Vertical Motions
General The calculating cycle requires intermittent vertical motions for the figure wheel axes, sector wheel axes, locks and warning axes. These vertical motions are derived from the eight pairs of conjugate cams located in the lower section of the cam stack. Cam followers operate three pairs of horizontal bars slung on the underside of the engine and one pair midway up the cam stack. The arrangement of cams and cam followers converts the continuous rotational motion of the cams into intermittent reciprocating motion of the horizontal bars. The reciprocating motion of the horizontal bars is converted to vertical motion by bell crank levers (163, detail on 160). These lift and lower the axes and locks. The overall arrangement produces four sets of intermittent vertical motions phased according to the timing diagram. The 'vertical motion' section covers the vertical drive for the locks, figure wheel axes, sector wheel axes, warning axes, as well as the cams, horizontal bars, bell crank levers, and additional framing support members. Description Drawing References:
Main drawings are 159, 160 and 177. Also 161, 163, 167, 168, and 169. Action of the Cams and Followers The vertical motion cams are the eight pairs of closely spaced cams in the lower section of the cam stack (160 and 163). Eight pairs of cam followers rock on eight separate pivots and are driven by the irregular shapes of the cams (159 shows all 16 cam followers and the pivot layout; 168 and 169 show clearer partial views). Bar levers, which are integral with the cam follower bosses, slot into the horizontal bars. Six of the follower pivots project through the lower framing plate of the cam stack to enable the bar levers to engage with the drive slots of the horizontal bars. The arrangement of cams, cam followers and bar levers, convert the continuous rotational motion of the cams into intermittent reciprocating motion of the bars. The levers operating each of the eight horizontal bars is driven by two cam followers on the same boss. Each of the cam follower arms is driven by a separate cam. Cams therefore occur in conjugate or complementary pairs with one pair of cams associated with each of the eight vertical motions. The purpose of the complementary cam arrangement is to provide positive bidirectional drive for the vertical motions. The motion of a cam follower boss is determined by the outside surface of the active cam of the pair and the rises acting on the leading and trailing arms dictate these motions. The leading arm provides drive in one direction and the trailing arm, driven by the mating cam, provides the return motion. The naming convention adopted is that the leading arms point in the clockwise direction as viewed from above (the cams rotate anticlockwise so viewed). The leading arm does not necessarily make contact with the active cam first. At any given time only one of the pair of cam followers is in direct contact with the cam. The other follower is provided with a clearance from the cam surface. The overall layout of the vertical motion cams and followers is shown in plan on 159 with additional partial views on 168 (left) and (right). 159 gives partial cam profiles with ail significant rises shown in local relation to active portion of the cam follower. The eight follower pivots are shown distributed around the cam stack on a fixed diameter pitch circle. The cams are shown with standard outside diameters. The shape of the cams deviates inwards from the standard outside diameter. In six of the eight cases the contact between the cam follower and the cam circumference is shown as a sliding contact (159). Only in the remaining two cases are the cam followers shown with a roller on one of the two arms and a sliding follower as the other. Roller-cum-slider followers are provided for odd and even sector motions only. This is shown on 159; the spaghetti junction of intersecting followers with rollers is clarified in 168. In all cases (i.e. sliders and rollers) leading arm rises are shown as straight and trailing arm rises as are shown curved on a 2.5" radius. The straight plane surfaces of the leading sliding arms are shown near-parallel to the rises on the cams (see for example the even warning follower on 159 top); in the case of the trailing arm rises, the shape of the inside of the arm roughly matches the curve of the rise (see even figure wheel follower on 159 lower right). It seems that Babbage was concerned to avoid point loading by distributing the load over a contact surface. The trailing arm rises are radiused to avoid fouling: if the trailing rise was straight, the top comer of the rise would foul the inner surface of the arm with the worst case occurring about half way through the rise. In all cases of sliding followers the distance from the centre of the cam follower pivot to the line of contact on the sliding arm is shown as 3.50". In the case of the roller followers the 3.5" is taken from the pivot centre to the centre of the roller rather than to the line of contact In the case of sliding contacts, the line through the pivot centre and the point of contact is in each case tangential to the outside diameter of the cam i.e. with the follower at the top