Have you ever wondered how many clicks a simple ball point pen will last for? To solve this question I designed a pen durability machine. What does this machine do? If you have watched the above video you might have a good idea of its function. Essentially all I designed my machine to do is repetitively click a pen, similar to a nervous office colleague who annoys the entire area with their constant pen clicking. This design is a little louder then Steve down the aisle but still accomplishes the same task.
Before designing and building anything you should always ask yourself what it is that you need, just the bare minimum to compete the project. For this machine all in need is for it to actuate the pen. The first task is to determine the easiest method to accomplish that. Thinking about machinery I have seen I was drawn to using a cam system. It is a simple method to turn rotary motion into linear motion. All I need is a small DC motor to turn a cam that will push the pen. Simple and easy to build in a weekend so I got to it.
The motors I had in my salvage bin would not have enough torque to actuate the pen at a descent speed so using my 3D printer and a 3:1 gear ration I was able to slow the motor speed down and gain enough torque to overcome the spring inside the pen. To keep the pen actuation as close to its actual application as possible I used a brass plunger as the wear item against the cam, this way the top of the pen will not wear due to the cam rubbing against it. Before designing the electronics to count the number of cycles I decided to test out the mechanics first. I used a optical interrupt switch and a oscilloscope to measure a cycle rate of about 6Hz. Knowing that I can get a rough estimate of cycles using the time elapsed. Using a 12VDC power supply I ran the machine for 4 hours, ~86K cycles. The pen survived and looks brand new, the cycling machine on the other hand did not fair so well. Lets look at the after photos.
The soft brass took a beating. Piles of brass powder was littering the area. The plastic gearing also had a fair amount of wear, sprinkling small bits of ABS dust into the brass powder. This is not desirable. I expected some wear on the machine itself but not to this extent. My hunch is that most pens should easily last for 1M or more cycles, it does not look like this machine will even last half that long. I need a better design with less moving parts, something more robust.
Pen Cycling Machine V2
I work with solenoids for my day job and they have this amazing ability of turning current into linear motion, either push or pull. So having a lot of familiarity with solenoids and their design my next attempt will incorporate a push solenoid. This should be relatively resistant to wear, there are many designs at work that reach durability cycles of 1M plus and still function like new. This will be the only moving part, aside from the pen, and will greatly decrease the wear components. I can also cycle them at about the same rate as the cam system. Join me soon as I cover the solenoid design.
SD Perry 