Patent Publication Number: US-6666187-B2

Title: Device for adjustably limiting the engine speed of a hand tool

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a device for adjustably limiting the maximum speed of an engine powered hand tool whose speed is controlled by a power-regulating device located in the handle of the tool. 
     While using engine powered portable hand tools, it is convenient to allow the engine to run at a constant speed that is less than the engine&#39;s maximum speed. By running the engine at a constant speed, the performance of the tool is improved and the load and wear on the engine is reduced. Operator comfort is also increased when the engine is run at a constant speed. 
     Hand held tools with engine speed limiting devices are available as are hand tools with handles that contain several different buttons to control different functions of the tool. For example, a handle with buttons for steering of the tool is illustrated in EP0968641. The handle described in EP0968641 has a power regulating device and a safety button, but the handle as described in EP0968641 does not allow the operator to limit the speed of the engine in order to make it possible for the operator to run the engine at a constant speed that is less than the maximum speed of the engine. 
     During use of the hand tool, is it desirable to increase the speed of the engine above the selected speed limit. This could, for example, be required when the amount of grass or bushes to be trimmed by the operator are higher than normal in a concentrated area. In these cases, it is not effective to limit the speed of the engine, and it is necessary to increase the speed of the engine to the maximum speed available to enable the operator to cut the concentrated area. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention achieves an increased speed for the engine above a selected speed limit. Under normal operation, an adjustable speed limiting device will stop the power-regulating device at the selected speed. However, if the operator needs to increase the speed of the engine above the selected limit, the operator can accomplish this by exerting an additional force on the power-regulating device. The adjustable speed limiting device will then be deactivated and the speed of the engine will be allowed to increase. The operator may then increase the speed of the engine up to the maximum engine speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described with reference to the accompanying figures in which: 
     FIG. 1 illustrates a device for adjustably limiting the maximum speed of an engine powered hand tool including an adjustable wheel located inside the handle of the tool for setting the engine speed; 
     FIG. 2 illustrates a cross section of the tool handle showing the speed limiting device not activated; 
     FIG. 3 illustrates a cross section of the tool handle with the speed limiting device activated and in a maximized position; 
     FIG. 4 illustrates a cross section of the tool handle with the speed limitation device activated and the power regulating device stopped at a selected speed limiting position; and 
     FIG. 5 illustrates a cross section of the tool handle including a device that prevents unintentional rotation of the wheel. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The device, as shown in FIG. 1, is incorporated into a hand tool handle that contains a casing  10  that surrounds the different parts that are needed in order to control and steer the different functions of the tool. The handle comprises a power-regulating device  11 , a button for adjusting the limitation of speed for the engine and a control that stops the engine. Inside the casing  10  are all the different parts that are needed to steer the different functions of the tool and connect the buttons on the handle with the appropriate part of the tool. 
     Referring to FIG. 2, the button for adjusting the limitation level for the power regulating device  11  is shaped like a wheel  13  and placed in the top of the handle where it is easy for the operator to reach it with their thumb. The wheel  13  is attached to the handle so that the wheel  13  can be rotated, but not moved in an axial direction relative the casing  10 . In the center of the wheel  13 , a nut  12  is placed. The nut  12  is provided with either two protruding parts or two grooves, one on each side of the nut, running in the axial direction of the nut  12 . The inside of the wheel  13  that is in contact with the nut  12  is provided with corresponding protruding parts or grooves, such that the nut can slide in an axial direction in the center of the wheel  13 . The protruding parts will steer the nut  12  in the wheel  13 . 
     The nut  12  is placed on a threaded shaft  15  and shaped such that a spring  14 , for example a helical spring, placed around the threaded shaft  15  is in contact with one side of the nut  12 . The nut  12  is provided with an opening that matches the diameter of the spring  14 . One end of the spring  14  is fastened in the nut  12  and the other end to support  16  located on the inside of casing  10 . The threaded shaft  15  is attached to the casing  10  in such a way that it is movable in an axial direction and within a selected range. 
     As shown in FIG. 3, the threaded shaft  15  is at the end of its range and as close as possible to the inside of the casing  10 . In this position, the threaded shaft  15  is an obstacle that stops the power regulating device  11  from being pushed any further inward, limiting the operator&#39;s ability to increase the speed of the engine to its maximum speed. 
     Referring now to FIG. 4, because of the threads on the shaft, when the wheel  13  and the nut  12  are rotated, the threaded shaft  15  moves in an axial direction in nut  12 . This movement of the threaded shaft  15  results in the shaft blocking the power-regulating device  11  at a selected point from further movement. If the operator wants to increase the speed over this selected level, the operator must press the power regulating device  111  with a force that exceeds the force of spring  14  that supports nut  12 . By doing so, nut  12  will slide in wheel  13  and threaded shaft  15  will move backward towards the inside of casing  10  and the speed of the engine will allowed to increase above the selected level and to its maximum level. 
     As shown in FIG. 5, to prevent wheel  13  and nut  12  from unintended rotation, a device  17  is attached in casing  10  and is in contact with a portion of the outer surface of wheel  13 . The device  17  retains wheel  13  in a selected position such that there will be no unintended change to the selected speed setting.