are assembled together as the intermediate body part 300 trapping all six wheels 30 in place they are secured together in assembled state by screw thread mounting of the top sub 100 to the upper end of the intermediate body part 300 and with the lower end of the intermediate body part 300 being threadedly coupled with the bottom sub 200 by screw threaded engagement of the intermediate body part 300 with the top and bottom subs 100 200 it is possible to completely avoid use of any grub screws or other means of locking the parts together however individual grub screws may additionally or alternatively be used for this purpose while still achieving a very marked improvement over the prior art arrangement of roller sub using only a pair of grub screws 34 one for coupling with the intermediate part 300 with the top sub 100 and the other for coupling with the bottom sub 200 the provision of the roller wheels 30 with their own integrally formed pivot pin means i e axle stubs 31 confers a number of technical benefits the axle stubs 31 of the roller wheels 30 occupy little volume in comparison to the axles and locking grub screws of the prior art this.