1.  Field of the Invention
This invention relates to a reduction apparatus for a tractor axle utilizing a planetary reduction mechanism. 
2.  Description of the Prior Art
Tractor axle reduction apparatus utilizing a planetary reduction mechanism have been proposed in Japanese Utility Model Publication Nos.  56-12433 and 56-12434.  These have basically the same construction and therefore the former is referred to herein. 
This prior art construction which is illustrated in FIG.  10 of the accompanying drawings is capable of a large reduction ratio since a planetary reduction mechanism is employed.  However, according to this construction, an axle 100 is in tubular form and contains a drive shaft 100A therein, which require large bearings 110 to support the axle 100.  Thus, the construction has a disadvantage of requiring a large axle reduction casing 104 for use in a tractor involving great torque transmission, and is therefore applicable only to small tractors involving small torque transmission. 
A carrier 107 is attached to an outer periphery of a wheel boss 106 mounting front wheels 118 and having a hollow portion 106a.  Planetary gears 101 are rotatably attached to the carrier 107.  Teeth of the individual planetary gears 101 are projected inwardly of the hollow portion 106a of the wheel boss 106 and outwardly of the outer periphery of the carrier 107, respectively.  A drive shaft 113 carrying a sun gear 116 projecting from a front gear case 104 engages the wheel boss 106 thereby to rotatably support the wheel boss 106 by the front gear case 104.  The planetary gears 101 are in engagement with a sun gear 116.  The front gear case 104 carries an internal gear 111 engaging the planetary gears 101.  At the wheel boss 106 there is formed a hollow portion 106a.  A ring-shaped carrier 107 is in engagement with a spline portion 106b defined in the outer periphery of the wheel boss 106.  Gear pins 108 are fixedly secured to the carrier 107 on a circumference having as its center an axis of the wheel boss 106.  The planetary gears 101 respectively are in rotatable engagement with the gear pins 108 via needle bearings 109 with teeth of the planetary gears 101 being projected inwardly of the hollow portion 106a of the wheel boss 106 and outwardly of an outer periphery of the carrier 107.  Outwardly of the front gear case 104, there is provided a ring-shaped inner gear 111 disposed in the direction of the outer periphery of the planetary gears 101 and meshing the planetary gears 101.  A leading edge of a drive shaft 113 rotated by a universal joint 112 is supported by the metal case 105 through a bearing 114 with the leading edge being projected into the hollow portion 106a of the wheel boss 106.  The metal case 105 along with the inner gear 111 is attached to the front gear case 104 by bolts 115.  A sun gear 116 meshing the planetary gears 101 inside the hollow portion 106a is attached to the drive shaft 113 inwardly of the carrier 107.