Why is the spring strengthened in the middle?
It doesn’t seem to affect the spring’s buckling characteristics.
My speculation is that it’s to reduce spring noise. That strengthened region at the middle is where the spring will buckle outwards most, resting against the barely visible side rails on the inside of the case. Instead of just one wobbly contact point it now has three rigid ones as a “skate” to reduce the stick-slip noise when opening and retracting the tip. Is this right?
(The pen is a Mitsubishi Uni-Ball Power Tank, pretty much my favorite model.)
This is one of my favorite models also. It will write well on thermal paper. I especially like how it’ll write on wet paper, which is a big bonus if you do a lot of plumbing
The way the spring is designed is very akin to taking two separate springs, and attaching them in series. On a highly technical level, this creates a quite different response to motion than a single spring would have. On a lower tech level, it’s approximately the same as regular spring with a different stiffness value.
I’d like to think that some engineers went down the rabbit hole, doing differential equations or modal analysis to solve some complex design problem. More likely it was just a cheap or easy way to solve a manufacturing or supply problem.
Edit: Upon further reflection, spring stiffness does not care about length. The primary concerns are thumb travel (deflection) and force to click (deflection times stiffness). I suspect that deflection is pretty well set by ergonomic factors. This design would reduce the effective spring stiffness by half (and therefore the force). The most obvious other way to reduce stiffness would be reducing wire thickness, which could reduce reliability.
Noise, on the other hand, is an expected result of clicky-pens. People don’t like products behaving unexpectedly. If the design made the pen quieter, you can bet they would advertise it on the packaging.
I kind of hate how much thought I’ve put into this.
Upon further reflection, spring stiffness does not care about length.
It does. If you have two springs with the same diameter made from the same wire, but one spring has the double amount of coils, i.e. the double length, the longer spring will have half the stiffness (or the double compliance) of the shorter spring.
This design would reduce the effective spring stiffness by half (and therefore the force)
In fact it’s the opposite. The “dead” coils at the end and in the centre increase the overall (compressive) striffness of the spring.
The most obvious other way to reduce stiffness would be reducing wire thickness, which could reduce reliability.
Slightly increasing (or decreasing) spring stiffness by changing the wire diameter is much harder to do, as the diameter of the wire enters the stiffness factor by the power of four
orders of magnitude, i.e. increasing the diameter from 0.4 mm to 0.5 mm increases the stiffness by factor 2.44. Thus, it’s much simpler to introduce ‘dead’ or fewer coils in the spring to increase its stiffness when the outer geometric design properties (spring diameter and length) are given by the design of the pen.When referring to length, a given spring does not care about total length (ignoring situations where you’ll introduce buckling or other modes of loading). Changing the number of coils means you’d be using a different spring design. For a given spring design, it’s the same stiffness whether it’s 5mm long or 10mm. That makes the math of splitting it in series simple.
I am curious about the full impact of the dead coils (I like that term). I was treating then simply as a rigid connections, effectively splitting the spring into series, reducing the effective stiffness. Can you elaborate on how they would work to increase the stiffness?
Your explanation on the effect of diameter makes a lot of sense, especially given how tight tolerances must be at this scale. I just assumed you wouldn’t want to get the spring too thin for strength reasons.
This prevents spring distortion by effectively creating two shorter spring segments, probably because the pen designer wanted a longer barrel and the mechanical engineer was told to “figure it out that’s why we pay you” and the truth is I have no idea it just sounds plausible enough.
My guess too. A longer spring would have more deflection in the middle. This design could reduce that and increase the spring force.
how would it deflect when confined in a plastic cylinder?
Could be that the machine they have to make springs is designed to make single length springs and they changed it up a bit to accommodate making a double-length version by only changing the cut frequency or something similar
This seems most plausible to me
I just have to say this is a cool photo.
The spring on its own:
Shenanigans. There’s no way people’s fingernails are that nicely trimmed. I call AI
It maintains better tension and will last longer. Great design honestly.
I think as a pen manufacturer you just buy bulk springs and then pull them to the desired length. In this case it’s either cheaper for them or they have an excess of longer springs.
You would think that but almost nothing is sourced that way. Easier and cheaper just to have them made to your engineering specs. For my business we make all the hardware in house so we get better part quality than to use off the shelf screws/bolts.
Mine doesn’t. Seems like an aesthetic feature, since the pen is transparent.
I just checked a couple of my clicky pens and springs are normal
If you can find another pen of the same type, you could see whether it’s consistent. My guess is that the spring “stuck” in that spot when they pulled/set it.
If not, it could be a noise thing.
