200 the distances from the origin om to arbitrary points ti and to of the internal and external walls 101 and 102 of the orbiting wrap element 100 are given respectively by where o slashed denotes the winding angle from the xm axis and is positive in the anticlockwise direction on the other hand the internal wall 201 of the stationary wrap element 200 is shaped with an envelope curve of locus circles of a radius r 0 which points on the external wall 102 of the orbiting wrap element 100 draw while the external wall 202 of the stationary wrap element 200 is shaped with an envelope curve of locus circles of a radius r 0 which points on the internal wall of the orbiting wrap element 100 draw the coordinates x y of arbitrary points pi and po on the internal and external walls 201 and 202 of the stationary wrap element 200 are given respectively by fig2 depicts the condition in which a portion of a convolution of the stationary wrap element 200 is shifted radially outwardly to come into contact with that convolution of the orbiting wrap element 100 which is positioned outwardly thereof as shown therein u03b1 i is an angle formed between a straight line connecting the origin om with a point qo lying on the external wall curve 102 and the normal to the external wall curve 102 at the point qo while u03b1 o is an angle formed between a straight line connecting the origin om with.