GB2287170A - Rotary lawnmowers - Google Patents

Abstract

In a lawnmower which comprises a chassis 2 a cutting blade 8 located at the end of a rotatable shaft 9 mounted to the chassis and adapted to extend perpendicularly to a surface of grass to be cut, and adapted to be rotated by a power unit 10, there is provided height adjustment means for automatically increasing the cutting height of the blade as the blade encounters increasing cutting resistance, the height adjustment means including a mounting (18) (Figure 2) for the shaft, the mounting being rotationally movable in guides (21) to rotationally lift the mounting with increasing cutting resistance above a predetermined limit in a rotational direction opposite the direction of rotation of the shaft and to rotationally lower the mounting with decreasing cutting resistance above the predetermined limit, with the mounting being biased towards its lower position. <IMAGE>

Description

LAWNMOWER INTRODUCTION TO THE INVENTION This invention relates to lawnmowers of the type having a blade rotating about an axis which extends perpendicularly to the lawn or grass to be cut.

BACKGROUND TO THE INVENTION Grass lawns are often of an uneven nature with grass level, grass density and underlying earth levels varying. When cutting grass with a lawnmower under these conditions the power plant of the lawnmower has to provide sufficient power at all times to overcome the load variations caused by the differing cutting conditions. Specifically when encountering a hummock or convex irregularity in the grass surface the mower motor may stall as the power available is unable to cut through the irregularity due to it's density. The reason for the increased load and uneven grass cut achieved when encountering a hummock (convex irregularity) is that the wheels of the mower may straddle the bulge in the lawn which results in the bulge intruding into the mower housing thus effectively reducing the cutting height of the lawnmower and even causing the cutting blade to impinge on the earth under the lawn. In the alternative, if the mower does cut through the grass under these conditions, the grass is often cut too low down to the grass root zone leaving a white patch on the lawn.

A common type of lawnmower consists of a housing which encloses the cutting blade, a power source mounted on top of the housing and centralised so that a blade mounted on the motor shaft will rotate within the housing in a horizontal plane. The mower may or may not have wheels and may or may not have a grass catcher attached to it to collect the grass clippings and is equipped with a pushing handle. The motor is normally rigidly attached to the housing.

Further, there is a device to adjust the cutting height of the lawnmower. This is achieved by a device attached to the wheels so that the wheels are raised or lowered relative to the housing and thus the mower blade, causing a change in the mower cutting height. Once set this cutting height is fixed unless the mowing operation is stopped and the cutting height is adjusted. The disadvantage of this system is that the cutting height of the mower is generally set by trial and error to the lowest possible cutting height so that when moving the motor will not be slowed down by the load and stall. This results, nevertheless, in the motor becoming overloaded when localised irregularities are encountered.

An object of this invention is to provide a lawnmower which caters for the problems described above.

SUNNARY OF THE INVENTION According to the invention a lawnmower comprises a chassis; a cutting blade located at the end of a rotatable shaft mounted to the chassis and adapted to extend perpendicularly to a surface of grass to be cut, and adapted to be rotated by a power unit; and height adjustment means for automatically increasing the cutting height of the blade as the blade encounters increasing cutting resistance.

Further, according to the invention, the height adjustment means includes a mounting for the shaft, the mounting being rotationally movable in guides to rotationally lift the mounting with increasing cutting resistance above a predetermined limit in a rotational direction opposite the direction of rotation of the shaft and to rotationally lower the mounting with decreasing cutting resistance above the predetermined limit, with the mounting being biased towards its lower position.

Still further, according to the invention, the height adjustment means includes a locking means for locking the adjustment means in a rotation position.

Still further, according to the invention, the shaft is the shaft of a power unit and the mounting is part of a housing for the power unit.

Still further, according to the invention, the power unit is an internal combustion engine or an electric motor.

Still further, according to the invention, the mounting is biased by the weight of the power unit.

