GB160447A - Improvements in hobs for cutting gear teeth - Google Patents

Abstract

160,447. Metropolitan - Vickers Electrical Co., Ltd., (Assignees of Labberton, J. M.). March 13, 1920, [Convention date]. Void [Published under Sect. 91 of the Act]. Milling-cutters.-The invention relates to hobs such as is shown in Fig. 3, with the angles a of inclination of its side faces equal to the pressure angle and its depth d equal to the height of the teeth cut, each hob being designed for cutting the spur. bevel, or helical involute teeth referred to in Specification 153,581, [Class 80 (i), Gearing, Belt &c.]. In determining the dimensions of the said involute teeth having given the usual particulars-the distance between gear centres, the gear ratio, and the diametral pitch-an assump-- tion of the desired amount of rolling-action be-- tween the gear teeth is made and is used as a basis in solving a series of equations based on a mathematical consideration of the action and construction of the gearing. A diagram, Fig. 1, is drawn embodying the pitch circles contacting at the point o, the base circles x<1> y<1>, the line of of pressure x, y and tooth curves S, T contacting at the point o. Points s', t' on the curves S, T, respectively, are chosen to represent the points at which the first and last contacts with the tooth points t<2>, s<2> are made. Tangents to the base circles are drawn from the points s<1>, t<2>, t', angles subtended at the gear centres y', y<2> being designated as shown. Calculations are then made equating the angle c' with quantities involving the ratio of the curve s' o to the curve t<2> o, the gear ratio, and the angle b. The angle C<SP>2> is similarly equated. An assumption of the required value of the ratio s<1> o : t<2> o, which represents the proportion of rolling taking placebetween the teeth and which should be rather less than unity, is then made, so that the angles c', c<2> in terms of b, or in terms of a, the pressure angle, as b=tan a, can be calculated. Further assumptions are then made that the length of contact surface t<1>t<2> and the total length of each tooth are each equal to half the circumferential pitch. Based upon the first of these assumptions, such calculations are then made as to produce an equation in which by substituting the values of c', c<2> previously obtained, the value of a, the pressure angle of the gears, is determined. The addendum of each wheel is then determined by subtracting the pitch radii' r<2>, r<1> from the lengths of the lines t<2>z<2> and s<2>z<11> (not shown), which lengths are obtained in terms of the angles shown and of the radii r<2>, r<1> from the lengths of the lines t<2>z<2> and s<2>z<1> (not shown), which lengths are obtained in terms of the angles shown and of the radii of the base circles. Fig. 2 shows a curve stated to be obtained by calculation and designed to give equal strengths of teeth in the two wheels, the abscissµ denoting the gear ratio R and the ordinates the relative thicknesses of the teeth on the pitch line.