Thursday, September 12, 2013

Designing the Cap

One of my pet peeves since USB flash drives first came about is loosing their caps. The first several flash drives I bought had separate caps with no way to store them when they were open. This inevitably led to the game of "try to remember where I put the cap" after using the drive. Later USB models either had a way to store the cap on the other end of the drive or no cap at all, like the slide-to-open SanDisk models.

One of the design requirements for our VIUD is some way to prevent the cap from becoming easily lost. There are a few ways in general of accomplishing this:

  • No cap at all (slide-to-open or flip-to-open)
  • Cap storage at other end
  • Lanyard attachment

The first option is not applicable for our design due to other requirements, notably the need to be waterproof. The last option is useful to have but is not a complete doesn't work if you don't have the USB drive on a lanyard. With only one option left we now have to figure a way of storing the cap on the other end of the drive when it is in use.

Cap Storage

The initial design from essentially the very start was to have the cap physically fit onto the other end of the VIUD by way of a properly sized o-ring (see below image).

Prototype #2 Showing the Cap On and Off
By carefully tuning the size of the groove and the o-ring size and hardness the cap snaps on and off easily and doesn't fall off by itself. While I particularly like this design it does have a few drawbacks. One is that the exposed o-ring is vulnerable to damage, particularly at high temperatures like in flame tests. Not a huge deal though, especially considering the o-ring is easily replaced and only costs a few cents.

The other, more significant, drawback is that it lengthens the body by around 3/8". The USB flash drive is too large to fit inside the part where the cap is stored and while it doesn't make it that much longer it is one factor of many that contributes the overall length of the device.

Another thing to consider is the long term reliability of this cap storage method. While the second and third prototypes have survived one year of use very well I did notice the cap storage became a little looser over time likely due to the o-ring wearing out.

Magnetic Cap

I was all set on using the prior design for cap storage until someone innocently suggested trying a magnetic cap storage. The idea is simple enough: a small magnet inside each end of the VIUD which permits the cap to magically "stick" to the end of the drive when open. Using a magnetic cap storage would eliminate all the draw backs of the previous method in addition to letting us make the overall design look more symmetric and good looking.

There are a few questions to answer before accepting this new concept:

  • What size magnets do we need to use?
  • How close to each other do they need to be?
  • Will the magnets affect the USB flash drive?

The first two questions were quickly answered by creating a few test pieces and doing some stress modelling. Ideally we'd like the magnets close to the surface but need to consider the strength of the body when under stress. Modelling and testing revealed that a distance of 0.07" would be acceptable. With 3/8" diameter rare-earth magnets their pull force is more than sufficient with a 0.14" separation and the body wall is still strong enough at this thickness.

On a whim I tried attaching the test magnet pieces to my fridge but they were just barely not strong enough to support the entire weight of the VIUD. Increasing the magnet size to 1/2" diameter made the magnet more than strong enough to hold the VIUD to a metal surface which is a neat feature.

In order to facilitate the magnetic cap attaching to the end of the drive a small recess was made in the cap that fits nicely to the chamfer in the other end of the body (see below image). 

Test Pieces for the New Cap Design With Internal Magnets

As for how the magnets may affect the USB flash drive: in theory they shouldn't affect the flash memory at all however I did test it. I placed two large rare-earth magnets at each end of a USB for one month. Testing before and after revealed no changes as expected. The only change was that USB connector became very slightly magnetized.

Temperature Issues

One thing I completely forgot about with regards to the magnets was the effect of temperature on them. Magnetic materials have a property called the Curie Temperature which is the temperature at which the material completely loses it permanent magnetism. A material's magnetism is actually gradually lost as you increase the temperature towards this point so in order to pass all our temperature ratings we will need to take a closer look at this effect.

The material used for the magnets are a Neodymium-Iron-Boron compound, the more common and cheaper form of rare-earth magnets. Neodymium magnets have a Curie temperature of 320 °C (610 °F) which seems high but they will actually begin to lose their magnetism at much lower temperatures depending on their size and strength. It is actually a rather complex thing to figure out but a small Neodymium magnet like the one we're using has a rated working temperature of only 80 °C (175 °F). If you exceed this temperature to around 100 °C (212 °F) the magnet's strength is reduced by up to 60%.

Now technically these temperatures are fine as it is unlikely that the VIUD would normally be exposed to temperatures high or long enough to affect the magnet significantly. Even if the temperatures were exceeded the worst case is the magnets losing some of their strength...hardly a major failure. However, it would be nice for our temperature ratings to be able to achieve higher temperatures for longer than permitted from the Neodymium magnets alone.

There are two solutions to the temperature issue: high temperature Neodymium and Samarium-Cobalt magnets. There are types of Neodymium rare-earth magnets that have a higher working temperature rating of 150 °C (302 °F) which should be good enough for our purposes. Even better are another type of rare-earth magnets, Samarium-Cobalt, which are rated up to 300 °C (570 °F). Which one we use mostly comes down to whether we can find a supplier for them at a reasonable cost.

Lanyard Attachment

While I wasn't going to have this option initially it would be good to add something to attach the VIUD to a key ring or lanyard like almost all the other rugged USB competitors have. Assuming the lanyard attachment will be on the cap there are a few possible methods of making the hole for it:

Choosen Lanyard Attachment Design
  1. Diagonal hole
  2. Two perpendicular holes
  3. A hole sideways with extra material removed

The first two are the simplest from a manufacturing standpoint but the main drawback is that they interfere with the magnetic cap storage. I ended up deciding on the last option and while it is slightly more complex (and thus expensive) it doesn't interfere with the cap storage and it looks pretty good.

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