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Originally Posted by [email protected]
(Post 10335690)
Since these duplex bearings are widely used in TR assemblies, their failure rate should be very low unless they are not installed properly.
I don't really see much of a functional difference between the duplex bearing and double or even single ball bearings which expands the installed base even further. |
Originally Posted by [email protected]
(Post 10335690)
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. 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 |
Basically, thanks for corrupting my correct post Old Farang. ;) But, the groove and the and the step are there specifically to accommodate the engineering needs of the bearing inner races, as I described. Corner radii are used as standard practice and are incorporated as required. If the bearing did not need the inner races to be precisely seated in this way, the shaft would not be waisted by the groove or the step.
OAP |
Onceapilot you are right. Groove at the right end of bearing surface is engineering need for manufacturing (free space for grinding wheel/tool for close tolerance work on shaft) and a stress relief, as sharp corners are not desired there - stress/cracking promotion. On the left side - same thing - manufacturing process requirement - transition to close tolerance surface-not a step for spacer. Anyway, spacer should contact inner race freely, to transmit clamping force from nut. Am surprised sketch on S2/2018 report is not correct, as Nodrama suggests.
From photo it is obvious there is a standard right hand thread provided for bearing nut. Would be of interest to hear what maintenance/servicing manufacturer put in Instructions for Continuing Airworthiness? As mentioned in my post#1123 A-109E had duplex bearing design and lubrication requirements changed considerably, Bell 212 and 412 requires lubrication of this bearing every 50 hours with warning - two shots of grease only. |
Thanks for your input hoistop. Between yourself, nodrama, Old Farang and myself, I think we got this covered!:) The next important thing will probably be some clarity from the investigators on why this failure occured. :sad:
OAP |
Originally Posted by hoistop
(Post 10337473)
Am surprised sketch on S2/2018 report is not correct, as Nodrama suggests.
The picture is from the Training School notes, not from an approved Technical Publication. |
The next important thing will probably be some clarity from the investigators on why this failure occured |
Originally Posted by Onceapilot
(Post 10337481)
Between yourself, nodrama, Old Farang and myself, I think we got this covered!:)
OAP |
Originally Posted by hoistop
(Post 10334832)
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. |
Originally Posted by [email protected]
(Post 10335466)
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?...
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Pozi, I`ll try to clarify a bit, p56 #1117; if you undo the nut at the `spider end’, and the gearbox back-plate, you should be able to remove the hyd. unit complete, leaving the t/r supported on the rotor mast, which has internal splines at its top-end #1102. The spider/slider have splines matching the mast-splines, but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well. The pitch control rod must run through the piston to the feedback link, and the piston limits its travel at each end of the jack... That`s how I see it, unless anyone can produce a full cutaway drawing. |
Originally Posted by sycamore
(Post 10338516)
Pozi,I`ll try to clarify a bit,p56 #1117; if you undo the nut at the `spider end`,and the gearbox back-plate,you`should be able to remove the hyd. unit complete,leaving the t/r supported on the rotor mast,which has internal splines at it`s `top-end #1102.The spider/slider have splines matching the mast-splines,but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well.The pitch control rod must run through the piston to the feedback link,and the piston limits it`s travel at each end of the jack...That`s how I see it,unless anyone can produce a full `cutaway drawing.
A sectional drawing of this particular unit would indeed be helpful, some of the examples posted have been "schematic" and don't show the arrangement of the control shaft and outer (hollow) shaft. Understand how the TR will stay where it is, supported by its own bearings. But wouldn't removing and replacing the hydraulic unit be complicated by the fit of the duplex bearing? However, this isn't really relevant to the incident. |
Originally Posted by Pozidrive
(Post 10338653)
Understand how the TR will stay where it is, supported by its own bearings. But wouldn't removing and replacing the hydraulic unit be complicated by the fit of the duplex bearing? However, this isn't really relevant to the incident.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox. |
What percentage on the #1 Hydro system will auto-close the TRSOV after fluid loss/leakage?
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Originally Posted by sycamore
(Post 10338516)
The spider/slider have splines matching the mast-splines, but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well. Yes, there is a ‘nut’ holding the tail rotor hub on (there is always a ‘nut’), though it doesn’t look like a nut in the conventional sense. |
Originally Posted by Sir Korsky
(Post 10338768)
What percentage on the #1 Hydro system will auto-close the TRSOV after fluid loss/leakage?
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Originally Posted by nodrama
(Post 10338753)
No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. There’s the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, a some electrical plugs. That’s it. Very straight forward. The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox. Only question left is the detail of the control shaft/outer shaft/hydraulic piston, which isn't really relevant. |
Originally Posted by Pozidrive
(Post 10338821)
So the control shaft is an easy sliding fit through the inner race of the duplex bearing? And it's the castellated nut (and spacer) that clamp the inner race to the step at the outer end of the control shaft?
. I can’t help you with a detailed diagram of the servo actuator system. I’m not involved with component strip and overhaul. |
Originally Posted by nodrama
(Post 10338753)
No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. There’s the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, and some electrical plugs. That’s it. Very straight forward. The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox. I appreciate your contribution, however there is something in your delivery that sets the weemonkey "hang on" alarm going... |
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