Up until a few years ago, I thought I knew all there was to know about diagnosing and replacing universal joints (U-joints)... I was wrong.
I had been mentored by older technicians 40 years ago when I was the young impressionable new guy in the shop, and I assumed they knew what they were doing. Over the years, I diagnosed and replaced U-joints in the same manner. Most of the time I thought I was successful, but there were times when I knew something was not right with my installation, but I did not know why.
A few years ago, while preparing for a manual drivetrain class I teach, I began researching the proper method of diagnosing, removing, and installing U-joints. I found that some manufacturer’s service diagnostic and replacement information was very limited while other manufacturers (including manufacturers of U-joints) give you detailed step-by-step service instructions, including specific tools to use and measurements to take.
Measurements? What measurements?
I had never been taught about centering a U-joint in the driveshaft yoke ears. I had never been taught to measure and adjust the axial end play of the U-joint with selective color-coded snap rings. I had never seen a U-joint with selective snap rings.
After a little more research, I found out that aftermarket U-joints do not come with selective snap rings. I had always purchased aftermarket U-joints. As it turns out, not all U-joints are equal. In this article, we will look at the potential differences in U-joints and how they can impact you and your customer.
I have a few questions to get you thinking about U-joints. We will answer each question in this article:
- What is the difference between a new $8.99 U-joint and a new $135.00 U-joint kit for the exact same vehicle application?
- Have you ever installed a new U-joint and had the customer complain of a vehicle vibration afterward?
- Why do the original factory-installed U-joints in vehicles seem to last forever?
- Why are a large majority of factory installed U-joints the “sealed” type without a grease fitting?
- Why are there colored snap rings on many of the factory-installed U-joints?
Historical content
Before we discuss U-joints, we need to clarify a little history and terminology. First, it is unknown who invented the original 2-axis U-joint, but it happened sometime in antiquity (thousands of years ago). Although today the name “Universal Joint” is defined by the Society of Automotive Engineers (SAE) in standard J901, here are the three most commonly used names for U-joints in service information, parts stores, etc.:
- The Cardan Joint- Incorrectly named in honor of Italian mathematician Hieronymus Cardano (1501-1576), who is credited with describing/inventing a swiveling gimbal with three degrees of freedom (for holding a ship’s compass level in the ocean waves) in 1557. A gimbal is not a U-joint and functions quite differently.
- The Hooke’s Joint- Correctly named in honor of English mathematician Robert E. Hooke (1635-1703), who in 1675 demonstrated that an angled shaft connected to a U-joint with two degrees of freedom does not rotate at a constant velocity. Hooke also discovered and demonstrated that connecting two U-joints together causes an angled shaft connected to them to rotate at a constant velocity. Today, this constant velocity joint configuration is incorrectly called a “Double Cardan Joint.” Hooke used his inventions in an attempt to display the time of day from a sundial onto a vertical wall, so people passing by could easily see the time of day.
- The “Polhem Knot” Joint -Incorrectly named after Swedish inventor Christopher Polhem (1661-1751), who, after visiting England and studying Robert Hooke’s work, went back to Sweden in 1697 and "re-invented" the U-joint under his name.
Prior to the rise in popularity of the horseless carriage (automotive industry) in the late 1800s, U-joints were primarily used in industrial applications to connect two machines together. These early U-joints required constant maintenance, cleaning, and lubrication.
in 1902, Clarence W. Spicer (1875-1939), engineer and inventor, invented an enclosure for U-joints to protect them and make them self-lubricating. He obtained 40 U.S. patents between 1903 and 1934 for various designs of improved U-joints and driveshafts. His inventions led to the replacement of the chain-driven axle with shaft-driven axles at the dawn of the automotive industry.
In 1919, Charles A. Dana (1881-1975), businessman, partnered with Spicer, purchased a controlling interest in the Spicer Manufacturing Company and managed the company while Spicer continued innovating and improving U-joints and driveshafts. Dana managed Spicer Manufacturing for 30 years.
in 1946, in honor of Dana, Spicer’s company was renamed to Dana Corporation, which has continued to innovate and produce top quality U-joints and drivelines under the “Spicer Drivetrain Products” brand. Spicer is still a major supplier of factory-installed U-joints and driveshafts.
