limited by the distance between the stops 64 and seal plates 66 of piston sections 18 at one extreme in which buttons 24 are fully retracted substantially as shown in fig3 the left end of piston 62 fits within central bore 72 of stop 64 and axially facing annular piston face 61 engages the facing end surface 65 of stop 64 at the other extreme in which buttons 24 are fully extended slightly beyond the position shown in fig4 the opposite end 63 of piston 62 engages the annular surface 37 of seal plate 66 surrounding recess 70 each cam 28 has an axial length slightly greater than the permitted axial travel of rod 14 the cams are positioned on the rod so that buttons 24 will engage the minimum diameter end of the conical cam surface 44 of the respective cam 28 when rod 14 is in the button retracted position and the other end of cam 28 e g the far end of the cylindrical cam surface 42 when rod 14 has moved to the button extended position springs 32 bias rod 14 towards the button extended position and spring seals 56 hold buttons 24 against their respective cams the number of belleville washers 46 in each spring 32 and the configuration of each washer is such that the force exerted by the spring remains substantially constant as rod 14 moves between its two extreme positions in practice washers having an h t ratio t being the thickness of the material of the washer and h being the free height of the washer less its thickness in the range of 1 2 to 1 7 typically 1 41 are used and the total number of washers.