Stepped Drum mechanism
Other wheel mechanisms
Toothed rack mechanisms
The various adding machines and calculators can be categorised in many ways - by size, complexity, input method, mechanism, etc. Below I set out the various categories that I feel are the most relevant, with examples of each category from my collection. Note that not every calculator will be listed here, so for the complete list you need to visit the main Mechanical Calculators Page.
A Direct Drive machine is one where the parts that display the digits of the result register are directly manipulated by the user. These are the simplest type of adding machines, in that they essentially consist only of a single register, often with a carry mechanism.
This category can be further subdivided by the input method used, which can be sliders, wheels, or dials.
The classic example of this type are Addiators, i.e. Troncet adders, such as the Pocket Adding Machine. The sliders that you push up or down also act as the register display.
On this type of machine the register consists of a set of wheels next to each other on the same axle. The Addimat / Schnellrechner is the obvious example. I consider the Golden Gem to be a direct drive adder with wheels, because its chains are essentially just extensions of the number wheels. A chain acts like a large flattened wheel that you manipulate.
Dials are wheels on an axis that is pointed towards you, so you have to do a circular motion to turn them. There are many dial adders such as the Calcumeter, Dial-A-Matic, Stephenson's adder, and the Kes-add. I would also include the Lightning Adding Machine, because its number wheels are directly and constantly connected to the dials. The dials do not have to be the same size, see for example the Webb Adder, and can be concentric, like in the BriCal.
An adding machine has Semi-Direct Drive when the results register digits are updated during the input of a number, but the input and the register are not continually connected. There is usually some kind of ratchet system between the two to allow the input mechanism to return to its initial state without undoing the changes that were made to the register. It is also possible that the ratchet system is in effect while setting the input, and that the transfer to the register happens when the input is cleared. Note that on machines in this category the input cannot be retained for repeated additions, because the either the setting or the clearing of the input drives the register directly.
This category can also be further subdivided by the input method used. In addition to sliders, wheels, and dials, this category also allows buttons or keys to be used.
The Resulta E7 is a good example of this type. Its predecessor, called the Minerva was still an adder with direct drive as it did not have a separate input register. The Triumphator KA also falls into this category, because its input levers are just a way of turning the wheels of the input register.
The only example of a semi-direct adding machine that uses dials is the Conto. Note that it adjusts the main register when the input dials are cleared rather than when the input is performed.
Key-driven adding machines fall into this category. The register is updated during either the downstroke or the upstroke of a key. Examples of this type are the Burroughs Calculator, the Comptometer, Sumlock and Plus Adders, the Direct II and the Contex A/B. There are also small counting machines with buttons, such as the Aderes, the Alexe, the Benton Tally Register, the Denominator, and the Pocket Counter.
With this type of machine the input is separate from the results register, and only applied by turning a crank or pulling a lever, whether by hand or by electric motor. The input is retained so that it can be added repeatedly, allowing for small multiplications. If the register and the input can shift relative to each other, then larger multiplications can be performed too. That makes these machines fully fledged calculators as opposed to adding machines.
In these machines the input method is not as important as the way the input is then transferred during operation. The registers generally have number wheels, so the input of a digit 0-9 needs to be converted to a varying amount of rotation. How this conversion is done gives a good way to further categorise these machines. These mechanism categories are unique to these indirect drive machines and are mostly not applicable to the semi-direct or direct types of adding machine.
The two most common and well-known mechanisms are the stepped drum, and the pinwheel.
The stepped drum was first successfully used in a calculator by Leibniz, but it wasn't until Thomas de Colmar's Arithmometer that stepped drum machines were produced in larger numbers. Consider one gear, driving another gear of the same size. One revolution of the first gear causes one revolution of the second. If you were to remove the teeth from one half of the first gear, then a full revolution would only cause half a revolution of the second gear.
