end 39a of lower track 39 along one half of the curved path about lower sprocket wheel 27 so that roller 23 comes into contact with track 38 while roller 24 is still in contact with track 39 it will be noted however that the adjacent ends of tracks 38 39 are spaced along the half of the lower curved path which is diagonally opposite the half of the upper curved path along which the adjacent ends of tracks 37 and 40 are spaced in like manner adjacent ends 41a 44a of upper tracks 41 44 are spaced along one half of the curved path about upper sprocket wheel 29 such that for either direction of carrier motion about the closed loop path roller 21 is momentarily in contact with track 41 while roller 22 is in contact with track 44 the end 42a of lower track 42 is spaced from the adjacent end 43a of lower track 43 along one half of the curved path around lower sprocket wheel 30 so roller 21 is momentarily in contact with track 42 while roller 22 is in contact with track 43 as in the case of the right side tracks the adjacent ends of tracks 42 43 are located along the half of the lower curved path diagonally opposite the half of the upper curved path along which adjacent ends of tracks 41 44 are located further since the views of both fig3 and fig4 are obtained looking to the right in fig1 it will be noted that the adjacent ends of left side tracks 41 and 44 are located along one half of the upper curved path while the adjacent ends of right side tracks 37 and 40.