Day 744, On balance

This should have been obvious, hard disk drive spindles have balance weights added to ensure smooth running.

It never occurred to me that they would have balance weights, but then I've never thought about it in any particular depth.  But hey, when did ignorance of a subject stop people from making massive assumptions.*

So why are there balance weights and why should it be obvious?

An imbalance can occur in a rotating object when there is a disparity between the centre of gravity of the rotating system and the centre of rotation for the rotating system.  An imbalance in a rotating system causes vibration.**

Let's consider this in the context of motor vehicles and their wheels.  A vibrating car wheel can be felt through the steering column and will be noticeable in uneven wear of the tyre, there is also a strong likelihood of earlier than normal failure of a wheel bearing due to the increase in stress and wear.

In a system such as a hard disk drive assembly, where there is are very fine tolerance of distance between read/write heads and the disk surface, a few nanometers,*** any vibration of the disk surface will lead to an increase in data read and write errors.  Over time there will also be greater wear on the hard drive spindle which will exacerbate the existing imbalance and lead to premature failure of the disk drive.

In both the case of the motor vehicle and the disk drive spindle small weights are added in places appropriate to remove the imbalance in the rotating system.  When you next walk along a pavement take a look at the shoulder of car wheels, just next to the tyre, and it won't be long before you start to spot the lead balance weights attached to the wheel rim.

If you've ever heard of wheel balancing before, for cars, bikes, buses, etc, then why would it not be obvious after a small amount of consideration that there might be a similar requirement in other types of rotating system.  Of course it would only be obvious if you have a particular amount and type of prior knowledge.****

The tiny weights below shown removed from the disk spindle, presumably colour coded to indicate different mass.

* You know what I'm getting at.
** Don't take my word for it, here are some experts explaining this at great length in a patent application.
*** Don't take my word for it, here's an article from IEEE Transactions on Magnetics where some experts explain it at great length and in great detail.
**** I actually went "oh look, balance weights!" and was momentarily (a few nanoseconds) surprised, thus giving me an excuse for an overly verbose blog.


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