Rolls Mechanical Calculator: What It Is & How It Works

The Rolls mechanical calculator is a brand of pin-wheel (or Odhner-type) calculating machine, manufactured primarily in England during the mid-20th century. You enter numbers using sliding levers, turn a crank to add, subtract, multiply, or divide, and read the result from two sets of dials—one for the accumulated number and one for a running count of crank turns. These machines were the desktop computers of their era, used in offices, banks, and engineering firms from the 1930s through the 1970s.

Formula

The Rolls calculator performs arithmetic mechanically, but the underlying math is simple. For addition and subtraction, the result is directly the sum or difference of the two entered numbers:

Sum = A + B
Difference = A − B

For multiplication, the machine repeats addition: A × B is the same as adding A to itself B times. For division, it repeats subtraction: A ÷ B is how many times B can be subtracted from A before reaching zero (or a remainder).

In the calculator block above, A and B are the two numbers you enter. The sum and difference are shown as whole numbers, matching the typical integer-only display of a mechanical calculator.

Worked Example

Using the defaults from the block: A = 123, B = 456.

• Sum: 123 + 456 = 579
• Difference: 123 − 456 = −333

The block shows Sum = 579 and Difference = −333, exactly as calculated. Note that mechanical calculators of this type usually handle negative results by showing a complement (e.g., 999667 for −333), but the digital model here gives the direct answer.

Background: The Odhner Pin-Wheel Design

The Rolls calculator is based on the pin-wheel mechanism invented by Willgodt Odhner in the 1870s. Instead of gears with teeth, it uses a set of retractable pins arranged around a wheel. Each pin can be extended or retracted to represent a digit (0–9). When you pull a lever, the pins engage with a counting gear, adding or subtracting the digit. The design was compact, reliable, and cheaper to produce than earlier stepped-drum machines like the Arithmometer.

Rolls (full name: Rolls-Mark) was a British manufacturer, active from the 1930s until the early 1970s. They produced a range of models, from small hand-cranked units to electric motor-driven versions. The machines were known for their solid cast-iron bodies, enamel paint, and distinctive curved shape. They were widely used in British offices and schools, and many survive today as collector’s items.

How the Rolls Calculator Works

Using a Rolls calculator involves a specific sequence of operations. Here is a step-by-step for a typical addition:

1. Clear the machine by pulling the clearing lever (usually on the left side).
2. Set the first number by sliding the levers on the front panel. Each lever corresponds to a decimal place.
3. Turn the crank one full turn forward (clockwise). The number appears in the result dial.
4. Set the second number on the levers.
5. Turn the crank forward again. The result now shows the sum.

For subtraction, you turn the crank backward (counter-clockwise). Multiplication is repeated forward turns (e.g., 3 × 4 means setting 3 and turning the crank 4 times). Division is repeated backward turns, with a separate dial counting the turns to give the quotient.

Key components:

• Input levers: Set digits for each column.
• Crank: Performs the operation; forward adds, backward subtracts.
• Result dial: Shows the accumulated total.
• Revolution counter: Shows how many times the crank was turned (used for multiplication/division).
• Clearing lever: Resets the dials to zero.

Common Problems and Maintenance

Decades after they were made, Rolls calculators often need attention. Common issues include:

• Sticky levers: Old grease hardens. Flush with a light solvent (like isopropyl alcohol) and re-lubricate with a drop of light machine oil.
• Pins stuck: Individual pins may not retract. Gently work them free with a small screwdriver.
• Incorrect results: Often due to a pin not engaging. Check each pin visually.
• Broken crank handle: Replacement handles are sometimes available from vintage calculator parts dealers.

Always avoid using heavy grease or WD-40 as a lubricant, as it can attract dust and cause more problems. If the machine is severely damaged, consider consulting a professional restorer.

Comparing to Other Mechanical Calculators

Rolls calculators are similar to other pin-wheel machines like the Brunsviga, Facit, and Odhner. The main differences are in build quality, layout, and brand reputation. Rolls machines are generally considered solid and reliable, though less common than the Swedish Facit or German Brunsviga. Compared to stepped-drum machines (like the Monroe or Marchant), pin-wheel calculators are simpler, quieter, and easier to maintain, but they are slower for heavy multiplication.

Electric models (with a motor that turns the crank) were faster and more expensive, but the manual hand-crank versions are more common and cheaper to collect today.

Costs and Collecting

Prices for Rolls calculators vary widely. On auction sites and vintage markets, a non-working example might sell for $20–50, while a fully restored, working unit can fetch $150–300. Rare models, especially electric ones in original boxes, can go higher. Condition is key: check for rust, missing parts, and whether the pins move freely. A machine that works mechanically is worth more than a decorative piece.

If you’re buying, ask the seller if the crank turns and if the levers move. Even if it’s stuck, it may be repairable with patience.

Alternatives and Legacy

By the 1970s, electronic calculators made mechanical ones obsolete. The first handheld electronic calculators (like the Texas Instruments TI-2500) were faster, quieter, and cheaper to produce. Today, you can simulate a mechanical calculator on your phone or computer, but many people still enjoy the tactile experience of a real Rolls.

If you’re interested in vintage computing, a Rolls calculator is a great entry point. It’s also a fantastic teaching tool to demonstrate how arithmetic works at a fundamental level.

Frequently Asked Questions

Is a Rolls calculator the same as an adding machine?

Yes, in common usage. An adding machine is a general term for a mechanical calculator that adds and subtracts. The Rolls is a specific brand of pin-wheel adding machine, which can also multiply and divide.

How do I clean a Rolls calculator without damaging it?

Use a soft brush to remove dust. For sticky parts, apply a small amount of isopropyl alcohol to a cotton swab and gently clean the pins and levers. Avoid soaking the mechanism. After cleaning, apply a tiny drop of light machine oil (like sewing machine oil) to moving joints.

Can I use a Rolls calculator for decimal numbers?

Most models only handle whole numbers. To work with decimals, you manually track the decimal point. For example, to add 1.23 + 4.56, you enter 123 and 456, add them to get 579, then place the decimal point two places from the right: 5.79.

Why does my Rolls calculator give wrong answers?

Check the pins: each digit lever should have exactly the right number of pins extended. If a pin is stuck, the digit will be off. Also, ensure the machine is fully cleared before starting a new calculation.

Are Rolls calculators worth money?

Working models can fetch $100–200 on collector markets. Rare models or those in pristine condition with original boxes can be worth more. Non-working ones are usually cheap, but may be repairable.

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in a | First number | - | 123 | 0 | 999999
in b | Second number | - | 456 | 0 | 999999
out sum | Sum | - | a + b | 0
out diff | Difference (a - b) | - | a - b | 0