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Notes to viewers

The paper version of this document was obtained from
Reg Crick of the Science Museum on August 6, 1999

Please recognize that scanning and OCRing and conversion to HTML is an imperfect art, and differences from the original must be considered to be present.

Please Note: Items in Italics were hand written and must be further reviewed by experts

The cover page is marked in handwritting
"2nd typing Corrected Draft Copy 1
latest on 13/1/93 - DS

Update history

  • November 17, 2000, Mike Gleen kindly sent some OCR and HTML corrections which have been incorporated. :-)) (My OCR program frequently interpreted "is" as "1s". :-(


to Operate and Maintain

Charles Babbage's
2nd Difference Engine

built by

Barrie Holloway and Reg Crick

June 1991

for the

Science Museum,
London SW7 2DD

Barrie Holloway and Reg Crick

June 1991

Section Title Page
Introduction 3
Engine Site and Moving of the Engine 4
Engine Case 5
Setting the Figure Wheels for a Calculation 6
Speed of Operation 8
Lubrication of Engine 9
Trouble Shooting 10
Spares 13


December 26th 1991 was the bicentenary of the birth of Charles Babbage. To mark that occasion, the Science Museum, in London, built Babbage's Second Difference Engine.

For eight months the public watched the construction of the calculating Engine, which was designed around 1847 by Charles Babbage, an English mathematician. The Engine was designed to calculate and print mathematical tables, and this current attempt was the first ever made to build this Engine or any other of Babbage's Engines. The fully operational Engine was completed in June 1991.

The Engine is designed to be set up and operated by hand, by a skilled and trained operator who is fully conversant with all the intricacies and problems of setting up. The operator should understand the contents of this instruction manual.

Only trained maintenance staff, who fully understand the running of the engine, should be allowed to carry out any form of repair or periodic maintenance work on the engine. When the engine is not in operation or gallery maintenance work is being carried out, the glass cases should be placed around the Engine and firmly locked up. This will reduce the amount of dust accumalating on the Engine.

1. Engine Site and Moving of the Engine

The Engine, complete with it's case, occupies an area of 12.65 feet by 6.65 feet and is 8.21 feet in height. Additional floor space is required to allow for the removal of the glass case and the subsequent storage when the engine is being demonstrated or worked on. Weight of the Engine is 5860 lbs / 2.62 tons. Adequate floor reinforcement is required at the six main jacking points and spreader plates are used under each jacking point. By using the jacking points, the Engine should be sited level. No other services are required to operate the Engine as it is fully mechanical.

To move the Engine to another site:-

Unlock and remove both halves of the glass case (section 2).
Remove the wooden plinth from around the Engine by unscrewing and removing the end sections and then removing the side sections. Underneath the wooden plinth is the steel base; located In the side are the tie rods, pivot blocks, pins and bolts for stabilising the Engine during the move. There are eight tie rods, four each of two lengths. Two of each length are used each side of the Engine. Mount the top steadies to the top of the Engine frame. Mount the pivot blocks, together with one end of each tie rod, to the base, secure the other end of each tie rod to the top steadies. Adjust the lock nuts to tension the rods. Screw down the six jacking bolts to lift the Engine clear of the floor.

Each bolt to be turned a couple of turns at a time to prevent distortion of the base. Once the base has been raised, it will allow sufficient space to enter with additional lifting gear and skates to allow the Engine to be moved. The Engine rail fixing bolts must be slackened before moving the Engine. Spreader boards should be laid in the path of the Engine to spread the load over a larger floor area. Once the engine has reached it's new location, remove all lifting gear and skate boards. By adjusting the six jacks, lower the Engine and level it. Re-tighten the rail fixing bolts. Remove all the stays, pivot blocks and top steadies and store along the side of the base. Check over the Engine to see that none of the mechanisms have been disturbed, especially the carry mechanisms, and that all the Sectors and Figure wheels are free on their axes.

Four outriggers are used at the four corners of the steel base to move the Engine in and out of the goods lift. These can also be used to assist with the levelling of the Engine.

Cycle the Engine (see Section 4) to check that all sequences are free to rotate and lift.

Check Engine by carrying out a calculation, y = x from x = -80 to x = +80 will check all the functions of the Engine and exercise all the carry levers. (see Section 3). If there is an error, check operation of each carry mechanism (see Section 6). If the calculation is correct, then continue with the final stages of positioning the Engine.

Replace the wooden plinth, firstly place the two sides and then screw the ends into position. Return the two case halves to the Engine and jack the cases until they seal. Lock into position.

