The present invention relates to a steering mechanism for a lawn tractor. It is advantageous for a lawn tractor to have a tight turning circle so that cutting around obstacles on a lawn can be made easy. Steering mechanisms that can provide such low diameter turning circles can also produce undesirable steering effects, such as heavy steering, or they can be overly complicated and thus expensive to manufacture and maintain. Consequently, there is a need for a steering mechanism for a lawn tractor that provides a tight turning circle, is easy for a user to operate, does not damage a lawn and which can be cost effectively manufactured and efficiently maintained.

Accordingly, the present invention provides a steering mechanism, for fitment to a lawn tractor chassis, comprising a pivotable sector gear having gear teeth that mesh with gear teeth on a rotatable steering gear, wherein the gear teeth on the sector gear are arranged in an arc on a forward facing edge of the sector gear, a nearside steering bar that is attached to the sector gear and to a nearside steering knuckle, an offside steering bar that is attached to the sector gear and to an offside steering knuckle, wherein the nearside steering knuckle pivots about a kingpin axis P-P of a nearside kingpin and the offside steering knuckle pivots about a kingpin axis P-P of an offside kingpin, wherein a wheel, comprising a wheel rim and a tyre, is rotatably attached to each of the nearside steering knuckle and the offside steering knuckle and wherein the nearside kingpin and the offside kingpin are each located outside of the wheel rim, wherein, in use, rotation of the steering gear causes the sector gear to pivot, whereby the nearside steering bar and the offside steering bar cause the nearside steering knuckle and the offside steering knuckle to pivot about the nearside kingpin and the offside kingpin respectively, resulting in the turning of each wheel.

Preferably, the arc on the forward facing edge of the sector gear is an arc of a circular sector, wherein the circular sector is concentric with the centre-point of a sector gear pivot aperture that is provided through the sector gear rearwardly of the forward facing edge, the centre-point is located on a longitudinal axis X-X of the sector gear, the arc extends by an equal angle either side of the longitudinal axis X-X and the sector gear pivots about the centre-point.

Preferably, the arc of the circular sector has a radius R1 of 115mm and an included angle 0 of 109 degrees. Preferably, the sector gear is provided with a nearside steering bar pivot aperture on the nearside of the longitudinal axis X-X and an offside steering bar pivot aperture on the offside of the longitudinal axis, wherein the nearside steering bar pivot aperture and the offside steering bar pivot aperture are located rearwardly of the sector gear pivot aperture and are arranged symmetrically about the longitudinal axis X-X.

Preferably, the centre-points of each of the nearside steering bar pivot aperture and the offside steering bar pivot aperture are located at either end of an arc with a radius R2 of 150mm and are each offset from the longitudinal axis X-X by an angle a of 55 degrees.

Preferably, the nearside steering knuckle and the offside steering knuckle are each provided with a stub axle to which a wheel is rotatably connected and the stub axle has a stub axle axis C-C, the kingpin has a kingpin axis P-P about which the nearside steering knuckle and the offside steering knuckle rotate, wherein the kingpin axis P-P is offset from a perpendicular to the stub axle axis C-C by an angle y of 7 degrees.

Preferably, the intersection point of the kingpin axis P-P and the stub axle axis C-C is located at a distance D1 of 115mm along the stub axle axis C-C inboard from the centreline of the wheel.

Preferably, the nearside steering knuckle and the offside steering knuckle are attached to the nearside steering bar and the offside steering bar at a steering bar pivot point and the steering bar pivot point is located at a distance D2 of 122mm along the stub axle axis C-C from a point where the kingpin axis P-P intersects the stub axle axis C-C and at a distance D3 of 220mm in a direction vertically perpendicular from the stub axle axis C-C.

Preferably, the nearside kingpin is attached to one end of an axle that is orientated transversely to the longitudinal axis of the lawn tractor chassis and the offside kingpin is attached to the other end of the axle, wherein the axle is provided with a pivot point aligned with the longitudinal axis of the lawn tractor chassis about which it pivots in use.

Preferably, the steering gear is attached to a steering wheel by a steering column, wherein, in use, rotation of the steering wheel causes rotation of the steering gear.