IMPORTANT: In 1982, the Spicer Driveshaft Division of Dana Corporation developed the first all-aluminum driveshaft. Today’s aluminum driveshafts require special zinc-phosphate coated steel bearing caps and snap rings in order to avoid severe corrosion caused by electrolysis.
Today there are several aftermarket companies which are in competition with Spicer, including many producing lower quality counterfeit/knock-off parts at greatly discounted prices. The use of aftermarket U-joints may cause additional challenges and troubles of which you may be unaware.
The difference between U-joints
As part of the U-joint research for my classes, I tested six different brands of aftermarket U-joints, two Original Equipment Manufacturer (OEM) U-joints, and two Spicer U-joints for the same application. These U-joints cost anywhere between $8.99 and $135.00 each. I tested each U-joint for the following characteristics:
- Weight of the complete U-joint with snap rings. Always replace both (all) U-joints as a set on the same driveline to maintain balance and reduce moments of inertia.
- The materials from which the U-joint was constructed
- The cross-span variation of the trunnion cross
- The cross-span variation of the trunnion cross with bearing caps
- The bearing cap diameter
- The design of the grease seal(s)
- The recommended bearing lubrication type
- Lubrication service intervals for U-joints with grease fittings
- Lubrication reservoir precautions for sealed U-joints
- Method of reduction of metal-on-metal friction from trunnion thrust surface to bearing cap
- Method of reduction of metal-on-metal friction from needle bearing ends to bearing cap
- Method of needle bearing retention
- Methods of corrosion protection
- The thickness of snap ring set(s)
Both $135 OEM U-joint kits contained a Spicer U-joint kit inside the box. The $35.00 Spicer U-joint kits contained the exact same instruction sheet and part numbers as the OEM kits.
All six brands of aftermarket U-joints were almost identical in appearance, but some had some serious quality and precision issues. To illustrate these issues, let’s look at the answer to the first question at the start of this article: What is the difference between a new $8.99 U-joint and a new $31.99 (or higher priced) U-joint for the exact same vehicle application?
The $8.99 U-joint
The graphic in Figure 2 depicts an $8.99 U-joint available today online and in many auto parts stores. At first glance, you may not realize that you are actually looking at technology from a 1968 Dana patent (US3369378A) that expired in 1985. When a patent expires, the technology is open for the rest of the world to use. It appears that many suppliers of aftermarket U-joints simply copy the technology from old, outdated, expired patents and hope that the general consumer will not know any different. In reality, today’s modern U-joints can outlast this old technology by a factor of 10 to 1.
Quality control?
Five out of the six aftermarket U-joints I tested had less than impressive quality controls regarding precision machine work. Some were worse than others. The worst can be seen in Figure 3. The cross-span variation measurement across the two sets of opposing bearing caps was 0.180mm (0.007”). By comparison, the same measurement on the OEM and Spicer U-joints was 0.025mm (0.001”).
Any cross-span variation can result in an offset u-joint cross centerline. An offset U-joint cross, and the resulting offset driveshaft can cause a vibration if the driveshaft runout was already on the borderline of specifications. In other words, installing this U-joint could cause a vibration. This is one of the answers to question 2 at the start of this article.
The typical contents of an aftermarket U-joint kit can be seen in Figure 4. This 51-year-old design has several disadvantages when compared to a modern Spicer design.
Single flexible rubber seal
This design of grease seal does a poor job of keeping the grease inside the bearing caps. It also does a poor job of keeping dust and moisture out of the bearing caps. Because this seal does such a poor job, periodic flushing of the old grease, dust, and moisture with new grease is required to maintain bearing life. This explains the need for a grease fitting.
Grease fitting
I used to think a U-joint with a grease fitting was a good thing, but I no longer think that way. The problem with using a U-joint with a grease fitting is remembering to have it greased properly (the flushing process). I personally have been to national chain stores for an oil change in my own vehicle. When they were finished servicing my vehicle, I asked them how many grease fittings they lubricated, and they told me my vehicle had sealed joints and no lubrication was necessary. I knew it had several grease fittings, and I had to tell them where they were. I am sure they hated having me for a customer.
Snap rings
All six aftermarket U-joint kits I tested came with one set (4) of snap rings. The thickness of the snap rings averaged 1.37mm (0.054”). The average typical snap ring thickness used with Spicer U-joints is 1.50mm (0.059”). One benefit to using snap rings which are too thin is they fit into the snap ring groove easier than thicker snap rings do.
