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I recently replaced the 2 universal joints on the drive shaft of my rear wheel drive Lancer station wagon (instead of the entire drive shaft). small welding tacks were done to keep them in place. Thereafter, there was a significant vibration of the entire car (presumably from a offset/misaligned joint) from about 60km/hr onwards. The tacks were removed, re-aligned and tacked again. Now the vibration appears at 80km/hr. Any suggestions on how to get perfect alignment with no weight offset, or sort out this issue?

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It looks like the driveshaft has rotary imbalance

Based on the description provided, the source may or may not be the welded tacks. Even if the tacks are symmetrical to one another, if one tack is heavier than the other(s) this can throw off the rotational balance of the driveshaft.

The greater the imbalance, the more pronounced the vibrations will feel. The speed at which the natural frequency occurs is a bit more complicated to figure out, but isn't strictly speed-dependent.

I realize it's a bit late for your case, but I'm not so sure that tacking on new U-joints is the way to go here. It is tricky to get right at best, and you would ideally want to put the driveshaft on some sort of test bench to prove that it is balanced to an acceptable tolerance before installing it on the vehicle.

It may be worth getting the driveshaft rebalanced through the careful addition of the right amount of mass to offset the overall imbalance, much like how wheels are balanced through the addition of lead weights.

Short of replacing the driveshaft, I cannot think of any other practical way to eliminate this imbalance issue.

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    Why did you need to tack weld the uj's? Normally they have internal circlips to keep them in place. Also, and more worrying, is the heat generated by welding may affect the heat treatment applied to the uj's. If they needed to be tacked due to previous damage then if balancing does not work you will need to have the shaft professionally rebuilt or source a new one. – Solar Mike Dec 19 '16 at 21:05
  • @Zaid. Thank you. Is there a machine like they use for wheel balancing, and if so what is it called? Balancing would probably be the best option now, instead of cutting and re-welding (which was already done once) and the resulting heat issues. Finding a drive shaft may not be feasible as this is a 1979 manufacture car (which I have been using for 20 years) – Dinesh Dec 21 '16 at 3:24
  • @solar mike. There were no circlips. The front one had been previously replaced and welding tacks put in. These had to be cut away to get the U-joint out. The rear one which did not have circlips was 'hucked' in place. As to the heat the welder was instructed to use the minimum time and heat, but yes it would affect. The mechanic says the joints may loosen a bit with running and the vibration may reduce. – Dinesh Dec 21 '16 at 3:26
  • There are companies that will build shafts to order - still used in machinery and agriculture, so getting a re-built shaft should not be an issue as long as the distance between the two flanges is known - does your shaft have a sliding joint? One possibility is that the shaft has been put together incorrectly - a 90 degree rotation can cause vibration. One solution for balancing is to use jubilee clip(s) and use that to add mass before welding weights on. Add a clip on the shaft, mark it, test drive, rotate it 90 degrees, test again etc... Good luck. – Solar Mike Dec 21 '16 at 13:26

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