may be integrally formed with the post 52 or may be separately formed and then attached to the post 52 by suitable means the hub 54 also defines a bore 60 as shown in fig1 2 and 4 the cross member 56 has two arms 62 connected by a substantially cylindrical central portion 64 disposed through the bore 60 in the hub 54 the central portion 64 and bore 60 are sized so that the cross member 56 and central portion 64 are free to rotate within the hub 54 the arms 62 of the cross member 56 each have longitudinal lobes 66 68 and 70 the lobes 66 68 and 70 also define contact surfaces 56 a 56 b and 56 c contact surfaces 56 a are formed from adjacent longitudinal lobes 66 and 70 such that when the cross member 56 is positioned such that longitudinal lobes 66 and 70 are in contact with the blade 14 contact surfaces 56 a are positioned immediately adjacent to the blade 14 similarly when adjacent longitudinal lobes 66 and 68 are in contact with the blade 14 contact surfaces 56 b are positioned immediate adjacent to the blade 14 finally when adjacent longitudinal lobes 68 and 70 are in contact with the blade 14 contact surfaces 56 c are positioned immediate adjacent to the blade 14 in.