Patent Publication Number: US-8528659-B2

Title: Pneumatic power tool with exhaust silencer

Description:
This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/SE2006/00393 filed Apr. 3, 2006. 
     FIELD OF THE INVENTION 
     This invention relates to a pneumatic power tool having a housing and a pressure air driven rotation motor which is connected to a pressure air inlet passage and via an exhaust air outlet passage to an outlet opening means in the housing, and a speed governor connected to the motor and having a valve element arranged to control the air flow through the inlet passage. 
     BACKGROUND OF THE INVENTION 
     A problem concerned with power tools of this type is the considerable exhaust noise from the motor. This is particularly annoying at idle running when no process noise is created. For instance in pneumatic grinders the process noise created during grinding is very loud and dominates completely over the exhaust noise from the motor, which means that even during operation when the motor is delivering full power the exhaust noise from the motor is no problem. When, however, the motor is relieved from load the speed governor will automatically start choking the pressure air inlet flow so as to bring down the power output of the motor and hence limit the idle speed of the motor. Still there is a considerable noise emanating from the motor exhaust, and since there is no process noise present the motor exhaust noise will be dominant and cause an annoying noise level at the working site. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to create a pneumatic power tool wherein the exhaust noise from the motor at idle running is considerably reduced without having a negative influence on the full power output of the tool. 
     Further objects and advantages of the invention will appear from the following specification and claims. 
     According to an aspect of the invention, a power tool includes a housing, a pneumatic rotation motor, an air inlet passage for ducting motive pressure air to the motor, an exhaust air outlet passage connecting the motor to an exhaust air discharging means in the housing, and a speed governor connected to the motor and arranged to control the air flow through the inlet passage. The outlet passage includes a first outlet duct and a second outlet duct extending in parallel with each other between the motor and the exhaust air discharging means. The first outlet duct continuously connects the motor to the exhaust air discharging means. An exhaust valve is arranged to control the air flow through the second outlet duct by being shiftable between a closed position and an open position. The exhaust valve is spring biased towards the closed position. And an activation means is arranged to shift the exhaust valve from the closed position to the open position at air pressure levels above a certain pressure level in the air inlet passage downstream of the speed governor. 
     A preferred embodiment of the invention is described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a longitudinal section through a power tool according to the invention. 
         FIG. 2A  shows, on a larger scale, a fractional section of the power tool in  FIG. 1  illustrating a full power condition of the tool. 
         FIG. 2B  shows a section similar to  FIG. 2A , but illustrating the tool in an idle running condition. 
         FIG. 3  shows a cross section along line III-III in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The power tool shown in the drawings is a pneumatic angle grinder having a housing  10  with a handle  11  at its rear end and an angle drive  12  with an output shaft (not illustrated) at its forward end. The output shaft is intended to carry a grinding wheel, and a protective wheel guard  14  is secured to the front part of the housing  10 . In the housing  10  there is supported a pressure air driven rotation motor  13  which is drivingly connected to the output shaft via the angle drive  12  and which via a pressure air inlet passage  15  and a throttle valve  16  is supplied with motive pressure air. The throttle valve  16  is operable by a maneuver lever  17  which is pivotally supported on the handle  11  via a hinge  18 . 
     The motor  13  comprises a cylinder  21  and a rotor  22  which is journalled in bearings mounted in two opposite end walls and which is connected to a speed governor  25  for controlling the air supply to the motor  13  in response to the actual motor speed. Only the rear end wall  23  of the motor  13  is visible in the drawings. The speed governor  25  comprises a tubular valve element  26  which is arranged to be displaced by centrifugal force activated balls  27  into an air inlet flow restricting position to thereby limit the idle speed of the motor to a predetermined level. In its flow restricting position the valve element  26  co-operates with an annular seat  28  formed by a shoulder in the air inlet passage  15 . This shoulder is formed by a neck portion  29  on a bell-shaped insert  30  secured to the motor end wall  23  by dowels  31 . 
