Overview
I was able to acquire a Marchant ACR8M “Silent Speed” mechanical calculator bought off of Ebay. This is the calculator after some light cleaning and repair.
The calculator includes its serial number and patent information on the bottom of the machine.
Marchant calculators were top of the line machines throughout the early 1900s. The Silent Speed was among the fastest of any mechanical calculators ever made thanks to a unique and ingenious proportional drive mechanism. Division was among the fastest as well thanks to decaded mechanism for anticipating an underflow, saving machine cycles.
The model number is listed for this machine as ACR8M-161734. Serial numbers 161,000 - 165,00 were manufactured in 1942. Serial numbers and manufacture dates for the Silent Speed, Figuremaster, and Figurmatic can be found on this page. The calculator was in very good condition. Outer panels have light scratching and flaking as well as decade old tape residue. The Marchant painted logos are still present in all places. With only minimal surface cleaning, the calculator looks quite nice. The tape residue is resistant to any cleaners I put on and is completely hardened. Attempts to scrape it off resulted in paint coming off as well so I have left it for now.
The carriage contains a 9 digit counter register and a 16 digit accumulator register. Both registers carry automatically. There multiple sliders for keeping track of decimals on both registers. An arrow shows above the current counter position (over the 8th digit in the picture). To the left of the counter register are 6 tab setting buttons labeled 1 - 8. The tab buttons, when pressed, cause the carriage to stop at a certain position after an operation or tab return.
The calculator has 8 key columns (8M in ACR8M) on the keyboard for digit entry. Each column consists of 1 - 9, with a 10th red key at the bottom. The bottom row of keys is used to clear or zero a individual column and set it back to zero. The keyboard only allows one key to be pressed at a time in each column. Pressing another key releases the previous one that was pressed in a column. The keyboard also contains 9 decimal indicators. They can be flipped over to a painted white side via the wheels below the zeroing keys. This is used to help keep track of decimal places. Above the keyboard is a display register that shows the currently pressed keys on the keyboard.
Beside the keyboard, there are control keys and another column of keys. In the top right, is a red auto division key. To the right, is the division stop key. Below that row is the addition and subtraction keys. To the right of the subtraction keys is the carriage shift-direction keys. There is a shift left, no shift, and shift right key. These keys determine which way the carriage moves during tabulation and during multiplication. Below those three keys is a reverse key. The reverse key, when set, does the opposite of the operation pressed (addition or subtraction).
In-between the reverse and addition keys is a small lever. This lever, when set downwards, causes the carriage return to a position determined by which shift-key is currently set and clear the accumulator register after division. If the shift right key is pressed and the lever is down, after a division the carriage will automatically return to the right and clear the accumulator register. If the lever is in the up position the carriage will not move or clear after a division.
In the bottom left is the keyboard clear button. This key clears all columns on the keyboard. To the right of that key, is the tab and clear keys. The clear tab key returns the machine to position determined by the shift-keys. Below the tab key are two carriage clear keys. A accumulator register clear key and a counter register clear key. All three keys can be pressed at the same time.
To the right of those keys, is a column of digits. This is the “on-the-fly” multiplier. Pressing these keys multiplies what is in the accumulator register by the key pressed, and increments the counter register by that amount. The carriage then moves in the direction determined by the shift key setting. If no shift is pressed, the carriage does not shift.
In the top right, beside the multiplier column, is another small lever. This lever reverses the counter register. When the lever is in the up position, the counter register increments with each operation. If the lever is down, the calculator decrements with each operation.
On the far right, are two “manual” carriage shift buttons. The buttons shift the carriage one position, or until the button is released. If the carriage cannot go any further and a carriage shift is initiated, the motor turns on for a second then the machine stops.
Operation
Addition
Single addition is simple and the same as on many other keyboard mechanical calculators. A first number is entered on the keyboard and the Addition key is pressed. This places the number in the accumulator register. The counter register is incremented by one. The keyboard is automatically cleared. Another number can be entered on the keyboard. When the addition key is pressed again, the product of the addition is in the accumulator register. For repeated additions less than 10 times, the number should be entered on the keyboard, then the addition key pressed at the same time as the multiplier row key for the number of successive additions you want. The carriage will not shift after this operation no matter of the shift key setting. The counter register is incremented by the number of additions.
Subtraction
Subtraction is essentially the same process as addition except using the subtraction key. Repeated subtractions use the same method as well.