2. Engine Case

The Engine is protected by a glass case, which, when unlocked, splits into two halves. Each half is on three wheels, which allows it to be pushed away from the Engine. The rear wheel is on a swivel joint, which allows steering. The joint between the two halves has a rubber seal which helps to prevent dust from entering. There are guide runners along each side of the wooden plinth, which guides the case when being removed or closed. There is also a locating pin on the top cross member which helps to guide the two halves together in the final stages of closing the case.

The case can be opened and closed by one person, but it is easier with two when trying to steer the case on to the plinth.

To unlock and remove the case:-
Either section of the case can be removed first, depending at which end the handle has been stored. In general the handle is stored at the cam stack end of the Engine.

  1. The case half over the cam stack end of the Engine will be removed first.

    Using keys 3 and 4 unlock the bolts on the end skirting. Rotate both bolts anti-clockwise until they can be removed. Remove the bolts and the end skirting. This provides access to the main case handle and the two jacking points. Remove the main handle and using the bolts connect the handle to the case chassis, using the outer holes.The skirting can now be placed across the two bolts between the handle and the glass. Using a tommy bar, slacken the two jacking points by turning them anti-clockwise until both are well clear of the floor.Remove the steel pads. The case half can now be pulled back, and once clear of the plinth can be moved to another location.

  2. The same procedure can now take place at the other end of the Engine, unlocking the bolts using keys 1 and 2. There will be no second handle under the Engine. If the case is just to be pushed back, this can be done without using the handle. The skirting can be placed with the first skirting on the first case half. If the case half ss to be moved to another location, then the handle is to be removed from the first case half and affixed to the second case half.

  3. To return the case halves to the Engine and to lock the cases, the reverse procedure is followed.

    Each case half is steered and pushed around the wooden plinth. The case half at the print end of the Engine should be secured first. Using the wooden blocks and bolts, the case half is purled up until it's stops are in contact with the plinth. Some adjustment of the jacking points may be required to align the bolt holes. Once in position, both jacking points should be fully screwed down on to their metal plates. Once jacked the bolts are released and the wooden blocks left in the end section of the base. The skirting can now be secured to the end and locked.

    When pushing the second case half over the Engine, ensure that the location peg in the top member locates correctly into the hole in the other case half. Secure the case half and fully jack up. Check that both side joins have closed before fixing the final skirting. If there is a gap between the side joins, adjust the lock nuts on the jacks and jack up further until the gap closes. Stow the handle under the wooden plinth before fixing the end skirting.

3. Setting the Figure Wheels for the Calculation

Babbage's Instructions (385) are incorrect. If followed all the figure wheels would not read zero and some locks would jam. This is due to the random movement of the figure wheels caused by the return motion of the figure wheel levers. Babbage apparently overlooked this problem. To overcome this, temporary setting locks are used. Each one is mounted to the cursors and are engaged into and disengaged from the wheels during the setting up cycle. By following the table below, the Engine will reach and stay on zero by the end of the first cycle. During the second cycle they will hold the set numbers until the sectors are re-engaged at the end of this cycle.

The procedures set out In the table below start at the completion of a set of calculations with the Chapter disc at zero.

FIRST 0 From front of Engine lift small lever at cam stack to disengage drive to sector bars. Also lift the two larger levers (sector lift levers) to lift the sectors out of mesh from the figure wheels. All setting locks should be disengaged.
. 10 Engage ODD setting locks. Odd axels at zero
. 35 Engage EVEN setting 1ocks. Even axels at zero

SECOND 20 Disengage ODD setting locks. Set required numbers on to ODD figure wheel columns.
Re-engage ODD setting locks.
. 45 Disengage EVEN setting locks. Set required numbers on to EVEN figure wheel columns.
Re-engage EVEN setting locks.
. 0 Release sector lift levers and re-engage the sector drive. Disengage all setting locks.
The Engine is now set up to calculate.

Note: All axels must ??? zero at the completion of the first cycle - if not the Engine will jam during the second cycle.

The Engine is now ready to perform a calculation.

The numbers to be set on the figure wheels are the coefficients of the differences of the polynomial equation to be calculated. Viewing the Engine from the front, all eight columns of figure wheels are displayed.

From left to right the axes are EVEN, ODD, EVEN, ODD, etc. The result is read on the first column from the left each time the chapter disc is at zero. The result is to thirty (30) significant figures, reading units, tens, hundreds, etc as you read up the axis.The number of axes used to calculate the results are one more than the level of the equation, eg x4 requires five axes, the first being the left column. The second from the left is the first difference, the third is the second difference and so on.

