Principle of Modified Gears in Hard-Toothed Gear Reducers and Selection Coefficients
2021-12
In the principle of profile-shifted gears for hard-toothed reducers, when using a rack-type tool to hob a gear, if the reference line of the rack cutter is tangent to the pitch circle of the gear blank, the resulting gear is a standard gear. However, if the reference line of the rack cutter is not tangent to the pitch circle of the gear blank, the resulting gear is a profile-shifted gear. The distance between the reference line of the cutting tool and the pitch circle of the gear blank is called the shift amount, denoted by xm; X is referred to as the shift coefficient. When the cutting tool moves away from the gear blank, X takes a positive value (known as positive shift); conversely, when the cutting tool moves toward the gear blank, X takes a negative value (known as negative shift). For helical gears, the relationship between the transverse shift coefficient and the normal shift coefficient is: X1 = XN.
After gear modification, the tooth profile of the modified gear still belongs to the same involute curve as that of a standard gear; however, the segments of this curve used by the modified gear differ from those of the standard gear. By carefully selecting the modification coefficient X, we can exploit this characteristic to obtain favorable segments of the involute curve, thereby improving the performance of gear transmission. The use of modified gears can prevent undercutting and enhance both the surface contact strength and the root bending strength of the teeth. Moreover, it improves the gear surfaces' resistance to scuffing and their wear resistance.
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