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curious about U-joints and drivelines

Discussion in 'Trucks and Trailers' started by Garet, Apr 4, 2001.

  1. Garet

    Garet LawnSite Member
    Messages: 157

    I was just wondering about how U-joints, when they go bad, cause the clunking noise you hear when you put it in gear? I can't picture how they could cause this. Maybe somebody can enlighten me on this matter.

    Also, are the front and rear drivelines supposed to be lined up in the exact same way. What I mean is are the u-joints supposed to be facing the same way? I ask this because I ran a lower gear in the front for just a little bit and it probably misaligned the front driveshaft a little. Before I took the front axle out the U-joints were in the exact same position as the back. Lined up the same I mean. Is it crucial to have them lined up or does it really matter? Also, how does one go about balancing the driveshafts? Is this a mechanic only job?


  2. reallyrusty

    reallyrusty LawnSite Member
    Messages: 40

    u-joints are to put it simply a cross or x
    the ends are ground cylindrical and a cap or cup filled
    with needle bearings ride against the crosses cylinders.
    There should be next to no space (clearance) with these asemblies. When they run dry(run out of grease) the cross ends wear the caps and needle bearing causing clearance so when you put it in gear you hear a clank (cross hitting cap because of the space) They are not that hard to change if you have a least a big vice. There is no backwards other than think about the position of the grease fitting (so you can get at it).
    Alignment is not done on u-joints.
    Balancing is done at a specialty shop.
    hope this helps
  3. 85w/350

    85w/350 LawnSite Member
    Messages: 244

    I have only had to replace on u-joint and i heard that noise for about 3 days and then i was returning home one evening and I pulled up a little too far...heard that clanck noise when i put it in reverse and I gave it some gas...next thing I heard was my drive shaft clanking aroudn underneath ...bed bottom...ground...bottom..ground....definately somethign to inspect when you get under there
  4. Garet

    Garet LawnSite Member
    Messages: 157

    so its ok for the front and rear drivelines to be at different angles in relation to the way the thing the u-joints attach to is facing? Even if it was facing in the exact same direction when I first took out the original front axle? This is what I was talking about, not the direction of the u-joints in relation to how they are installed, but the direction of the yoke(I think it is called that)


  5. reallyrusty

    reallyrusty LawnSite Member
    Messages: 40

    Maybe somebody else can back me up but i would say yes.
    The u-joint is there because the front/rear moving constantly changes alignment. The u-joints compensate for the angle changes.If i understand now what your asking.

    Some guys actually rotate the front axle slightly up if they have installed a lift kit because the greater shaft angle can wear the u-joints faster.
    hope this helps and somebody else can back me up
  6. Garet

    Garet LawnSite Member
    Messages: 157

    Kind of hard to exlpain what I am getting at so I'll try again:) I am not talking about the angle that the axles are facing if this is what you are talking about. I am talking about the thing the driveline bolts into on the front and rear axles. The thing it bolts into faces a certain direction and you are not supposed to turn the driveline when you take the rear end out because of alignment issues with that thing. The same goes with the front end. Since I drove the front end at a different ratio than the back for a while it changed the way the thing that the driveline plugs into is facing in relation to the rear's one. Before I touched anything they were facing in the exact same position. To sum it up once again I am talking about the thing on the front and rear axle that has the bolt holes for the straps which hold the drivelines on. Are these supposed to be turning in a synchronous manner. Not one pointing up and down and one pointing left and right.

  7. 75

    75 LawnSite Senior Member
    Messages: 992

    OK - I believe what you're referring to is known as the "yoke".

    I'm not 100% certain on the subject of orientation BUT I don't think the front & rear axle yokes being in different positions is a problem.

    What's important is the orientation of each driveshaft to its corresponding yoke. This is the reason all the manuals say to mark the parts before disassembly, so that the driveline balance is maintained.

    Even a small difference in gear ratio between front & rear axles puts extra strain on the driveline (because front & rear shafts are turning at different speeds) and can contribute to accelerated U-joint wear, among other things. Tires that aren't the same diameter on all 4 corners of the truck can have the same effect too.

    Hope this info clears up some of the mystery!
  8. Garet

    Garet LawnSite Member
    Messages: 157

    Thats what I was talking about 75. By the way roughly how much would it be to have the drivelines balanced?
  9. 75

    75 LawnSite Senior Member
    Messages: 992

    Glad that was of some help then Garet, on the subject of how many $ to balance the driveline I don't know - never had it done.

    I'd suggest searching out a shop that specializes in driveshaft/driveline work (possible places to ask for leads might be a transmission rebuilder or heavy truck repair shop) for a quote. I don't imagine it would be a huge amount of money - anyone out there ever have this done?
  10. Alan

    Alan Member
    Messages: 1,185

    Why would you need to have the driveshaft balanced? They are balanced when they are made and unless there is physical damage there should be no need to rebalance them. As far as having the u-joints "in phase" between front and rear, that is an impossible situation. Since front and rear wheels follow constantly varying ,and different, tracks down the road the driveline can never stay synchronized. Also, how would you keep them in phase when you were in two wheel drive? It is, however, important that the joints on any shaft be in phase. There is an acceleration/deceleration that occurs in every half revolution of the shaft, due to the changing angles of the driveshaft joints. I don't even come close to understanding the physics that would explain it, but I DO know that if you get a shaft misaligned by one spline when you put one back together it will shake itself apart. Been there, done that.

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