Herringbone Gears1

HERRINGBONE GEARS1 The Design and Construction of Double Helical Gears on the Wuest System BY PERCY C. DAY That the helical principle in toothed the load is carried near the point of the gear gearing is ideal from a theoretical point of tooth, that tooth is subjected to a maximum view is well known. From a practical stand bending stress along its whole length. Dur point herringbone gears have been less sat ing the first phase, the portion of the pin isfactory than straight-cut spur gears be ion tooth near the root is sensibly sliding cause, until recently, no method was devised over the outer portion of the gear tooth ; for producing them with the requisite speed that is to say, two metallic surfaces of small and accuracy. Within the last six years a area are sliding under heavy compression. method has been found and developed, in The action during the second phase more England, to a high degree of perfection. nearly approaches ideal conditions. The teeth Herringbone gears made by this method are are engaged near their respective pitch lines called Wuest gears, after the name of the in and very little sliding takes place. ventor, and can be produced with even great During the third and final phase, the pin er accuracy than cut gears of the spur type. ion tooth is subjected to a maximum bend The distinction between these gears and ing stress, while the tooth surfaces again those of the ordinary herringbone type is slide over each other, this time with the outer that the teeth of the former, instead of join portion of the pinion tooth engaging the ing at a common apex at the center of the gear tooth near its root. face, are stepped half the pitch apart and do The point to be noted is that while those not meet at all. This arrangement of the portions of the mating teeth which are near teeth does riot affect the action of the gears, the pitch lines transmit the load with rolling but it facilitates their commercial production contact, those which are more remote have and admits the use of precision methods in to transmit the same load with sliding con their manufacture. tact. The inevitable result is that the points Action of Spi/r Gearing and roots of all the teeth tend to wear away In the action of a straight spur pinion more rapidly than the portions near the pitch driving a gear, there are three distinct phases lines, so that the involute tooth curves, neces of engagement : sary to the preservation of even angular 1. The root of the pinion tooth engages the velocity, lose their form and the motion be point of the gear tooth. comes uneven. 2. The teeth are engaged near the pitch It may be suggested that the sliding ac line. tion can be got rid of by shortening the 3. The point of the pinion tooth engages teeth so that they engage only the phase of the root of the gear tooth. rolling contact. This has been tried with a Assume that the teeth are accurately cut to certain measure of success in the stub- involute form, so that if the pinion moves toothed gear, but it cannot be carried far with even angular velocity it will produce enough without curtailing the arc of con corresponding evenness of motion in the tact so that continuity of engagement is lost, gear; also that the pinion has sufficient teeth thereby introducing more serious trouble to allow the engagement of successive teeth than that which it is desired to avoid. to overlap. Distortions of gear teeth from involute At the beginning of the first phase, while form, whether due to inaccurate cutting or subsequent wear, give rise to all kinds of 1Froni a paper read at the New York meeting of the American Society of Mechanical Engineers, Dec. 6, 1911. trouble. The average angular velocity may JANUARY, 1912. 27 28 INDUSTRIAL ENGINEERING and be uniform, and yet the passage of each pin load, and the effect of wear on the ends of ion tooth through its brief engagement with the teeth merely tends to throw more load the mating gear may be accompanied by suc on the center portions ; in other words there cessive retardation and acceleration which, is a tendency to concentrate th-e load near though small in itself, takes place in such a the pitch lines. The ends of the teeth, in short interval of time that it may cause in stead of wearing awav to an ever-increasing teracting stresses many times greater than extent from their original involute form, are the average working load on the teeth. These relieved of some of the load from the mo internal stresses are very difficult to deal with, ment that wear commences to take place. because they are indeterminate. They cause As •;oon as the load on these ends has been noise, vibration, crystallization and fracture. partially relieved and transferred to the mid dle portion, the wear becomes enualized all Action of Herringbone Gears over the teeth and thev do not tend to dis Herringbone gears completely overcome tort further from their original shape. all these difficulties, but only when they are It is quite clear that an immeasurable accurately cut. amount of wear on the tooth ends will be Two exactly similar pinions with straight sufficient to relieve them of all the load, so teeth, placed side by side on one shaft, with that the distortion from original form will the teeth of one pinion set opposite the be practically nothing. The minute extra spaces of the other, form what is known as wear that does take place at the ends is only a stepped-tooth pinion. If this pinion is the amount necessary to transfer such pro meshed with a composite gear made up in a portion of the load near the pitch lines that similar manner, the action is modified so that the wear is equalized all over the surface of there are always two phases of engagement the teeth, those portions in sliding contact taking place simultaneously. Such gears are carrying less than those in rolling contact. commonly used for rolling mill work, be Thus the teeth keep their involute form and cause they stand up to heavy shocks better motion is transmitted from ninion to gear in than the plain type. a perfectly even manner, without iar. shock, The helical gear is the logical outcome of or vibration. Although herringbone teeth the stepped gear carried to its limit, and built may not be intrinsically stronger than up from infinitely thin laminations. Since the straight teeth, the elimination of all shock steps have merged into a helix, there must pud indeterminte internal stresses renders be a normal component' of the tangential them capable of dealing with far heavier pressure on the teeth, producing end thrust transmitted loads. The concentration of the on the shafts. To obviate end thrust, the major portion of the load on the parts of helical teeth are made right-hand on one the teeth in rolling contact eliminates fric side of the face and left-hand on the other. tion to a marked extent. Such gears, with double helical teeth, are Since all phases of engagement occur sim known as herringbone gears. ultaneously, the transference of the load from The fundamental principle of the action of one pinion tooth to the next takes place herringbone teeth lies in the circumstance gradually instead of suddenly. This is the that all phases of engagement take place second principle of herringbone gearing, and simultaneously. This holds good for every may be termed continuity of action. In position of pinion and gear, provided only straight gears the continuity of action is a that the relationship between pitch, face function of the number of teeth in the pin width, and spiral angle is such as will insure ion. Straight pinions with less than twelve a complete overlap of engagement. teeth are seldom made, and more than that Since all phases of engagement occur to number must be used if the drive is to be gether, it follows that the load is partly car even moderately satisfactory. ried by tooth surfaces in sliding contact and In herringbone gears continuity depends on partly by surfaces in rolling contact. The the relationship between the face width and result is curious and interesting. Those por the number of teeth in the pinion. Pinions tions of the teeth farthest from the pitch with as few as five teeth have been used with line, which engage with sliding action, tend success by merely increasing the face width to wear away more rapidly than the por to suit such extreme conditions. The feat tions nearest the pitch line. But the pitch ure, which is peculiar to herringbone gears, line portion is always carrying part of the has made practical the adoption of extremely JANUARY, 1912. THE ENGINEERING DIGEST 29 high ratios of reduction hitherto considered by end mills; (c) generating by shaping or impossible. planing methods; (d) generating by hobs. The third principle of herringbone gear The hobbing process as applied to ing is that the bending stress on the teeth straight-cut gears has proved so successful docs not fluctuate from maximum to mini that it is not difficult to understand why it mum as in straight gears, but remains always has sprung into prominence in a compara near the mean value. This feature is of spe tively short time. It is essentially a rational cial importance in rolling-mill driving and process.

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