X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.121] verified) by logan.com (CommuniGate Pro SMTP 5.2.2) with ESMTP id 2874566 for flyrotary@lancaironline.net; Sun, 27 Apr 2008 17:18:07 -0400 Received-SPF: pass receiver=logan.com; client-ip=75.180.132.121; envelope-from=eanderson@carolina.rr.com Received: from edward2 ([75.191.186.236]) by cdptpa-omta01.mail.rr.com with SMTP id <20080427211729.GWNA19888.cdptpa-omta01.mail.rr.com@edward2> for ; Sun, 27 Apr 2008 21:17:29 +0000 Message-ID: <000b01c8a8ac$512d8c80$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Post Mortum on RD-1B Date: Sun, 27 Apr 2008 17:19:02 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0008_01C8A88A.C9D45C30" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3138 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198 This is a multi-part message in MIME format. ------=_NextPart_000_0008_01C8A88A.C9D45C30 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Thanks for the Post-Mortum, Tracy. Wow! An impressive bit of Holm's detective work with clue leading to = clue and deduction conclusion to conclusion. Clearly, attention to = details in the assembly and mounting of the redrive is crucial - just = how crucial is readily apparent. Ed ----- Original Message -----=20 From: Tracy Crook=20 To: Rotary motors in aircraft=20 Sent: Sunday, April 27, 2008 4:07 PM Subject: [FlyRotary] Re: Post Mortum on RD-1B Here's my shop notes from the "this can't be good" incident. =20 Post Mortum on RD-1B SN # 35B Background on unit: Drive had very little testing on installation (ground only). Builder = reported some oil leaking from rear seal and requested a new seal which = was subsequently sent to and installed by builder. Engine experienced = various problems during testing (coolant found in rotor chambers) and = was rebuilt. The builder was not able to identify the source of the = stray coolant. Builder decided to examine drive while it was off during = engine rebuild and noticed that the input shaft thrust bearing had = failed (roller cage was melted and all rollers were loose in drive). = Unit was returned to RWS for analysis and repair. In addition to the thrust bearing, the following problems were found = during inspection: 1. Input shaft gear drive pin had sheared off and the gear had spun = on the shaft causing the thrust bearing oil passage to clog. 2. Sun gear pilot bearing had seized on the prop shaft pilot and = stayed there when the drive was disassembled. 3. The rear input shaft thrust bearing race had spin welded and = seized on the shaft with evidence of extreme heat in that area (blued = metal). I noted that the race was more or less in its normal position = on the shaft and had not seized due to moving forward onto a larger = diameter portion of the shaft as would be expected when the rollers and = cage vacated their normal location. This indicates that the rear = bearing race seized prior to the destruction of the roller bearing cage. = 4. The seized race had spun against the adapter plate causing the = aluminum to heat and extrude into the oil seal cavity. This allowed the = input shaft to move further back against the adapter plate. 5. The oil seal was completely destroyed by the heat of the spinning = race. 6. The moly impregnated nylon snubber was crushed on one side with = evidence of heating due to friction. =20 7. The loose thrust bearing rollers had bounced around inside the = planet carrier housing leaving the housing and adapter plate with the = appearance of being shot peened. Surprisingly, it appears that none of = the rollers got past the planet carrier bulkhead and into the gears. = There was no evidence of damage to any of the gear teeth. =20 There are fairly obvious explanations for each of the individual = failures observed but the likelihood of all of them being present and = manifesting themselves at the same time is very small. The analysis of = the root cause reduces the problem to finding a single factor which = would fit all of the observations and failures. The key piece of evidence in this failure turns out to be the pinched = nylon snubber washer. It would be possible for an error in setting the = input shaft end play at time of manufacture to cause the snubber to be = crushed when assembled but this would result in a symmetrical crush = around the entire circumference of the snubber. Operating the drive in = this condition would cause the snubber to quickly overheat and melt = until the interference was eliminated. It is probable that the drive = would operate normally at this point except for the harsh metal to metal = contact between sungear and ring gear bulkhead under negative (abnormal) = torque conditions such as engine backfire. Insufficient endplay alone = would not explain any of the other failure points. The only possible explanation I could come up with that would explain = the asymmetrical force sufficient to deform the snubber (it takes a = force measured in tons to do this) is for the rear race of the input = shaft thrust bearing to drop off the pilot on the shaft and get trapped = between the