Patent Publication Number: US-2010119368-A1

Title: Double seal with pressurised lip

Description:
The present invention relates to the field of sealing gaskets, in particular radial-friction gaskets. 
     More particularly, the present invention relates to a sealing device for providing sealing between a casing and a shaft rotatably mounted in said casing, the device comprising first and second annular lip gaskets for placing axially side by side between the casing and the shaft. 
     Below, the adjectives “axial” and “radial” are relative to the direction of the axis of rotation of the shaft. 
     Traditionally, a pair of lip gaskets is used to provide an enclosure with sealing by making contact with the shaft. 
     Because of the friction that exists between the shaft and the lips of the gasket, rotation of the shaft gives rise to wear of the lip gaskets and requires the gaskets to be changed, in particular to avoid leaks of oil. 
     Such leaks of oil are harmful to the environment and they may also lead to damage to rotary parts such as the gearwheels that are no longer properly lubricated. 
     When such a device is mounted in a helicopter turbine engine, the helicopter needs to be taken out of action in order to change the gaskets, which presents a cost that it is desirable to avoid. 
     An object of the present invention is to provide a sealing device having a lifetime that is longer than that of the prior art. 
     The invention achieves this object by the fact that the sealing device of the invention further includes means for delivering a stream of pressurized gas into an annular casing defined by the first lip gasket, the second lip gasket, and an outside surface of the shaft, such that during rotation of said shaft, the gas stream is suitable for causing at least one of the two lip gaskets to lift off a little from the outside surface of the shaft in order to flow out from the cavity, the means for delivering the pressurized gas stream further including a diaphragm for limiting the flow rate of the pressurized air in the event of one of the annular lip gaskets becoming damaged. 
     Thus, during rotation of the shaft, at least one of the two lip gaskets, and preferably both of them, lift(s) off from the outside surface of the shaft as a result of the gas stream flowing between the lip gaskets and the outside surface of the shaft, thereby advantageously eliminating friction between the shaft and the sealing device. 
     In spite of the lip gasket(s) lifting off, the sealing function is advantageously preserved by the gas stream flowing out from the cavity and tending to keep the external particles outside the cavity. It can thus be understood that particles of oil or dust cannot pass through the sealing device in one direction or the other. 
     As a result, the sealing device in accordance with the present invention wears substantially slower than does a prior art device, thereby having the effect of increasing its lifetime. 
     Furthermore, when the shaft is not rotating, sealing is achieved merely by the fact that the annular lip gaskets come into contact against the outside surface of the shaft. There is no need to pressurize the cavity at this time since there is no friction between the shaft and the sealing device. 
     Preferably, the diaphragm is placed in the channel or at one of its ends. 
     During normal operation of the sealing device of the invention, the flow rate of gas is limited by the lift-off distance of the lips of the lip gaskets. 
     Should one of the two lips become damaged, then the gas flow rate could increase suddenly giving rise to an undesirable loss of gas. 
     By virtue of the diaphragm, the flow rate of the gas is advantageously limited in the event of one of the lip gaskets being damaged. 
     Preferably, the first lip gasket includes a first lip, while the second lip gasket includes a second lip, and the first and second lips are designed to extend in the axial direction of the shaft while extending away from each other. 
     Thus, it is the first and second lips that lift off from the outside surface of the shaft when the gas stream flows out from the cavity. 
     Advantageously, the means for delivering the pressurized gas stream comprise a channel disposed between the first and second lip gaskets, said channel being connected to a source of pressurized gas. 
     Preferably, the channel extends radially between the two lip gaskets. 
     The present invention also provides to a helicopter turbine engine including a casing and a shaft rotatably mounted in said casing, said turbine engine further including a sealing device in accordance with the present invention. 
     Advantageously, the turbine engine of the invention further includes a source of pressurized gas for feeding the means for delivering a stream of pressurized gas to the annular cavity. 
     In preferred, but non-exclusive manner, the source of pressurized gas is a take-off located at the outlet from the compression stage. 
     It is also possible to provide an external source of pressurized gas without going beyond the ambit of the present invention. 
    
    
     
       The invention can be better understood and its advantages appear more clearly on reading the following description of an embodiment given by way of non-limiting example. The description refers to the accompanying figures, in which: 
         FIG. 1  is a detailed view of a helicopter turbine engine casing having a rotary shaft mounted therein, the turbine engine including a sealing device in accordance with the present invention; and 
         FIG. 2  shows a turbine engine provided with a sealing device of the present invention. 
     
    
    