of a rise. The effective angle between the follower arms on the same boss is therefore constant. In the case of the two roller-curn-slider followers the line through the point of contact of the roller and the pivot centre is tangential. Here the effective angle between the follower arms differs from that of arms with sliders only. It is not immediately obvious why rollers were preferred for the sector vertical motions. The sector wheels represent the largest deadweight to be lifted, while the locks, though lighter, represent the largest shock load to the drive. It is possible that Babbage sought to reduce wear by using a roller on what he considered to be the most demanding load. This view is supported by the fact that in the case of the circular motions, rollers are preferred on all the cam followers, and sliding contact is avoided entirely. Though the loads for the circular motions are lighter, the duration of the actions tend to be more sustained. The rises on the cams are therefore longer and the period of active sliding contact, and therefore of wear, is correspondingly greater. It is possible that Babbage preferred rollers to sliders wherever possible. However, rollers are less space efficient they require greater separation between the cams than do sliders and a lower stacking density of the cams would be required to accommodate them. The compressed space of the lower cam stack assembly as drawn prohibits the general use of rollers. However, the tight pitch of the cam spacing is relaxed in the case of the sector cam followers because they require a free vertical motion to disengage during setting up. It is likely that Babbage exploited this additional space to provide rollers where he could i.e. one for each of the sector followers. An alternative to the general use of sliders would have been to reduce the overall stacking density of the vertical motion cams and expand the lower section of the cam stack. Without an additional stage of lining this would increase the overall height of the machine and the knock-on effect would be to put the upper figure wheels out of normal reach. It seems that Babbage preferred sliders to an overall increase in cam stack height Lifting and Lowering The lifting and lowering action for the figure wheel axes, warning axes, and the locks is provided by bell crank levers which are slotted into the horizontal bars below. The bell cranks for the sector axes are operated from above (160) (163 shows the even sector bar but not the bell cranks). The bell cranks for the figure wheel axes, warning axis and sector axes have forked ends which fit into bobbins pinned to the lower end of the axes (160). The fork and bobbin arrangement allows independent vertical and circular motion to be imparted to the axes without conflict the axes are lifted by the forked crank in the bobbin and rotated in the fork by the racked drive (see section on Circular Notion). The basic action of the bell crank and horizontal bar is cleariy shown for the figure wheel axis bearing the tabular result (extreme left hand axis on 163 with detail on 160). The lower end of the axis projects below the bobbin through a bearing hole in a plate fixed across the underside of the front and rear framing members (160 lower left). The arrangement for the locks is slightly different. Unlike the axes, the locks do not rotate so a bobbin drive is not called for. Instead the lifting and lowering action is transmitted by a connecting rod pivoted at each end to the lock and at the other end to the bell crank (160). The connection between the sector bars and the sector bell cranks is shown as a mix of pivots and slots. 163 shows the even sector bar with pivots for the first and last bell cranks, and with slots for the two intermediate cranks. 160 confirms pivots for the last odd and last even crank drives. The layout of the bell cranks is shown on 177. The bell cranks for a figure wheel axis, sector axis, and warning axis share a single shaft for each of the eight colurnn positions (160 and 177). The lock cranks each have a separate shaft. The bell crank shafts are located at each end in bearing holes in the horizontal framing members. The longer of the forked bell crank levers drive the sector axes; the shorter ones drive the figure wheel axes (middle) and warning axes (rear) (177). The figure wheels are set to their initial values manually. To free the figure wheels for setting up they must be disengaged from the sector wheels. Two lifting handles are provided for this purpose. The arrangement is shown on 163 and 177. Lifting the left hand handle operates shaft to which the bell crank lever for the first odd sectors is pinned (pinning shown on 161 ). This bell crank lever drives the odd sector bar to lift the remaining odd sector axes. The right hand handle lifts the even sector axes in the same way. As well as disengaging the sector wheels from the figure wheels when setting up, it is also necessary to disengage the sector bars from the horizontal drive. This prevents the drive for the vertical motions conflicting with the now immobilised sector axes when the engine is turned over during the setting up cycle. Details of the mechanism for disengagement are given on 159 centre, and 