Monroe Model K, S/N KA-161-77676
Technology: Stepped drum, hand cranked.
Digits: 8 keyboard, 8 counter, 16 accumulator
Dimensions: 11-1/2"W x 15"D x 8"H, carriage width 18-1/2"
Weight: 29 pounds (13kg)
Manufactured: Orange, New Jersey, 1920s.
This page gives a brief description of the internal mechanism of the Monroe full-keyboard rotary calculator. Although this Model K machine dates from the early 1920's, the principle of operation is identical throughout most of the range.
The Monroe mechanism is based on the stepped drum or "Leibnitz wheel". The essential features of the selector mechanism can be seen in this drawing from Frank Baldwin's US Patent of 1908 (No. 890888). The vertical lever 29 is pivoted at its lower end, and is rocked sideways when the ramps at the bottoms of the key stems 31 press against the horizontal bars 25. The upper end of lever slides the stepped drums along the drive shaft so as to bring the corresponding number of teeth into alignment with the register (not shown).
The first production version of the mechanism is described in US Patent 1080245, submitted by Baldwin in 1912 and assigned to the Monroe Calculating Machine Company of New York. Monroe himself is said to have played a significant part in the development of the production version, and his name appears as inventor on several of the subsequent patents. Development continued under Edgar Phinney until around 1920, and then under George Chase into the 1950s and Herman Gang into the 1960s.
The Monroe Model K calculator was the first major revision of Baldwin's original 1912 design. It was first introduced as a hand-cranked machine in 1921, with the motorised Model KA following a year later.
The machine illustrated was originally built as a Model KA, but the motor and control mechanisms have been removed at some time in its 85-year history. The machine is now equivalent to the hand-cranked Model K, and is still in good working order.
This view shows the chassis of the machine with the keyboard and carriage removed.
The rocking selector bars can be seen in the front section of the machine, with the actuator and idler gear shafts behind. The carriage mounts on the two supports at the top rear corners.
The three large gears drive the actuator shaft and the carry drum (hidden) at the rear of the machine.
This underneath view of the keyboard shows the double ramps on the ends of the keystems, which engage with the pairs of rocking bars to position the stepped-drum mechanism.
Baldwin's original design used straight rocking bars and graduated (machined) ramps on the keystems. This version greatly reduces the cost and complexity by using identical pressed-metal keystems, which engage with graduated flanges on the pressed-metal rocking bars.
The stepped drum mechanism
The rear ends of the selector rocking bars engage with the stepped drums on the actuator shaft (across the diagonal in this view), and slide them sideways to bring the required number of teeth into alignment with the idler gears behind. The idler gears drive the accumulator register in the carriage, which mounts directly above.
The "drums" are actually little more than segments of discs, and operate in "bi-quinary" rather than decimal fashion. There are two drums for each column, one with four graduated teeth, and one with five plain teeth. The pairs of rocking bars slide either or both of these drums into alignment with the idler gear, to advance the register by 0 to 9 places.
The upward-pointing teeth of the carry sensing levers can be seen at the top right of this view, to the left of each idler gear. The levers are pressed and latched downwards whenever a register dial passes zero (in either direction).
The carry drum
The carry "drum" at the rear of the machine is built as a light-weight skeleton frame of cut-away discs. Between each pair of discs are two sets of spring-loaded sliding tabs. If a carry sense lever is latched down, it will push the sliding tab sideways as the drum rotates. The end of the tab will then engage with the idler gear of the next column and advance it by one place.
The Monroe mechanism rotates forwards and backwards for addition and subtraction, so there are two sets of tabs arranged in staggered fashion to allow for ripple carries in either direction.
This underneath view of the carriage shows the accumulator register at the top and the counter below.
The clearing mechanism is at the right-hand end. The single handle clears either register, depending on the direction of rotation. The larger shaft between the two registers carries the clearing stops, and the smaller carries a small cam (centre left) which raises the carriage during clearing.
Part of the carriage shifting rack is visible at the top left.
The numeral wheels are mounted on a series of flanged collars which are keyed to a long shaft. A small spring-loaded pin (above the shaft) provides a detent.
The large pin to the left of the star wheel (near the 3) trips the carry sense lever when the displayed numeral passes zero. The smaller pin to the right of the 9 is a zero stop for the clearing mechanism.
To clear the register, the mechanism first raises the carriage clear of the idler gears. The lower shaft is rotated slightly to bring its teeth into the path of the stop pins on the numeral wheels, and the register shaft is then rotated a full turn. The detent pins drive the numeral wheels around until they strike the stops, and then click over until the shaft returns to its home position.