adapter plate and input shaft. This can happen for a number of reasons. Failing to push the input = shaft all the way through the damper spline and into the crankshaft = pilot is the most obvious. Too much grease in the crankshaft pilot hole = can also prevent the shaft and bearing assembly from seating firmly = against the adapter plate. It is a close tolerance fit between input = shaft pilot and the crankshaft pilot hole and the grease will not easily = extrude past the clearance. Even if none of this has occurred and the = shaft assembly is in fact seated against the plate when the assembly is = performed, the close fit of the pilot and the grease will compress air = in the cavity causing it to push the shaft back out if the builder is = not observant and does not hold the shaft in long enough for the air = pressure to bleed off. Whatever the cause, if the shaft and bearing is = not against the adapter plate, the rear race can fall off the shaft and = get stuck between the shaft and adapter plate. In the event that this happens and assembly continues, there is one = more chance for the builder to notice that something is wrong. When the = main gear housing is slid onto the 12 mounting bolts, it will not quite = seat all the way onto the planet carrier housing. This might be hidden = by the recess in the planet carrier housing if not checked closely. The = 12 mounting bolts will however, have plenty of exposed thread to fasten = the nuts to. When they are tightened, they will exert a force of many = tons on the pinched bearing race. This in turn causes the input shaft = to be pushed forward into the snubber seated against the ring gear = bulkhead. Making matters worse, since the pinched race is only on one = side of the shaft, it is cocked sideways causing the sungear pilot = bearing to contact the prop shaft pilot at an angle. The stage is now = set for a tragic set of events when the engine is started. =20 The exact sequence of events is in question but in order of most = likely occurrence, the following things happen. The snubber is caught = between two surfaces rotating in opposite directions and is abraded in = the contact patch area. This relieves some of the pressure but it is = not enough to make up for the extra 4 mm thickness of the pinched race. = The engine fires and rpms increase rapidly. The badly cocked sun gear = pilot bearing seizes on the prop shaft momentarily causing the drive to = lock-up. The firing chambers of the engine and inertia then shears the = sun gear drive pin which serves the same purpose as the propeller drive = pin in an outboard engine, it saves the drive train and engine from = catastrophic failure in the event of a prop strike. At some point, the = Loctite sleeve retainer holding the pilot bearing in the sun gear looses = its grip and the gear starts spinning on the OD of the bearing. The = input shaft is also now free to spin inside the sun gear causing the = oiling passage to be clogged with metal and Loctite debris, shutting off = oil supply to the thrust bearing.=20 The engine is now free to unleash its power on the hapless thrust = bearing with its pinched race behind the input shaft. This assembly = thrashes about until friction causes the race to stick in various places = which causes the input shaft to machine itself into a crude tapered = pilot and at some point causes the race to self-center on the spinning = input shaft. The compressive forces on the assembly are finally = relieved. Things appear to work normally for awhile. However, the = thrust bearing pilot on the input shaft has been badly mangled at this = point and is no longer a precision fit inside the rear race. In = addition, it has been deprived of the forced lubrication because of the = blocked oil passage in the input shaft. It soon friction-welds itself = to the input shaft and begins spinning against the adapter plate where = it generates large quantities of heat. The first thing to feel the = effects of this heat is the oil seal which is pressed into the adapter = plate directly behind the race. It is soon destroyed but the welded = race probably keeps much oil from escaping, especially since not much = oil is being sent in its direction anyway. The very last thing to go is the thrust bearing roller cage. It is = made of a high temperature polysulfide material but it finally succumbs = when enough heat is transferred from the race spinning against the = adapter plate. The rollers are flung out of the cage but at this point = nothing changes other than the peening of the surrounding housing from = the flying rollers. The rear race is firmly welded to the shaft and is = not depending on the rollers to support it. The final event would have = been the eventual bouncing of one or more rollers into the gear train = but it was spared this due to the fortuitous timing of the builder's = inspection. End of report. Tracy (will take about as long to repair drive as this took to write = ) ------=_NextPart_000_0008_01C8A88A.C9D45C30 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Thanks for the Post-Mortum, = Tracy.
 
