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Digit Readout and Addition

(Nine of each assembly are required)

Addition in the difference engine proceeds in two phases. In the first phase, numbers are added digit by digit, and then in the second phase, any carries that were generated in the first phase are added to the result. The pictures below show the assemblies needed to accomplish the digit by digit addition. The carry mechanism is covered on a separate page. The axes which carry the digit wheels rotate four revolutions per cycle of the machine. On one of the four revolutions the mechanism is engaged to sense the position of the digit wheel and advance the adjacent wheel by the corresponding number of positions.

The pictures below show the construction of one of the readout assemblies. A pair of face plates is bolted together by 1 1/8" bolts. The whole assembly will be fixed to the rod once installed in the machine. A fish plate is bolted to the top side of each of the face plates over one of the slotted holes to provide a bearing for a 2" rod. This rod carries a 1" gear seen in the picture to the right, and a pawl. Note that the bolt holding the lower fish plate needs two washers or the protruding end of the bolt will foul the threaded pin on the digit wheel after assembly. A 2½" rod carries a ½" pinion meshing with the 1" gear, and a crank which can just be seen at the lower right of the left picture below. The top of this rod carries a second pawl, and the lower end a collar held in place by a 3/8" bolt. The head of this bolt will contact the threaded pin on the digit wheel to disengage the mechanism after the right number of counts have been added. These two rods are constrained to turn through only a very small angle. A threaded pin stops the crank at the outside, and on the inside it is stopped by the main axle. A detent is formed by a second crank mounted on a 1½" rod. In the slotted hole of this crank a ½" pulley is loosely mounted on a pivot bolt. The pulley is held against the end of the first crank by a small spring made from about 1" of spring cord with a hook for spring cord screwed in each end. One hook is located in the round hole of the crank, while the other is slipped over the edge of the upper face plate.

Careful adjustment of a number of elements is needed here. First, the amount of motion is controlled by the position of the threaded pin in the slotted hole of the face plate which can be seen in the right picture above. Second the position of the pivot bolt in the slotted hole at the end of the second crank needs to be such that there are two positive locations the mechanism will settle in. The positions of the two pawls and the 3/8" bolt in the lower collar can only be set once the whole assembly is together in the machine. Finally, the position of the bolt holding the upper fish plate needs to be set so that a ratchet wheel on the central rod will not foul the washer, but at the same time the bolt head must not prevent the pawl from fully engaging the ratchet wheel.

The final element of the readout mechanism is shown to the left and consists of a 2½" gear wheel and a ratchet wheel connected by a socket coupling. This assembly must be free to turn on an axle so careful selection of components and alignment is needed to make sure it does not bind. The ratchet wheel will engage with the inner facing pawl when the mechanism is engaged. This causes the gear to be carried around with the cage, until the bolt in the collar encounters the threaded pin in the digit wheel and kicks the pawl out of engagement. The choice of gearing ensures the pawl can be disengaged without the bolt tending to drag the digit wheel around.

The pictures below show two different views of the complete assembly of the digit wheel, readout cage, and ratchet assembly. Three washers need to be placed between the digit wheel and the lower face plate of the readout cage.

The picture below shows the components assembled in the machine. The upper 2½" gear of the ratchet assembly meshes with the 57t gear wheel to the left, but not with the one on the right. As mentioned above, washers under the digit wheels of alternate axes are used to provide vertical offset. In operation, once every fourth turn of the digit axis, the outwardly facing pawl will be pressed inwards by an assembly on the axis behind (the bolting axis, to use Babbage's terminology), engaging the inner pawl with the ratchet wheel. The 95t gear is then carried round, driving the 95t gear of the digit wheel on the axis to the left at the same rate, through the intermediate rod with the two 57t gears. Depending on the position of the digit wheel, the mechanism will be disengage by the bolt hitting the threaded pin after the appropriate number of steps.

Adjustments have to be made to the exact position of the ratchet wheel relative to the pawl as it engages so that the content of the digit wheel is accurately added to the digit wheel to the left. Likewise, the position of the bolt in the lower collar relative to the threaded pin is critical to correct disengagement.

Because the result axis does not need readout apparatus, a total of 9 copies of the readout cage and ratchet assemblies are required, four in the first difference column, three in the second difference column, and two in the third difference column.

Addition commences on a column when the pawls at the top of the readout assemblies are pressed inwards by the "bolting" assemblies. These assemblies are carried on the bolting axes, one of which is located behind each difference axis. The bolting axes turn at one fourth the rate of the digit axes with odd and even difference columns having the bolting assembles set one half turn apart. Thus, once every fourth turn of the driving crank, the readout cages of the odd numbered difference columns are engaged, then two turns later, the even difference columns are engaged. The pictures below show the bolting assembly.

A total of nine of these assemblies is required. A face plate with boss downwards carries on top a bell crank mounted on a pivot bolt. In one end of the bell crank a short threaded pin is mounted which carries a ½" pulley without boss. There is a washer below the pulley which is held in place by a collar. To the other end of the bell crank, a tension spring is attached by a 3/8" bolt. The tension spring runs over another ½" pulley again mounted on a short threaded pin in one of the inner round holes of the face plate and it fixed to another 3/8" bolt mounted at the outer end of one of the slotted holes. A stop is provided by an angle bracket, mounted through its round hole to one of the slots in he face plate. Note from the picture the approximate place this should be adjusted to.

Once mounted on the bolting axis, the bolting assembly must be positioned vertically so that the pulley on the end of the bell crank aligns with the pawl on the readout cage. The spring loading of this pulley is designed to handle the case that a digit wheel to be added stands at the value zero. In this case nothing should actually be added to the adjacent column. As the bolting assembly rotates and contacts the pawl, if the digit wheel stands at zero, then the bolt projecting from the collar at the bottom of the readout cage will contact the threaded pin in the digit wheel. Instead of the pawl engaging, the bell crank on the bolting assembly will be depressed and the pulley will ride over the pawl without causing it to engage.

Since the bolting assemblies are mounted on the same axes as several of the components of the carry mechanism they must be constructed before the bolting assemblies can be installed in the machine. The picture to the left shows the bolting assembly viewed from the rear of the machine in a position where the pulley on the bolting assembly is just about to make contact with the pawl on the digit readout assembly.

Framework
Arrangement of axes and drive system
Decimal digit storage
> Digit readout and addition
Carry detection and propagation
Setup and adjustment
Operation
Parts list

Last modified: 16 July 2004
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