Understanding Universal Joints: Types and Applications

The universal joint, often called a U-joint, is a crucial component in many mechanical systems. It connects two rigid shafts, transmitting power while accommodating angular misalignment. This article delves into the world of universal joints, exploring their types, applications, and maintenance.

Introduction to Universal Joints

The universal joint (U-joint) is a mechanical coupling that connects rigid shafts whose axes are inclined to each other. It is commonly used in shafts that transmit rotary motion. Universal joints are essential parts of a vehicle’s drivetrain and are often taken for granted until they start to wear out or need replacement.

Basic Structure and Function

A universal joint, in its simplest form, consists of two shaft yokes at right angles to each other and a four-point cross that connects the yokes. It consists of a pair of hinges located close together, oriented at 90° to each other, connected by a cross shaft. The cross rides inside the bearing cap assemblies, which are pressed into the yoke eyes. At the end of each output and input shaft is either a yoke and/or a flange. The yokes that hold the u-joint bearing caps are sometimes referred to as ears. The bearing caps of the u-joint are pressed into the yoke (ear) and held in place with a c-clip (half circle), an internal snap ring, or a full-circle snap ring. Vehicles that see heavy off-road use should use an internal or full-circle snap ring.

U-joints are designed to accommodate minor driveline angle and suspension ride height changes as the vehicle rolls down the road. A universal joint (U-joint) is a mechanical joint used for connecting shafts that rotate and are aligned at different angles to each other. The U-joint compensates for the misalignment between the shafts in any direction.

Types of Universal Joints

There are several types of U-joints for various applications. The two main types are the cross and roller type and the ball and trunnion type.

1. Single Cardan Joint (Hooke's Joint)

This is the most common type of universal joint. It consists of a single cross-shaped connecting piece that links the two yokes together. It allows the drive shaft to bend or pivot at the U-joint. The single Cardan joint is a term for a driveshaft with one universal joint at each end of the assembly. It is commonly known for its use on automobiles and trucks.

2. Double Cardan Joint (Constant Velocity or CV Joint)

The double cardan u-joint uses a pair of u-joints connected by a coupler that allow a greater working angle. Double Cardan is a term used when describing a one-piece driveshaft with three (or more) universal joints. A double Cardan joint consists of two universal joints mounted back to back with a center yoke; the center yoke replaces the intermediate shaft. What a double Cardan will do, is split a universal joint operating angle into two separate angles that are exactly one half of the original angle.

Normally a Double-Cardan (a.k.a. Constant Velocity or CV) style driveshaft is used in applications where it is not possible or practical to properly align the ends of a driveshaft for a single-Cardan setup. Examples include where the operating angle would be too great over a single Cardan joint (see below) a double-Cardan allows the operating angle to be split across the two halves of the joint.

It is also possible to use two CV joints on a driveshaft which is commonly used where it is not possible to align either end of the driveshaft, such as when both vertical and horizontal misalignment occurs, or when mismatched operating angles are present, such as in front-wheel drive vehicles, where both up and down motion is present from the suspension travel as well as rotation about a vertical axis due to steering action. CV joints are designed to transmit power at a constant rotational speed regardless of the angle between the input and output shafts.

3. Ball and Trunnion Type

The ball and trunnion type is another type of U-joint used for automotive applications.

4. Constant Velocity JointThis type of universal joint is designed to maintain a constant rotational velocity between the two shafts, even as their relative orientation changes.

5. Hooke's JointThis type of universal joint uses a series of curved arms, or levers, to link the two yokes together.

6. Cross and Bearing JointThis type of universal joint uses a cross-shaped connecting piece and a series of bearings to link the two yokes together.

U-Joint Sizes and Series

U-joints are sized via "series," and you’ll often hear people say that they need a 1310 u-joint. These four-digit series codes are how cap diameter and overall joint width are categorized. The roller U-joints range in size from a 1210 series with a 2.438″ width and a 1.062″ cap diameter to a 1555 series with a 4.965″ width and 1.375″ cap diameter. While there are other series of u-joints within the driveline components industry, the three most common u-joint by size for passenger cars and light trucks are the 1310, 1330 U-joint, and 1350 U-joint types. These types of u-joints are comprised of a specific body dimension and cap diameter and must be used with the matching driveline flanges/yokes for proper and safe operation.

Most passenger cars use 1310-series U-joints. However, more powerful, performance-oriented vehicles may use the stronger 1350-series joints. Most factory u-joints up until the mid-1970s were of the 1310 series, so with performance upgrades that provide more power, and more importantly more torque, to the driveline an upgrade to 1330 or 1350 series u-joints should really be considered.