To set the numbers on the figure wheels during the setting procedure, you turn each wheel by hand in either direction, setting the number required under the arrow. At the setting, to set odd or to set even, the warning axes are rotated away from the figure wheels. When the wheel is turned by hand through 9 to 0 no warning is set up. When calculations are being carried out on the Engine, each turn of the chapter disc past zero is the completion of one calculation. The next turn Increments the value of x In the equation being solved. If the Operator stopped turning the handle when the chapter disc reached zero, he could come to the front of the Engine and read the result on the first column. Babbage intended the Operator not to stop turning the handle but to continue turning until a successsion of calculations had been completed. The results, in theory, should be automatically printed out in table form and only when a page of tables is completed should the Operator stop. The Engine will automatically cut out the drive to the handle when the end of a page is reached. The Operator can then change the printing tray and re-set the drive to continue with the calculations.

The carry levers can be set in the inoperative position if not required. This is done at two different parts of the cycle.

At zero on the Chapter disc, the even carry levers can be set to their inoperative position. At 25 on the Chapter disc the odd carry levers can be set.

To move the carry levers at these settings, use a large screwdriver and slacken the three ?fixing? screws that fix the detect reset st??? to the figure wheel supports (of axels to be adjusted), Do not remove the screws. These fixing ??? in slots. Lever up the detect reset stops so that the screw fitings now sit at the bottom end of the slots.

The carry lever can now be rotated to the inoperative position so that the detect locates into the fourth notch. When all habe been set into inoperative position the detent reset stops can then be lowered, carefully checking that all carry levers are behind the stops.

Once the detent ??? stops have been fully lowered, the three fixing screws can then be secured.

When the Engine is next cycled, those carry levers in the inoperative position will not function even if a figure wheel passes from 9 to 0. The lever on the back of the carry lever is now clear of the rib ?a? the figure wheel.

Once the carry levers have been returned back to their operational position, the engine must be cycled one revolution of the Chapter disc before setting [text ran off the Xeroxed sheet]

4. Speed of Operation

Steady turning of the handle is required at a speed of approximately 8 to 10 calculations per minute (or 30 to 40 revolutions of the handle per minute). Slower or faster speeds will result in the Engine jamming. The twin tooth drive at the rear of the phasing gear strikes the impact tooth at the rear of the register pinion, before the phasing gear meshes with the register pinion. The twin tooth and impact tooth are made of high grade steel (EN 16). This engagement is to absorb the shock load of engagement that takes place as the gear and pinion are of cast iron.

If the speed of operation is increased to more than 10 calculations per minute, the register pinion overruns and the teeth mesh jams during the next cycle. With slower or erratic turning of the handle, the sectors fail to mesh with the figure wheels correctly, due to insufficient momentum for engagement. The sector wheel teeth sit on top of the figure wheel teeth (See Trouble Shooting for how to clear jams).

5. Lubrication of Engine

The Engine should be oiled and greased on a regular basis.

If the Engine is being demonstrated on a daily basis, lt should be oiled and greased at least once a week. If no demonstrations are taking place, then the Engine should be oiled and greased on a monthly basis, but the handle should be turned at least twice a week to cycle the mechanisms.

Grease : "Alvania" grease or it's equivalent should be used.


  1. all vertical motion cam profiles only and their levers.
  2. all bevel gears above and below the cam stack.
  3. all bevel gears on the carry axes and those on the carry drive shaft.
  4. the phasing gear, register pinion, "Impact tooth and the tw-in tooth drive.
  5. the pawl wheel and crank pinion.

Oil : "Nevis 7" oil or it's equivalent should be used. This is a non-staining oil.


  1. top of shaft positions on figure wheels, sectors, warnings and carries. Fill counterbores on all eight axes.
  2. middle shaft positions on figure wheels, sectors, warnings and carries. Fill counterbores on all eight axes.
  3. top lock and zero stop slide positions on all eight axes.
  4. middle zero stop slide positions on all eight axes.
  5. bottom lock slide positions In frame on all eight axes.
  6. yoke positions on zero stops, top and bottom, on all eight axes.
  7. all the bottom sets of levers, at oil holes, joints connections to lever bars and at yoke connections to axes shafts.
  8. bottom bearing bars at shaft entry at eight positions.
  9. link joints at bottom of each figure wheel lock.
  10. vertical motion bars in bearing slide plates.
  11. counterbalance springs and mechanisms for figure wheel locks.
  12. cam lever engagements into lever bars at bottom of cam stack.
  13. circular motion cam followers on cam stack.
  14. print shaft bearings in four positions.
  15. crank handle at access screw.
  16. top cam bevel at screw position.
  17. top bearing mounts on top of cam stack.
  18. cross shaft on top of cam stack, one screw position.
  19. carry axis drive shaft bearing blocks, two positions.
  20. sector engage/disengage handle spindles at front of Engine.
  21. all rack slides for all axes, including four oil holes in top warning casting.
  22. all con rod pivot pin joints.
  23. teeth mesh between sectors and figure wheel on all eight axes.
  24. the wipe faces of the carry levers and their detents on all seven axes.

