The Physics of Computer Keyboards

Ergonomics Rubber Dome Switches
Mechanical Switches

Types Of Mechanical Keyboards


What makes a mechanical keyboard a mechanical keyboard?

Well, there are a couple significant differences that specifically mechanical keyboards have over your standard rubber dome keyboard.

1. Mechanical keyboards do not require a key to be fully depressed, or bottom out, in order to actuate. Most use some sort of mechanism that allows the circuit to complete before the key is fully depressed, resulting in less force exerted on the fingers, as well as a significant increase in typing speed for those who have trained with lighter key actuation force.

3. Mechanical are often much more durable than rubber dome keyboards. Rubber domes degrade overtime and are unserviceable when a key no longer functions. However, mechanical keyboards are often made with discreet modules or standardized parts which were designed to be replaced. Also, due to the properties of the materials being used (plastic and metal vs rubber), they degrade at a much slower rate. Rubber domes last about 10 million key presses per key, but mechanical switches often last more than 50 million key presses. Some have even been rated up to 30 billion keypresses!

3. Mechanical keyboards come in a very wide variety of mechanisms or switches, usually variations of one of two designs, The Cherry MX design (the most common) or Alps Electric design. Some make an audible sound to notify the typist that the key has actuated, while others have a bump that is noticed after the key has passed the actuation point. Other switches use rubber to muffle the sound and use grease to ensure the plastic on plastic surfaces don't have a scratchy noise for a silent experience. Its quite staggering the number of different switches out there.

So, for simplicity's sake, we will focus on three different types of mechanisms: The Cherry MX switch, the Alps Electric SKCM switch, and IBM Buckling Spring switch.

Cherry MX Switches


US Patent 4467160A

This is what is commonly referred to as the Cherry MX switch. If you were to go buy a "mechanical keyboard" or "gaming keyboard", chances are you would find something that uses this switch, or at least a derivative of its design. The Cherry MX switch design is composed of a couple core parts.

The "keycap", the piece of plastic that has the letters or numbers printed on it. Usually, keycaps are changed for cosmetic purposes, but sometimes it brings functional benefits, such as allowing light to pass through if there is an LED back light. The only real purpose they serve is to provide an idea of what key executes what letter when pressed.
2. The slider, which would serve a similar function to the stem referred to in the rubber dome page. However, in modern designs, the stem is given a standardized shape in order to replace the keycaps. The slider may also be comprised of multiple parts, such as the "click jacket" which is in the above design. The "click jacket" produces the click sound, usually after but rarely synchronized key actuation.

3. A spring, which provides the return force for the switch. The springs can be weighted differently to improve actuation speed via reduction in actuation force.

4. Metal contacts, which connect to the PCB that the switches are attached to, as well as provide a path for the electrical signal to pass through.

US Patent 4467160A

So, you can see that in this design, bottoming out isn't required for the key to actuate. Since the slider redirects the downward force horizontally with angled contacts like so:


This is pretty significant, seeing as rubber domes require bottoming out. This means that the typist is less susceptible to mashing the keys and exerting more force than necessary once they are used to the actuation force. While it would take a bit of time to break the habit of bottoming out the keys, another extremely useful feature is the "click jacket", which produces a clicking noise after a certain amount of force is applied. By associating key actuation with a sound or physical change in key feel, the typist is able to improve typing speed.

However, the more significant part of this switch is the spring! So, unlike the most common keyboards, which are comprised of one large rubber dome mat that are not adjustable, Cherry MX switches come in several different configurations. This includes springs of different spring constants!

* NOTE *:For sake of simplification, keyboard actuation force is generally measured in gram-force (gf), so springs are generally described in terms like "55 gram weight" or "55 gram springs" (Fylladitakis, 2016). This doesn't have to do with the spring's actual mass, but rather describes the spring constant in terms that are easier to conceptualize as opposed to the typical unit of N/kg which usually describes the spring constant per Hooke's Law (F= -k * x, from Knight, 2018)). In reality, gram force is somewhere around 0.98 centi-newtons (1 cN = 0.01 N) per 1 gram-force or 1.02 gf per 1 cN. Calculations are much easier if we assume 0.01 N = 1 gram-force.