1310 U-Joint: Typically for medium Torque/ horsepower applications in cars. Between 400lb/ft & 600 lb/ft. Depending on the use of the vehicle.
1330 U-Joint: Typically for medium Torque/ horsepower applications in cars. Between 400lb/ft & 600 lb/ft. When it comes to 1310 vs 1330 u-joint, the 1310 and 1330 joints share the same cap diameter, but the 1330 has a larger body.
1350 U-Joint: Typically for High Torque/ horsepower applications in cars. Note the cross is the same size as 1330 to its left but the bearing caps, and therefore the trunnions that they pivot on are larger, i.e. Meanwhile, the 1330 and 1350 u-joint have the same body dimensions, but the 1350 uses a larger cap diameter. It’s easy to see that the 1350 u-joint hp rating would be the highest, but in the real world you may not need the capabilities of a 1350 series u-joint.

As the u-joint series moves up in size you’ll see that only certain aspects of the dimensions change. However, these changes net a significant increase in torque capabilities.

Determining the Proper U-Joint Size

Determining the proper u-joint for your needs will include factors such as horsepower, tire diameter, tire compound or “stickiness,” gear ratio, and vehicle weight. Your typical muscle car or hot rod (and many two-wheel drive pickups) use a standard single cardan u-joint at each end of the driveshaft to compensate for angular misalignment as the rear axle moves up and down with the rear suspension.

Greasable vs. Sealed U-Joints

For most any application you will find both a greasable and a solid or sealed U-joint option. When it comes to greasable vs sealed U-joints, there are some considerations you must factor into your driveline’s U-joint needs, including accessibility, use case, strength needs, and more.

Greasable U-Joints: For example, if you’re building a nice street cruiser that will rarely see rain and isn’t being launched hard on slicks or drag radials, you could easily use a greasable U-joint, provided you incorporate greasing the joints into your regular chassis maintenance.
Solid U-Joints: Alternatively, consider that when it comes to greasable vs solid U-joints that a solid U-joint may be the better solution. This is especially true for higher horsepower applications, or you’re looking for a maintenance free solution. The solid body of a non-greasable U-joint is stronger, as it does not have the cross-drilled grease passages of the greasable version. These drilled passages take away material and overall strength, no matter the U-joint size.

Your use case will also determine greasable vs solid U-joints and what is best for you. Building an off-road rig that’s going to see lots of mud and water up to the wheel hubs?

Measuring U-Joint Size

Using the correctly sized u-joint in your driveline is critical for proper fitment of your driveshaft to prevent vibration, breakage, and even possible ejection of your driveshaft! There are two measurements you will need to make/confirm to determine the correct u-joint for your application. These measurements are the overall width of the u-joint assembly (from cap to cap) and the cap’s diameter. These two measurements will help confirm what series of u-joints you have using our u-joint chart so you can obtain the proper replacement u-joints.

Note that you can have a u-joint with two different bearing cap sizes. These types of u-joints, commonly referred to as conversion or combination u-joints, usually can be had as a U-joint 1310 to 1330 or U-joint 1330 to 1350 pairing of bearing caps to aid in driveline conversions. In some instances, the use of a U-joint 1310 to 1350 may be called for. This special U-joint 1310 to 1350 can solve mix and match driveline issues. This is important to remember and is why we recommend always measuring a bearing cap off each “cross” of the u-joint for proper u-joint identification, as the difference between a bearing cap for a 1310 u-joint vs 1350 bearing cap is only 1/8-inch (.125”). The use of calipers or a micrometer is highly recommended for accurate measurements. When we are discussing the various bearing cap sizes of a u-joint it is important to note that the measurements between the different u-joint cap sizes are minimal and can be tough to confirm with a tape measure or ruler, which is why we recommend a set of calipers for precise measurements that are easy to confirm.

Bearing Caps

But just what is a bearing cap on a u-joint? The bearing cap, sometimes referred to as a bearing case, on the standard single cardan u-joint is found at the end of each leg of the main u-joint body. These bearing caps feature a machined pocket that holds hardened needle roller bearings (and grease) for the bearing cap to rotate around the u-joint body. The bearing cap is a press fit into the driveshaft yoke and is retained by internal or external snap rings.

Note: Ford used a 1330 “big cap” u-joint that used standard 1330 caps on one pair of caps and a larger 1330F “big cap” size of 1-1/8-inch (1.125”) on the other pair. This thicker set of bearing caps were used in the differential pinion flange location to resist deformation by the bearing cap Drive Shaft U-joint straps from being overtightened. Generally, these joints will only be needed if you’re using a 9-inch center section sourced from a late ’70s to early ’80s Ford truck or Bronco. Aftermarket 9-inch center sections will not use these. Also of note is the S44/3R U-joint often used in older GM and Mopar applications.

Measuring Without an Existing U-Joint

But what if you don’t have a u-joint to measure? For example, you’re doing an LS swap in a classic pickup with a new Ford 9-inch axle and need to measure for u-joints to add to your custom driveshaft order. For pinion flanges or transmission slip yokes you will measure the distance between the u-joint bearing cap retaining lugs or from the edge of the snap ring retaining lip to the opposite retaining lip.

Snap Rings

Speaking of snap rings, you will find u-joints are secured to the driveshaft yokes via inside or outside snap rings.

