Repair of Gyroscopic Instruments

Peter wrote:

In the last KI256 the repairer reported a “worn shaft” which I don’t quite get since surely the only wear should be in the bearings?

We now know from the part number that the radial play is to be in the range of 0.0002" to 0.0004". The bearing will need to be a light press fit into the housing and probably onto the shaft as well. It would only take a tiny amount of wear on the shaft to make the fit slightly sloppy – where “slightly sloppy” is more than the specified play in the bearing.

OK; yes. The other “legal” aspect is that lubrication (and presumably the use of loctite) is banned per the MM, so the only option is to replace any affected parts. But since this type of servicing would be done, ahem, in the field, none of that is relevant.

I reckon this thread is going to be extremely helpful to a lot of people, especially as HBK move all repair work to themselves in the USA which will render repairs of all this stuff uneconomical to do legally, yet most owners of older hardware will not be able to rip it out and fit “glass” for 20k upwards.

I do not know of any ball bearing that has NO play in itself. But in cases like that or any other machinery that is required to operate at minimum noise you set a tiny bit of axial load on one bearing with slide fit in the case. So you get zero play in the assembly and that is what I´d do: Put some thin spring washer (wave washer?) under one bearing and it´ll be allright. No clearance in a bearing will normally lead to a blued bearing at high revs. These small bearings have a much higher rev limit than required in your case. With a typical 400 Hz generator you will get 24 000 revs at a three phase motor so I guess that is the kind of range we will expect here. Fitting an open bearing in this instrument is intentionally limiting the life of it – great for the manufacturer !!! Finding a ceramic type would help a lot as this is standard for extra high revs applications elsewhere but that would be a special in the specific size here. Another way for setting the internal clearance in the standard bearing is to experiment with the press fit of the steel sleeve that has to go over the 5/16" bearing o.d. to get the final 1/2" size – a bit tricky but doable. The best idea would be to find that extra space in the instrument to fit a closed 2 Z bearing at 1/8" width but I have no idea about the conditions therein – some more pictures , Peter ? Of course you will never ever fit this modified gear into a certified aircraft , this is certainly only for personal experiments . . . . Vic

The context in this case is a KI256 which is a vacuum driven horizon, so the bearings see a ton of sh*it blown in; everything below a certain size according to the filter spec. I have never seen the inside of a KI256 but I bet it is pretty dirty.

Electric gyros should last a lot longer but rumour has it that – in some cases at least – they don’t. They have a DC input and use an inverter driving a 3 phase motor so the frequency could be anything at all…