This is my project for Mr. Tabora's AP Physics C class. Intended to "produce a product that will communicate my understanding of the physics of the device", it'll show how to take apart and understand some of the mechanisms in an Underwood No. 5 typewriter, and give ideas for improving it. Click each image for a more detailed explanation.

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"Underwood No. 5" - The Classic Typewriter Page

Step 1: Gathering Materials

To take the typewriter apart, you'll need thin flathead screwdrivers in different sizes. Gloves will help, but may easily be caught and torn in one of the typewriter's many rollers and levers.

Step 2: An Overview of Typewriter Physics

Before we get started, let's get an overview of how a typewriter works.

The basic action is pretty simple: you press a key, and a character swings up to hit an ink ribbon against the paper, making a typed letter. Under the hood, however, things get complicated pretty quickly. A complex system of levers converts the force of your fingers pressing down a short distance to a less powerful force over a longer distance. Momentum builds up in the typebar(1) as it swings. When it hits the ribbon it applies an impulse strong enough to compress the ribbon against the paper, marking a single letter.

Meanwhile, a separate mechanism is working in the back of the typewriter. Each time a character is typed, the carriage(2) moves one space to the left. Once the user reaches the far right of the paper, a bell rings to alert them to press the carriage return lever, moving the paper up and winding the carriage back to the left side of the page.

Also included are keys (and thus mechanisms) for shifting to uppercase letters, backing up a space, and switching between red and black ink. This typewriter also has adjustable tab spacing and page margins, but we won't cover that.

typebar: Also known as "type hammer". A lever with a character on the end that swings up to hit the ribbon.
carriage: The roller mechanism responsible for holding and advancing the paper.

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Step 3: The Key Mechanism

Before taking anything apart, let's take a last look at the key lever and typebar.

Step 4: Disassembly

First, we'll remove the front plate. Unscrew each of the four corner screws and lift it out. We are now able to see the strip of soft material the type falls back on when it's released. This typewriter is around a hundred years old, and it's in pretty bad shape.

Step 5: Removing the Tab Marking Plate

Now we'll take out the two screws holding in this margin-marking plate. With it removed, we can see the ratcheting mechanism that allows the margin stops to only be moved in one direction without squeezing their handles. A closer look shows us that the stuck right margin has no physical obstacle - it's just stuck in place. A strong pull frees it, but it would need cleaning and lubricant to be moved easily by the operator.

Step 6: Finding Damaged Areas

Removing the topmost decorative plate, we find rust creeping in. The platen, the large rubber cylinder that holds the paper, has hardened with age, and has been damaged by type hitting it directly. It can no longer do its job of providing a soft backing; with enough force, letters will now tear the paper (visible on the 'C's in earlier photos).

Step 7: Replacing the Platen -- or Not.

To recondition the platen, we could remove it and treat its surface. We could also replace it with a similarly sized tube of rubber. Removing the platen requires removing both side knobs, the indenting mechanism from the left side, and the carriage return lever. Unfortunately, the flathead screws are badly rusted and prone to slipping. Without something to help extract the jammed screws, we can't proceed.

Incidentally, getting this far exposes the mainspring mechanism. A flexible strip attaches the coiled mainspring to the carriage. The force from this spring pulls the carriage, either freely when hitting "Tab" or one tick at a time when typing.

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Step 8: Final Thoughts and Possible Improvements

This typewriter could be salvaged, but it's in very bad shape. Antique typewriters still hold some value in good condition, but because so many were made (remember, this is "the most successful typewriter design in history") one with this much damage is probably worth less than its thirty pounds of metal as scrap.

Typewriters in this style enjoyed a relatively brief existence. Although this form of front-striking, open-faced typewriter spawned many imitators, continuous improvements eventually made them obsolete. The typewriter has many obvious problems: it's heavy, it's loud, and print quality is inconsistent simply because a typist can't hit the keys with the exact same amount of force every time. Although the QWERTY keyboard reduces their incidence, key jams when two keys are pressed at the same time are still a problem.

These problems and more were steadily dealt with: lighter, portable typewriters were produced, "silent" typewriters were designed, and electric typewriters like the IBM Selectric struck each key with the right amount of force, producing even text. Ultimately, the word processor and versatile personal computer did away with the single-purpose, inconvenient typewriter, although some are still produced today for use in lower-income countries.

If I were to improve this typewriter's design (without transforming it into something else entirely), I would make a simple change: a clamp or screw to hold ribbon spools of different sizes. As typewriters shrank, so did their ribbons, and the limited types of ribbon that can be found in an ordinary office supply store are too small for the imposing Underwood No. 5. I would also consider an auto-advancing ribbon cartridge instead of a user-adjustable ribbon to save time and improve the sometimes-blurry letters that ribbon can produce.

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