3 sheared generator shafts in a week

[Rotorhead84]

Bell 206 w/ 250c20. We have one particular ship that is eating generator shafts like candy.

We have only had it a few years. The first few years were fine, I don't believe it ate any shafts (we run electric pumps with the generator so we tend to break one every season).


Not sure why this old girl ate 3 in a week. Any word of bad shafts going around? We tend to buy a handful of shafts when we run low. I think the last time we bought any was a few years ago. Anyone ever experience this?


This is a dry spline set up.

[Salusa]

What's your voltage regulator set to?

[Hughes500]

Are you just replacing the shafts in same started gen or are these freshly overhauled complete units ?
If just replacing the shaft then it sounds like the bearings the shaft runs on are u/s .

[misterbonkers]

Are you switching the generator on at less than 70% NG? This can cause shearing too.

[Ascend Charlie]

Are all your electrical switches (radio master, lights) in the ON position, waiting for the generator? The sudden load-up might be the problem. Try turning on the loads one at a time.

[parabellum]

Apart from all the above, which make a lot of sense, could it be that something is slightly out of alignment and applying forces where forces are not supposed to be? Just my thoughts. (engine failure over the jungle 1968 in a 206A! :))

[whoknows idont]

My money is on what Hughes500 said.

[onetrack]

Where is the failure occurring in the shaft? In the same place every time? At the end of the splines?

There are a number of things that can contribute to or be responsible for, any shaft failure under load.

1. Poor quality material selection for the shaft. Upgrading to a superior grade of steel alloy with constituents such as nickel, molydenum, manganese and chrome, provides a material that has a much higher tensile strength and ability to absorb peak loads.

2. Poor machining. If the break is consistently in the same area, inspect the machining/spline cutting, for poor finish or wayward cuts by a machine tool, that are creating stress zones and fracture initiation.

3. Shaft overloading by applying additional high torque stresses when the shaft is already under load. Shock loading is also possible where a high load is applied rapidly, resulting in a sharp peak in torque. Overloading or shock loading creates stress zones that initiate shaft fractures.

4. Shaft misalignment. Misalignment will create bending stresses in the shaft, creating stress zones that initiate fractures. Ensure the shaft is running true by checking alignment of components.

[Rotorhead84]

Thanks all for the replies.

What's your voltage regulator set to?
Not sure to be honest. From what I remember the voltmeter reads ~28v when the gen is on just like the other ships we run. Haven't flown this particular aircraft in a while.

Are you just replacing the shafts in same started gen or are these freshly overhauled complete units ?
If just replacing the shaft then it sounds like the bearings the shaft runs on are u/s .

After the first shaft sheared we just figured it was due to break because like I said we go through shafts every so often due the work they are doing. After the second shaft broke, we swapped in a different starter/generator, and it broke the shaft either at the end of the day when I shut down, or the next morning when I first hit the starter button because it worked fine the entire day and the next morning when I hit the starter it wouldn't turn the engine, just made a "whirrrrr" noise because the shaft was sheared and not engaging.

Are you switching the generator on at less than 70% NG? This can cause shearing too.

No, I always roll up to 70% N1 before engaging the generator. And I am the only person flying this aircraft so I know that is not the issue.

Are all your electrical switches (radio master, lights) in the ON position, waiting for the generator? The sudden load-up might be the problem. Try turning on the loads one at a time.

No. Everything is off minus the fuel boost when the generator is engaged.

Where is the failure occurring in the shaft? In the same place every time? At the end of the splines?

There are a number of things that can contribute to or be responsible for, any shaft failure under load.

1. Poor quality material selection for the shaft. Upgrading to a superior grade of steel alloy with constituents such as nickel, molydenum, manganese and chrome, provides a material that has a much higher tensile strength and ability to absorb peak loads.

2. Poor machining. If the break is consistently in the same area, inspect the machining/spline cutting, for poor finish or wayward cuts by a machine tool, that are creating stress zones and fracture initiation.

3. Shaft overloading by applying additional high torque stresses when the shaft is already under load. Shock loading is also possible where a high load is applied rapidly, resulting in a sharp peak in torque. Overloading or shock loading creates stress zones that initiate shaft fractures.

4. Shaft misalignment. Misalignment will create bending stresses in the shaft, creating stress zones that initiate fractures. Ensure the shaft is running true by checking alignment of components.

The shaft always breaks in the same spot. in the tapered weak spot that was designed to shear to keep the gen from locking up the gearbox.

http://i.imgur.com/rgsWsgJ.jpg


We do apply load rapidly to the generator. We are running electric pumps (we do agricultural spraying). We do not cycle the pumps every pass, only once per load. But the gen still sees a 50amp spike probably 40 times a day. And we break them from time to time, but never this frequently.


When the first shaft broke, it broke at an angle in such a way that it was still actually turning. (the one with the large disc/plate in the picture) When it broke initially, I heard a soft "whack" noise and got an immediate vibration in the aircraft. Mostly in the pedals. No generator m/c light. I flew to base immediately and when I turned the generator off at shut down the vibration stopped immediately. I then tried to turn the generator back on, but it did nothing, and once the generator switch was placed to "on" the m/c light came on. The vibration did not return. We found that the clamping band had rotated all the way around and the locknut was against the fuel control and inaccessible. Which is why we had to cut it off.

[Widewoodenwingswork]

Try replacing the aluminium adapter plate that is fixed to the AGB that the starter clamps to. (The one you probably should have taken off to remove the clamp that you couldn't undo the nut on so that you didn't have to destroy a clamp).

Best of luck!

[RVDT]

I notice in your pic of broken parts that there is no friction ring present.

It goes between the dampener and dampener back plate.

You do have one right?

