bolt 31 is threadedly connected to socket 30 thereby fixing the end of central shaft 12 firmly to the arm 29 via pin 32 and socket 30 fig2 shows a preferred shape of shaft 10 in cross section it is of course not necessary to the smooth working of this invention that shaft 10 be of any particular cross section except that shaft 10 must move axially up and down in the directions of double arrow 36 a smooth circular outer surface of shaft 10 is eminently workable it is however a characteristic of the apparatus of this invention that the outer surface of shaft 10 where it is contacted by lock washer 23 will eventually become marred as will the inner edges of lock washer 23 over long periods of use if the damage to shaft 10 or washer 23 is great enough the damaged piece must be replaced while a prudent choice of materials of shaft 10 and washer 23 will confine the wear to the softer material this may not be sufficiently economic one solution to this problem is shown in fig2 where shaft 10 is made with two opposed ledges 38 of larger diameter than the remainder of shaft.