Steering gear for a motor vehicle with a helical gear transmission

Abstract

Steering gear for a motor vehicle with a helical gear transmission, wherein the helical gear transmission comprises a helical pinion (1) which has a globoidal design, characterized in that the helical gear transmission has an axis crossing angle of 70° to less than 90°; that the globoidal design of the helical pinion (1) is derived from the tooth geometry of a reference counter gear, which in turn is derived from an actual counter gear in the steering gear and has several geometric modifications compared to it, wherein the number of teeth of the reference counter gear is greater than the number of teeth of the actual counter gear.

Description

[0001] The invention relates to a steering gear for a motor vehicle with a helical gear transmission.

[0002] Electrically assisted steering systems typically use worm gear drives with steel worms and plastic-rimmed helical gears to couple the power steering assistance. These drives enable a relatively high torque transmission in a single stage from an electric motor at the worm to a steering output shaft at the helical gear. Kinematically, these are helical gear drives with large shaft crossing angles (70° - 90°) and a contact pattern consisting of individual point contacts between the worm and helical gear flanks. The gear teeth are involute and cylindrical to ensure good insensitivity to variations in center distance. This somewhat unfavorable, wear-prone contact geometry is accepted because it provides high insensitivity to positioning errors during gear assembly.Due to the low modulus of elasticity of plastics, even small loads at each point contact cause a corresponding deformation of the plastic tooth flanks and a widening of the points into elliptical contact surfaces, so that an acceptable load-bearing capacity can still be achieved.

[0003] Despite this increase in the bearing surface area, the plastic tooth flanks typically undergo load-dependent plastic deformation and / or abrasive wear over the gearbox's service life. This behavior, known as settling, is detrimental to the steering gear's function and performance for various reasons. One approach to improving this wear and settling behavior is to increase the total bearing surface area by using a globoidal worm gear. However, in its conventional design, a globoidal worm would be very sensitive to positioning errors.

[0004] The invention is based on the objective of providing a helical gear drive with a readily manufacturable, globoidal worm gear that is similarly insensitive to positioning inaccuracies as a cylindrical worm, but which, under increasing load, bears the load across more and larger flank areas, thus achieving a better distribution of the total load that relieves the individual flank engagements. This should be possible not only for the shaft crossing angle of 90° typical in conventional worm gear drives, but also across the entire angular range from 90° to at least 70°.

[0005] The problem is solved by deriving the globoid worm gear from the gear geometry of a reference counter gear, which in turn is derived from the actual counter gear in the steering gear and has a number of suitable geometric modifications compared to it.

[0006] The globoidal screw can then be manufactured by gear skiving using a skiving wheel that corresponds to the end-section profile of the reference counter gear. The flank line modifications of the reference counter gear are replicated by the machine kinematics as the tooth width is traversed.

[0007] The reference counter wheel can have various features, either alone or in combination, which are briefly described below.

[0008] The reference counter gear, like the actual counter gear, has an involute tooth pattern.

[0009] The reference counter wheel and the counter wheel match in their rack reference profile.

[0010] The reference counter-gear has no backlash relative to the worm gear.

[0011] The number of teeth on the reference gear is greater than or equal to the number of teeth on the actual gear.

[0012] The reference counter gear has profile corrections that, during the rolling process of generating the worm gear teeth, produce the desired profile corrections that influence the contact pattern. For example, this can be a tip recess on the worm resulting from a negative root recess on the reference counter gear.

[0013] The reference counter gear has flank line corrections that produce advantageous flank corrections on the worm.

[0014] The invention is explained in more detail below with reference to an embodiment shown in the drawing. The drawing shows: Fig. 1: A helical gear drive according to the invention in a side view

[0015] The Fig.Figure 1 shows a helical gear drive according to the invention, comprising a globoidal pinion 1 and a helical gear 2, which is cylindrical and has straight teeth. Due to its design as a helical gear drive, the axes of rotation 3, 4 of the pinion 1 and the helical gear 2 form an axis crossing angle other than 90°. The helical gear drive is intended as a steering gear for a steering system of a motor vehicle.

Claims

[1] Steering gear for a motor vehicle with a helical gear transmission, wherein the helical gear includes a helical pinion (1) which has a globoidal shape, characterized by , that the globoidal design of the screw pinion (1) is derived from the tooth geometry of a reference counter wheel, which in turn is derived from an actual counter wheel in the steering gear and has several geometric modifications compared to it. [2] Steering gear for a motor vehicle according to claim 1, characterized by that the reference counter gear has an involute tooth profile. [3] Steering gear for a motor vehicle according to one of claims 1 or 2, characterized by , that the reference counter gear and the counter gear match in their rack reference profile, with the reference counter gear having no backlash relative to the worm. [4] Steering gear for a motor vehicle according to any one of claims 1 to 3, characterized by , that the number of teeth of the reference mating gear is greater than or equal to the number of teeth of the actual mating gear. [5] Steering gear for a motor vehicle according to any one of claims 1 to 4, characterized by , that the reference counter gear has profile corrections which, during the rolling generation of the worm gear, produce the desired profile corrections that serve to influence the contact pattern. [6] Steering gear for a motor vehicle according to claim 5, characterized by , that a head retraction on the worm gear is generated from a negative foot retraction on the reference counter gear. [7] Steering gear for a motor vehicle according to any one of claims 1 to 6, characterized by , that the reference counter gear has flank line corrections which produce advantageous flank corrections on the worm. [8] Steering gear for a motor vehicle according to any one of claims 1 to 7, characterized by, that the screw pinion (1) is produced by means of gear skiving with a skiving wheel corresponding to the end section profile of the reference counter gear, wherein the flank line modifications of the reference counter gear are mapped by the machine kinematics when traversing the tooth width.

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