Rubber Dome Keyboards Now you're probably thinking, why are you bringing this up again? Didn't you just complain about these? You would be right! I did complain quite a bit about these. However, I do want to explain a little bit more about these. Why? Because honestly, you will probably just end up using these for the rest of your life rather than getting a mechanical keyboard, so you should at least know a thing or two about them. With that in mind, lets take another look at the diagram from the previous page. From DeskAuthority.com user codemonkeymike We are aware that the free body diagram looks something like this Adapted from DeskAuthority.com user codemonkeymike Can this design be improved? Indeed, it can! take a look at this. This is a keyboard attached to an old Lenovo Thinkpad T61 Laptop. Can you guess what kind of mechanism it's keyboard uses? Yup! Rubber domes, just like before! However, these guys are just a tiny bit different. This design actually improves on the previous rubber domes in a couple ways: improved key stabilization and reduced actuation force. Since we are already familiar with the forces at work regarding actuation force, we already know that the force required to actuate the key is reflected back on the fingers as a reaction force like so Adapted from DeskAuthority.com user codemonkeymike Now, one of the ways we can reduce the actuation force is by reducing the distance between the top of the dome and the bottom of the dome. This means there is less distance to travel, requiring less force overall. While laptop keyboards are made with fairly stiff rubber so there is less accidental actuation, the force required to actuate a key is still reduce to around 0.5 N. This means that the daily amount of force that our programmer's fingers were subjected to are now (0.5 N) * (5 letters + 1 space) * (60 words per minute) = 180 N per minute or 3.0 N per second 8 fingers * x + 2 fingers *(0.5x) = 3 N per second 9 fingers * x = 3 N per second x = 3 N per second/ 9 fingers = 0.3333 N per finger per second (0.3333 N per finger per second) * (9 fingers) * (60 seconds / 1 minute) * (60 minutes / 1 hour) * (8 hours) = ~86.40 kN per day This is a significant drop from the previous 103.68 kN per day! There is also another key difference in this laptop design. You see those white brackets? Those guys stabilize the keys so they don't wobble. As it turns out, most keyboards on laptops these days tend to not have these stabilizers, leading to a rather mushy and wobboly as well as inconsistent typing experience! But why are they necessary you ask? Consider the following: as you are masterfully touch typing on your keyboard, your finger accidentally slips and presses on the very edge of the key like so Adapted from DeskAuthority.com user codemonkeymike This causes the key to rotate ever so slightly along the edge of the "stem" or "neck" of the key. In physics it is typically called torque, in engineering its typically referred to as "moment".  So, when this torque is created about the edge of the stem, another force must be applied to keep the key from rotating and not actuating properly. Equations adapted from Physics for Scientists and Engineers, 4th Ed. (Knight 2018) Adapted from DeskAuthority.com user codemonkeymike In desktop keyboards, getting around this is fairly simple: make the stem thicker so that the force is in line with the stem. Most of the time, a normal keystroke will hit somewhere above the stem rather than beside it, meaning this won't be an issue for most people. When it does happen, however, the friction force increases dramatically, which usually results in keys binding stuck. Equations adapted from Physics for Scientists and Engineers, 4th Ed. (Knight 2018)  But on a laptop, whose main goal is to be thin and light, this may not be a viable option. A stem that is nearly as thick as the key itself may interfere with the function of the keyboard by preventing the dome from collapsing all the way. So, they make use of the "scissor switch" mechanism, which takes any force on the top of the key and distributes it evenly along the key. This results in the key not experiencing torque while it is being depressed, eliminating most of the binding issue. However, since it is still a rubber dome, one can be prone to mashing the keys in order to get the computer to register a key press, eliminating the biggest advantage this design has over desktop rubber dome keyboards of reduced actuation force. Also, while key binding is an annoying issue when it happens, most modern keyboards are not as prone to this issue. So, how do we properly address reduced actuation force in a way that will meaningfully improve productivity? Simple! Change the point at which the key activates.

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