Sealing device for temporary pressure sealing
TECHNICAL FIELD 
     The present invention relates to a sealing device for temporary pressure sealing between a shaft and a bushing surrounding the shaft, of the type in which an annular seal ring is moved by applied fluid pressure into sealing contact with the shaft; 
     BACKGROUND ART 
     Such a sealing device of a known type is shown in FIGS. 1 and 2 on the accompanying drawing, in which FIG. 1 shows a section through the center line of the device; and FIG. 2 shows an enlarged view of a portion of FIG. 1. 
     In FIGS. 1 and 2, an inner sleeve 1 and an outer sleeve 2. are made of metal and together form a bushing or support for a pump shaft 3. They define an annular slot 4, the opening of which faces the shaft 3. The slots walls comprise two conical annular surfaces 5 and 6, which converge in a radially inward direction so that slot 4 is trapezoidal in cross-section. A sealing ring 7 of rubber or a similar resilient, elastomeric material is arranged in the radially inner part of the slot 4. Radially outside the sealing ring, the slot 4 is connected to a channel 8 for the supply of a pressure medium, for example oil. The rubber ring 7 is formed with ring surfaces which are pressed against the conical ring surfaces 5 and 6 of the slot when channel 8 is connected to a pressure source. At the same time the radially inner surface of the rubber ring is pressed against the pump shaft 3 and seals thereagainst. Thereafter, the space above or below the rubber ring can be evacuated. When the pump is to be started, the rubber ring 7 is first relieved of pressure to avoiding abrasion thereof. 
     The drawback of this prior art seal is that during long periods of pressurization the rubber ring will creep in such a manner that part of the rubber material, initially disposed within the edges of the slot, will successively pass beyond these edges and accumulate outside the slot, as suggested in FIG. 2. Then when the pump is to be started and the contact between the rubber ring and the pump shaft is to be broken, this may not be possible because of the above-mentioned creep of the rubber. When the shaft 3 rotates, the rubber ring is destroyed. 
     DISCLOSURE OF THE INVENTION 
     The above-mentioned drawback is avoided in a sealing device according to the invention, in which the radially inner portion of the seal ring remains always spaced from the walls of the slot so that creep of the seal ring material does not occur which would prevent retraction of the seal ring upon depressurization. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to FIGS. 3 and 4 on the accompanying drawing, in which 
     FIG. 3 shows a device according to the invention with the rubber ring in pressure-relieved state; and 
     FIG. 4 shows the same device with pressurized rubber ring. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the device according to FIGS. 3 and 4, a sealing ring 9 of rubber or a similar material is used. The other components of the device are identical with the corresponding components shown in FIGS. 1 and 2 and are designated with the same reference numerals. Sealing ring 9 has a radially outer portion which is provided with two conical, annular sealing surfaces 12 and 13, which make contact with the conical ring surfaces 5 and 6, respectively, of the slot 4. Further, the sealing ring has an approximately equally long radially inner portion which does not make contact with the slot walls and which is defined by two surfaces 14, 15 facing separate slot surface 5 and 6 and together with these defining separate annular gaps 10 and 11, respectively. Gaps 10 and 11 lie radially outside the narrowest portion of the slot also when the sealing ring 9 is fully pressurized by source 16. Thus there is no risk that an annular portion of the sealing ring lying radially inside the narrowest slot portion shall be able to creep through the narrowest slot portion to prevent the expansion of the sealing ring when the pressurization ceases. 
     INDUSTRIAL APPLICABILITY 
     The invention may be used in any application where it is desired to provide a temporary, pressure activated seal around a shaft, the seal being adapted to remain in place when not pressurized and to withdraw to a position substantially out of contact with the shaft.