168 top right (163 shows only the support bracket on the cam stack side of the right upright). The sleeve, cam follower and bar lever for each of the odd and even sector bars are an integral assembly which is free to slide upwards on the cam follower pivot Operating the release lever raises the two sleeves and lifts the bar levers out of their drive slots. Two detents for the release lever spring hold the lever in the released and unreleased positions. Implementation Support for Horizontal Bars The six lower horizontal bars, which provide the vertical motions for the figure wheel axes, warning axes and locks, span the full length of the frame. The bars are supported at each end by slotted supports fixed front to rear to the main frame uprights. Five of the bars pass under the cam stack and are and these have an additional support on the underside of the right hand side of the cam stack. This slotted support, fixed to the right hand print shaft bearing mounting, is shown in 167, 163 and on 159 (faint dotted). The five longer bars are associated with the vertical motions for the odd warning, odd and even figure wheel axes, and the odd and even locks (159). There is very little separation between the horizontal bars for the even figure wheel motions and the odd lock motions (159) and 167 shows the bar support with a thin tongue separating the two bars. This was considered a weakness and, given the material (phosphor bronze), difficult to manufacture. The thin tongue was omitted in all four bar supports and single broader slots provided for the two bars. Separation was instead provided by five spigotted phosphor bronze spacers. These are located in equally pitched holes along the length of the odd lock bar. The original separation remains unchanged. The spacers are 5/8" diameter and 0.082" thick (E323). During assembly it was found that the full 62" span between the main uprights was too long to avoid downward bowing of the horizontal bars. An additional intermediate support was added for the six bars at the approximate midpoint of the span between the main uprights. The extra support is identical to the two fixed across the uprights. Counterbalancing the Locks During assembly it was found that even with a four-to-one reduction in the drive the engine could not be turned past the points in the cycle where the locks are released. The weight of the locks, friction in the angled slot-bearings, sideways pressure from the figure wheels, and the suddenness of the motion, make releasing the locks the most demanding load in the cycle. The 45o pressure angle on the lock cam presents a shock load to the drive which was too great to overcome with the original arrangement. The solution adopted was to counterbalance the weight of the locks using springs. The original design (163) shows a single figure wheel axis projecting above the upper bearing plates and passing through a spring to counterbalance its weight. This single feature was assumed to be generic and the technique was used to counterbalance all figure wheel, sector wheel and warning axes. However, counterbalancing the locks from above in the same way poses difficulties. The locks are not shown projecting above the upper bearing plates and the shock load problem was encountered after manufacture i.e. during the build. Extending the locks would have meant remaking the locks and modifying the bearing plates. An additional difficulty is presented by the fact that the motion of the locks has a sideways as well as vertical component: the locks are lifted by the bell crank levers and slide in angled bearing slots which give a sideways motion so as to withdraw from the figure wheels (160,161). In addition, the locking mechanism for the first column (seventh difference, extreme right) was modified to immobilise the first column during a part of the cycle in which it is unsecured (see below, pg. 6). The correct operation of this mechanism relies on the weight of the lock. If each lock was counterbalanced individually, this additional lock would need to be excepted. These difficulties weighed against counterbalancing the locks using individual springs acting on the upper bearing plates. Instead, a single spring-loaded assembly was devised to act on the horizontal drive bars to counterbalance the locks via the bell cranks. This relatively small assembly is visually discrete and for the most part passes unnoticed. The counterbalancing mechanism consists of a set of three springs placed end to end. The springs are in compression and kept in axial alignment by a forked tie rod (E411) threaded along its length and a threaded sleeve (E419) passing through the centre of the springs. The tie rod acts on a lever which is trapped in a recessed block (E414) screwed to the side of the horizontal lock bar. The effect is to bias the bar to the left i.e. in the direction of lift. The compression force is adjusted by turning the threaded sleeve and shortening or lengthening the effective length of the composite spring. There are two identical mechanisms housed in the one assembly - one for the odd axis locks and one for the even axis locks. The six springs are of the same type used on the upper bearing plates to the counterbalance