Empty drip trays as and when required.

To empty drip trays remove the end sections of the wooden plinth. Unclip the strip which holds the two trays together. Slide each tray out, empty and then return. Re-clip the trays and replace the wooden end sections.

6. Trouble Shooting

a) Engine Jamming b) Calculation Errors c) Removal of Axes

a) Engine Jamming

  1. 1) Jamming of the phasing gear to the register pinion on the carry drive.

    This is due to erratic turning of the handle or turning the handle too fast, giving rise to an over-run of the register pinion. This prevents the phasing gear from meshing at the next commencement of the intermittent drive cycle. To clear the jam a large screwdriver is required to rotate the register pinion anti-clockwise, thus releasing the jam. The Engine will continue with no loss of calculation numbers or error induction.

  2. Jamming due to the figure wheel locks striking the tops of the figure wheel teeth
    OR the sectors failing to mesh with the figure wheels

    This is due to erratic turning of the handle or turning the handle too slowly. Either the lock jams by striking the figure wheel teeth which has failed to fully rotate or the sectors have rested on the top of the figure wheel teeth due to one or the other failing to fully rotate. The error in rotation is only one to two degrees and is generally due to the lack of momentum to complete the rotation and engage. To clear the Jam establish which reason has caused the problem. To check the figure wheel locks, try lifting them at the bottom link. If they are free there will be a small amount of movement. If one has jammed it will feel solid. To clear the jam, remove the shoulder screw and washer at the bottom of the lock and lift the lock up. Check the columns of figure wheels to see which one has been slightly displaced. Turn the displaced wheel to align the number to the arrow. Do not over-rotate the wheel otherwise the wrong number will be set up and an error created in the calculation. Lower the lock and check that it now engages into the figure wheel teeth. Re-connect the shoulder screw and washer. Re-check all the figure wheel locks again before attempting to turn the handle. To check the sector/figure wheel mesh, try pulling down each column of sectors. If they are free there will be some movement. If one has jammed it will be solid. To clear the jam, move each sector on the column until you find' the one that is solid due to resting on the figure wheel. Using a large screwdriver, lever up the column of sectors enough to take the load and free the sector/wheel mesh. Release the column of sectors and pull down on the top to check there is now movement. Repeat the procedure if still solid, as there may be more than one sector involved. Re-check all the other sector columns. With care the jam can be cleared without interrupting the calculation cycle and the calculation continued without errors.

  3. Jamming due to sectors catching the sector zero stop

    Carry out the same procedure as (11) except you are now looking at the sector/zero stop relationship. If this jam cannot be cleared you may have to remove the sector zero stop to free the jam and then return the stop back to it's original position. To remove the sector zero stop, remove the two screws on the top bearing plate and the two screws in the bottom bearing plate that holds the stop. Again there should be no interruption of the calculation cycle.

b) Calculation Errors

Errors In calculation are either due to the Operator setting the wrong coefficients on to the figure wheels (see Section 3) or a carry lever that has failed to operate correctly.

When the Engine is moved to a new site it is suggested that the calculation of y = x' from x = -80 to x = +80 be carried out to check the accuracy of operation. If the Engine is being used on a regular basis this check should be carried out at the start of every week. If there is an error in the calculation then the Engine should be set up to carry out two sets of calculations simultaneously. Calculate y = x from x = -80 to +80 as before. With x = -80 or +80 the result has only fourteen significant figures. Therefore the two calculations can be set one above the other, y = x' across the axes using wheels 1 to 14, y = x using wheels 15 to 28 and setting the counter on wheels 29 to 31. The results can be compared and checked to see if they are correct after each calculation. If one answer is wrong, then the coefficients at that setting can be compared with those of the other equation. By this means you can identify which carry has failed to function. That carry lever can then be adjusted using the tool provided and the operation repeated. This procedure is repeated until the full set of calculations has been carried out without error. (N.B. Care must be taken not to overstress the carry levers when adjusting otherwise the solder joint of the lever will break.)

c) Removal of Axes

It may be necessary, from time to time, to remove an axis so that adjustments can be made or damaged parts replaced. The initial procedure is common to all four axes, warning, figure wheel, sector and carry. Ladders, small and large screwdrivers, alien key, rule, file, hammer and a small pin punch will be required.

