I have an embarrassing admission to make; up until a few years ago, I thought I knew all there was to know about diagnosing and replacing universal joints (u-joint), I was very 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 that 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 which 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, all u-joints are not equal. In this article, we will look at the potential differences in u-joints and how they can impact you and your customer.
|Figure 1 – 2017 RAM 2500 Factory U-Joints come with green and blue selective snap rings|
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?
Warning! Historical Content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 or whatever it was called, 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.