Auto Repair Help


by Kyle McFadden

The driveshaft provides a torque transfer link, between the transmission and the differential. The design of the driveshaft allows it to transfer torque while compensating for the movement of the rear suspension, as it reacts changes in the road surface. The driveshaft consists of a length of hollow tube, usually constructed of steel, composite material, or aluminum, with yokes welded on either end. Universal joints are pressed into the yokes and serve as a flexible connection for the transmission and the rear differential. Most universal joints are cross shaped with rotating caps, placed on each end of the cross. The caps contain small needle shaped bearings, packed in special grease, to allow low friction movement between the cap and universal joint body. A short stubby shaft, known as the slip yoke, is attached to the driveshaft, using the front universal joint as a connector. The slip yoke connects the driveshaft to the transmission output shaft. It has a polished outer surface, to provide a sealing surface for the transmission tailshaft seal, and inner splines, that slide over the splines of the transmission output shaft.
Driveshaft UJoint

Both the slip yoke and the universal joints allow the driveshaft to counteract the normal movement of the rear suspension, as it reacts to changes in the road surface. When the rear suspension extends, or moves down, the driveshaft must become longer. When the rear suspension compresses, or moves upward, the driveshaft must become shorter. The slip yoke is designed to slide back and forth, on the transmission output shaft, to compensate for the change in distance, between the differential and the transmission. In addition, as the rear suspension moves, the angular relationship of the rear differential and the transmission output shaft also changes. The universal joints allow the driveshaft to react to the change in angles, between the differential and the transmission.

Universal joints can only efficiently operate over a small angle. Because of this, the relationship between the rear differential and the transmission output shaft, is carefully designed. The rear differential is generally set, at such an angle, to align with the transmission output shaft. In short, the rear differential is aimed at the transmission output shaft, to provide a good average working angle. When this relationship is altered by excessive weight, or suspension height change, the driveshaft angle can exceed working design. The usual result is driveshaft vibration, as the universal joint must speed up and slow down, each rotation, due to excessive driveline angle. Also, when the suspension height of a vehicle is raised beyond normal limits, the driveshaft length may become too short, due to the excessive distance between the rear differential and the transmission. The driveshaft may become easily disconnected, due to the distance the slip yoke must extend to compensate for the suspension height.

Some vehicles use a double universal joint assembly, at the rear of the driveshaft. This type of universal joint assembly is known as a constant velocity joint. By using two universal joints, the driveshaft can operate at a greater angular range, than a single universal joint design. These are mostly equipped on larger passenger cars such as station wagons, or luxury cars, to provide a smoother ride, especially under heavy loads. Light duty trucks and large passenger cars can be equipped with two piece drivelines. This type of driveshaft will use a center support bearing, mounted on a frame crosspiece, to hold up the front half of the driveshaft. With the two piece driveshaft, the slip yoke is not used at the transmission. Instead a slip joint is placed between the front and rear halves of the driveshaft, at the center support bearing. The slip joint works in the same manner as the slip yoke, sliding in and out, to compensate for suspension height changes.

Most universal joint failure is a result of lack of lubrication. When the universal joint lubricating grease breaks down, or is contaminated by dirt or moisture, the needle bearings rapidly wear. This can result in a squeaking sound, as the vehicle slowly accelerates, and may result in driveshaft vibration. Most vehicles today do not have lubrication fittings on the factory equipped universal joints. However, almost all replacement universal joints are equipped with lubrication fittings.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

5 responses to “Driveshaft Ujoint”

  1. Randy says:

    how to put drive shaft back in time for 2004 bwm 330i & with new rubber coupling & new center bearing

  2. RicardoMarine says:

    Kyle McFadden, I enjoyed reading your write up just now.
    The reason that I found your artacle was due to a search I was doing to see if the term “Cordial Action” is being used to describe the vibrations caused when a bearing cross has reached it’s maximum angle of articulation.

    I’m an old school mechanic beginning in the mid 60’s. I was somewhat of Tri-Five Chevy enthusiast and owned a beautiful 1955 2 dr hardtop way back when.

    Being a stickler regarding technical terminology, I noticed that you refered to bearing crosses as “u-joints”. In using rather strict terminology, a U-joint or Universal joint is the sum of all components. I.E., the two bearing crosses (aka cross bearings or spiders) both end yokes and the center drive line component.

    Heck… even the parts counter guys call these “u-joints”. I know… I know, the world ran with the term “U-joint”, but it is incorrect!

    I now own a very small Volvo Penta stern drive business. These drives run under full power while the univeral shaft is not articulating the bearing crosses at much of an angle. These bearing crosses must articulate in order to roll the bearing cap needle bearings across the trunion surface. If not, this causes the needle bearings to “point load” against the trunion surface. I can tell you that no amount of additional lubrication renders the wear any less.
    This may not be such an issue with a car/truck drive line as these do articulate.


    • Albert Hall says:

      The term u joint has been used for over 400 years. If you look on Moog’s web site you will see the bearing is called a u joint for universal joint. The bearing has been know by four different names all by the last name of the inventer.


      • Ken Dandurand says:

        I agree with your response. I have been a mechanic for 50 years (retarded now) and have always called them what my dad called them (he was a mechanic from 1920 – 1985) and while I understand Ricardo’s statement above, it reminds me of a friend who finelines the terms for clips or magazines. I still call them clips as I sure don’t want to lengthen the reply time when I ask my partner for another Magazine 5.56 vs “Gimme a Clip!”. The other thing that ticks me is the name for most any component on a vehicle. Book calls it one thing, parts stores call it something else, and dealer parts labels call it something different all together.

        OK, now for my question. I helped my grandson change out the “U-joints” on the front driveaxle for his 09 Rubicon. The following weekend he came back and said they were not grease was coming out of the caps. He was rigth, no zerks and no reason for it to come out. When I looked at the yoke, you could see sliding marks on the inside of the yokes where the caps are sliding in and out. The clips are in place (his statement as my back was giving me fits and I could not get down to look at them) so he used a dril hammer (Short stubby thing) to drive them back in all the way with a rod and the hammer. This morning he said the driver’s side was still tight and not moving but the passenger side is moving as before. What are your thoughts? Clips? Size wrong from O’Reilleys, ???

  3. Jalu Sakti says:

    It’s so interesting how this system of different parts can compensate for road surfaces that are not very flat, like you said. Aside from this driveshaft u-joint system, are there any other systems found in cars these days? I have always been fascinated with mechanics and construction. One day I want to build my own car, so I have to understand things like this.

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