In The Shop
Feb 12, 2011
You may already know these tidbits about repairing or maintaining driveshafts and u-joints, but I ran into folks with questions this week, so maybe a refresher would be useful.
-When reassembling a two-piece driveshaft, the u-joints on the ends must be "timed." The yoke arms on one end must be in alignment with the yoke arms on the opposite end. If a splined driveshaft is slid together with the yoke arms out of alignment, the driveshaft will have significant vibration. This isn't a problem with a one-piece driveshaft, like the driveshaft on a rear-wheel-drive car, because the yoke arms can't get out of alignment.
-Don't overtighten the small bolts that hold in place the end caps on u-joints. If those bolts are overtightened it puts stress on the needle bearings inside the caps, creates heat, and shortens the life of the u-joint. Proper torque for most vehicle and farm-related u-joint bolts is 30 foot-pounds or less. Pat Fagen, the owner of FastShafts in Des Moines, jokes that he tells customers who don't have a torque wrench to, "put grease on their hand, hold the driveshaft with that greased hand, then torque the bolts till the driveshaft starts to slip."
-Greasable u-joints are one place where it's okay to pump the heck out of the handle on a grease gun. U-joint manufacturers recommend greasing u-joints until fresh grease purges from the rubber seal on each arm of the u-joints "cross" assembly. That way old, contaminated grease is replaced with fresh grease, extending u-joint life.
-And this one is a pet peeve, not necessarily good mechanical advice: Driveshaft/u-joint couplers don't need to have their splines greased. A dry pto shaft is not a bad thing. A dry-lubed pto shaft--with graphite or silicone spray--is a good thing. A pto shaft smeared with axle grease is...a messy nuisance, an example of lubrication overkill guaranteed to make a mess of your hands, your pants, and the inside of your tractor cab. But that's just my opinion.