Multiplication
The Silent Speed features a dedicated multiplier row for fast multiplication. The multiplier can be entered forwards or backwards. To enter the multiplier forward, the left-shift key should be pressed. If the carriage is not in the most right position, the Clear Tab key can be pressed to move the carriage all the way to the right. The multiplicand should be entered on the keyboard. Then the multiplier is entered digit by digit using the multiplier row. The carriage automatically shifts after each multiplication. The Silent Speed also features a mechanical buffer system, meaning the next digit of the multiplier can be entered before the previous operations has even finished. This combined with the speed of the machine means the multiplier can be entered continuously with no waiting for the machine to finish an operation. If the Right-shift key is pressed, the carriage will move to the right, so the multiplies will need to be entered from the last digit to first. The result is stored in the accumulator register, with the multiplier in the counter register.
Division
The Silent Speed has one of the fastest auto-division mechanism of any mechanical calculator ever made. First, the carriage must be placed all the way to the right via the manual carriage shift key or a Left shift key set then Clear Tab. First, the dividend is entered on the far left side of the keyboard for the most accuracy (via more decimal places). This number is placed in the accumulator register with the addition key. The divisor should be entered on the keyboard next. The counter register shows a 1 but is automatically cleared before division. The Auto divide is then pressed. The machine will automatically perform the division. The quotient is in the counter register. If the Division return lever is in the down position, the carriage will automatically return home and clear the remainder from the accumulator register. Otherwise, the division stops when the carriage has reached its maximum right position.
Outer Panels Removed
The outer covers on the body of the machine are easily taken off via 12 screws. First the back panel is removed followed by the two side panels.
The carriage has three panels easily taken off the machine as well.
Removing the panels shows a glimpse of the mechanical complexity inside the machine.
Repair and Some Inner Workings
Initially the, keyboard on the calculator arrived with multiple keys pressed in a single column. The keys were popped up using a large flat-head screwdriver and were fine after that. No control keys were pressed except for the manual left shift key. When the machine was plugged up, the carriage continually tried to shift left even though it was at the maximum right position. The manual shift left button was stuck down. Applying some oil to the two nuts on the left-hand side of the following photo and pulling on the button released it.
The carriage stopped attempting to shift but the motor still ran continuously. Inspecting the machine revealed the machine was at the end of a cycle but the control latches were still in place keeping the machine running. The culprit was the main clutch at the top-back of the control unit.
Removed from the machine, the clutch looks like this:
The clutch has a visible spring loaded cam that should disengage at the end of an operation. This cam allows for the spring loaded drive pawls on the clutch to be engaged or disengaged. Several clutch dogs rest against these drive pawls and release control unit latches, specifically one keeps the motor engaged or disengages it. The main clutch is also connected to a reverse clutch and many other drive lines. Here is another view of the main clutch assembly.
Popping the cam forwards with a screwdriver, oiling the clutch, and repeating the process freed it up properly after about three tries.
Multiplication got stuck often or didn’t move as well. Playing with the multiplication and more disassembly eventually showed several parts that were stuck thanks to old oil or grease. The first part was this piece connected to the multiplication column.
At the left-center of the picture are two pieces diagonally connected downwards to the same shaft. These had a large amount of buildup on them.
Another piece that was also stuck can be seen on the bottom of the control unit and freed up after some oiling. The piece can be seen left of center with a large flat bolt head.
Occasional register sticking was occurring, so it’s a good practice to regularly oil the carriage dials. The Marchant Silent Speed machine carriage digits have a unique mechanism from other calculators. Thanks to the proportional drive mechanism, another digit carry mechanism was needed. Each digit in a register is connected to the next left digit in the register. One full rotation of a digit rotates the next digit 1/10th, and the next left digit 1/100th, and so forth. Because a digit can be not centered thanks to the proportional gearing, a selector unit is in place to ensure each digit is properly determined, centered, and displayed. This, the clearing mechanism, and the division trip mechanism results in a lot of gears and moving parts that could get stuck.
Division Video
A video showing multiple angles of the machine performing division with the outer panels removed can be found on my YouTube channel.
Division Slow Motion Video
Also, a slow motion video showing multiple angles of the machine performing division with the outer panels removed can be found on my YouTube channel as well.
More Pictures
The Westinghouse motor.
Setting gears, clearing levers, main motor input drive with safety clutch, and the contact to turn on the machine.
Motor input and safety clutch removed.
Multiplication column removed from the machine.
Removable button plate with addition, subtraction, and keyboard clear levers.
Partially disassembled control unit taken off the machine.
Right side of machine with control unit removed.
Carriage shift gear and maximum shift lever seen above the carriage dial contact gears.
Carriage contact gears.
Carriage clearing arms, underside of carriage, and digit closeup.
Bottom of the machine with baseplate removed. Keyboard selector levers can be seen as well as the main setting line for the machine.