the figure wheel, sector wheel and warning axes. Modification to the Sector Bars The bar driving the sector bell cranks is shown supported only by the bell crank levers (160 and 163). The cranks for the first and last even axes have pivot fixings and the two intermediate cranks are shown with slot drives. The arrangement is assumed to be repeated for the odd sector axes (in 163 the odd sector bar is masked by the even sector bar). As drawn, the sector bars would ride in a slight arc determined by the length of the bell crank arm, and the intention seems to be that the intermediate cranks would lose the vertical component of motion in the slots. This arrangement gave rise to two concerns. The effective length of the crank with the slotted drives is shorter than that of the pivoted cranks, and the vertical motions imparted to the first and last sector axes would therefore differ slightly from those imparted to the intermediate sector axes. Deepening the slots to equalise the lever lengths would weaken the bar to an unacceptable extent and would risk fouling the slots at the extremities of the travel. A further concern was the risk of vertical bowing due to the upthrust from the bell cranks. The sector bars and associated bell cranks were modified to take account of these concerns. The extreme left hand pivot connection between the bell crank and the bar was retained as shown in the original. All other slot drives, including the two closest to the cam stack (odd and even sector bars), were replaced with pivots and slightly elongated holes in the bars. With this arrangement, the vertical component of motion imparted by the arc of the bell crank lever tends to diminish over the length of the bar. Elongating the holes in the bars avoids any contention that might arise from small differences in the lengths of the levers, and the use of pivots in preference to slots avoids the risk of disengagement from upward bowing. An additional mechanism was provided to control the operation of the lock for the first odd figure wheel axis (see below 'Modified Seventh Difference Lock'). A further advantage of replacing the right hand fixed pivots with elongated pivots is that the downward motion imparted to the even sector bar by the additional first odd lock mechanism (E22) does not conflict with the upward arc that would otherwise be imparted by the bell crank. An additional slotted support was fixed to the right hand uprights to provide front-to-rear support for the sector bar close to the cam follower. The purpose of this modification was to spare the bell cranks taking any side thrust from the cam follower lever acting in the profiled slot at the end of the sector bar, and also to reduce the risk of disengagement during normal operation. Modification to the Drive for the Warning Axis The modification of the axes layout for mirroring [see Calculation section] only affects the position of the carry axes so the layout of 177 is unaffected with the exception of the bell crank for the first odd warning axis (seventh difference, top right on 177) which was omitted. The drive slot for the horizontal bar for the axis was also omitted. Cross-over Bar Levers There are three instances in which the bar levers which drive horizontal bars need to cross over bars in the same plane. These are the bar levers for the odd locks, even warnings and even figure wheels (159). The bars for these three motions have raised projections with slots for the bar levers as shown for the even sector bar on 163. Raising the drive slots allows the bar levers to clear the intervening bars and provide drive to the otherwise obstructed bars. The lower cam stack framing plate is drawn 1/2" thick ( 160) with just sufficient clearance for the vertical projections on the three bars. The annotation on 160 indicates that the framing plate should be thickened from 1/2" as drawn, to 1". If the upper surface of the framing plate is taken as the reference and the plate thickened downwards, there would be insufficient space for the slotted projections on the horizontal bars (E324, E342, E334). This was resolved by following the vertical layout arrangement on 163 which is drawn with the plate thickened and with sufficient clearance for the bars. Disengaging the Sectors for Setting Up Babbage's annotated instructions for setting up BAB[F]385 suggest that the sectors should be 'lifted to their highest point' during the setting up procedure i.e. the sectors should be taken out of engagement. The sectors are disengaged during setting up by operating the two sector lifting handles (163, 177). No provision is made on the original drawings for locking these in place to hold the sector axes in their raised (disengaged) positions so as to relieve the operator of the load during setting up. Two pull-out plungers were added to the front horizontal framing members (F361A). During normal operation the plungers are locked in their home positions by bayonet locks. During setting up they are pulled out by hand after the levers have been lifted and provide fixed resting support for the levers. Babbage would perhaps have simply jammed the levers in their raised positions. |
Go to next section Drive
Go to top of Babbage Technical Description