Failed dual mass flywheel analysis

Discussion in 'VW Mk4 Jetta, Golf, New Beetle, Passat TDI forum' started by Keith_J, Jan 6, 2012.

  1. Keith_J

    Keith_J New Member

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    A month ago, I started noticing a general roughness at idle and a grabby clutch. As this is a 2k Jetta ALH, I knew the DMF could be going out. So I planned and fortunately, replaced before it catastrophically failed.

    Being the neurotic mechanical engineer, I had to tear it open to see why. So many dismiss the DMF as unneeded but the NVH superiority were the reason I replaced with a LUK DMF kit.

    First, the old unit was unbolted to gain access to flywheel bolts. I already knew the release bearing was toast and the two small spring steel clips found in the housing were the release bearing's retainers.

    [​IMG]

    Now onto the pressure plate:
    [​IMG]
    There are 8 spots on the ID that show cracking.

    [​IMG]

    I drilled the 8 rivets holding the flywheel's friction surface to the driven hub. You can see where the two rubbed each other, indicating inner bearing failure:
    [​IMG]

    Next, I cut the case open with a 14" abrasive chop saw. Messy, noisy and hot. Cut along the edge of the starter ring gear. Once open, I saw the problem. There are supposed to be 8 planet gears on the 8 bosses for the bearing, these pivot on the flywheel and engage an epicyclic ring gear on the driven hub (which is riveted to the flywheel's friction surface). In this one, only TWO planet gears are left and they are partials, most teeth worn out. They are at the 11:00 and 2:00 positions. The rest are fragments, found in springs and grease. The springs are all fine, not an issue of abuse.

    [​IMG]

    The root cause of failure? Shattered planet gear bearings. These are made of Nylon. While this may be quiet, the issue of using a soft/hard gear should always be with the SMALLER gear being the harder material. Since the speed here is slow, both should be metal, not Nylon/steel.

    The springs and guides worked just fine. Adding a center bearing would also be a good idea.
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  2. Seatman

    Seatman Active Member

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    I think you might might have to much time on your hands lol :D


    I don't know what the benefits of the dmf are meant to be but my last car had the AGR with a smf as standard and now I've put the engine in the golf with the dmf (it was new so thought I'd make the most of it), I think I prefered the feel of the smf and will be going back to it later in the year. I'm going to fit the VR6 sachs kit but with the standard G60 rather than one of these extra light things.
  3. Keith_J

    Keith_J New Member

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    Time? Took longer to post the images and write this thread than to cut it apart. I have a neurosis of root cause analysis, a condition my former employer instilled in my brain. Part of their total quality management brainwashing.

    Plus I have pretty good metalworking tools. And an old 14" abrasive blade which was 9" but is now close to 6". I hate this blade as it makes nasty burrs, my newer blades cut faster without burring.

    Too much banter on the failures, not enough root cause analysis. So I lit a candle.

    Now, why a DMF? Noise, vibration and harshness. Plus a DMF can be lighter, making the engine feel sprightly. I also have 1:0.658 5th gear which also helps lower interior noise. Here in Texas, we have a few Interstate Highways with 80 MPH speed limits, most of the noise I hear is from wind.

    The DSG transmission uses a DMF for similar reasons and also for geartrain longevity. Torsional vibration requires gears to be over-sized. This increases the inertia which slows shifting. Also, more gears in less space. But in the manual transmission on the earlier vehicles (5 speed and lower), the gears are designed for SMFs. Not so in the 6 speed.
  4. Seatman

    Seatman Active Member

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    Plus the smf for the 6spd is crazy money (at least over here), you're talking about £1000! Nice box though, nice gear changes. A friend has the 6spd in his gt tdi. My previous car was the seat cordoba which is where the smf came from, it was only at around 2000 rpm at 80 mph, shame the golf isn't like that and actually feels like it could do with a 6th gear. On the other hand I find the golf to be a lot quieter overall though so that's a bonus.

    Anyway where's your revs at if you're doing 80mph with the altered 5th gear?
  5. Keith_J

    Keith_J New Member

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    At GPS indicated 80 MPH, tachometer is reading 2400 RPM. Before with the 1:0.756 5th gear, it was 2800 RPM. The speedometer is off 5-8 MPH, odometer is spot on. Go figure, guess that could be calibrated with VCDS but never bothered.
  6. Seatman

    Seatman Active Member

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    Yeh I've seen the calibration mentioned on the tdi club but if you follow it the way it is there's less chance of getting caught speeding so I'm not messing with mine.
  7. chittychittybangbang

    chittychittybangbang Administrator

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    Thanks, I'll add these pics to the clutch FAQ. Very interesting, I have the 2006 flywheel sitting in my garage but it seems too hard to cut it open. I'm not clear on how you cut the starter ring gear off, you cut it along the inner diameter?