     The motor  13  has one or more air inlet ports (not illustrated) in the rear end wall  23  communicating with the air inlet passage  15 , and a number of exhaust ports  32  in the cylinder  21 . The exhaust ports  32  communicate with exhaust outlet openings  33  in the housing  10  via a first outlet duct  34 , a second outlet duct  35  and an exhaust chamber  36 . The exhaust chamber  36  and the outlet openings  33  form an exhaust discharging means, and the first and second outlet ducts  34 , 35  extend in parallel with each other from the exhaust ports  32  of the motor  13  to the exhaust chamber  36 . The first outlet duct  34  is located at one side of the housing  10 , whereas the second outlet duct  35  is annular in shape and surrounds the motor  13 . The first outlet duct  34  contains a filling  38  of a porous sound damping material. 
     At the downstream end of the second outlet duct  35  there is provided an exhaust valve  39  which comprises an annular valve element  40  movably guided on the insert  30 , a valve seat  41  formed on a wall element  42  (see  FIG. 2A ) in the housing  10 , and a spring  44  is arranged to bias the valve element  40  towards the seat  41 . As the valve element  40  co-operates with the seat  41  the second exhaust duct  35  is closed. The first exhaust duct  34 , though, is always open to communicate with exhaust chamber  36 . 
     The exhaust valve  39  further comprises an activation means in the form of a number of piston elements  45  movably guided in the insert  30 . At their one ends the piston elements  45  contact the valve element  40 , and by their opposite ends the piston-elements  45  extend into the inlet passage  15  at a point downstream of the speed governor valve element  26  to, thereby, be exposed to the air pressure in the inlet passage  15  at that point. The total end surface area of the piston elements  45  and the bias force of the spring  44  are chosen so as to maintain the valve element  40  in closed position as long as the pressure in the inlet passage  15  is low due to a closed or almost closed speed governor valve element  26  at idle speed condition of the motor. In other words, when the tool is relieved from a working load and the motor speed increases the speed governor  25  gets into action which means that the valve element  26  is urged by the balls  27  into co-operation with the seat  28  to restrict the pressure air inlet flow and limit the idle speed of the motor  13 . Thereby, the pressure downstream of the speed governor valve element  26  is reduced, which means that the force acting on the end surfaces of the piston elements  45  is reduced and will not be able to maintain the valve element  40  in the open position against the action of the spring  44 . Thereby, the valve element  40  is displaced by the spring  44  to its closed position in contact with the seat  41 . See  FIG. 3B . 
     Accordingly, in the idle running condition of the tool the exhaust valve element  40  occupies its closed position which means that the second outlet duct  35  is blocked and the exhaust air flow from the motor  13  can only reach the exhaust chamber  36  via the first outlet duct  34 . The exhaust flow through the first outlet duct  34  is restricted and damped by means of the filling  38  and will not create any annoying noise when finally leaving the tool housing  10 . 
     In the condition illustrated in  FIG. 2A  the power tool is working at full power output as a working load is applied on the output shaft. In this condition the motor speed is brought down below the predetermined level where the speed governor  25  is set to restrict the pressure air inlet flow. This means that the speed governor valve element  26  is in its rest position at a distance from the seat  28 , thereby leaving a full flow opening past the governor  25 . This also means that the air pressure downstream of the speed governor valve element  26  is high, thereby exerting a high enough pressure load on the piston elements  45  to make them move the exhaust valve element  40  to open position against the action of the spring  44 . 
     In this full power working condition of the tool both the first outlet duct  34  and the second outlet duct  35  are open to permit an unrestricted exhaust flow from the motor  13 . In this condition the exhaust air flow through the outlet openings  33  in the housing  10  will be rather noisy, but the noise of the ongoing working process is far higher and will dominate completely over the exhaust noise. 
     Accordingly, the invention suggests a pneumatic power tool which by a two-way exhaust passage, namely one constantly open and noise damped duct for idle running and another valve controlled duct open at full power operation only, provides a low noise idle running without having any power restricting effect at full power operation. 
     It is to be noted though that the embodiments of the invention are not limited to the above described example but can be freely varied within the scope of the claims. For instance, the activation means for the exhaust valve may be designed differently, i.e. the separate piston elements may be exchanged by a single annular piston.