Wow!  An impressive bit of Holm's = detective=20 work with clue leading to clue and deduction conclusion to = conclusion. =20 Clearly, attention to details in the assembly and mounting of=20 the redrive is crucial - just how crucial is readily=20 apparent.
 
Ed
----- Original Message -----
From:=20 Tracy=20 Crook
Sent: Sunday, April 27, 2008 = 4:07=20 PM
Subject: [FlyRotary] Re: Post = Mortum on=20 RD-1B

Here's=20 my shop notes from the "this can't be good" incident.  =
 
Post=20 Mortum on RD-1B SN # 35B

 

Background=20 on unit:

 

Drive=20 had very little testing on installation (ground only).  Builder reported some oil = leaking from=20 rear seal and requested a new seal which was subsequently sent to and=20 installed by builder.   Engine experienced various = problems=20 during testing (coolant found in rotor chambers) and was rebuilt. The = builder=20 was not able to identify the source of the stray coolant.  Builder decided to examine = drive while=20 it was off during engine rebuild and noticed that the input shaft = thrust=20 bearing had failed (roller cage was melted and all rollers were loose = in=20 drive).  Unit was = returned to RWS=20 for analysis and repair.

 

In=20 addition to the thrust bearing, the following problems were found = during=20 inspection:

 

1.  Input shaft gear drive pin = had sheared=20 off and the gear had spun on the shaft causing the thrust bearing oil = passage=20 to clog.

2.  Sun gear pilot bearing had = seized on=20 the prop shaft pilot and stayed there when the drive was=20 disassembled.

3.   The rear input shaft = thrust=20 bearing race had spin welded and seized on the shaft with evidence of = extreme=20 heat in that area (blued metal). =20 I noted that the race was more or less in its normal position = on the=20 shaft and had not seized due to moving forward onto a larger diameter = portion=20 of the shaft as would be expected when the rollers and cage vacated = their=20 normal location.  This = indicates=20 that the rear bearing race seized prior to the destruction of the = roller=20 bearing cage.

4.  The seized race had spun = against the=20 adapter plate causing the aluminum to heat and extrude into the oil = seal=20 cavity.  This allowed = the input=20 shaft to move further back against the adapter plate.

5.   The oil seal was = completely=20 destroyed by the heat of the spinning race.

6.  The moly impregnated nylon = snubber was=20 crushed on one side with evidence of heating due to friction. 

7.  The loose thrust bearing = rollers had=20 bounced around inside the planet carrier housing leaving the housing = and=20 adapter plate with the appearance of being shot peened.   Surprisingly, it = appears that=20 none of the rollers got past the planet carrier bulkhead and into the=20 gears.  There was no = evidence of=20 damage to any of the gear teeth.  =20

 

There=20 are fairly obvious explanations for each of the individual failures = observed=20 but the likelihood of all of them being present and manifesting = themselves at=20 the same time is very small.  = The=20 analysis of the root cause reduces the problem to finding a single = factor=20 which would fit all of the observations and failures.

 

The=20 key piece of evidence in this failure turns out to be the pinched = nylon=20 snubber washer.  It = would be=20 possible for an error in setting the input shaft end play at time of=20 manufacture to cause the snubber to be crushed when assembled but this = would=20 result in a symmetrical crush around the entire circumference of the=20 snubber.  Operating the = drive in=20 this condition would cause the snubber to quickly overheat and melt = until the=20 interference was eliminated.  = It=20 is probable that the drive would operate normally at this point except = for the=20 harsh metal to metal contact between sungear and ring gear bulkhead = under=20 negative (abnormal) torque conditions such as engine backfire.  Insufficient endplay alone = would not=20 explain any of the other failure points.

 

The=20 only possible explanation I could come up with that would explain the=20 asymmetrical force sufficient to deform the snubber (it takes a force = measured=20 in tons to do this) is for the rear race of the input shaft thrust = bearing to=20 drop off the pilot on the shaft and get trapped between the adapter = plate and=20 input shaft.

