Patent Publication Number: US-7722323-B2

Title: Axial flow turbine with overspeed preventing device

Description:
This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/SE2005/001757 filed Nov. 23, 2005. 
   FIELD OF THE INVENTION 
   The invention relates to an axial flow turbine motor of the type including a rotor provided with one or more peripheral rows of drive blades and a peripheral band-shaped wall element radially clamped against the tops of the drive blades, and a stator provided with inlet nozzles for a motive pressure medium and having an annular inner surface surrounding the rotor. 
   BACKGROUND OF THE INVENTION 
   A problem concerned with turbine motors is the difficulty to accomplish a simple yet reliable overspeed preventing safety device by which the rotor is prevented from reaching hazardous high speed levels where damage to equipment and operator injuries may be caused. Normally a safe and suitable speed is provided by a speed governor of any suitable type, but in case of a speed governor malfunction there has to be an independently operating overspeed preventing device fitted to protect the operator and other equipment from damage and injury. Since turbine motors operate at very high speed levels it is very difficult to design a reliable and safely operating overspeed preventing device of the type comprising a mechanical speed responsive actuator and a pressure medium shut-off valve. Such an actuator/valve arrangement is also rather complex in design and costly to manufacture as it contains many parts which require expensive machining. 
   OBJECT OF THE INVENTION 
   It is the main object of the invention to create an axial flow turbine motor with a simple and quite cheap yet effective overspeed preventing safety device. 
   Further objects and advantages of the invention will appear from the following specification and claims. 
   Preferred embodiments of the invention are described below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawing: 
       FIG. 1  shows a fractional view of a pneumatic turbine having an overspeed protecting device according to the invention. 
       FIG. 2  shows a perspective view of a drive blade covering band according to the invention. 
       FIG. 3  shows a diagram illustrating the strain in the drive blade covering band in relation to rotor speed. 
   

   DETAILED DESCRIPTION 
   In  FIGS. 1 and 2  there is shown an example illustrating the invention. Accordingly, the drawing figures show a pneumatic single stage turbine including a housing  10  with at least one pressure air nozzle  11 , and a rotor  12  journalled in the housing  10  and comprising a single circumferential row of drive blades  13 . The housing  10  is formed with an annular inner surface  14  which surrounds the rotor  12  and the rotor blades  13 , and the air nozzle  11  is arranged to direct a substantially axial flow of pressure air onto the drive blades  13  at an inlet end of the latters to thereby drive the rotor  12 . The pressure air leaves the blades  13  at an outlet end thereof at a lower pressure. 
   The turbine motor according to the invention is equipped with a non-illustrated speed governor of any suitable type to keep the rotor speed at a desired level. This is essential for example in grinder applications where the rotation speed of a grinding tool shall be kept at a certain level for obtaining the most efficient grinding operation and the longest possible service life of the grinding tool. In such applications it also very important that the rotor speed is safely prevented from reaching any higher speed levels, since there will be a risk of grinding tool disintegration or explosion. In order to ensure that such high speed levels will not be reached, even if the speed governor should malfunction, the turbine has to be provided with an independently operating overspeed preventing device which will come into action only at speed levels well above the speed level normally ensured by the speed governor. 
   On the periphery of the rotor  12 , i.e. on tops of the drive blades  13 , there is mounted an endless cover band  16  forming a wall element around the drive blades  13 . One purpose of this band  16  is to accomplish a favourable air flow through the rotor blades  13  to enhance the efficiency of the turbine. In order to obtain an expanding air flow through the drive blades  13  the latters are formed with a larger radial extent at their outlet ends, which means that the cover band  16  has a larger inner diameter at the outlet end of the drive blades  13 . Accordingly, the cover band  16  has a large diameter portion  19  at the outlet ends of the drive blades  13  and a reduced diameter portion  20  at the inlet ends of the drive blades  13 . On the outside of the reduced diameter portion  20  On its outside the cover band  16  is formed with a number of circumferential ribs  17  for reducing air leakage around the rotor  12 . 
   According to the invention, the band  16  is intended to form an overspeed protective device by being formed of a suitable material and having suitable dimensions to yield under centrifugal action at rotor speed levels exceeding that normally provided by the speed governor. When a too high speed level is reached the band  16  will yield and expand in such a way that it looses its contact with the drive blades  13 . Then, the band  16  is displaced by the pressure difference across the drive blades  13  in the direction of the air flow such that it will get jammed between the annular surface  14  of the housing  10  and the drive blades  13 . Thereby, the band  16  will generate a braking force on the rotor  12  and prevent the latter from reaching hazardous high speed levels. As illustrated in 
     FIG. 2  the band  16  could be provided with weakening grooves  18  to enhance yielding of the band  16 . In  FIG. 2  only one groove  18  is visible, but there is at least one more groove located so as to ensure a perfect balancing of the band  16 . 
   The curve in the diagram shown in  FIG. 3  illustrates how the strain related expansion of the band  16  increase as the rotation speed of the rotor  12  increases, and at a certain point Y above 10000 rad/sec the band  16  suddenly starts yielding, suitably in one or more of the grooves  18 . This means that the expansion of the band  16  suddenly starts increasing more rapidly. This is indicated by a discontinuity of the curve which is quite significant and makes it possible to determine with a good accuracy the speed level at which the overspeed preventing device will come into action. This speed level could easily be separated from the speed level normally provided by the speed governor, which in this example is below 10000 rad/sek. 
   As a result of passing the yield point Y for the cover band material yielding or even rapture of the band  16  will occur, which means that the band  16  expands and separates from the drive blades  13 . Then, the band  16  is displaced axially by the pressure drop across the drive blades  13  and gets jammed between the blades  13  and the housing surface  14 . Instead of reaching any hazardous high speed levels the rotor  12  is now braked down to a safe speed level or even stopped. 
   In the illustrated example the band  16  is made of an aluminium alloy and is clamped onto the drive blades  13  by a shrinking process. However, the band  16  could be made of a plastic material, and instead of shrinking the band  16  could be mounted on the rotor drive blades  13  by a press fit.