Still further, according to the invention, the chassis includes a housing for the blade and the housing includes a grass discharge chute.

Still further, according to the invention, the chassis and the housing are provided by a single unit.

Still further, according to the invention, the chassis is a wheeled chassis and includes height adjustment for adjusting the height of the chassis with respect to a grassed surface to be mowed.

Still further, according to the invention, the lawnmower includes a handle for manual pushing and control.

According to a further aspect of the invention, a lawnmower comprises: a chassis; a cutting blade located at the end of a rotatable shaft mounted to the chassis and adapted to extend perpendicularly to a surface of grass to be cut, and adapted to be rotated by a power unit; and height adjustment means including a mounting for the shaft, the mounting being rotationally movable in guides to rotationally lift and lower the mounting.

Further, according to this aspect of the invention, the height adjustment means includes locking means for locking the mounting in a rotation position.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention described by way of example only follows with reference to the accompanying sketches in which: Figure 1 is a partial sectional side elevation of a lawnmower; Figure 2 is an enlarged sectional elevation of a mounting for the lawnmower shaft and motor; and Figure 3 is a plan of the lawnmower.

DETAILED DESCRIPTION OF THE DRAWINGS In this embodiment of the invention a household type of lawnmower (1) having a substantially horizontal rotating blade is provided.

The lawnmower (1) has a metal housing (2) which functions both as a housing and as a chassis. The housing (2) is open at the bottom and includes an integrally formed discharge chute (3) at the rear thereof.

In known manner the housing (2) includes four wheels (4), two at the front (5), and two at the rear (6).

The wheels are mounted to the housing and include a height adjusting mechanism (not shown) to adjust the height of the housing with respect to a lawn to be mown. Such mechanisms are well known in currently available lawn mowers.

At the rear (6) of the housing is a handle (not shown) mounted to pivotal brackets (7).

An elongated blade (8) is mounted at its centre in known manner to the end of a shaft (9) of an electric motor (10). The electric motor (10) rotates the shaft in a clockwise direction "X" when the lawnmower is viewed from above as in Figure 3.

The electric motor (10) and hence the shaft (9) are mounted to the housing (2) as described below.

A motor housing (11) having an upper part (12) and a lower part (13) is provided.

The lower part (13) includes a generally dish-shaped portion (14) having a central aperture (15) through which the shaft (9) can project and an outwardly extending peripheral flange (16). The dish-shaped portion (14) passes through a circular aperture in the top of the housing (2) and the flange (16) is secured to the housing by means of bolting.

From the flange (16) is an upstanding peripheral skirt (17) which has a locating formation for the top portion (12) of the housing which is in the form of a cover.

Fixed to the flange (16) is an annular mounting member (18). This mounting member (18) includes a flat portion (19) which is adapted to be fixed to the flange (16) and an upstanding guide member (20).

In the upstanding guide member (20) are three equally spaced guides (21) in the form of slots extending at an acute angle of about 20"to the flat portion (19). The higher ends of the slots is spaced from the lower ends in a counter clockwise direction.

Located in the slots are followers (22) which are fixed to a support member (23) which, in turn, is fixed to a housing (24) for the motor (10).

The motor (10) is rotatable in the guides (21) between the limits set by the ends of the guides.

The weight of the motor and the angle of the guides is such that the weight of the motor biases it to a position in which the followers are located at the lowest part of the guides.

This bias is furthermore strong enough that when the lawnmower is operating in a normal cutting mode the motor and hence the blade rotated by it remains in its lowest position with the followers (22) at the bottom of the guides.

Should the blades encounter cutting resistance above a predetermined minimum the motor will tend to rotate in the opposite direction, that is, counterclockwise.

When the blade is rotating in clockwise direction "X" and when the cutting resistance becomes sufficient the rotational reaction of the motor will overcome the biassing weight of the motor and rotate the motor in a counterclockwise direction. The followers (22) will thus move up along the guides (21) thereby lifting the motor (10) and hence the blade (8).

