If you've read the page in the Glossary about the action then you'll know just what a complex thing it is. You'll also be aware that any problems associated with the action tend to be somewhat expensive to fix.

But...you're sitting next to a brand new instrument...or perhaps you've assumed that as the keys go up and down then they must surely be working.

Oh dear.
Unfortunately the standards of manufacturing aren't always what they ought to be, and I've seen brand new horns with what appears to be ten year's worth of key wear built in...at the factory! And then some instruments resist wear better than others - Yamaha's instruments are very good in this respect - but as long as your keys go up and down they are going to wear.

So while you're quaking in your boots let's turn the heat up and see how you score on the 'Wheel of Misfortune'!

You'll need your instrument for this next bit.
Study the pictures below...they show the two common types of pivot/key combinations. The first is a rod screw key, the second is a point screw key ( note the bare point screw showing above the key - this is what the key fits over ).

The two pivot types wear in slightly different ways. Rod screws may rub along their entire length, thus wearing the entire barrel of the key. Point screws only contact on the ends of the barrel, so the wear is confined here ( much cheaper to sort out too! ).
Find a rod screw key first; for sax players you can do no better than your low F key, clarinettists grab your speaker keys and flautists stand by your G keys.

The diagram above shows the three types of wear commonly found on keys that are pivoted on rod screws.

Now, pinch the key as near to its centre on the key barrel as you can - and try to move it from side to side...left to right, as it were...
Is there any movement? Does the key have room to move between the pillars? Sure, there will be SOME movement, but it ought not to be much more than a tiny fraction. If there is much more than this then you have longitudinal wear - i.e. the ends of the key barrel have been worn away.
Now grab the key cup and try rocking the key from side to side again with a slightly circular motion. What you're testing for here is end play...which means the bore of the key barrel is worn at its ends. Again, there will be a little play there...but not much.
Lastly, grab the cup and push/pull it against the pivot. Not too hard mind - there should be no play at all. If the key moves then that indicates lateral wear along the length of the key barrel. It could also mean wear in the pillars.
Try some other keys...sax players head for the top B key, clarinettists head for the low F, flautists for the thumb keys.

Now lets have a go at the point screw keys.
Sax players have to start with the Bis B key ( the one with the small pearl, below the top B key ), clarinettists with the low E and flautists with the low D ( though I'll come back to flutes in a moment...with even more bad news! ). This time you need to test each end of the key barrel. Pick an end, grip the key wherever you can and give it a wobble...left to right, back and forth, round and round. Any movement? Try the other end of the key, same tests.
Point screw wear can sometimes be hidden. The diagram above shows that wear can take place both inside the barrel and on the end - but it's often the case that the barrel wears inside and not on the ends...so that although the key seems to sit snugly between its two pillars, it can still show movement due to internal wear in the barrel.

Flutes are a slightly different matter - some of the keys are joined together to form what's known as a 'compound key'. This means that two or more keys are pivoted on a rod...which is then pivoted itself on points. Double trouble!
Typically there are three such keys on a flute...the C/A key, the right hand stack ( low F to D ) and the trill keys. You need to be a little bit careful when testing these compound keys for wear as what might seem to be free play on, say, the F key may well be free play at the point screw at the D key. Once you've wiggled the keys about a bit you'll start to notice the difference.

Let's just scare the sax players a little bit more! Try the low B/Bb key, don't forget to wobble the keys at BOTH ends.

By doing these simple tests all over your instrument you'll gain a rough idea as to the ( excuse the pun! ) state of play! This is just a guide though, you might perhaps feel that a little wobble is insignificant when in fact it could be quite a problem.

And that problem is that whenever a key is pressed down it tries to take the path of least resistance. With a well maintained action the keys have no choice but to go where they're meant to go...straight down onto the tone hole. Chuck in a bit of wear on the pivots and a key will try to take up this free play as soon as it can. This might be at the beginning of its stroke or at the end.
If it does so at the beginning then the action will feel clunky, and you cannot expect that key to reliably bring down any others it is connected to ( and they might be worn too...thus doubling the effect ). If it occurs at the end of the stroke as the pad hits the tone hole then it will lift slightly, creating a leak...usually at the back of the pad, nearest the pivot, where it can't be seen.

All is not lost - this free play can be removed ( though with varying degrees of difficulty ). Rod screw keys can have the key barrels 'swedged' ( pronounced swayged, as in wages ). This is done using a special tool that compresses the key barrel - and let me tell you, it can be hard going!
Point screws are much easier, the pillars are reamed out a fraction to allow the screws to reach further into the pillar. If necessary, beefier screws can be fitted and the holes in the key barrels can be re-bored too if required
Those instruments that have those nasty parallel point screws fitted will have built-in free play from the off...but it's a simple matter to change these for proper point screws.

There are still other methods. Some point screws have no shoulder ( see Point Screws ) and need only be screwed in further. Some rod screw keys have little or no key barrel to compress, so they may require filling in and re-boring, or an oversized rod screw ( always a pain, as you have to do the pillars too - and any other keys that sit on the same pivot ).

Some repairers take up end to end play with little teeny washers. This works a treat...for longitudinal play, but the tragedy is that this kind of play is nowhere near as bad as lateral play, which washers do b****r all to correct! They also look rather ugly.

I should imagine that some of you are wondering why you can't just take a screwdriver to your instrument and tighten all the point screws up yourself. Well, you can - but depending on the type of screw ( shouldered or cylindrical ) you'll either make no difference at all and possibly bust the head of the screw finding out, or you'll lock the action up solid.
With plastic bodied clarinets in particular you might find that all goes swimmingly well - and when you next take the instrument out of the case the action has locked solid!
Contraction! It's a pain, isn't it!