First Steps: Analysis and review of previous designs, customer needs

I've gotten started by glancing through data compiled by previous engineers on this project. Various issues have prevented most of them from succeeding, and I hope to build off their successes, and failures. Much preliminary work is out of the way, leaving me the fun parts. Out of five or six designs suggested previously, most work recently has concentrated on the "Zany Penguin" design. While reminiscent of a linux distribution, it's surprisingly lacking in kernels. What it does have, however, is the basic theory of a pusher/grabber mechanism removing condoms one at a time from the bottom of a stack. This isn't the traditional method -- Typically, gas-station style machines (I'll refer to them as that from now on, i think) rely on a helix coil with condoms placed within -- similar to those big black and glass snack vending machines that are ubiquitous in every office building ever constructed. While this method was carefully considered -- if it ain't broke, don't fix it -- consensus of my predecessors was that the system has a few irreconcilable problems:

• Mechanically, it's complex. And difficult to source parts for -- complex gear drive, specifically designed helix inventory rack, etc.
• From an end-user perspective, it's fairly time-consuming and troublesome to maintain. Each time it's refilled, condoms have to be individually added to each "slot" in the helix, and lined up just so to ensure it vends reliably. Our goal is to get condoms to people as easily and cheaply as possible, rather than for profit, and those stocking the machines will be local business-owners and individual volunteers, not Pepsi Salesmen. 

Other designs were interesting -- there's some novel approaches, involving circular drums and such, but the one which shows the most potential remains Zany Penguin.

I was initially hopeful upon viewing the current (when I joined) CAD models: Things looked pretty well fleshed-out and ready for prototyping. However, as I examined the model more deeply, I noticed a few major oversights. (Fig. 1)

 

 

 

Fig.1: Zany Penguin, alpha build

What you're looking at is four rows of sheet metal square 'tubes', which I've taken to calling inventory tubes. The bottom of each tube is positioned above the axle, which rotates, engaging the bottom condom with a large, square block. This pushes the condom forward, and out to the customer.

Clearly this design isn't as finished as it first appears. Firstly, there's no system for getting the condom from the axle to the customer -- some kind of chute is needed. Also, the coin mechanism still needs to be integrated, a case design finished, etc. These are minor details, however, compared to the big issue -- will the device actually move condoms from the inventory tubes to their final destination?

One of the most valuable things an engineer knows is that, while every gadget works perfectly in his head, unfortunately, his mind does not encompass the entire world. The large problem here lies with physics, and typically involves friction. Both of these concepts were developed by physicists and scientists, and, as usual, it's their fault, not ours. However, we still have the problems to deal with.

A really nice tool for dealing with this problem is mathematics, specifically the kind done by a piece of silicon, not the engineer. I'm referring to computer-aided simulation. While the condom-inventory tube-axle mechanism is rather complex, some reasonable simplifications can get it to the point of easily tweaking with it and seeing what happens for various designs. The largest issue with the Zany Penguin Alpha is one of friction. A large stack of condoms wishes very much to stay a large stack of condoms, due to the weight pushing on the bottom condom. When the axle encounters the condom and suggests that it, and none of its friends, leave for the cold cruel world, the condom takes offense. Either it tries to bring ten of its closest condom-friends and jams the mechanism, or it politely declines the axle's suggestion, and the axle slips right past, to try again a rotation later.

What needs to be checked, is that the amount of force applied to slide the bottom condom out is enough to make it budge. However, it has to be applied just right, or else multiple condoms are going to start moving.

This is where the gas-station companies stopped, had a coffee break, and someone noticed a snack vending machine. The rest is history.

Of course, being stubborn and viewing this problem as a personal affront to engineers everywhere, I decided to wrestle with it and see what happened.

I started from scratch, for a few reasons. Firstly, I hate Solidworks. I've played with it a few times, and never really liked the taste it left in my mouth. I'm much more comfortable in Siemens NX, which, while a bit finicky and quirky, is actually much better at some things than Solidworks -- and pitifully terrible at most others. Of course, Murphy's Law also states that any CAD project must switch formats at least once throughout the design process.

Also, there's another issue with physics in Alpha design -- there's four inventory tubes, with an axle peg placed every 90 degrees around the axle. That means the device can only move through a quarter rotation per vend, and thus the axle stub can go from essentially -45 to 45 degrees, relative to a line normal to the face of the condom. I felt that it'd be likely that much more surface area than this would be required to contact in order to slide the condom out.

My first gross over-simplification, for the purposes of just feeling things out, was to treat condoms as a nice, homogenous, square box. This let me get a basic mechanism down, and proved it was feasible.
(Figs. 2 and 3: Views with case partially hidden/partially transparent)

Fig. 2: Front View

Fig. 3: Trimetric View

 

 

 

I've made a few changes to the overall design worth noting. Firstly, placing the inventory tubes offset from the rear of the device allows a longer crankshaft arm length -- providing a larger radius arc, and more tangential force application to the condom over a longer duration. I also played with placement and determined that offsetting the rod to the front slightly improved vending, as the boxes were fully pushed out -- with a centered rod, I ran into issues in simulation where boxes would get caught by their back corners under the next box as it fell.

I used mechatronics concept simulator for this first rough simulation, which shows just how well the geometry of the pushrod works out -- notice how the box is engaged, then the pushrod moves it forward, simultaneously lifting the stack above it up behind the vend slot. I'm quite satisfied with this design, and feel that it would work incredibly well with perfect little square boxes.

Actually, nice square boxes are one type of condom packaging. However, given the unpopularity of that form factor, we're designing for foil wrappers, which are much more common.

I experimented with varying the box dimensions, the "scatter" of the stack of boxes, and received very consistent good results. I then began experimenting with contacting only the bottom surface of the boxes, which will be a requirement if negligible- or variable-thickness, deformable foil packs are to be used. More on that next time.