The quill shaft is driven from the back of the generator and the torque loading spike is dampened by the twist of the quill shaft and the dampener friction ring.

If you do not have the friction ring and it is metal to metal it will most likely snap as there is no dampening.

50 Amps isn't much. 1.6 HP?

http://i.ebayimg.com/00/s/NzIwWDk2MA==/z/pPcAAOxyF0pTh2L9/$_1.JPG

[Sevarg]

Years ago Bell came out with a transistorised voltage regulator, this responded so fast to changes in load that it sheared the shafts. Normally after an internal start with a battery that wasn't the best in the world. The answer was to refit the carbon pile regulator. This worked every time. The transistor reg is a Golden (might be silver) coloured box and carbon pile is rounded with fins (photos anyone?).
So I would check Volt Reg type and that the voltage is correct for the temp you are operating in ( use a good voltmeter), Battery state.
I must say I have never heard of a shaft shearing and no gen warning, My first thought would be shearing on start, rubbing compressor???
Hope this help.

[vaqueroaero]

I also do ag work, running an Isolair pump and have never had a problem breaking generator shafts. Once the tank is empty I shut the pump off so depending on what we are doing will cycle it a bunch of times during the day.

As a side note the aircraft battery will power the pump for two 100 gallon loads at 2 gallons/acre. Discovered that when I forgot to turn the generator on......

[Saint Jack]

In addition to all of the very relevant and detailed replies above, I'd like to add the following:


1. You said "...We tend to buy a handful of shafts when we run low". Where do you buy them from? Are they genuine Lear Siegler parts or PMA items? Do they come with the correct paper-work?


2. Do you use external power (GPU) for engine start, particularly the first start of the day? If you do and it's not by using a bank of large batteries what is the starting current limiter set to? If the limiter is set too high it will result in excessive 'wrenching' torque when you initially press the starter. GPU's with a 'soft-start' function whereby there is a quick but gradual build-up of starting current when the starter is pressed are ideal.

[lamanated]

remember, the 70% is a max, to keep you from shearing it, but I've seen guys doing it at 100 with no load and no breaking. you can try turning it on at 66 etc, with a light (battery load) to test the theory , its to keep the engine from quitting if selected too low also , are you running a Ni Cad, and waiting a min for it to take the charge? or a passable Gill (yuch) or Concorde battery? I havent seen a sheared shaft in decades, is it on the same starter? same repair shop ?I've seen back yard overalls, with the nut have tightened..

[cockney steve]

I've seen back yard overalls, with the nut have tightened..
I found wearing a size or two larger helps, plenty of room for the nuts, then.:}

[whoknows idont]

I havent seen a sheared shaft in decades

I have seen it 5 weeks ago. It does happen, not an urban myth...

[Rotorhead84]

I notice in your pic of broken parts that there is no friction ring present.

It goes between the dampener and dampener back plate.

You do have one right?

The quill shaft is driven from the back of the generator and the torque loading spike is dampened by the twist of the quill shaft and the dampener friction ring.

If you do not have the friction ring and it is metal to metal it will most likely snap as there is no dampening.

50 Amps isn't much. 1.6 HP?

I'm not sure I've ever seen a friction ring. We have 3 58s and none of them have friction rings that I know of. Do you have a picture of one?

I must say I have never heard of a shaft shearing and no gen warning, My first thought would be shearing on start, rubbing compressor???
Hope this help.

It was strange for sure. It sheared at about a 45 degree angle and it was able to still drive itself.

In addition to all of the very relevant and detailed replies above, I'd like to add the following:


1. You said "...We tend to buy a handful of shafts when we run low". Where do you buy them from? Are they genuine Lear Siegler parts or PMA items? Do they come with the correct paper-work?


2. Do you use external power (GPU) for engine start, particularly the first start of the day? If you do and it's not by using a bank of large batteries what is the starting current limiter set to? If the limiter is set too high it will result in excessive 'wrenching' torque when you initially press the starter. GPU's with a 'soft-start' function whereby there is a quick but gradual build-up of starting current when the starter is pressed are ideal.

I'm not sure of the brand of shaft. We purchased them from Duncan Aviation. Unsure about the paperwork. I'd have to ask our mechanic about that.

We don't use a GPU very often. A few times a year maybe. Normally we start off the aircraft battery. When we do use a GPU is just a battery cart with a couple of 12v batteries wired in series.

are you running a Ni Cad, and waiting a min for it to take the charge? or a passable Gill (yuch) or Concorde battery? I havent seen a sheared shaft in decades, is it on the same starter? same repair shop ?I've seen back yard overalls, with the nut have tightened..

We have a mix of Concordes and Gills. We are phasing out the Gills. In fact we just got rid of the last one. Complete crap. We tend to blow at least one shaft every season. The first two were on the same generator, then we swapped generators and it popped again. Same mechanic. He worked on 58s in the army for quite a long time. He is local. We don't have a repair shop so to speak, we are a small ag outfit.

[helofixer]

http://helimart.com/images/products/23032-027.png

friction ring is a small brownish or black round thin piece of material sandwiched between the two large silver discs just in front of the mesh area on the mounting side of the gen. If you have another in stock look at it from the side and you should see a thin dark washer like object between the silver large discs...if that makes any sense. look at the drawing posted a few posts up and youll see the friction ring in the drawing.

[cockney steve]

If the generator has developed a fault, such that it sees "full field" when it reaches cut-in speed, that would tend to apply an immediate full load....the illustration appears to be a straightforward friction-clutch,which, if correctly preloaded, would slip under a snatch-load and save the drive-shaft.
I have no personal experience in this field, but general engineering principles hold good. I don't see how a binding compressor would make a jot of difference to the generator shaft....it is being asked to transmit more torque than it's design-limit will allow....it is the weakest link, whereas the friction washer should serve that function.