Different U-Joint Sizes Explained

Different U-Joint Sizes Explained

Once you've understood the purpose of u-joints, you'll need to use other classifying factors to find the proper one for your job. Identifying the type of u-joint will also be determined by the application in which it will be used. After having the different u-joint sizes explained, you will be able to decide on the best joint for the project at hand.

Before finding the universal joint you need for your job, you will need to know some essential facts. Sizing coincides with the type of machine you are implementing the joint into. It’s beneficial to understand what these joints do and their purpose, along with types of joints and how to measure them. Understanding u-joint body types will aid in the decision-making process for the products you choose. Following a few general pointers or even speaking with a professional can help shed light on these key elements.  

What Are U-Joints

Universal joints, or u-joints, are used to allow driveshafts to move with the suspension. They ensure that power can be transferred from the transmission and drive-wheels without breaking. U-joints are typically placed at either end of a driveshaft, allowing for the transfer of mechanical energy between two moving parts. This leaves both parts room for flexibility without harming or breaking vital components within machinery. In vehicles, the u-joint is a cross-shaped piece in between two yokes.


The purpose of a universal joint is to allow the movement of driveshafts simultaneously with the machine's suspension. U-joints permit the axes to be at an angle from each other. It compensates for the fact that the driveshaft isn't necessarily always in-line with the component that it’s connected to. If the driveshaft is between a stationary object and a moving one, a u-joint would be needed to allow free movement between those items.

U-joints can be located along driveshafts, 4WD transfer case shafts, and even axle shafts. U-joints are different from constant velocity joints. They are consistently under immense pressure and friction that can wear out without proper lubrication. Operating at improper angles or abnormal conditions are contributing factors in deterioration. Rough vibrations or odd sounds such as clunking or squeaking of the driveshaft may indicate an impending u-joint failure.  

Some Common Types of U-Joints

  • Hooke-Type Joint: a universal coupling that connects axes to rigid rods commonly used in the transmission of rotary motion.
  • Double-Hooke-Type Joint: similar to constant velocity, the double-Hooke joint involves two Hooke's joints with a shaft in the middle to help eliminate discrepancies in the angular dislocation and velocity between driving and driven shafts.
  • Birfield Joint: a 100% true constant velocity (CV) joint. Dissimilar to the function of a u-joint where vibrations and spinning unequal transfer amounts of power, the Birfield joint creates an even and steady transfer rate of torque.

Understanding U-Joint Sizes

Understanding what size of universal joint you need for your job requires knowing many things. If deciding on the u-joints for a vehicle, keep in mind that different makes of cars and trucks require different sized u-joints. DriveShaft of Tulsa offers extensive information on how to identify u-joints as well as comprehensive information on driveshaft services. Measuring joints will help you identify the proper u-joint size and type.

How To Measure U-Joints

Most universal joints will have one side that is pushed into the welded-in yoke, while the other side connects to the other yoke. C-clips are used to hold the pressed in caps of the u-joint in place. When measuring a universal joint, you will need to see if it is an inside or outside-locking joint.

Most of the time, these u-joints are not interchangeable, so you'll need to identify which type you require. They will be measured in different ways. You may use a tape measure, but a digital caliper can be more efficient and accurate, as well as more time effective. Refer to outside and inside-locking u-joint charts to help determine the series you are working with:

  • Measuring Inside-Locking U-Joints: To obtain the proper length of an inside locking u-joint, measure from groove to groove.
  • Measuring Outside-Locking U-Joints: To obtain the proper length of an outside locking u-joint, measure from cap end to cap end.

Two Types of U-Joint Bodies

There are two types of universal joint bodies, solid-body or non-greaseable, the latter of which is often referred to as "lubed for life" as they do not contain grease fittings. They do not have stress risers by the opening of the fitting, deeming them stronger. Greaseable joints must be lubricated regularly. People often fail to routinely grease the greaseable joints, which should be lubricated at least every 5,000 miles.

There is a lot of disagreement amongst tradespeople on whether the non-greaseable joint is indeed stronger than its greaseable counterpart. Of course, something that doesn't require routine maintenance always seems to be a better candidate for lasting life and staying intact longer. However, there is not a lot of evidentiary support to prove it. Some companies make a grease fitting at the end of the joint cap rather than in the body to make greasing easier and less invasive.

Making the decision about whether to go with the greaseable or non-greaseable joint depends on how honest you are with yourself about your commitment to maintaining your driveshaft. Asking yourself how often you'll grease your driveshaft will be necessary if you're leaning toward a greaseable joint. If you know that you won't be greasing your driveshaft routinely, opt for the non-greaseable joint, as it is better sealed than a greaseable joint. Non-greaseable joints are also designed to resist grease, dirt, and water infiltration.

No joint lasts forever. Knowing how to maintain your joint in order to get the most life out of it is important. Some decisions about your joint are in your control. Other choices may depend solely on where the joint is being used and what it is being used for. By following this basic guide of different u-joint sizes explained as well as other key features about them, you'll be better educated and more prepared to complete your project.

This is a general guide, and there is always more to learn about the different types of u-joints. If you are unclear at any time, ask a professional. They will be able to determine the necessary next steps regarding both products and procedures accompanied with machinery that requires a driveshaft and universal joints.

Different U-Joint Sizes Explained