471 in the nature of piston rings the rings 471 are of t shaped in cross section the stem is very small the sealing ring 471 is formed of plastic material which is relatively hard yet somewhat elastic for example a high strength polyamide or similar material polytetrafluoroethylene is particularly suitable in the circumference of each of the vanes 435 there is a corresponding groove 470 of t shaped cross section the vanes 435 are preferably also made of plastic material for example polyamide and after they are placed on the relatively wide shaft extensions 236 they are secured by screws 238 the t groove 470 is sufficiently deep that an elastic element can be inserted under the sealing ring 471 this can be formed in any way in particular it is desired to use an elastic spring strip 474 formed of elastomeric material having for example a hardness of 60 shore c all of the vanes 435 are provided with t grooves 470 sealing rings 471 and spring strips 474 these are pressed into the grooves 470 from outside that is possible becuase the vanes and the sealing rings are formed of elastic material and the stems 472 are small the spring strips 474 constantly.