With the lifting of the blade (8) less grass will be cut and the resistance to rotation of the blade will decrease until a new equilibrium will be reached with the biassing weight of the motor (10).

It will be appreciated that the lawnmower will cut a lawn in such a manner that the cutting blade will follow the contour of the lawn or alternatively when encountering thicker grass conditions will cut less grass (will leave a longer grass blade). The second effect results in a faster mowing of the lawn and fewer motor stalls. The speed of pushing the mower also can be increased as a faster pushing speed, which results in an increased motor load, results in an automatic reduction in the amount of grass being cut. In the event that the user finds this to be undesirable (the varying amount of grass cut) the lawnmower external height adjuster will be used to lift the mower to a higher minimum setting. The motor will then tend to remain at higher minimum setting. The motor will then tend to remain at it's lowest setting. It will be appreciated that the mower will operate at it's optimal power delivery in that if the external height adjuster is set slightly lower (that is, to cut more grass) than the motor can handle then the invention will result in the motor lifting away from the housing and consequently lifting the blade away from the surface to be cut. The motor will then operate at it's peak power delivery, that is, it will cut the grass as low as the power unit will allow.

Other embodiments are envisaged within the scope of the invention including other types, shapes and configurations of lawnmower. Particularly, it is envisaged that the height adjustment means described above could be an independent means to be adjusted by a user using a suitable lever for example. The lever would be lockable in a chosen rotational position of the lawnmower. Alternatively, the height could be adjusted by manually rotating the motor.

Claims (15)

1. a lawnmower comprising a chassis; a cutting blade located at the end of a rotatable shaft mounted to the chassis and adapted to extend perpendicularly to a surface of grass to be cut, and adapted to be rotated by a power unit; and height adjustment means for automatically increasing the cutting height of the blade as the blade encounters increasing cutting resistance.

2. A lawnmower as claimed in claim 1 in which the height adjustment means includes a mounting for the shaft, the mounting being rotationally movable in guides to rotationally lift the mounting with increasing cutting resistance above a predetermined limit in a rotational direction opposite the direction of rotation of the shaft and to rotationally lower the mounting with decreasing cutting resistance above the predetermined limit, with the mounting being biased towards its lower position.

3. A lawnmower as claimed in either of claims 1 or 2 in which the height adjustments means includes a locking means for locking the adjustment means in a rotation position.

4. A lawnmower as claimed in any of the preceding claims in which the shaft is the shaft of a power unit.

5. A lawnmower as claimed in claim 4 in which the mounting is part of a housing for the power unit.

6. A lawnmower as claimed in any of the preceding claims in which the power unit is one of an internal combustion engine and an electric motor.

7. A lawnmower as claimed in claim 2 in which the mounting is biased by the weight of the power unit.

8. A lawnmower as claimed in any of the preceding claims in which the chassis includes a housing for the blade.

9. A lawnmower as claimed in claim 8 in which the housing includes a grass discharge chute.

10. A lawnmower as claimed in claim 8 in which the chassis and the housing are provided by a single unit.

11. A lawnmower as claimed in any of the preceding claims in which the chassis is a wheeled chassis and includes height adjustment for adjusting the height of the chassis with respect to a grassed surface to be mowed.

12. A lawnmower as claimed in claim 11 in which the lawnmower includes a handle for manual pushing and control.

13. A lawnmower comprising: a chassis; a cutting blade located at the end of a rotatable shaft mounted to the chassis and adapted to extend perpendicularly to a surface of grass to be cut, and adapted to be rotated by a power unit; and height adjustment means including a mounting for the shaft, the mounting being rotationally movable in guides to rotationally lift and lower the mounting.

14. A lawnmower as claimed in claim 13 in which the height adjustment means includes locking means for locking the mounting in a rotation position.

15. A lawnmower substantially as herein described with reference to the accompanying drawings.