 

  This can happen for a number = of=20 reasons.  Failing to = push the=20 input shaft all the way through the damper spline and into the = crankshaft=20 pilot is the most obvious.  = Too=20 much grease in the crankshaft pilot hole can also prevent the shaft = and=20 bearing assembly from seating firmly against the adapter plate.  It is a close tolerance fit = between=20 input shaft pilot and the crankshaft pilot hole and the grease will = not easily=20 extrude past the clearance.  =20 Even if none of this has occurred and the shaft assembly is in = fact=20 seated against the plate when the assembly is performed, the close fit = of the=20 pilot and the grease will compress air in the cavity causing it to = push the=20 shaft back out if the builder is not observant and does not hold the = shaft in=20 long enough for the air pressure to bleed off.   Whatever the cause, if = the shaft=20 and bearing is not against the adapter plate, the rear race can fall = off the=20 shaft and get stuck between the shaft and adapter plate.

 

In=20 the event that this happens and assembly continues, there is one more = chance=20 for the builder to notice that something is wrong.  When the main gear housing = is slid=20 onto the 12 mounting bolts, it will not quite seat all the way onto = the planet=20 carrier housing.   = This might=20 be hidden by the recess in the planet carrier housing if not checked=20 closely.  The 12 = mounting bolts=20 will however, have plenty of exposed thread to fasten the nuts = to.   When they are = tightened, they=20 will exert a force of many tons on the pinched bearing race.  This in turn causes the = input shaft to=20 be pushed forward into the snubber seated against the ring gear = bulkhead.   Making matters worse, = since the=20 pinched race is only on one side of the shaft, it is cocked sideways = causing=20 the sungear pilot bearing to contact the prop shaft pilot at an = angle.   The stage is now set = for a=20 tragic set of events when the engine is started. 

 

The=20 exact sequence of events is in question but in order of most likely=20 occurrence, the following things happen. =20 The snubber is caught between two surfaces rotating in opposite = directions and is abraded in the contact patch area.  This relieves some of the = pressure but=20 it is not enough to make up for the extra 4 mm thickness of the = pinched=20 race.   The engine = fires and=20 rpms increase rapidly.  = The badly=20 cocked sun gear pilot bearing seizes on the prop shaft momentarily = causing the=20 drive to lock-up.  The = firing=20 chambers of the engine and inertia then shears the sun gear drive pin = which=20 serves the same purpose as the propeller drive pin in an outboard = engine,   it saves the drive = train and=20 engine from catastrophic failure in the event of a prop strike.  At some point, the Loctite = sleeve=20 retainer holding the pilot bearing in the sun gear looses its grip and = the=20 gear starts spinning on the OD of the bearing.   The input shaft is = also now free=20 to spin inside the sun gear causing the oiling passage to be clogged = with=20 metal and Loctite debris, shutting off oil supply to the thrust = bearing.=20

 

The=20 engine is now free to unleash its power on the hapless thrust bearing = with its=20 pinched race behind the input shaft.   This assembly thrashes = about=20 until friction causes the race to stick in various places which causes = the=20 input shaft to machine itself into a crude tapered pilot and at some = point=20 causes the race to self-center on the spinning input shaft. The = compressive=20 forces on the assembly are finally relieved.  Things appear to work = normally for=20 awhile.  However,  the thrust bearing pilot on = the input=20 shaft has been badly mangled at this point and is no longer a = precision fit=20 inside the rear race.  =  In addition, it has been = deprived of=20 the forced lubrication because of the blocked oil passage in the input = shaft.  It soon = friction-welds=20 itself to the input shaft and begins spinning against the adapter = plate where=20 it generates large quantities of heat. =20 The first thing to feel the effects of this heat is the oil = seal which=20 is pressed into the adapter plate directly behind the race.  It is soon destroyed but the = welded=20 race probably keeps much oil from escaping, especially since not much = oil is=20 being sent in its direction anyway.

 

  The very last thing to go is = the=20 thrust bearing roller cage.  = It is=20 made of a high temperature polysulfide material but it finally = succumbs when=20 enough heat is transferred from the race spinning against the adapter=20 plate.   The = rollers are=20 flung out of the cage but at this point nothing changes other than the = peening=20 of the surrounding housing from the flying rollers.  The rear race is firmly = welded to the=20 shaft and is not depending on the rollers to support it.   The final event would = have been=20 the eventual bouncing of one or more rollers into the gear train but = it was=20 spared this due to the fortuitous timing of the builder's=20 inspection.

 

End=20 of report.

Tracy  (will take about as long to repair drive as this = took to=20 write )

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