The present invention will be described here with reference to the following figures: Figure 1 is a plan view of a steering mechanism according to the present invention attached to the chassis of a lawn tractor;

Figure 2 is a perspective view of a sector gear, steering gear and steering bars of the steering mechanism of Figure 1 ;

Figure 3 is a frontal view of the steering mechanism showing the front axle assembly;

Figure 4 is a plan view of a sector gear, steering gear and steering bars of the steering mechanism of Figure 1 ;

Figure 5 is a cross-sectional close up frontal view of the nearside steering knuckle of the steering mechanism of Figure 1 ;

Figure 6 is a plan view of the front axle and steering knuckles steering straight ahead, with the front wheels not shown;

Figure 7 is a plan view of the front axle and steering knuckles steering fully to the left, with the front wheels not shown;

Figure 8 is a plan view of the front axle and steering knuckles steering fully to the left, with the front wheels shown; and

Figure 9 is a frontal cross-sectional view of the steering mechanism showing the positive camber applied to the stub axles.

Figure 1 shows a steering mechanism 1 according to the present invention attached to a lawn tractor chassis 3 for steering of a pair of front wheels 5. A sector gear 7 and a steering gear 9 are each pivotally mounted to the chassis 3 for rotation about vertical axes Y-Y. The sector gear 7 has spur gear teeth 11 which mesh with spur gear teeth 13 on the steering gear 9, as shown in Figure 2, and the steering gear 9 is coupled to a steering wheel 15 via an articulated steering column 17. A nearside steering bar 19 is coupled between the sector gear 7 and a nearside steering knuckle 21 and an offside steering bar 23 is coupled between the sector gear 7 and an offside steering knuckle 25. The term nearside is used to mean the right hand side of the lawn tractor chassis, when viewed from the front, and the term offside is used to mean the left hand side of the lawn tractor chassis when viewed from the front. The nearside steering knuckle 21 and the offside steering knuckle 25 are each rotationally attached to a front axle 27 by a kingpin 29. The front axle 27 is pivotally attached to the chassis 3 about an axis that is longitudinal to the chassis. The front wheels 5 rotate around stub axles 31 that are provided on the nearside and offside steering knuckles 21 ,25.

The sector gear 7 is formed from a flat plate and has an upper face 33 and a lower face 35 that are parallel to each other and orientated horizontally. The sector gear 7 is symmetrical about a longitudinal axis X-X, which is aligned with the longitudinal axis of the chassis 3. A circular sector gear pivot aperture 37 passes through the sector gear 7 between the upper and lower faces 33, 35 and a sector gear pivot pin 39, that is attached to the chassis 3, passes through the sector gear pivot aperture 37. The sector gear 7 comprises four integrated portions. There is a central portion 41 , that is located around the sector gear pivot aperture 37, and a spur toothed sector portion 43, a nearside steering bar support arm 45 and an offside steering bar support arm 47 that are each located around the central portion 41 and extend away from it. The vertical axes Y-Y of the sector gear 7 and the steering gear 9 are placed rearwardly of the front axle 27 and on the longitudinal axis of the lawn tractor chassis 3.

The sector portion 43 is located forwardly of the sector gear pivot aperture 37, i.e. towards the front of the chassis 3 and the front wheels 5. The sector portion 43 has a curved edge 49 located at its forwardly directed face. The curved edge 49 corresponds to the arc a sector of a circle of a radius R1 of 115mm (measured to the tips of the spur gear teeth 11), wherein the sector has an included angle 0 of 109 degrees and the circle is concentric with the sector gear pivot aperture 37. The curved edge 49 extends by an equal distance either side of the longitudinal axis X-X. The spur gear teeth 11 are arranged along the length of the curved edge 47.

The nearside and offside steering bar support arms 45, 47 are located rearwardly of the sector gear pivot aperture 37, i.e. towards the back of the chassis 3. A nearside steering bar pivot aperture 51 and an offside steering bar pivot aperture 53 pass through the nearside and offside steering bar support arms 45, 47 respectively and are located at a radius R2 of 150mm measured from the centre of the sector gear pivot aperture 37 and at an angle a of 55 degrees from the longitudinal axis X-X.

