# Basic knowledge of gear (3)

1. How to determine the minimum number of teeth without undercutting for standard external gear?

Answer: it is determined by Zmin = 2H * A / sin2 α.

1. What is a shift gear?

Answer: the gear whose index circle tooth thickness is not equal to the width of the tooth slot and the gear whose tooth top height is not the standard value are called displacement gears. In machining, a gear whose center line of rack tool is not tangent to the graduation circle of the gear to be machined is called a modified gear.

1. What are displacements and coefficients and minimum coefficients?

A: displacement: the vertical distance that the center line of the tool is shifted by the position of the machining standard gear.

Modification coefficient: the coefficient required to express the modification amount with standard modulus.

Minimum modification coefficient: the minimum value of modification coefficient required for undercutting of involute gear.

xmin = h*a （Zmin – Z）/ Zmin

1. Compared with the standard gear, which sizes of the modified gear with the same number of teeth have changed, which sizes have not changed, and why?

Answer: the number of teeth, modulus, pressure angle, graduation circle, base circle, graduation circle pitch, full tooth height remain unchanged, and the tooth top circle, tooth root circle, graduation circle tooth thickness and tooth slot width have changed.

Reason: the machining method of the modified gear with standard gear cutter is the same, that is, the correct meshing condition is the same, so the module and pressure angle of the graduation circle are the same. Therefore, it can be seen from the formula that the pitch circle and the base circle are the same, and then there are the calculation of the tooth root height, the tooth top height, the tooth root circle and the tooth term circle. The benchmark is the pitch circle. When machining the modified gear, if the center line of the standard tool moves from the pitch circle to the outside, the tooth root circle becomes smaller, and the tooth root circle becomes larger. If the whole tooth height is to be kept constant, the tooth top height becomes larger, and the tooth top circle becomes larger, because the tool moves outward at the gear pitch circle The thickness becomes smaller, that is to say, the tooth slot processed becomes smaller, and the tooth thickness becomes thicker because the pitch circle is the same.

1. How is the profile of helical involute curved surface of helical gear formed?

Answer: when the generating surface of the involute rolls on the base cylinder, a line with a β B angle with the generatrix of the base circle on the generating surface forms the involute spiral surface of the helical gear teeth.

1. Is the helix angle of each coaxial cylindrical surface helix where the helical gear tooth profile is located the same? Why?

Answer: the helix angle is different. Because the helix angle β I is determined by the lead L and the diameter Di of the cylinder, the lead is the same, and the diameters of each circle are different, so the helix angle is different, and the relationship is TG β I = L / π di

1. What are the characteristics of helical gear engagement?

Answer: (1) the tooth profile of the two wheels starts to contact from the point, and the contact line grows from the short to the long, and then shortens, until the point contacts, and then disengages from the engagement. Unlike the straight tooth cylindrical gear transmission, the contact suddenly disengages along the entire tooth width, but gradually enters into the engagement and gradually disengages from the engagement. This kind of impact is small and the transmission is stable.

(2) Large coincidence ε = ε α + ε β

1. Which is the standard parameter surface of helical gear and which is the standard involute? Explain why.

Answer: the normal surface is the standard parameter surface. Theoretically, the end face is a standard involute, because the involute is formed by the pure rolling of the generating surface on the base cylinder, and the track of the oblique line on the generating surface is an involute. In machining, the normal surface is the standard involute, because the standard cutter for machining the helical gear profile is used for machining the straight cylindrical gear, its cutting movement direction is tangent to the helix, and the cutter surface is on its normal surface, so the normal surface is the standard Zhejiang Kaixian line.

1. What is the relationship between the end face of helical gear and the geometric parameters of normal face, and why is the end face parameter required?

Answer: Mn = MT cos β, TG α n = TG α t cos β B, h * at = h * ancos β, c * t = C * ancos β

Because the geometric dimensions are DT, DBT, DAT, DFT, Pt, PBT.

1. What are the correct Meshing Conditions and continuous transmission conditions for a pair of helical gears?

Answer: correct engagement condition: m N1 = m N2 = m α N1 = α N2 = α

External engagement β 1 = – β 2 internal engagement β 1 = β 2

Continuous transmission condition: ε = ε α + ε β ≥ 1 