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SUMMARY OF CURRENT RESEARCHES RELATING TO 
The manner in which the driving motion is communicated from the 
screw to the two travelling supports is peculiar. A device was needed 
which would admit of the travelling supports being independently moved 
to any positions when not clamped to the screw, and of being clamped in 
any position to the screw, so as to be driven by it without backlash. 
These conditions wore finally fulfilled in the following manner. Upon 
each half of the screw is placed 
a massive gun-metal nut, about 
50 mm. long, and of square section 
25 mm. in the side. The lower 
face of this nut slides upon the 
upper face of the lower girder, and 
this prevents it from turning ; it is 
also prevented from turning by the 
interposition, between its upper 
face and the lower face of the 
upper girder, of a long slotted 
rectangular flange of brass which 
constitutes part of a clamping bar. 
Each nut is bored out with a short 
cylindrical cavity. Within this are 
inserted a spiral spring of steel 
wire, and a second nut which, 
though capable of longitudinal 
play, is prevented from turning by 
the insertion of a key into a key- 
way. The introduction of this 
second nut, with a strong spring 
between it and the main nut, is a 
mechanical device originally due 
to Sir William Thomson, P.E.S., for preventing backlash. The slotted 
clamping bars mentioned above are of brass, each 176 mm. long. 
Each is, as may be seen from figs. 15 and 16, of L-shaped section, 
fitting over, and in front of the square nut. The upper horizontal 
flange which comes between the top of the nut and the under side of 
the higher girder is of accurate fit, and is slotted out to receive beneath 
it the head of a clamping pin. This pin, passing vertically through a 
projecting lug of the travelling support, enters the threaded shaft of 
a clamp-screw which bears at its top a milled head. When this is turned, 
the head of the clamping pin is raised and clamps the slotted bar to 
the lug of the travelling support. The vertically-situated flange of 
each of the L-shaped clamping bars fits accurately into the space 
between the upper and lower girders, and it also is slotted to admit 
through it a clamping pin w’hich projects horizontally from the square 
nut. A clamping screw with milled head screws on to this pin, and 
clamps the slotted bar to the square nut. It is shown in fig. 16 project- 
ing to the left. In order that the turuiug of the long screw may drive 
the two travelling supports, it is requisite that each should be clamped 
to its slotted bar, and that each slotted bar should be clamped to the 
square nut. The object of using such long clamping bars instead of 
mere short pieces is to enable the travelling supports to be clamped 
Fig. 16. 