Replacing the original snap rings with these thinner snap rings would result in an increase in the driveshaft runout of 0.13mm (0.005”) because of the additional axial end play in the U-joint assembly. This condition will cause the driveshaft to orbit rather than rotate on a centerline. This additional runout can cause a driveshaft vibration. This is also one of the answers to question 2 at the start of this article.
Now, imagine the vibration caused by the wrong combination of an offset U-joint cross and these snap rings. The worst-case scenario with this U-joint could have an additional 0.30mm (0.012”) of driveshaft runout just from changing a single U-joint! Many driveshafts have a maximum runout specification of 0.51mm (0.020”). Almost all driveshafts I have tested for runout measured at least half of their specification, even on brand new vehicles. It would not take much more runout to exceed the maximum allowed.
Friction producing design
All six aftermarket U-joints I tested were designed with the thrust end of the trunnion cross rubbing metal-to-metal on the bottom of an unmachined bearing cup. Additionally, the bottom of the needle bearings makes metal-to-metal contact with the bottom of an unmachined bearing cup.
Misleading advertising/markings
One of the six aftermarket U-joints I tested came from a major auto parts chain here in the U.S.A. “Made in China” was printed on the outside of the box, yet the letters “USA” were cast into the U-joint cross. It makes me wonder if this is a knock-off U-joint.
The $35.00 to $135.00 U-joint
The patented technology used in OEM and Spicer U-joints allows them to last ten times longer than the $8.99 U-joint. Let’s look at this technology.
Triple lip seal design
As shown in Figure 5, the inside lip seal faces the bearing cap and keeps the grease inside the bearing caps and lubrication reservoir. The other two lips are facing the trunnion cross and keep dust and moisture out of the bearings. Because this seal does such a good job, no external grease fitting is required for grease flushing, and bearing life is extended. This answers questions 3 and 4 at the start of this article.
Seal guard
The seal guard protects the triple lip seal from damage from mud or road debris as you drive. The seal guard is an important improvement in U-joint design.
Quality control
The two OEM and the two Spicer U-joints I tested had impressive quality controls regarding precision machine work. As can be seen in Figure 6, the cross-span variation measurement across the two sets of opposing bearing caps was a consistent 0.025mm (0.001”) or less.
Corrosion protection
As mentioned before, aluminum driveshafts require special zinc-phosphate coated steel bearing caps and snap rings in order to avoid severe corrosion caused by electrolysis. OEM and Spicer U-joints may have just two of the bearing caps coated with zinc-phosphate (dark brown/grey coating). Those two bearing caps are to be installed into the aluminum yoke ears of the driveshaft. The steel bearing caps are to be installed into the steel companion yoke ears. When replacing a U-joint in an aluminum driveshaft, be sure to use the coated bearing caps and coated selective snap rings.
Thrust spacer
These kits were designed with a nylon spacer on the thrust end of the trunnion cross to prevent metal-to-metal friction with the bottom of the machined bearing cap.
Thrust washer
These kits were designed with a nylon thrust washer installed between the bottom of the needle bearings and the bottom of the bearing cup. The purpose of the thrust washer is to prevent metal-to-metal friction.
Snap ring sets
The OEM and Spicer U-joint kits contain three sets of four color-coded selective snap rings. U-joint kits for aluminum driveshafts also contain three sets of selective snap rings, but they are not color-coded since they are coated with zinc-phosphate.
There are two reasons for using selective snap rings (from question 5 at the start of this article):
- As seen in Figure 8, the snap rings are used to adjust the axial end play in the U-joint. Axial end play is the space between the trunnion thrust surface and the bottom of the bearing cap and is measured with a dial indicator. Axial end play is adjusted by changing to a thicker or thinner snap ring on each side of the same U-joint cross.
- The snap rings are also used to center the U-joint cross in between the yoke ears of the driveshaft. You must use the same thickness of snap ring on each side of the same cross to keep the U-joint centered in between the driveshaft yoke ears. Always measure each snap ring and keep track of where you install it.
Snap rings are easily damaged and should not be reused if they do not spring back to their original dimensions (compare to a new snap ring).
Summary
Hopefully, you have learned enough to know what to look for, and what look out for, when purchasing and servicing U-joints. As for me, I will always use the better quality, more expensive Spicer U-joints in any vehicle of mine. Best wishes!
