More about this adder below the image.
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The photographer's stamp on this photo reads: HALL & CO. Commercial Photographers 20 Hunter Street Sydney
The adding shafts with their epicyclic gear arrangement, escapement wheels and solenoids that are activated by ticket issuing machine pulses are at the back of this adder.
The storage screw has been described in the previous image in the Photo Gallery so I will pick up the description of the Variable Speed Friction Gear from there. Neither storage screw is visible in this image however the storage screw position sensing rods can be seen extending to the right past the pulleys on the right hand side. The upper rod has a mechanism attached to the end of it, that drives the variable speed friction gear mentioned in George's paper.
I have repeated an extract from George Julius' paper he presented to the Institution of Engineers Australia in 1920 here, as this extract relates to the storage screw and in particular the variable speed friction gear. The movement of this screw is so arranged that it also controls a variable speed friction gear through which the counters are driven. During any period of acceleration in the issue of tickets, the screw is withdrawn in the nut faster than the counter operates, and this through the friction gear speeds up the counter, and the nut, in an endeavour to overtake the movement of the screw, and a condition of balance is ultimately established. If the issue of tickets is retarded or ceases, the nut immediately gains on the screw and brings it forward, thereby picking up all the stored-up records, and by means of the friction gear gradually slowing down the counter until when all the records are recorded, it quietly comes to rest. The rotation of the nut also is utilised to continually rewind the coil spring which operates the epicyclic gears, and thus ensure a steady driving effort on these gears.
At the end of the rod that projects out of the right hand side of the upper storage screw, which is part of the storage screw position sensing mechanism, there is a hinged arm projecting down from a mounting above, which has a hinged tip at the bottom end making contact with the right hand end of the storage screw position sensing rod. I think this is part of the control mechanism for the variable speed friction gear that George has described. I drew an analogy to this mechanism, an electronic device called a closed loop servo system, in the text associated with the previous image in the photo gallery. In this analogy, the storage screw is part of the closed loop servo system and the screw is the part being controlled. The mechanism attached to the right hand end of the storage screw is part of the sensing mechanism in the feedback path of the closed loop servo system.
There is more than one method of implementing the variable speed friction gear however, in this permutation of this mechanism, we have the lever mentioned that connects to the right hand end of the upper storage screw. A rod, which is the fulcrum of the lever connecting with the end of the storage screw position sensing rod is mounted between two projecting arms from the adder chassis. The rotation of this rod between the arms transfers the movement of the clearly visible lever sensing the storage screw proximity to its rest position, transfers the motion of the lever to another lever aligned with and hidden by the near side rod supporting arm. This hidden lever is clearly visible in the previous image of the photo gallery. It is unclear from any of the photographs what this hidden leaver connects to however two things are visible. Firstly there is a rod descending vertically down from a rounded section in the near rod supporting arm and in the previous image in the Photo Gallery, this descends to a position close to the spindle of the nearest pulley and disk. Secondly there is what looks like a long threaded rod immediately to the left and parallel to the previous descending rod and from the previous image in the Photo Gallery this could have something to do with the lever which has no clear connection in the available photographs. What is clear is that both these descending rods end up in the vicinity of what I think is George's variable speed friction gear.
On the right hand side of the adder in the image there are four pulleys visible. The upper of the two nearest pulleys has a disk attached close behind it. The lower of the nearest two pulleys has a disk attached to its spindle at a distance placing it behind the first disk. Between these two disks can be seen a wheel oriented at 90 degrees to the two disks. This wheel contacts the visible side of the second disk and the opposite side of the first disk. This wheel transfers the motion from what I presume is the main drive shaft associated with the lower pulley, to a driven shaft associated with the upper pulley. It looks like this wheel is not only capable of rotation around its spindle but also vertical motion as its spindle is moved up and down. This is indicated by the scrubbed shiny section of the disk attached to the main drive pulley. This shiny section clearly indicates the extremities of the vertical movement of the transfer wheel. This transfer wheel is what I think George is referring to as the variable speed friction gear. It is not a gear in the sense that it has meshing teeth to transfer motion, however as the name implies using friction to transfer the motion of the drive disk to the transfer wheel and from the transfer wheel to the driven disk and it does transfer motion at variable ratios. The vertical motion of the transfer pulley gives different gear ratios as it moves up and down.
Looking at the limits of vertical motion, with the transfer pulley at the bottom of its range of travel, at the bottom of the shiny section of the disk on the main drive pulley spindle and the closest it gets to its centre, it will be at the nearest point to the perimeter of the driven disk and this is the most geared down position imparting the least angular velocity to the driven disk. With the transfer disk at the opposite end of its travel at its nearest point to the perimeter of the main drive disk and also nearest the centre of the driven disk, this is the maximum geared up position imparting the most angular velocity to the driven disk. I have likened this mechanical control system to an electronic device called a closed loop servo system. This is the control section of the servo system and is at the end of the feedback loop. The sensing for the feedback loop is done at the storage screw position sensing rod which provides acceleration information when the storage screw is in the vicinity of its rest position. In its rest position nothing moves and the adding shafts are locked. Outside the vicinity of the rest position the slow to accelerate side of the machinery reaches and maintains its maximum rotational velocity which allows it to catch up with the storage screw. As indicated from the above paragraph, although it is not clear exactly how the storage screw sensing mechanism finally connects with the transfer wheel, this sensing mechanism does lead to the vicinity of the transfer wheel but is obscured by the pulleys. In terms of the servo system analogy we have not determined exactly how the feedback path is closed in this permutation.
It is interesting to note that in the vicinity of the transfer wheel, the two disk surfaces that the transfer wheel is in contact with, are travelling in opposite directions. However as in the case of the drive disk this point is above the spindle and in the case of the driven disk it is below the spindle, both spindles are rotating in the same direction.