    I wonder why a bad flywheel would lead to cracking on the PP, I'm guessing it was somehow slipping and overheating but how? Was the friction surface of the flywheel stuck to one side? If not, how far to the side could you rotate it?
  8. Keith_J

    Keith_J New Member

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    EUREKA!!! I think the issue with the Sachs DMF is precession. I've always had an issue with 1-2 shifts during "spirited" driving, acted as if the synchronizer for 1-2 wasn't doing its job. With the LUK clutch, the problem is gone.

    Precession is the wobble of a spinning object. Consider a spinning toy top. While the primary mass(crankshaft connected) is rigidly damped by the rear main bearing, the secondary (friction surface and pressure plate) is only arrested by the bearings between the primary and secondary masses. The same ones that failed in my case.

    Further thought on this failure centered around the rather thin epicyclic bearings, about 4mm thick. Any wear could cause these bearings to jump off track during precession.

    This is a design issue with the Sachs. Since I don't have a cut-away view of the LUK, I cannot tell if it has the same weakness. Seeing how LUK has had good service life, I have to be optimistic. The only modifications to my 2k ALH are Bosio PP520 injector nozzles.
  9. Keith_J

    Keith_J New Member

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    The vehicle had an Upsolute tune with 205 injectors. This had issues with clutch slippage which probably led to the pressure plate fractures.

    As far as cutting it open, the unit is welded circumfrentially with either Plasma Arc or Electron Beam welding (PAW or EBW). These welding procedures are automated and result in very narrow heat affected zones, to the point they are less than 1mm wide in this application. I simply set my chop saw up with the saw fully open, then welded a jig so the flywheel could rotate on an arbor while I applied the blade to the area just behind the ring gear. Sparks flew everywhere, I used a block of wood to resist the flywheel's rotation. The blade's motion fed the rate which I modulated with pressure on the saw. I knew when it was near when the Nylon guides were being cut or heated as the smell changed. Nylon has a specific odor, sort of like hair burning but less intense. Nylon's diamine-dicarboxylic nature makes it a kind of synthetic protein.

    Rotation? Even with 500mm leverage, the most rotation I could get was less than 10 degrees. I would suspect this was due to bearing fragments literally jamming the cogs. It was just miles from total failure:eek

    If those two partial bearing cogs were to fail, only the very thin inner flange would have kept the secondary mass together with the primary.:dunno

    Yes, these images should be useful for those contemplating a clutch change. I would say the wear of the friction surfaces is slight for the mileage, the other components are far from replacement. Dual mass flywheels wear regardless of engagement cycles. Every time a cylinder fires, the bearings undergo movement. Conventional clutches only have the sprung hub to move.

    I can only hope the engineers have incorporated changes due to failure analysis.
    Last edited by a moderator: Jan 7, 2012
  10. YMZ

    YMZ Super Moderator

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    Unfortunately it's the bean-counters who usually make the decisions...

    Thanks for the most informative post !!!

    Yuri
  11. LEZ

    LEZ New Member

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    I had to look all this clutch and flywheel stuff up DMF etc interesting another expensive piece of kit to go wrong and cause grief yeeee ha.
    Lez
  12. Keith_J

    Keith_J New Member

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    You really know how to push my buttons :ugh

    I've locked horns with accountants after they have tried to substitute parts based on cost alone. They had no ownership rights to make such changes.

    I'm going to have to get drunk and forget this happened. Again.
  13. Keith_J

    Keith_J New Member

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    LUK has been making DMFs since 1985. They made significant market penetration, I'm certain they have the engineering down.

    Sachs was probably specified based on cost alone. The root cause of the failure is the aging of the Nylon epicyclic bushings, something which accelerated age testing didn't take into account.

    Cogging of the bushings assures rotation which prevents flat-spotting. And being cast, cost is low. Weight is also a function.

    The LUK design dispenses with the Nylon cog and spring bushings, using only two long springs instead of 10. And full diameter plain bearings, probably metal on metal but lubricated with the MoS2-silicone grease common to these designs. We shall see the life span but history has shown good results.

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