192 clutches and power-stop control – Clutches – Axially engaging
Reexamination Certificate
1999-03-03
2001-03-27
Bonck, Rodney H. (Department: 3681)
192 clutches and power-stop control
Clutches
Axially engaging
C192S09300C
Reexamination Certificate
active
06206162
ABSTRACT:
The present invention relates to a clutch mechanism for a power tool and in particular, to a clutch mechanism for a chain saw.
A chain saw comprises a cutting chain which is driven around a chain bar by a motor. The motor can be either an internal combustion engine or an electric motor. The chain saw is supported by an operator in use by two handles, a first rear handle located at the rear of the main body of the chain saw and a second bail handle located on the side of the main body of the chain saw.
However, the operation of a chain saw to cut material can be hazardous. One well known hazard is that generally referred to as “kick back”. During use, the moving chain cuts through the material. However, the chain can become snagged or caught on the material thus preventing the chain from moving relative to the material. This results in the chain saw being thrown by the force of the motor trying to drive the chain, upwards towards the head of the operator which is known as “kick back”. It is therefore desirable to stop the chain as soon as possible when “kick back” occurs.
Current European safety standards require that the chain on a chain saw has to stop within a pre-determined period when “kick-back” occurs. The stopping process is commonly triggered by the forward pivotal movement of a pivotal handle guard located in front of the bail handle. The handle guard is configured so that the forward pivotal movement of the handle guard interacts with the chain driving mechanism in order to stop the chain. Pivotal movement of the handle guard can be caused by the back of the hand of the operator, holding the bail handle, hitting the handle guard as the chain saw “kicks back”. Alternatively, the handle guard can be designed so that it has a sufficient moment of inertia to cause it to pivot forward when the chain saw “kicks back”.
Chain saws are commonly constructed so that the chain is driven by the motor via a clutch. When the pivotal handle is pivoted forward due to “kick-back”, the movement of the pivotal handle guard interacts with the clutch in order to disengage the clutch allowing the chain to run to a halt due to friction generated between the chain and the chain bar. A known type of such a clutch mechanisms that of a dog clutch. However, known designs of dog clutch are complicated. They are often difficult to machine and therefore expensive to manufacture. Furthermore, they are bulky, taking up substantial and valuable space within the chain saw.
With a large number of such designs of clutch, braking mechanisms are needed in order to assist in the stopping of the chain and therefore reduce the time taken for the chain saw to stop. A common type of brake is that of a band brake. However, the addition of such brakes further complicates the design increasing costs and size.
It is an object of the present invention to provide a simple but reliable design of dog clutch which overcomes or at least mitigates the above problems.
According to a first aspect of the present invention, there is provided a dog clutch for a power tool comprising a first gear mounted on and attached to a rotatable spindle, a second gear rotatably mounted adjacent the first gear, and an engaging mechanism characterised in that the second gear is rotatably mounted on and axially slidable along the spindle and is biased towards the first gear, and the engaging mechanism is rotatably mounted adjacent to the second gear about and axially slidable along the longitudinal axis of the spindle and is configured so that rotational movement of the engaging mechanism translates into an axial sliding movement of the engaging mechanism wherein rotational movement of the engaging mechanism moves the second gear into and out of a driving engagement with the first gear depending on the direction of rotation.
This provides a compact, simple and relatively small design of dog clutch which is reliable in operation. Furthermore, due to its simplicity, the dog clutch is easy and cheap to manufacture. The design provides easy disengagement of the motor and chain, thus allowing the chain to stop quickly. Therefore, additional braking is avoided which keeps the design simple. The design further provides easy engagement and disengagement requiring little physical effort by the user to operate the dog clutch.
Preferably, the engaging mechanism comprises a cam ring having a plurality of cam surfaces which co-operate with corresponding surfaces to translate rotational movement of the cam ring into an axial sliding movement of the cam ring.
Preferably the engaging mechanism is rotatably mounted on the second gear. By having the engaging mechanism and second gear concentrically mounted about the spindle the size of the clutch is further reduced and the design of clutch is further compacted. In addition, it provides a simple and easy way of mounting the engaging mechanism within the dog clutch.
The second gear can be biased towards the first gear by a helical spring.
Preferably the engaging mechanism is mounted in an axially slidable but non-rotatable fashion within a gear actuator which is capable of pivoting about the longitudinal axis of the spindle between a first position where the second gear drivingly engages the first gear and a second position where the second gear is disengaged from the first gear.
By mounting the engaging mechanism within the gear actuator so that it is able to axially slide within the gear actuator, but is unable to rotate relative to the gear actuator, the gear actuator only needs to move in a pivotal motion only to engage or disengage the dog clutch. Therefore, this permits a simple interconnection between the dog clutch and to separate switching mechanisms. Switch mechanisms are able to be connected to the gear actuator so that movement of the switching mechanisms pivotally move the gear actuator to engage or disengage the dog clutch. Furthermore, the design of dog clutch is kept compact by concentrically mounting the gear actuator about the engaging mechanism.
The gear actuator can be biased, for instance, by a helical spring, towards the second position, the biasing force being of a sufficient strength to overcome the biasing force which biases the second gear towards driving engagement with the first gear. This ensures that the dog clutch is biased towards disengagement.
In one particular design the first gear can comprise an outer peripheral surface which meshes with a series of ramped dogs on the second gear, when the second gear drivingly engages the first gear.
The use of ramped dogs on the second gear, allows the gears to continue to rotate relative to each other if the two gears are not aligned, because the ramped dogs are able to slide smoothly until they become aligned with the peripheral surface at which point the ramped dogs will drivingly mesh with the peripheral surface, thus preventing relative rotation movement between gears. This prevents damage to the gears during engagement.
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Riley Martyn
Stones Kevin
Black & Decker Inc.
Bonck Rodney H.
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