two intersecting axes. (2) The gear distribution on the cone shrinks from the big end to the small end. (3) The large end face is the standard parameter face. (4) The tooth profile curve is spherical involute.

1. What are the correct Meshing Conditions for straight bevel gears?

Answer: M1 = M2 = m, α 1 = α 2 = α R1 = R2 (R is the taper distance) of large end face

1. What is the back cone, equivalent gear and equivalent number of teeth of the bevel gear?

Answer: the cone tangent to the graduation circle on the big end spherical surface of the bevel gear is called the back cone of the bevel gear, and the sector formed by the projection and expansion of the parallel conical generatrix on the big end surface of the bevel gear to the back cone is called the sector gear. The straight cylindrical gear equivalent to the large end profile of the bevel gear is called the equivalent gear of the bevel gear, and its number of teeth is called the equivalent number of teeth.

1. What is the purpose of equivalent gear and equivalent number of teeth?

Answer: a pair of equivalent gears of bevel gear are used to study the meshing principle of bevel gear, such as coincidence degree and correct meshing conditions. A single equivalent gear is used to calculate the minimum number of teeth without undercutting and to select the tool number and calculate the bending strength of bevel gear when machining bevel gear with profiling method.

1. How to calculate the pitch circle diameter, transmission ratio and equivalent number of teeth of the bevel gear?

Answer: D = 2rsin δ; i12 = ω 1 / ω 2 = Z2 / Z1 = D2 / D1 = sin δ 2 / sin δ 1; zv = Z / cos δ

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