Adjustable height anti-tip wheels for a power wheelchair

[0002] The present invention relates to an assembly for supporting anti-tip wheels on a wheelchair. More particularly, the assembly incorporates a locking mechanism which allows adjustment of the relative height of anti-tip wheels above the ground.

[0003] Some members of society have difficulty walking due to health problems. To provide mobility to these people, wheelchairs and power wheelchairs have been developed. These wheelchairs rotate forwards or backwards as torque is applied to the wheels of the wheelchair, or as the chair is accelerated or decelerated. To counter the forward or backward rotation of the wheelchair, anti-tip wheels have been combined with wheelchairs to limit the distance that the wheelchair can rotate forwards or backwards.

[0004] The height of front- or rear-mounted anti-tip wheels mounted on a wheelchair may limit the height of an obstruction which can be traversed, by causing the main wheels to be lifted off of the ground during traversal of the obstacle.

[0005] However, the greater the height of the anti-tip wheels, the greater the amount the wheelchair may rotate forwards or backwards before the anti-tip wheels engage the ground. Adjusting the anti-tip wheels to be closer to the ground alleviates this problem; however, it also limits the size of obstructions which can be overcome. The height at which the anti-tip wheels are mounted on the wheelchair, therefore, is a balance between the size of obstacles expected to be encountered and the degree of tipping one is willing to tolerate.

[0006] The present invention is an anti-tip strut assembly which allows adjustment of the height of an anti-tip wheel above the ground. A preferred embodiment of the assembly incorporates a cam mechanism to allow the initial positioning of the anti-tip wheels to be adjusted relative to the ground over which the wheelchair is expected to travel. The assembly includes an adjuster plate mounted to the strut. The adjuster plate has a pivoting connection to the strut at one end of the adjuster plate, an anti-tip wheel pin at the opposite end, and a slot between the two positions. A cam is mounted to the strut, and extends through the slot in the adjuster plate. Changes in the angle between the adjuster plate and the strut force rotation of the adjustment cam. A mechanical fastener allows the cam to be locked against the adjuster plate or the strut, preventing the cam from inadvertently rotating when an obstruction is encountered.

[0007] For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

[0008] FIG. 1 is a partial view of a powertrain and frame of a power wheelchair having a connected front anti-tip wheel. FIG. 2 is a perspective view of a strut embodying the present invention.

[0009] FIG. 3 is an illustrated parts breakdown of an anti-tip assembly of the present invention.

[0010] FIG. 4 is a cross-sectional view of a cam adjustment mechanism.

[0011] FIG. 5 is a cross-sectional view of a cam adjustment mechanism.

[0012] The preferred embodiment of the present invention is an anti-tip assembly for a powered, mid-wheel driven wheelchair. Such a wheelchair is shown in U.S. Pat. No. 5,944,131, incorporated herein by reference. More particularly, the preferred embodiment is for an anti-tip strut for use in conjunction with an active suspension for a mid-wheel driven power chair, as shown in U.S. Pat. No. 6,129,165 and commonly owned U.S. patent application Ser. No. 09/166,303, herein incorporated by reference.

[0013] The active suspension for a mid-wheel driven power chair described in the above referenced patent applications uses two motors to drive the mid-wheels. The mid-wheels are connected to the motors, which are mounted to the anti-tip struts. The anti-tip struts are rotatably connected to the frame of the wheelchair. At least one spring element is connected between the strut and the frame to limit the rotation of the strut. The torque applied to the mid-wheels in order to accelerate the wheelchair is countered by the elastic elements supporting the struts. The force on the elastic elements causes the anti-tip wheels to raise further above the ground. Deceleration causes a reverse reaction, rotating the anti-tip wheels towards the ground. This system improves the balance between the capability to overcome obstacles and the allowable forward pitching motion, but is limited by the need to provide elastic elements stiff enough to provide an acceptable ride quality.

[0014] The preferred embodiment of the present invention is shown adapted for use with the active suspension power chair described. Referring now to the drawings, wherein like reference numerals illustrate corresponding or similar elements throughout the several views, FIG. 1 shows a portion of the structure of a power wheelchair. The wheelchair structure which is generally designated by the numeral 20 includes a frame 22 which provides mounting points for a strut 24. A motor 26 is attached to the strut 24. A driven mid-wheel 28 rotates about an axle 30 which is driven by the motor 26. The strut 24 is pivotable about a strut pin 32 which connects the strut 24 to the frame 22. Spring 34 limits the rotation of the strut 24 about the strut pin 32, as well as provides a resilient suspension for the front anti-tip wheel 36. Wheel 36 is mounted to an adjuster plate 38, which is in turn pivotably connected 40 to the strut 24. A locking means 42 fixes the angular position of the adjuster plate 38 relative to the strut 24.

[0015] Also shown in FIG. 1 are anti-tip wheels 36 superimposed to show higher 44 and lower 46 positions available within the range adjustment of the adjuster plate 38. With the anti-tip wheels 36 in the lower position 46, the ability of the power wheelchair to rotate forward is limited by the proximity of the anti-tip wheels 36 to the ground. When the anti-tip wheels 36 are adjusted to a higher position 44, larger obstructions can be cleared. This adjustability allows the occupant of a wheelchair to vary the compromise inherent in positioning anti-tip wheels 36, optimizing the position of the anti-tip wheels 36 based on the obstructions 50 expected to be encountered.

