1. Field of the Invention
The invention relates to a differential apparatus for vehicles and, more particularly, to a bias ratio levelling apparatus of both wheels for a worm gear type differential which is used for a differential limiting apparatus.
2. Description of the Prior Art
When one wheel falls into the mud, snow or similar type road conditions in a vehicle provided with a torsion-type differential, which is a type of differential limiting apparatus, there is a torque bias effect in which a transmitting torque is increased from a wheel of a high grip side to a low grip side by a frictional force and a gear thrust force of an inner gear of the differential. However, a distorted direction of a worm gear of a differential inside is the same at both wheels, so that the difference at a worm gear thrust force is generated upon the generation of the differential rotation. The thrust force influences greatly on a mechanism which generates a frictional resistance within a differential apparatus. Accordingly, the frictional resistance is generated at both wheels (left and right), this means a difference is also generated at a torque bias ratio. As a result, the driving stability of vehicle is damaged. U.S. Pat. No. 4,191,071 is an example of the abovementioned worm type differential, and is shown in FIGS. 8 and 9.
In 8, an axle shaft 16 is connected to a worm gear 6 through a spline connection and an axle shaft 17 is connected to a worm gear 10 through a spline connection. Worm wheel gears 7, 11 attached to a differential housing 14 via shafts 4, 5 are utilized for rotating worm gears 6, 10. Rotational force is transmitted to the differential housing 14 via the ring gear 15 and the rotation of the differential housing 14 is transmitted as a rotational force of worm wheels 7, 11 via shafts 4, 5. The worm wheel gear 7 rotates the worm gear 6 and the worm wheel gear 11 rotates the worm gear 10. When there is no difference in rotation of axle shafts 16, 17, the differential housing, worm gear, and worm wheel gear are integrally rotated, i.e., as a unit. Any rotational differences in the axle shafts are transmitted to the worm wheel upon turning and is synchronized with another worm wheel via spur gears 9, 13 attached to both sides of the shafts 4 and 5. Thus, when the rotation of the worm gear is decreased at one side and the speed is increased at the other, it is quickly differentiated.
However, when one wheel slips on an extremely low friction road surface, a double action of torque is generated by the frictional resistance of the teeth surface and the thrust force in the torsion-type differential and the torque is biased at a side of the differential which is gripping or is experiencing less slip. In FIG. 8, spacers 20, 19 are inserted in the worm gear side. However, this arrangement causes the worm gears to interfere with each other at opposite sides thereof.
In FIG. 9, a thrust needle bearing 24 is inserted at one side of the housing so that the thrust force between the worm gear 10 and the differential housing A is decreased and the bias at left and right outputs or wheels is equal, however, there is no equal or levelling effect if the thrust direction is a direction of the axle shaft 16 (in the upward direction in FIG. 9) As a result, a difference in thrust force is generated and the driving stability is damaged or otherwise adversely effected.