And now it makes sense :ugh:, many thanks nodrama for your professionalism and to crabs for the diagrams :ok:

Just trying to clarify an earlier post which said the control shaft was the hydraulic piston. Also to understand why manual control isn't possible - in spite of a direct link from lever to spider via the control shaft.

Pozidrive - The input lever is attached to the end of the control shaft (with the nut that did come undone) - then the shaft runs through the TR servo assembly with the piston section as an integral part of the shaft - the shaft then runs out the other side, through the TR drive tube, through the duplex bearing and is attached to the spider with the nut that didn't come undone.

A TR control input moves the input lever, but only enough to displace the pilot/spool valve in the servo - this directs the hyd fluid to move the piston (which is part of the control shaft) which moves the spider and changes the TR pitch.

The feedback/followup movement of the control shaft recentralises the spool/pilot valve in the servo, fluid stops being directed and the TR pitch stops changing.

The whole system is designed to have hydraulic assistance and that allows the autopilot to create inputs to the spool/pilot valve without the TR pedals needing to be moved.

I don't think the input lever has enough mechanical advantage/leverage to move the control shaft in a manual mode, even if you and the control runs were strong enough to move the pedals.

Thanks, crab - I understand well, how follow-up hydraulic servo control mechanism works, believe me.
I am (still) wondering if piston is trully part of control shaft, or control shaft passes thru the hydraulic cylinder and is clamped/attached... to (hollow)piston somehow - in other words, did rotation of (undone) control shaft strain seals of hydraulic cylinder in a way they are not designed to or not? It is an important question.
If answer is yes, it means that in case this failure occured in cruise and pilot managed to somehow maintain (some) control, he might be soon faced with massive hydraulic leak - loss of hyd. system as a consequence. If this servoactuator is powered by both hyd systems, (two pistons one on top of other and dual spool valves as in standard dual servoactuators) both systems would probably soon leak hyd. fluid, rendering helicopter uncontrollable.

Hoistop - as I mentioned earlier, look at a MR servo jack - the piston is part of the control rod that moves the swash plate - why would you make it more complex by having to clamp a piston to a control rod when you can just machine it in one piece?

If there had been evidence of a hydraulic leak, it would have been mentioned in the initial report.

Hoistop - As I've said before, I've never seen the inside workings of one of these servo actuators.
The diagram is a schematic of what I think the TR actuator looks like judging by the shape of the one fitted to an AW169. I've also provided an AW169 'technical' description that I have found. I am assuming that 'connected by a flange' is describing how the cylinders are connected together.

https://cimg3.ibsrv.net/gimg/pprune....dabbc1eda4.jpg
"A Linear Tandem Hydraulic Actuator, consisting of a single jack with two pistons housed in two different cylinders mutually connected by means of a flange; the two cylinders receive the control flows from the two control valves and provide the force necessary to drive the pitch control mechanism of the tail rotor blades."

After all the excellent information about duplex bearings and preload offsets , we are still left to find a reason for the bearing failure and the excess torque on the spider end nut.

Unless the split pin at the spider end had come adrift, the control rod can't have rotated relative to the nut to tighten it - is the initial report wrong when it says that the torque is a result of that rotation and if so, how?

Unless there is a clear explanation of that process you come to the conclusion that the nut had been overtightened either on assembly or during servicing. Whether that excess torque directly affected the operation of the duplex bearing or just exacerbated an existing problem with it is one of the questions the AAIB will hopefully answer.

"Elimination or reduction of ball skidding"


These people should be producing underpants.

Surely the bearing failed first?, so the inner race turned on the shaft, tightening the nut and shearing the pin? A bit of extra torque on the nut would not make any difference to the bearing clearance unless it deformed the inner races, which is unlikely. As above, the clearance is set by the machining process, the nut only has to apply enough pressure to hold the inner races firmly together.

I don't know the inner workings of exactly how the inner shaft is coupled to (and therefore pushed/pulled by) the hydraulic actuator and whether rotating the inner shaft could cause a leak, however the Hydraulic system #1 has an automatic TR shut off valve (TRSOV in the diagram). If pressure drops at the TRA pressure switch, or the system level drops to a pre-determied level the TRSOV closes to maintain the remaining system #1 hydraulic fluid within the main rotor actuators to maintain main rotor control.

I.e. a catastrophic loss of the TR hydraulic servo would not in itself lead to a loss of main rotor hydraulics. Such a leak would however lead to a loss of system #2 pressure as there is no TRSOV on that system, so you would lose MR hydraulic redundancy.

https://cimg7.ibsrv.net/gimg/pprune....abafe42b6f.png

It was the pin and nut on the other end - not the spider/duplex bearing end - that failed and the excessive Tq was on the spider end nut which didn't fail and the split pin was intact. How did the excess Tq happen?

Aucky - just like the 139.

Crab,

The excess torque could be for a number of reasons and it is only the effort required to remove the nut.

It is easily conceivable that the clamp up was correct.

At first glance in the picture in the report, the TR end of the control shaft there appears to be burnt grease that has migrated along the shaft.

It woudnt surprise me if it migrated to the nut end as well. As you can see it can get in to very small places as there are traces between the outer part of the inner race and the shaft.

The thread seems to be full of it as well. You dont need much friction in the thread to change the torque requirement in the nut. Ever used Loctite? It doesn't have much inherent strength but spread over the area of the thread?

Nodrama,did you get my email??

Yes. I replied by email yesterday?

I'm not really one for speculating, so the picture below is inferring nothing (except that I drew on it with my foot!). The diagram of the spider assembly to the pitch change rod (shaft) in the report isn't entirely accurate. It doesn't show the metal spacer that is between the nut and the duplex bearing. My picture shows that- nut, washer, spacer, duplex bearing. Note the step, where the spacer couldn't move any further despite what torque was applied to the nut. Maybe this is purposely designed to prevent crushing of the inner race?

https://cimg5.ibsrv.net/gimg/pprune....2f33802d48.png

RVDT - I see what you are saying.

Agree. Already covered in my post #1096

OAP

nodrama,
The step is probably not designed as a stop, it is far too narrow. If you look, there also is a similar step undercut at the inboard end of the bearing seating. This is probably designed so that the inner races very slightly overhang the seating at each end and so, there is no chance of the inner race being wedged up on a radius at the inner end and at the outer end the spacer can also fit against the whole outer end face of the inner race. These bearing races are very hard but quite brittle. Any unequal or point loading can cause fracture.

OAP

Thanks OAP. That makes sense to me.

So what caused the failure?

Since these duplex bearings are widely used in TR assemblies, their failure rate should be very low unless they are not installed properly.