       FIG. 1  shows a detail of a casing  10  of a reduction gear  11  of a turbine engine  52  for a flying vehicle such as a helicopter, the casing having a sealing device  12  in accordance with the invention mounted therein. Clearly this figure shows merely one non-limiting example of how the device of the invention can be used. 
     As can be seen in  FIG. 1 , a shaft  14  presents an axis of rotation A and is mounted to rotate in the casing  10 , in particular by means of a bearing  16 . 
     Specifically, the casing  10  corresponds to the casing of the reduction gear  11  of the turbine engine, i.e. the end  18  of the shaft  14  beside the bearing is designed to be coupled to gears, while the opposite end  20  is a power take-off for coupling to a shaft that transmits torque to the rotor of the helicopter. 
     In other words, the power take-off end  20  is situated outside the turbine engine  52 , while the end  18  beside the enclosure  21  of the reduction gear  11  is situated inside the turbine engine  52 . 
     In order to lubricate the rotary elements situated within the enclosure  21  of the reduction gear  11 , oil is injected therein, such that this portion of the turbine engine  52  contains an air/oil atmosphere. 
     Both for environmental and for mechanical considerations, it is appropriate to prevent oil from escaping from the casing  10  of the reduction gear  11 . 
     It is also appropriate to avoid dust or other undesirable particles penetrating into the enclosure  21  of the reduction gear  11 , since otherwise there would be a risk of the gearwheels  53  of the reduction gear  11  being damaged. 
     In order to do this in accordance with the invention, the sealing device  12  placed between the casing  10  and the shaft  14  serves to prevent both loss of oil and penetration of external particles into the enclosure of the reduction gear  11 , while also presenting a lifetime that is longer than a prior art sealing device. 
     For this purpose, the sealing device  12  comprises a first annular lip gasket  24  and a second annular lip gasket  26  that are disposed side by side between the casing  10  and the shaft  14  while also lying on a common axis, it being understood that their common axis corresponds substantially to the axis A of the shaft  14 . 
     Preferably, the annular lip gaskets  24  and  26  are radial contact gaskets and they are preferably made of elastomer. 
     The first and second annular lip gaskets  24  and  26  are preferably fastened to a sleeve  28  placed axially in a bore  30  of the casing  10 , the sleeve  28  itself being held securely to the casing  10  between an end plate  32  that is secured to the casing  10  and the bearing  16 . 
     As can be seen in  FIG. 1 , the first and second annular lip gaskets  24  and  26  have respective first and second lips  34  and  36  that extend in the axial direction of the shaft  14  while also extending away from each other. 
     Furthermore, the lips  34  and  36  are shaped to present a first position, drawn in dashed lines in  FIG. 1 , in which each of them comes into contact with the outside surface  22  of the shaft  14  in order to provide sealing for the enclosure  21  of the reduction gear  11 . 
     In accordance with the invention, the lips  34  and  36  are in their first position preferably while the shaft  14  is not rotating. In other words, in their first position, the lips  34  and  36  provide static sealing between the casing  10  and the shaft  14 . 
     It will be understood that in their first position the first lips  34  prevent external particles from penetrating into the enclosure  21 , while the second lip  36  prevents droplets of oil from escaping from the enclosure  21  of the reduction gear  11 . 
     In particularly advantageous manner, the lips  34  and  36  are suitable for taking up a second position, drawn in continuous lines in the figure, in which position, the lips  34  and  36  lift off from the outside surface  22  of the shaft  24 . 
     Preferably, the lips  34  and  36  take up their second position when the shaft  14  is rotating. 
     To do this, an annular cavity  38  defined by the first lip  34 , the second lip  36 , and the outside surface  22  of the shaft  14  is pressurized by means  40  for bringing a stream F of pressurized gas into said cavity  38 . 
     Said means comprise a channel  40  formed in a rib  42  inside the sleeve  28 , said rib  42  occupying a plane that is orthogonal to the axis A of the shaft  14  such that the channel  40  extends substantially radially. 
     With reference to  FIG. 1 , it can be seen that a first end  44  of the channel  40  opens out into the annular cavity  38 , while a second end  43  of the channel  40 , opposite from the first end  44 , is connected to a coupling  46  via a radial pipe  48  provided in the casing  10 . 
     The coupling  46  is connected via tubing  45  to a pressure source which, in the present example, is a take-off point  49  situated at the outlet from a compressor  50  of the turbine engine  52 , as shown in  FIG. 2 . 
     In other words, the gas in this example corresponds to an air fraction taken from the air compressed by the compressor  50 . 
     An advantage of using the outlet from the compressor  50  as a source of pressure is that it makes it possible to omit using an external pressure source, although that remains entirely possible within the ambit of the present invention. 
     In accordance with the invention, the gas stream F delivered to the cavity  38  is at a pressure that is sufficient to enable it to lift the lips  34  and  36  off from the outside surface  22  of the shaft  14 . 
     It can thus be understood with the help of arrows shown in  FIG. 1 , that the gas stream F lifts the lips  34  and  36  off from the outside surface  22  of the shaft  14  so as to flow away from the cavity  38 . 
     More precisely, the gas stream leaving the cavity  38  is preferably constituted by a first annular stream F 1  flowing axially out from the turbine engine  52  and by a second annular stream F 2  flowing axially towards the inside of the enclosure  21  in the opposite direction to the first annular stream F 1 . 
     It can thus be understood that by means of the invention, the first stream F 1  prevents external particles from entering the enclosure  21  of the reduction gear  11 , while the second stream F 2  prevents oil droplets from escaping from the enclosure  21 , with sealing thus being ensured in spite of the fact that the lips  34  and  36  are lifted off when they are in their second position. 
     As mentioned above, because of the lips  34  and  36  advantageously being lifted off the shaft  14  while it is rotating, the annular lip gaskets  24  and  26  wear substantially less since there is no friction while the shaft is rotating. 
     The sealing device of the invention thus presents a lifetime that is longer than in the prior art. 
     Advantageously, the sealing device of the present invention also includes a diaphragm D serving to limit the flow rate of pressurized gas in the event of one or the other of the lips  34  and  36  becoming damaged.