risers but other thread shapes can be used as appropriate the increasing diameter and tapering aspect of the helical threaded members cause top shell 14 and bottom shell 16 to move apart as described above the contact at the tops of the risers 52 60 is made at the minor diameter of the helical threaded member thrust bearings are provided to limit the axial direction motion of the drive shafts within shell 12 as shown in fig9 a thrust bearing 62 comprises a two piece yoke configuration that mate together and press fit around ends of the shafts the top part 64 of the thrust bearing yoke defines openings for receiving a round portion 66 of the shaft ends in fig9 c square shaft 42 has a rounded portion 66 of lesser diameter than the square portion of the shaft a mating piece 65 of the thrust bearing engages with top part 64 to encircle the rounded portion 66 of drive shaft 42 pin elements 68 in the top portion 64 and bottom portion 65 engages a corresponding holes 69 in the mating piece to provide a press fit of the thrust bearing around the shaft journal grooves 67 can also be provided in thrust bearing 62 shaft 42 can have an annular ridge 63 around its rounded portion 66 which is received in journal groove 67 as shown in fig9 c a thrust bearing is provided at each end.