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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 14th Dec 2018, 10:48
  #1141 (permalink)  
 
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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.
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.
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Old 14th Dec 2018, 12:19
  #1142 (permalink)  
 
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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?
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Old 14th Dec 2018, 12:34
  #1143 (permalink)  
 
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Nodrama,did you get my email??
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Old 14th Dec 2018, 13:00
  #1144 (permalink)  
 
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Yes. I replied by email yesterday?
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Old 14th Dec 2018, 13:13
  #1145 (permalink)  
 
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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?

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Old 14th Dec 2018, 13:14
  #1146 (permalink)  
 
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RVDT - I see what you are saying.
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Old 14th Dec 2018, 14:21
  #1147 (permalink)  
 
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Originally Posted by RVDT View Post
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?

Agree. Already covered in my post #1096

OAP
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Old 14th Dec 2018, 14:42
  #1148 (permalink)  
 
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Originally Posted by nodrama View Post
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?

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
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Old 14th Dec 2018, 14:50
  #1149 (permalink)  
 
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Thanks OAP. That makes sense to me.
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Old 14th Dec 2018, 17:11
  #1150 (permalink)  
 
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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.
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Old 14th Dec 2018, 19:14
  #1151 (permalink)  
 
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Originally Posted by [email protected] View Post
Since these duplex bearings are widely used in TR assemblies, their failure rate should be very low unless they are not installed properly.
They are used in vast numbers everywhere in engineering.

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.
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Old 14th Dec 2018, 20:27
  #1152 (permalink)  
 
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Originally Posted by [email protected] View Post
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.
Well, all bearings have some sort of life / failure profile. Off the cuff, I would venture that this application was quite arduous for a relatively small, sealed lubricant bearing so, I would presume that this bearing could fail quite quickly in use after loss of lubricant, or other causes of failure. Can anyone with type knowledge comment if any method of monitoring is used on this bearing?

OAP
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Old 15th Dec 2018, 03:29
  #1153 (permalink)  
 
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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
This is basically correct. But the "grooves" are stress relieving grooves ground or cut on a radius to prevent a sharp right angle on the shaft material. The outer edge of the inner race of the bearing also has a radius for a similar reason, in addition to ensuring that the pre-load clamping covers the flat face of the bearing without putting any radial force to it which could lead to misalignment.
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Old 15th Dec 2018, 12:44
  #1154 (permalink)  
 
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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
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Old 17th Dec 2018, 09:58
  #1155 (permalink)  
 
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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.
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Old 17th Dec 2018, 10:05
  #1156 (permalink)  
 
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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.

OAP
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Old 17th Dec 2018, 10:58
  #1157 (permalink)  
 
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Originally Posted by hoistop View Post
Am surprised sketch on S2/2018 report is not correct, as Nodrama suggests.
Not a suggestion, fact.

The picture is from the Training School notes, not from an approved Technical Publication.
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Old 17th Dec 2018, 13:52
  #1158 (permalink)  
 
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The next important thing will probably be some clarity from the investigators on why this failure occured
which will surely come down to installation issues or maintenance issues unless there is a bad batch of bearings out there...then the lawsuits will start flying!
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Old 17th Dec 2018, 19:11
  #1159 (permalink)  
 
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Originally Posted by Onceapilot View Post
Between yourself, nodrama, Old Farang and myself, I think we got this covered!

OAP
damn, I am so glad you all got this...where would we be without the A team.
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Old 18th Dec 2018, 13:17
  #1160 (permalink)  
 
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Originally Posted by hoistop View Post
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.
That was my original point, which still hasn't been answered. The bulletin clearly says what you say - "the control shaft passes through an outer shaft, which forms part of the piston"
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