Cutlery – Cutting tools – With blade moving means
Reexamination Certificate
1999-11-10
2001-10-16
Payer, Hwei-Slu (Department: 3724)
Cutlery
Cutting tools
With blade moving means
C056S012700
Reexamination Certificate
active
06301788
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a battery powered vegetation trimmer of the type in which a rotating line, string or wire (hereafter referred to as line) is used to cut vegetation, particularly grass.
BACKGROUND OF THE INVENTION
In known battery powered vegetation trimmers of this type, a battery pack powers an electric motor. A rotary output of the electric motor directly drives a line support structure in a rotatable manner. The line is supported by the line support structure so that an end section of the line extends from the structure. As the structure rotates, the end section of the line is rotated and will cut any light vegetation or grass which comes into the path of the line.
A vegetation trimmer is designed so that the weight of the trimmer is supported by the user. The height, orientation and direction of movement of the cutting head of the trimmer, when in use, is determined by the way in which the user holds and moves the trimmer. As a vegetation trimmer has to be supported by the user, it is important to keep the weight-of the trimmer to a minimum. This is achieved by minimising the weight of the various component parts of the vegetation trimmer, for example, the weight of the battery or motor.
A problem with known battery powered vegetation trimmers is achieving an acceptable run time before the battery requires recharging. In order to achieve acceptable run times large (and thus heavy) batteries or relatively expensive batteries have to be used, neither of which are ideal. Therefore, when designing a battery powered vegetation trimmer, there is always a balance between run time and either weight or expense.
One way of overcoming this problem is to use two interchangeable batteries to power a vegetation trimmer. When one of the batteries becomes discharged through use on the trimmer it is replaced by a second battery, which itself can be replaced by a third battery or by the first battery which is recharged while the second is in use. The disadvantage to this solution is that at least two batteries must be purchased by a user, which adds cost. Also, replacement of the batteries interrupts the use of the trimmer.
SUMMARY OF THE INVENTION
The present invention overcomes the above problems by providing a battery powered vegetation trimmer which provides acceptable run times without requiring either expensive, heavy or multiple batteries.
According to the present invention there is provided a hand held battery powered vegetation trimmer which in use is substantially supported by a user above the ground in a cutting position, the trimmer comprising;
a battery pack which powers an electric motor to generate a rotary output, and
a cutting line which is adapted to be rotatably driven by a line support structure,
characterised in that;
the rotary output drives the line support structure via a reduction gear stage.
By introducing a reduction gear stage between the motor output and the line support structure the speed at which the line is rotated is reduced which substantially reduces the power consumption of the trimmer without reducing the cutting performance of the line. This is a surprising result as cutting performance would be expected to drop as the speed of rotation of the line is reduced. Power consumption is reduced by such an extent by using a reduction gear stage that a less powerful motor may be used in the trimmer and still give an acceptable cutting performance. Use of such a motor will reduce the size, weight and cost of the vegetation trimmer. Ideally, the weight of the motor is less than 1 kg and preferably less than 0.5 Kg. This enables the size and weight of the trimmer to be kept within reasonable proportions in order for the users to support and operate the trimmer.
The large reductions in power consumption are achieved mainly by a reduction in the aerodynamic energy losses because of the reduction in the speed of rotation of the line.
As a further added advantage the increased torque with which the line support structure rotates the line, because of the use of a reduction gear stage, allows the trimmer to cope in difficult cutting conditions. The trimmer according to the present invention does not slow down so much when cutting tough or heavy vegetation and is more difficult to stall.
Preferably, the reduction ratio is between a 2:1 reduction and a 4:1 reduction. This range of reduction ratios gives a good reduction in power consumption while not reducing the line speed so much that cutting performance of the line is effected. More preferably, the reduction ratio is between 2.5:1 and 3:1.
The gear reduction stage ideally comprises a pair of intermeshing gears. Such gear arrangements include but are not limited to epicyclic, parallel axis and bevel gear arrangements. Alternatively, the gear reduction stage may comprise a belt drive arrangement.
Good cutting performance at low power consumption can be achieved when the speed of rotation of the line support structure is preferably between 5,000 and 9000 rpm or more preferably between 6,000 and 9000 rpm, or ideally between 7,000 and 8,500 rpm when there is no cutting load on the trimmer.
When a reduction gear stage is used between the motor and the line support structure, as compared to an equivalent trimmer in which the line support structure is directly driven by the motor, the motor will run much faster. This results in the kinetic energy of the motor being much higher and thus the total mechanical energy of the system available to cut is higher which increases the stall torque and makes the trimmer harder to stall. Thus, the speed of rotation of the motor when there is no cutting load on the trimmer is preferably between 10,000 rpm and 28,000 rpm or more preferably between 13,000 and 28,000 rpm and ideally between 19,000 and 23,000 rpm.
The batteries used in the trimmer may be Nickel/Cadmium batteries which are robust, have a high discharge current capability, have a longer life and a low weight. Alternatively, Lead/Acid batteries may be used as they are cheap and provide a good improvement in trimmer run time for reductions in power consumption in the ranges achieved by the introduction of a gear reduction stage on a direct drive cordless vegetation line trimmer.
The vegetation trimmer may be arranged with a handle portion located at a first end of a support tube and a cutting head, which houses the line support structure, located at a second opposite end of the support tube. The battery pack may be releasably attachable to the handle portion in order to allow charging of the battery with the battery either attached to the trimmer (on-board charging) or detached from the trimmer (off-board charging). The electric motor is housed in the cutting head and drives the line support structure also located in the cutting head via the reduction gear stage.
The battery powered vegetation trimmer may have a support guide. The support guide can take the form of a skid bar or wheel located at the back of the cutting head of the trimmer. It can assist the user in directing and positioning the cutting head of the trimmer when carrying out certain tasks such as edging.
The battery powered vegetation trimmer may have a 30.5 cm (12 inches) cutting swathe. The power required to drive the cutting line of a string trimmer is dependent on the length of the line which extends from the line support structure. The introduction of a reduction gear stage between the motor output and the line support structure reduces the power consumption of the string trimmer whilst increasing the torque by which the line support structure rotates the line. Therefore, by using a gear reduction stage, it is possible to have a battery powered string trimmer which has a cutting line which extends further from the line support structure and hence has a larger cutting swathe, than it is possible with battery powered string trimmers without a gear reduction stage whilst still having an acceptable cutting performance. By having a larger cutting swathe, the string trimmer is able to cut a larger area in each sweep.
REFERENCES:
pat
Black & Decker Inc.
Harness & Dickey & Pierce P.L.C.
Payer Hwei-Slu
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