Alignment Problem - Suspect Velvet Drive Coupling

Discussion in 'Gas Engines' started by matscot, Mar 24, 2018.

  1. matscot
    Joined: Oct 2006
    Posts: 1
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    Location: Huntington, NY

    matscot New Member

    Need advise diagnosing a prop shaft alignment issue. I now suspect the cause to be the transmission coupling or possible worn splines on the reduction gear output shaft, but it could be something simple I am missing.

    Some background: Purchased the boat last year and replaced the engine but kept the 71C velvet drive transmission. Could only get the coupling gap down to 0.006 but ran the boat slow for about 15 hours last summer with no issues except for a noticeable vibration starting around 2000 rpm. Could also see a visible wobble at the stuffing box.

    This winter decided to cover all my bases to resolve this so I have replaced the shaft, cutlass bearing, shaft and trans couplings. (Prop is fine).

    I 've been able to get the gap closer, down to .004, but spec says 0.003 or less. Also the trans coupling gap moves as I rotate the coupling. So I bolted everything together and decided to check run out with a dial indicator. I am getting a 0.006 run-out on both the inner flange of the trans coupling and the shaft ( at just aft of shaft coupling). Does this sound right? I know the shaft and shaft coupling are good, so I am suspecting that the trans coupling is somehow not seating properly in the splines of the output shaft. (Its a 1.91 reduction gear)

    Any advise greatly appreciated! I'm spending far too much time on this.
     
  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Loosen the coupling bolts half a turn and check wobbling at idling rpms. If wobble is reduced, the drive flange and/or output shaft is bent or out-of-round. Installing an elastic coupling may be a better medicine than pulling the transmission apart.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Agree with above, though alignment should be roughed on the hard, but completed in the water. Doing either or will not work.
     
  4. Lepke
    Joined: Sep 2015
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    Location: Oregon to Alaska

    Lepke Junior Member

    Another potential problem could be flexible motor mounts. My experience is big boats and ships where engines are directly fastened to the hull structure. When working on small boats, I often found alignment changes as the motor mounts get older. As PAR: final alignment has to be in the water. If you troll with the boat, shaft misalignment can affect fishing, even with a flexible coupling.
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Can't be emphasized enough, alignment is a two step process; get it close on the hard, then final adjustments on the water once the hull settles in.
     
  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Ensuring that the gap between the coupling faces are parallel is critical but your comment that the gap changes when you rotate the shaft might indicate that the shafts are not collinear
    Ie shaft faces parallel but the axis of the shafts slightly out of line

    Without a proper alignment tool to check this there are a couple of things to watch for

    I will assume that the coupling faces require a bolt with a nut as compared to a bolt running through the flange into a threaded flange on the out put shat flange

    These bolts require a shoulder, the non threaded part to extend through the width of the first flange and most of the way into the second flange. When you are putting the bolts through the flanges they should slide through without have them to
    be driven through.

    We had a similar problem with a new engine/V drive install
     

  7. Lepke
    Joined: Sep 2015
    Posts: 81
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    Location: Oregon to Alaska

    Lepke Junior Member

    You can get an alignment better than most yards with a feeler gauge. You don't need dial indicators or other special tools. You slack the coupling bolts until you can make a slight gap. With misalignment, the gap will be greater on one side. With a feeler gauge set, measure the difference (if any) 180° apart. Rotate the shaft slightly and check again. Make sure the gap stays constant and in the same direction from the shaft. I can adjust couplings within .002" which is near perfect. The position of the gap tells you how the engine needs to be moved. If the gap is on the top, the engine needs to be moved up. That means slightly up on the rear motor mounts and up more on the front motor mounts. And so on. It may be side movement or a combination. A misalignment over time can ruin transmission bearings or stern bearings. It also makes noise fish detect. I use to troll for salmon commercially and alignment can ruin fishing.
    If you do the alignment on land, it needs to be checked again when the hull is supported by water.
     
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