that no part of the follower arm 38 needs to pierce the sealed outer housing hub 10 thus there is no need to seal such an aperture or risk that the seal will wear over time and thus leak with particular reference to fig7 note that there is an outer housing hub 10 linked via bolts 58 to the follower arm 38 to rotate therewith the follower arm 38 supports a roller shaft 32 on which spins the follower roller 30 the roller 30 is held on shaft 32 via a lock washer 46 and a nut 60 as is evident from fig8 the outer hub 10 retains an array 42 of radially outer magnets this can be an array of six 60 u00b0 arcs of alternating polarity magnets they can be held in place via an outer magnet shield 18 similarly an inner housing hub 14 supports an array 44 of inner magnets as shown in fig8 these can also be six 60 u00b0 arcs of alternating polarity magnets there is also an inner shield 16 to retain the array 44 of inner magnets in place and protect them the south polarity magnets in array 42 will seek to rotationally align with the north polarity magnets in array 44 and the north polarity magnets.