Patent Publication Number: US-6908277-B2

Title: Speed governor for a pneumatic rotation motor

Description:
This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/SE01/02834 filed Dec. 20, 2001. 

   The invention relates to a speed governor for a rotation motor having a rotor and a stator with a pressure air inlet passage, wherein the governor comprises a flow controlling inlet valve with a valve element disposed in the inlet passage, and a speed responsive activating device operatively connected to the inlet valve. 
   In particular, the invention concerns an improved speed governor suitable for fast rotating motors where conventional type speed governors are difficult to use due to high centrifugal forces and occurring balancing problems. Conventional speed governors are also mechanically rather complicated and are difficult to make operate in a safe way unless they are of a certain size. This, in turn, makes it difficult to obtain an accurate and safe speed governor operation at small size motors, for instance in hand held power tools, where the available space for a speed governor is very limited. 
   The above mentioned problems are avoided by a governor according to the invention wherein a pressure air inlet valve is controlled by an electrically governed pneumatic pilot circuit which includes an electromagnet actuated bleed-off valve which is activated by a motor speed responsive output voltage of a motor driven generator. 
   Further characteristic features and advantages of the invention will appear from the following specification and claims. 

   
     A preferred embodiment of the invention is described below in detail with reference to the accompanying drawings. 
     On the drawings 
       FIG. 1  shows a schematic illustration of the speed governor according to the invention. 
       FIG. 2  shows, on a larger scale, the inlet valve and air bleed-off valve according to an alternative embodiment of the invention. 
   

   The device illustrated in  FIG. 1  comprises a pneumatically driven axial flow turbine having a single-stage turbine wheel  11  with a circumferential array of drive blades  12 , and a stator  10  provided with a number of air nozzles  13  for directing a motive pressure air flow onto the drive blades  12  of the turbine wheel  11 . 
   Pressure air is supplied to the nozzles  13  from a pressure air source  14  via an inlet passage  15  and a flow control valve  16 . The latter comprises a valve element  18  which is movably guided against a spring  19  in a valve housing  17  and disposed in the inlet passage  15  for controlling the air supply to the nozzles  13 . To this end, there is provided a pilot circuit comprising an activating surface  20  on the rear end of the valve element  11 , a small area opening  21  through the valve element  18  connecting the activating surface  20  with the air inlet passage upstream of the valve  16 . The valve element  18  is arranged to be balanced between the air pressure in the inlet passage  15  and the actual pressure acting on the activating surface  20 . 
   The pilot circuit also comprises a bleed-off valve  24  which is connected to the activating surface  20  of the flow control valve  16  and arranged to open up or close, respectively, an air bled-off opening to the atmosphere. The bleed-off valve  24  comprises a small size aperture  25  and a ferrous disc shaped valve element  26  which is operated by an electro-magnet  27  to alternatively open and close the aperture  25 . The electro-magnet  27  is operated by an electric voltage delivered by a turbine integrated generator  30  via a rectifier  31 . 
   The generator  30  comprises a permanent magnet  32  rigidly secured to a rear extension  33  of the turbine wheel  11 , and a stator including a toroid shaped winding coil  34  surrounding the rear extension  33  and the magnet  32 . The output voltage from the winding coil  34  is an alternating current which via the rectifier  31  is transformed to a direct current voltage. The magnitude of the voltage delivered by the generator  30  is directly proportional to the rotation speed of the turbine wheel  11 , which means that the activating force developed by the electro-magnet  27  is proportional to the turbine speed as well. The result is that a low turbine speed and, hence, a low output voltage from the generator  30  makes the electro-magnet  27  generate a low lifting force on the disc shaped valve element  26  such that a relatively large bleed-off area is left open. This results in a decreased air pressure acting on the activating surface  20  such that the flow control valve  16  will leave a relatively large air flow area open and, thereby, let through a relatively large air flow to the turbine nozzles  13 . 
   If, however, the turbine speed should increase above a predetermined desired level, the magnitude of the output voltage from the generator  30  will exceed a certain level which will make the electro-magnet  27  lift the valve element  26  to an air bleed-off restricting position, thereby accomplishing an increased pressure acting on the activation surface  20  of the valve element  18 . As a result, the flow control valve  16  will restrict the air supply to the turbine nozzles  13 , and the turbine speed will be limited to the predetermined desired level. 
   In  FIG. 2  there is shown an alternative design of an air inlet flow control valve  56  which is built together with the electro-magnet controlled air bleed-off valve  54 . As in the previously described example, the flow control valve  56  comprises a valve element  58  which is movably supported in a valve housing  57  and arranged to control the air flow through the inlet passage  15 . In this embodiment of the invention, the valve element  58  is cup-shaped and has an outer waist portion  60  and a number of lateral apertures  61 . The waist portion  60  is arranged to communicate air passed an annular shoulder  63  in the valve housing  57 , whereas the apertures  61  are arranged to connect the inside with outside of the cup-shaped valve element  58 . In the closed position of the flow control valve  56 , the outer end of the cup-shaped valve element  58  engages sealingly a seat  64  in the valve housing  57 , and the shoulder  63  prevents the waist portion  60  from communicating pressure air from the upstream part  15 ′ of the inlet passage  15  to the downstream part  15 ″. 
   At its inner end, the valve element  58  is secured to a bellow  68  which encloses an air chamber  69  and an activation surface  70  on which an activation force is applicable on the valve element  58 . The chamber  69  communicates constantly with the inside of the cup-shaped valve element  58  via a restricted opening  72 . Since the upstream part  15 ′ of the air inlet passage  15  is connected to the pressure air source, the inside area of the valve element  58  as well as the chamber  69  are constantly pressurized. 
   At the lower end of the valve housing  57 , there is mounted a electro-magnet  75  with a winding coil  76  connected to the generator  30 . Through the winding coil  76  there extends a central tube with an air passage  77  which at its upper end communicates with the chamber  69  and which at its lower end is provided with a restricted air bleed-off opening  78 . The bleed-off opening  78  is controlled by a disc-shaped valve element  79  which is freely movable in a narrow space between the electro-magnet  75  and a cover plate  80 . This narrow space communicates with the atmosphere via a radial exhaust opening  81 . 
   In operation, the actual magnitude of the voltage delivered by the generator  30  determines to what extent the electro-magnet  75  shall make the valve element  79  cover or uncover, respectively, the bleed-off opening  78  to thereby adapt the air pressure in the chamber  69  and, hence, the activation force on the valve element  58 . At too a low turbine speed the voltage magnitude is below a determined certain level, which results in a low lift force from the electro-magnet  75  on the valve element  79 . The result is a relatively unrestricted bleed-off flow through the opening  78 , and due to the restricted area of the opening  72 , a large bleed-off flow from the chamber  69  results in a lowered activating force on the valve element  58 , which means that the latter is displaced in its opening direction providing a larger air inlet flow to the turbine. 
   In its open condition, the valve element  58  of the flow control valve  56  occupies a position wherein pressure air is able to pass the valve in two ways, namely A) from the upstream part  15 ′ of the inlet passage  15  to the inside the valve element  58 , through the apertures  61 , out through the outer open end of the valve element  58  past the seat  64  and into the downstream part  15 ″ of the inlet passage  15 , and B) from the upstream part  15 ′ of the inlet passage  15 , through the waist portion  60  past the shoulder  63  and into the downstream part  15 ″ of the inlet passage  15 .