Now imagine that you have an axle on which there are ten gears, each with a different number of teeth, ranging from zero to nine. So you have one "gear" with no teeth, one gear with only one tooth, one with two teeth, and so on until the one with nine teeth. All of these are next to each other on a shared axle. On a parallel axle you have another gear, but you can slide it so that it can interact with any one of the ten gears on the first axle. In this way you can use a sliding motion to select how much the second axle is to be turned by the first axle. A stepped drum is essentially the set of ten gears on the first axle, but fused together into a single object. In its simplest form it is a cylinder with ridges of different lengths, but parts of the cylinder where there are no ridges can be cut away to reduce weight.
In the Thomas Arithmometer and its descendants such as the Unitas and the Archimedes, there is one stepped drum for each digit of the input. The input is through a slider or keyboard, and it sets the position of the gear that is driven by the stepped drum. It is of course also possible to move the stepped drum relative to a fixed gear. This even allows the stepped drum to be made of two halves, one half whose number of teeth range from 0 to 4, and the other half which is set to supply either 0 or 5 extra teeth. This is done in the Monroe calculators.
A much more sophisticated stepped drum is used in the Curta. This drum is shared by all the input digits, which are arranged at regular intervals around the drum. Instead of ridges, the Curta's drum has small pins, similar to what you would see on a cylinder of a music box. The drum has 20 different tracks, alternating between adding and subtracting, one pair of tracks for each digit. Pulling the crank up for subtraction mode simply shifts the drum by one step, shifting from the additive tracks to the subtractive ones.
The pinwheel first became commercially successful with the machine designed by Wilgott Odhner. Although there were previous attempts to use the mechanism going back almost two centuries, only a few machines had been constructed using the principle.
A pinwheel is a gear with teeth that are retractable or extendable. Usually it has a lever attached to it that the user can move to select how many teeth the pinwheel gear has, ranging from 0 to 9 teeth.
The pinwheel has the disadvantage that when the pinwheel is turned, the setting lever turns with it. This means that there should be some kind of locking system to ensure the setting lever does not get moved inappropriately when the wheel turns. But more than that, the setting lever usually is short and thin because it has to go inside the body of the machine. Examples of these are the Brunsviga, Minerva, and Walther
Some machines were made that had setting levers that disconnected from the pinwheels when the crank was turned (e.g. Brunsviga model J) but that was complicated and expensive. There are also machines that use an external keyboard to set the pins of the pinwheel, and Facit machines are a good example of this.
Both the stepped drum and the pinwheel are essentially ways of selecting the number of teeth that are present on a gear. Instead of changing the number of teeth of a wheel, it is also possible to change what fraction of a full revolution the wheel is engaged.
One way is to have a fixed camwheel with a setting lever with which the profile of the cam wheel can be changed. The rotating gear wheel next to it has a cam follower that changes the behaviour of the gear wheel as it goes through the rotation. The Hamann Manus has wheels in which the teeth are on a ring that becomes connected or disconnected as the axle rotates ("switching latch" mechanism). The Marchant XL has wheels in which a toothed section is retractable during part of the rotation ("adapting segment" mechanism).
Instead of a wheel in which we select the number of teeth, it is also possible to have a linear rack with a variable number of teeth. That is what happens in the Thales KA - each key column is a rack, and the state of each key adds or removes one tooth. Pressing the lever pulls the number wheels along the racks, turning them appropriately. On the return journey the wheels travel at a lower height along a second set of racks which handle carries between the wheels.
One could also have a rack with a fixed number of teeth, but simply vary the distance that the rack moves to get the amount of rotation corresponding to each digit. The Brunsviga 90 KA has toothed racks that are pulled until they are blocked by barriers that were set up by pressing input keys on the keyboard. The Corema uses something similar. The Contex 10 uses a curved rack at one end of a lever, and the other end of the lever travels until it hits a pin that was set up by pressing a digit key.
The only mechanism I know of that does not fall into any of the above categories is the one used in the later Marchant calculators, such as the Silent Speed and Figurematic models. Instead of a stepped drum for each digit, there is a 10-speed gearbox. Unlike all other calculators, this makes the number wheels turn at different speeds, but for the same length of time. Combining this with a carry mechanism that is a simple 10:1 gear ratio, the number wheels turn directly to the answer in one smooth motion instead of the start-stop motion of other machines.
© Copyright 2019 Jaap Scherphuis, mechcalc a t jaapsch d o t net.