For example, if we put a spring that requires 0.55 N to actuate, then our ever diligent computer programmer experience

(0.55 N) * (5 letters + 1 space) * (60 words per minute) = 198 N per minute or 3.3 N per second

8 fingers * x + 2 fingers *(0.5x) = 3.3 N per second
9 fingers * x = 3.3 N per second
x = 3.3 N per second/ 9 fingers = 0.367 N per finger per second

0.367 N per finger per second) * (9 fingers) * (60 seconds / 1 minute) * (60 minutes / 1 hour) * (8 hours)
95,040 N or 95.04 kN per work day

Now, while this particular spring results with more force than the laptop rubber dome calculation, this result is still lower than the standard desktop rubber dome, while also being more consistent over time. This is due to the allowance of "over-travel", or the space between key actuation and bottoming out, resulting in the fingers being cushioned instead of running into a stiff resistance upon actuation, as well as less time needed to return the key to its initial state to be actuated again.

While the actuation distance can also be changed, from the typical 2mm of key travel to 1.2 mm key travel found in some "gaming keyboards", both modifications serve a similar purpose. Springs, in fact, are actually much easier to replace than adjusting the design of the metal contacts. Some keyboard enthusiasts even replace each spring individually to fit their preferences!

However, the Cherry MX switch is not the only mechanism with these advantages.

Alps SKCM Switches

*Note*: This is an Alps Clone switch called the NEC Blue, which share an extremely similar mechanism.

US Patent 4642433A

The Alps design, while mechanically different from Cherry MX switches, offers very similar advantages and a few others that Cherry MX lacks.

The basic parts are nearly identical, simply having different designs, such as the metal contacts being on the same side of the switch housing rather than opposite sides, or the slider being rectangular rather than a cross point.

Lets examine that slider a bit more. Now, while I don't have a great picture of an actual Alps Switch slider, the very first picture shows an Alps switch clone that has a slider that is inspired by the Alps switch. Notice how the stem is elongated and has a thin plastic post in the middle. This stem offers a key advantage over the much thinner Cherry MX stem in that it reduces torque like the laptop scissor switches before.

* Note *: This is a rubber dome switch, but the concept of torque is the same

By making the stem wider, torque is reduced by spreading the force evenly and directly above the slider, reducing friction. While Cherry MX tends to have less friction than rubber domes, many enthusiasts claim that all MX designs "feel scratchy", referencing the uncommon key binding that some have experienced with the switch
(Fylladitakis, 2016). Modern Cherry MX switches sometimes use lubricant to alleviate this, while the Alps Switch's wider stem and simpler construction, such as using a leaf spring to provide tactile feedback along with the click sound rather than a deformation on the slider, seem to solve this issue. Check it out below.

US Patent 4642433A

The Alps Design was not without flaws though. Though it was fairly easy to service (or modify for enthusiasts), the switch was rather sensitive, sometimes with only a little powder rendering a switch defunct permanently unless it was meticulously cleaned. Despite lacking the preference of typists, Cherry MX became more mainstream and edged Alps Electric, the company which owned the original Alps Switch patent, out of the keyboard space. The only modern Alps switches are clones.

However, there is a switch that exists which many consider to both designs, despite being older than both!

IBM Buckling Springs


US Patent 4118611A

The IBM Buckling Spring is perhaps the most mechanically interesting switch of the bunch. Why? Because it offers the common advantages of the previous two designs using only two moving parts! You see, the downward force of the key press is directed horizontally via the bent spring, which pushes the middle of the spring outward. This causes the spring to buckle sharply, rotating a plastic flapper which connects traces on the PCB via a thin plastic membrane. Check it out in the free body diagram below!


Due to the rotational motion as a result of the buckling spring, the flapper rotates rather fast, creating a clicking noise once it hits the PCB and plastic membrane. From a mechanical standpoint, its pretty cool how the previous designs required around 6 to 8 parts, this one does it with a fraction of the parts! Shows how well a simple understanding of physics and design can go a long way! It's durable, serviceable, and doesn't suffer from the same friction issues from previous designs to to the massive stem thickness. The only real disadvantage was that it was more susceptible to water damage due to the lack of liquid drain holes.

 However, the switch was not made as discreet module, as well as being much louder than other switches, which was less preferred by office workers who wanted a quieter work space. Ultimately, IBM phased out this design in favor of the less expensive and quieter rubber domes. It was a sad time in keyboard history, but most definitely a step toward the quiet working spaces of today.

So, hope fully you learned a little bit about keyboards and physics! If you are still interested in other goodies keyboards sometimes have, checkout the Extras! page.

Javier De Leon
Physics 211 - Fall 2018
University of Alaska Fairbanks
Background: "Blue" by karenatsharon is licensed under CC BY 2.0