Protect the wooden plinth with board before placing ladders on it's surface. Set the Engine at "0" for even axes and "25" for odd axes. Remove the three top collars of the axes of the difference being attended to. Remove springs and cups. Undo and remove all the fixing screws for the bearing plate, large screws which fix it to the main frame, medium screws which hold the figure wheel supports, small screws which hold sector zero stop and the figure wheel lock slide block. The phosphor bronze components can remain attached to the bearing plate. Lift and remove the bearing plate.

Carry Axes:

To remove the carry axis, unscrew the bottom bevel screw and lift the axis out from the middle bearing plate. In removing the axis, the bottom bevel, key and spacer will be released. To repair any carry arms, they can be removed from either end of the axis. Release the top or bottom collar by removing the taper pin and then slide off the carry arm until the one that requires attention is removed. The carry arms must be returned in exactly the reverse order and the carry axis returned to the Engine in exactly the same position. Check the pitch position by comparing it with the other axes.

Warning Axes:

To remove the warning axis you must first remove the carry axis. Remove the carry reset arm. Move all the carry levers into the fourth notch position, ie the inoperative position. Release the bottom bobbin and release the bottom carry lock. The axis can now be lifted up unitil it is clear of the middle bearing plate. In removing the axis, the bobbin, two sleeves and the quadrant will be released. Leave these in position. Remove the top two collars on the main axis and secondary spindle. Rotate the top carry lever until It is free to lift off. Remove carry lever. Remove spring housing, spring and detent lever. Remove detent support arm. Keep all in one set. This procedure should be repeated for each carry lever until the faulty one has been reached. Return all the carry lever assemblies by the reverse procedure. Return the axis back to the Engine, ensuring that the quadrant is correctly meshed with it's rack. Return the carry axis, bearing plate, etc. Each carry lever must then be checked in turn against the operation of It's four nibs. Any removal of the warning axis must result "in the re-checking of all carry levers on that axis. Two people are required to carry out this test.

Disconnect the figure wheel lock and raise it on to packers. The figure wheels are now clear of the lock.

One person on the front of the Engine will rotate the figure wheel from 9 to 0 four times, thus checking all four nibs. The second person at the rear of the Engine will check that the carry lever warns for each nib, resetting between each operation. Any that fail to warn correctly should be adjusted using the tool provided. Once the axis is correct, a calculation check should be carried out.

Figure Wheel Axes:

The shaft on the figure wheel axis does not have to be removed. To gain access to the figure wheels one must first remove the carry and warning axes as described in the previous paragraphs. Remove the sector zero stop and rotate all sectors 180 degrees. Remove the figure wheel lock by releasing the bottom shoulder screw and lifting the lock out of the Engine.

Remove one screw top and bottom of each figure wheel support and twist the figure wheel support until all the figure wheels have been released. Remove the top collar on the figure wheel shaft. Remove the figure wheel zero arm and then the figure wheel. Continue this process until you reach the required figure wheel. Keep all the zero arms and wheels in their pairs. Once the required maintenance is completed re-assemble the axis. Return all other axes, figure wheel lock, zero stop, etc. Once the Engine has been fully assembled, the carry levers must be re-checked as described under warning axes, to ensure that each carry is activated by it's nibs.

Sector Axes:

As before, the top bearing plate has to be removed. Remove the figure wheel lock. Similar to the figure wheel axis the shaft does not have to be removed, neither does the figure wheel axis, warning axis or carry axis. Remove the sector zero stop. Remove the top collar on the sector shaft and then remove the sector zero arms and sectors, similar to the process adopted for the figure wheels. Keep all the zero arms and sectors in pairs. Once the required maintenance is completed, re-assemble the axis. Return all the other components to the Engine. Once fully assembled acalculation should be carried out on the Engine to ensure correct working. Follow the procedures for checking carries if the calculation results are incorrect. It is not necessary to check the carries in the first instant as these have not been disturbed.

7. Spares

A number of components should be held as spares to replace parts that have broken, worn or lost when arrangements are dismantled from the Engine. Detailed below is a suggested list of parts, time will determine if other parts should be added to the list.

337C 349 Spring 2 off Flexo Springs, pt no.444048
337C 354 Spring 5 off Flexo Springs, pt no. 112804
337A 126A Even Detent Lever 3 off
337A 126B Odd Detent Lever 3 off
337A 127 Spring Housing 5 off
337A 122A Even Carry Lever 5 off
337A 122B Odd Carry Lever 5 off
337A 133A Odd Sector 2 off
337A 133B Even Sector 2 off
337A 133C Last Even Sector 1 off
337C 357A Figure Wheel 2 off
337C 357B Figure Wheel 2 off

end 14 April 1992

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