[0016] As shown in FIG. 2, the locking means 42 is located between the adjuster plate 38 and the strut 24. A wheel stud 52 extends from the adjuster plate 38 providing a mounting point for an anti-tip wheel 36. The strut 24 has a pair of co-axial apertures 54 which allow the strut 24 to be rotatably mounted to the frame 22 (shown in FIG. 1).

[0017] The adjuster plate 38 is connected to the strut 24 at the adjuster plate pivot 56. The adjuster plate pivot 56 rotatably connects the adjuster plate 38 and the strut 24. The adjuster plate pivot 56 consists of a bolt 58 passed through the adjuster plate 38 and the strut 24. The end of the bolt 58 is threaded, and a nut 62 retains the bolt 58 to the assembly. When the nut 62 is tightened, it pulls the adjuster plate 38 into contact with the strut 24. This contact creates friction between the adjuster plate 38 and the strut 24, and thus prevents rotation of the adjuster plate 38 relative to the strut 24. When the nut 62 is loosened, the adjuster plate 38 can rotate about this connection relative to the strut 24.

[0018] As shown in FIG. 3, the adjuster plate 38 has a slot 64 located between the adjuster plate pivot 56 and the wheel stud 52. The locking means 42 includes a cylindrical cam lobe 66. As shown in FIG. 4, the thickness 70 of the cam lobe 66 is slightly less than the thickness 68 of the adjuster plate 38. Referring again to FIG. 3, a cam pin 72 extends from one side of the cam lobe 66. The cam pin is 72 mounted to the cam lobe 66 such that the long axis 74 of the cam pin is displaced from the center axis 76 of the cam lobe 66. When the cam lobe 66 is rotated about its center 76, the cam pin 72 rotates eccentrically about the cam lobe center 76. As assembled, the cam lobe 66 is within the slot 64 in the adjuster plate 38 and the cam pin 72 extends from the cam lobe 66 though a cam aperture 78 in the strut 24. The end of the cam pin 72 is threaded and retained in the aperture by a washer 80 and nut 62.

[0019] A retaining plate 82 is attached to the cam lobe 66 on the side opposite from the cam pin 72. The retaining plate 82 is preferably circular in shape, and has an outer diameter 84 (see in FIG. 4) greater than the width 86 of the slot 64 in the adjuster plate 38.

[0020] The nut 62 retaining the cam pin 72 within the cam aperture 78 can be loosened to allow the cam pin 72 to rotate, or tightened to bring the retaining plate 82 into abutting contact with the adjuster plate 38. When the retaining plate 82 is in abutting contact with the adjuster plate 38, friction between the retaining plate 82 and the adjuster plate 38 prevents the cam lobe 66 from rotating relative to the adjuster plate 38, and thus locks the position of the adjuster plate 38 relative to the strut 24.

[0021] As seen in FIG. 4, the diameter 88 of the cam lobe is substantially the same as the width 86 of the slot 64 in the adjuster plate 38. When the nut 62 retaining the cam pin 72 is loosened to allow rotation, the cam lobe 66 is free to spin about the long axis 74 of cam pin 72 within the slot 64. The displacement of the long axis 74 of cam pin 72 from the center axis 76 of the cam lobe 66 allows the cam lobe to move forward and back in the slot 64 along line 92. Because the cam pin 72 is rotatably fixed to the strut 24, the movement of the cam lobe 66 along line 92 in the slot 64 results in a change in the angle between the adjuster plate 38 and the strut 24.

[0022] The inclusion of the locking means 42 between the adjuster plate 38 and the strut 24 provides a lock which withstands large forces before the relative positioning of the adjuster plate 38 to the strut 24 will slip. The angle between the adjuster plate 38 and the strut 24 cannot change unless the cam lobe 66 is rotated within the slot 64. By utilizing a retaining plate 82, the friction between the locking means 42 and the adjuster plate 38 is at a location where the moment arm of the friction which resists any twisting of the cam lobe 66 is larger than the moment arm of any force tending to rotate the cam lobe 66. As such, there is a mechanical advantage present, which ensures that the adjuster plate 38 cannot pivot relative to the strut 24 when the cam pin 72 is tightened. The locking means 42 can work without the retaining plate 82 based on friction between the cam lobe 66 and the strut 24. However, the common outer diameter between the resulting friction location and the cam lobe 66 does not benefit from a mechanical advantage.

[0023] The locking means 42 comprising the cam lobe 66, retaining plate 82, and cam pin 72 is preferably made by forming the cam lobe 66 and retaining plate 82 from a single piece of weldable material. A lobe aperture 94 is drilled through the combined cam lobe 66 and retaining plate 82, and a cam pin 72 is passed through this aperture 94. The cam pin 72 is then welded to the combined cam lobe 66 and retaining plate 82. This provides a hexagonal head 96 which extends from the retaining plate 82, allowing a user to apply a wrench (not shown) to the hexagonal head 96 and, when the cam pin 72 and adjuster plate pivot 56 are loose, to adjust the position of the adjuster plate 38 by turning the hexagonal head 96.

[0024] Although the embodiment disclosed above places the slot 64 in the adjuster plate 38, and between the wheel stud 52 and the pivotable connection 40, this location is not essential. The slot 64 can be located in the strut 24, and/or opposite from the wheel stud 52 relative to the pivotable connection 40.

[0025] The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.