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Rear Driveshaft Vibration problems (Long!!!)

Discussion in 'The Garage' started by WY_75, Apr 9, 2006.

  1. WY_75

    WY_75 Registered Member

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    I have a bad vibration due to something in the rear driveshaft in my 1975 K20 longbed.

    The truck is smooth under 40 mph. At about 40 a vibration begins and gets really bad at 50 or so. I yanked the rear driveline and drove the truck in 4-HI and it is smooth as silk. No vibes at all. So I look at the rear driveshaft. This is a 1-piece shaft with no center support. The front U-Joint(near transfer case) is original and is 30 years old. The rear U-joint has been replaced at some time, but the grease seals on 3 of the 4 caps are ripped open and the roller bearings in the caps have started to wear into the inner surface they ride on. I also find that the slip yoke was installed improperly in relation to the front portion of the shaft. The slip yoke was 1 spline off from where it should have been, so the U-joints were phased slightly incorrectly.

    Trying to fix problems, I then installed 2 new U-Joints and changed the rotation of the rear portion of the slip yoke so the U-Joints were phased correctly. I did however notice that there is some side-play at the slip joint. Possibly because it was not greased regularly.

    So I re-installed the rear driveshaft and test drove it. The vibration is still there at 40 mph but it is less than it was. The vibration is only about 80% as strong as it was previously, so the new U-Joints and slip yoke adjustment must have helped some. What else can I do?:confused:

    I am thinking the looseness at the slip yoke may be contributing to a slight "Jumprope" effect at a higher driveshaft RPM. Is this possible?

    I was thinking of turning the driveshaft end for end and putting the slip yoke near the transfer case so the torsional loading on the splines of the slip yoke would be in the opposite direction of the current loading.

    Is this a bad idea? Does it matter if the splined slip yoke on the rear driveshaft is nearer to the rear axle or nearer to the Transfer case? Is one position better than the other? What was the factory position? Front or rear?

    Do you folks have any other suggestions on what to look for to cure this vibration?

    Thanks in advance, and sorry this was sooooo lengthy.
     
  2. 1977k5

    1977k5 3/4 ton status Vendor

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    Your problem is probably with the slip part of the shaft. Sometimes the pocket where the u-joint caps press into can wear out and the cap will spin in the driveshaft.
     
  3. justhorsinaround

    justhorsinaround 3/4 ton status

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    Yup. Pull up and down on the slip yoke part to see if there's play. If'n there is, go buy a used shaft and take the crap shoot or go to your favorite drive line shop and know you're gettin a known good product.

    If there is no play then hmmmmmmm.......
     
  4. Zeus33rd

    Zeus33rd Smarter than you GMOTM Winner

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    Are there any balance weights on it? If there are none, I would maybe suspect that it's out of balance. When a one piece shaft starts getting that long, they're tricky to blance out. Working for HAD, I've built and balanced some 65" long+ one piece shafts for Ford Excurions. Alot easier to straighten and balance a 65" long 2 piece shaft. Try taking it to a shop in your area to get the balance checked.

    It doesn't matter which end the slip yoke is positioned at. Factory was with the slip at the top.

    Thats about the only thing new I can come up with. :wink1:
     
  5. WY_75

    WY_75 Registered Member

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    Yes, it looks like all the factory weights are there. No indication of any missing weights.

    Howevere, there is some play in the slip yoke. That may be my trouble.
     
  6. MUDSTEADY

    MUDSTEADY Newbie

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    How much lift you running. I am guessing your rear u-joint is running at to much of an angle compared to your rear pinion.
     
  7. WY_75

    WY_75 Registered Member

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    Sorry, should have mentioned that I was running NO LIFT!!!! Truck has all factory 3/4-ton springs and sits at factory height. Scary huh?:D

    I want to add a shackle flip in the rear from DIY4X.com, but I figured I better get my vibration issues fixed before I attempt a lift.
     
  8. roadnotca

    roadnotca 3/4 ton status

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    Is the D-shaft tube dented?
     
  9. WY_75

    WY_75 Registered Member

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    No dents anywhere that I can find.

    Like I said, I am going to put the slip yoke end of the drive line like it should be. Right now it is near the rear axle. Maybe applying drive torque to the slip yoke splines in the opposite of the current direction will tighten the slip yoke up and help the vibration.

    If that doesn't help, I will turn the slip yoke 180* in its splines. If that doesn't help, its time for a new driveshaft:rolleyes:
     
  10. roadnotca

    roadnotca 3/4 ton status

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    Maybe short of calling Tom Woods or HAD, run it past a local D-shaft shop for their "expert" (free) opinion.:D
     
  11. MNorby

    MNorby 3/4 ton status Premium Member GMOTM Winner

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    If you need a hand looking at it I get to Cody fairly often.
     
  12. roadnotca

    roadnotca 3/4 ton status

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    There ya go:D
     

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