Edit> Calculated some times. A 0.3g key takes 0.067s to rise 4mm with a constant 55g spring force.
A 10.9g key takes 0.45s. Nearly half a second!
Hmmm - been awhile since I took Physics (much longer than I care to admit), but pretty sure that's not quite right.
If I am correct:
First, we have to convert everything into meters, kilograms and newtons for distance, mass and force units to be correctly applied.
Key Travel (Displacement) = 4mm = 0.004 meters
Mass of Original Plastic key = 0.3 g = 0.0003 kg
Mass of New Metal key = 10.9 g = 0.0109 kg
Nominal Spring "Force" on key measured as weight to actuate, discounting key weight = 55g** = 0.055 kg
** Plus or minus the 0.3g of the plastic key is well inside the margin of error of the Rip-O-Meter so seems reasonable to take the full measurement as the weightless strength of the spring in this case.
Converting the kg of the Spring "Force" measurement to Newtons (actual unit of Force) we multiply by conversion factor of 9.80665 Newtons per 1 kg (at Earth Sea-Level):
F = 0.055 x 9.80665 = 0.539 newtons
Acceleration from A = F / M:
Original key = 0.539 / 0.0003 = 1796.67 meters per second per second
Metal key = 0.539 / 0.0109 = 49.45 meters per second per second
Now, to take into account Gravity's effect, subtract 9.8 meters per second per second for each (assuming, of course, that the keys are vertical, though the effects of slightly angling the keys is relatively minor if only a few degrees off):
Original key Net Acceleration = 1796.67 - 9.8 = 1786.87
Metal key Net Acceleration = 49.45 - 9.8 = 39.65
To get how much time it takes to travel from full down to full up assuming a constant force from the spring:
Time squared (in seconds) = Displacement (in meters) / one-half Acceleration
(above from the simplified formula S (distance traveled) = (1/2 x A) x (T squared) - - since initial Velocity is 0)
So, plugging things into this formula gives the following Key return times:
Plastic Key = 0.00212 seconds = approx. 2 milliseconds
Metal Key = 0.0142 seconds = approx. 14 milliseconds
So even the metal key would give a theoretical limit of around 35 characters per second on a given key if the down and up time are identical and with no time between strokes (or 7 words per second = 420 wpm).
Mind you, all of this assumes a constant acceleration force from the spring for the whole distance, with no friction, and that the keycap is the only mass being accelerated (the key stem/plunger should be weighed and added to the keycap to achieve more accurate results).
EDIT: It occurred to me that perhaps I should have used only the net spring force after the weight of the keys themselves were deducted (55g and 44g for Plastic and Metal respectively) as that is all that would be available to provide the acceleration force. If this is the case, then the revised results are:
Plastic key (taken as baseline and essentially the same) = 0.00212 seconds
Metal key = 0.0164 seconds (only a 2.2 millisecond difference and still giving 30 characters per second)
The revised figures for Metal Key:
F = 0.044 x 9.80665 = 0.4315 newtons
A = 0.4315 / 0.0109 = 39.59 meters per second per second
Net A = 39.59 - 9.8 = 29.79
Time to travel up 4mm = 0.0164
EDIT - EDIT: Wow, I am rusty. Don't know what I was thinking trying to put the gravitational stuff in there when it is already accounted for if you net the opposing forces of spring and weight. So last "Edit" approach is correct but without the spurious 9.8 m/s
2 held over from first try. (Note, however, that the initial 55g force measurement for the spring is essentially already a net force for the original plastic key if the measurement was taken with the key in place.)
Final revisions result in (by coincidence basically the same as the first try):
Plastic key = 0.00211 = approx. 2 milliseconds
Metal key = 0.01420 = approx. 14 milliseconds
