Click pens.
You click them on, you click them off.
How do they work?
And what creates that distinctive click?
There are many types of click-to-write pens.
A simple latch mechanism locks this ink cartridge in place; it’s released by pressing a button.
And in this pen an angled-tooth cam locks or releases the ink
cartridge.
And here a triangular cam that tilts from side-to-side
to lock and release the cartridge.
In this video I detail the operation of this pen: a Parker
Jotter, introduced in 1954.
It’s one of the first retractable pens.
Now, listen carefully and you’ll notice that it emits four clicks.
A click when I push the button ... a second
click when I release it …And then when I push the button to retract the
ink cartridge, a third click ... and release… a fourth click.
To understand why these clicks occur let’s look inside.
The pen’s barrel houses an ink cartridge with
the ballpoint tip, two springs—one at the top and one at the
bottom—and a plastic tube which is press fit inside the metal barrel
of the pen so that it is fixed in place.
Inside that tube is where all the action happens.
A Parker Pen Company patent describe the three parts that produce the pen’s click: the
cam body, the tubular plunger and the stop members.
The cam body is affixed to the top of the replaceable ink cartridge.
During operation this cam body rotates and slides up and down.
The tubular plunger does not rotate; it only slides up or down.
The stop members are shown as separate pieces here,
but keep in mind they’re part of the press-fit plastic tube and so
never move.
The pen can be in either of two positions: the retracted,
non-writing position where the tip of the cartridge is inside the
barrel.
Or, in the extended, writing position where the cartridge’s
tip projects from the barrel.
To see how the plunger, cam body and stop members interact I’ve broken the cam
movements into eight steps.
First, push the button to drive the plunger and the cam body down the
barrel until the top of the cam body drops below the fixed stop
members.
Second, remember that the spring at the bottom of the pen
pushes the cam body up and so keeps it in contact with the plunger.
That axial spring force shoves the cam body up the barrel, but also
rotates the cam body because it mates with the plunger at an
angle.
The cam body rotates 45 degrees until it strikes the surface of
the plunger.
That very fast motion creates the first click.
Third, I release the button and a spring between the
plastic tube and the lip of the plunger forces it to the top of the
pen.
When the tips of the plunger’s fingers clear the cam body, the
cam body rotates another 45 degrees until it strikes the blue stop members.
That’s the second click of the pen!
Fourth, I release the button completely and now the
cam body remains locked in place.
It cannot rotate nor move upward and so the pen is ready to write.
To retract the pen I press the button to drive the plunger into the cam body, and to
push it past the stop members.
The cam body rotates until it strikes the plunger.
That’s the third click.
I release the button so the top spring forces the plunger
upward.
When the plunger fingers clear the cam body, it slides and
rotates 45 degrees until it strikes into the stop members.
That’s the fourth click.
Once the button is completely released stop members slot
into the cam body so that when the plunger moves up, the cam body
moves with it to retract the ink cartidge.
In extending and retracting the ink cartridge the cam body rotates 45
degrees four times — or 180 degrees total.
The cam body has 180 degree rotational symmetry so the
next extend/retract cycle repeats the same sequence of steps.
This particular model of pen has sold over three-quarters
of a billion units since its debut.
It took a team of sixty-six people to design this pen.
And although it’s a mechanical marvel, the Parker company
worried that this retractable ballpoint ink pen would damage their
reputation — Parker pens were fountain pens — and so the company left
off their iconic “arrow” clip.
But within four years the pen earned its arrow.
And inspired other pen makers to design click and lock
mechanisms — all different to avoid patent infringement.
The clicking of a pen demonstrates how a single engineering
problem can have many engineering solutions ranging from robust,
to cost efficient, to unique.
I’m Bill Hammack, the EngineerGuy.
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