The nearside and offside steering bars 19,23 are pivotally connected at their rearward ends to the nearside and offside steering bar support arms 45, 47 by steering bar pivot pins 55. The forward ends of the nearside and offside steering bars 19,23 are respectively pivotally connected to the nearside and offside knuckles 21 , 25 by steering knuckle pivot pins 57. Figure 5 is a cross-sectional close-up view of the nearside steering knuckle 21 with a front wheel 5 attached. The following description is made with reference to the nearside steering knuckle 21. The offside steering knuckle 25 has all of the same features as will now be described.

The nearside steering knuckle 21 has a stub axle carrier 59 to which the stub axle 31 is attached. The stub axle carrier 59 has an axle hub socket 61 within which is located one of the axle hubs 63 that is provided on each end of the front axle 27. A kingpin 29 attaches the steering knuckle 21 to the front axle 27 by passing through an upper kingpin aperture 65 and a lower kingpin aperture 67, each of which is provided in the axle hub socket 61 , and through a kingpin hub aperture 69 in the axle hub 63. The nearside steering knuckle 21 pivots around an axis P-P of the kingpin 29.

The axis P-P of the kingpin 29 is orientated at an angle y of 7 degrees to the vertical and at the point where it contacts the ground it defines, along with a line passing through the centre of the front wheel 5, a scrub radius Rs of 90mm, (as shown in Figure 5). The axis P-P intersects the longitudinal axis of the stub axle 31 C-C at an offset point 71 which is located at a horizontal distance D1 from the centre of the front wheel 5. The distance D1 is of a magnitude such that the king pin 29 is located outside of a wheel rim 73 of the wheel 5, and such that the axis P-P of the king pin does not pass through any point of the wheel rim 73 or a tyre 75 of the wheel 5.

The nearside steering knuckle 21 also has a steering bar attachment plate 77 which is located above the stub axle carrier 59 and through which a steering bar pivot aperture 79 passes. One of the steering knuckle pivot pins 57 is located in the steering bar pivot aperture 79 to pivotally connect the nearside steering bar 19 to the nearside steering knuckle 21. The steering bar pivot point 81 is located at a horizontal distance D2 of 122mm and at a vertical distance D3 of 220mm from the offset point 71. The distance D4 between the ground and the axis C-C of the stub axle 31 is 210mm.

As shown in the plan view of Figure 6 the steering bar pivot point 81 is horizontally offset in a forwards direction by a distance D5 of 5mm from the intersection point 71 between the kingpin axis P-P and the stub-axle axis C-C. Figure 6 shows that a toe-in is applied to the stub axles to assist with centring of the steering mechanism 1.

Figures 7 and 8 show the steering mechanism 1 rotated fully to the left. The turning angle of each of the inside and outside front wheels 5 are different. The inside front wheel 5 (on the nearside) is turned away from the straight ahead position by an angle that is greater than the angle at which the outside front wheel 5 (on the offside) is turned away from the straight ahead position. The geometry of the steering mechanism 1 minimises the forces experienced by the outside front wheel 5 and compensates for any slack in the steering mechanism 1 so that the outside front wheel 5 does not scrub. In addition, any locking-out or over-centring of the steering mechanism 1 is prevented by the selected geometry.

Figure 9 illustrates that the geometry of the steering mechanism 1 applies a positive camber to the front wheels.

In use, the front wheels 5 are turned by turning the steering wheel 15. Rotation of the steering wheel 15 causes the steering column 17 to rotate and rotation of the steering column 17 causes the steering gear 9 to rotate. If the steering wheel 15 is turned clockwise, in order to steer the lawn tractor chassis 3 to the right, the sector gear 7 rotates anti-clockwise, the nearside steering bar 19 moves rearward (towards the back of the lawn tractor chassis 3) and the offside steering bar 23 moves forward. Rearward movement of the nearside steering bar 19 causes the nearside steering knuckle 21 to rotates clockwise and forward movement of the offside steering bar 21 causes the offside steering knuckle 25 to rotate clockwise as well, thereby turning the front wheels to the right.