U-Joint Maintenance, Servicing, and Replacement

U-joints can last for decades or break when the vehicle is still relatively new. Their lifespan is partly determined by the quality of the U-joint, the type of vehicle, and how that vehicle is used and maintained. But even then, it’s hard to predict how long a U-joint will last.

When u-joints go bad you’ll typically have a squeak coming from the driveline at first (indicative of dry needle bearings) which often turns into driveline vibration, and ultimately u-joint failure. The best course of action is to service your u-joints regularly (if greaseable) or replace them at the first sign of noise. Symptoms of a bad U-joint include a squeaking sound while moving at slow speeds or when accelerating from a stop, a clunking noise when shifting into any gear, a shuddering or vibrating sensation when accelerating or driving, clicking noise near the front of the vehicle when turning, or evidence of rust coming out of the grease seals. If your vehicle shows any of these symptoms, have it inspected as soon as possible by an ASE certified technician. The most common cause of U-joint failure is the lack of or loss of lubrication.

U-Joint ReplacementWhen it comes time to replace your driveshaft’s u-joints the job is not difficult, though it does take a couple of extra hands, a press, large C-clamp, or a vice, and suitable work area to support the length of the driveshaft. Using a drive shaft u-joint diagram will help in keeping all the parts going back together correctly. A press or large vice works best, but you can always use the hammer and socket method of extracting a u-joint.

In preparation for u-joint replacement it is best to mark your driveshaft yokes to the trans slip yoke and differential pinion yoke to ensure everything is reassembled in the same manner. To remove your worn u-joints the snap rings must be removed with the appropriate snap ring pliers first. Often, a quick smack with a hammer on the yoke itself or with an old socket or proper diameter steel shaft directly on the snap ring face will help to unstick the snap ring in the groove, allowing it to become free enough to be removed with snap ring pliers. Once the snap rings are removed the u-joint can be pressed through the yoke until the opposite bearing cap is free to be removed by hand.

To install your new U-joint, carefully remove the bearing caps and place the bare u-joint body into the yoke and all the way to one side. This will allow you to reinstall that side’s bearing cap over the u-joint’s trunnion (the machined surface the bearing cap and needle bearings ride on) and then use a press or vice to apply pressure to the bearing cap until seated below the snap ring groove. You want to do this carefully to not unseat any of the needle bearings. Install the new snap ring supplied.

U-Joint LubricationWhile many OEM types of u-joints are sealed, and often it is preferable to use a non-greaseable, solid body u-joint for its extra strength, for many street going vehicles a u-joint with grease fitting is perfectly acceptable and will last a long time with proper servicing. Access to the u-joint’s grease fitting can be hindered by the driveshaft yoke at certain ride heights. Often simply moving the rear suspension up and down while the vehicle is on jack stands will provide enough clearance to insert a grease gun tip into the location.

For greaseable types of u-joints it is preferred to flush the old grease and contaminants out by injecting new grease into the u-joint grease fitting until you see the new, clean grease exiting all four bearing caps at the seal area. Pump the grease gun slowly to not damage the seals and do not use a battery or air powered grease gun for this operation. Once all four bearing caps show new grease being pushed past the seals you can wipe away the old/extra grease and clean the u-joint of any exterior dirt and grease. As mentioned above, for greaseable types of u-joints the small bit of maintenance required pays off in longer life of the u-joint. Using a manual u-joint grease gun works best for filling/flushing the u-joint’s grease.

Don’t forget extra grease fittings (or “zerks” as some people refer to them as). While new joints come with fittings, older joints may have damaged/deformed or plugged grease fittings that will not allow fresh grease to be added. Simply replacing the u-joint grease fitting with one from our grease fitting assortment will have you fixed up and ready for many more miles on the road.

Causes of U-Joint Failure

Several factors can contribute to U-joint failure:

Improper maintenance or lubrication: Some u-joints need regular maintenance, which includes greasing the u-joints about every 5,000 miles. Without lubrication, the U-joints will eventually dry up and are more likely to bind. Eventually, this causes the U-joint to seize or break. Not keeping up with general maintenance in a greaseable unit or contamination of the lubricant in a sealed, non-greaseable joint.
Bad alignment: If you change the vehicle’s ride height, either by raising or lowering the vehicle, it could cause the U-joint to bend too far.
Normal wear and tear, especially on vehicles that are often used for heavy towing or off-roading.
Misalignment, corrosion, or excessive vibration.

U-Joints in Off-Road Vehicles

In the typical off-road vehicle, a suspension lift is done to increase clearance and allow larger tires to be installed. To compensate for the larger diameter, lower gears are installed in the axles. A suspension lift increases the angle of the shaft, and the lower gears mean the shaft has to spin faster for a given axle speed, both things are working in the wrong direction.

When talking about strength and durability of front axle u-joints one must remember there is a torque multiplication factor at work here. It comes into play when the front driveline experiences angles and at a maximum angle of 40 degrees 30% more torque can be projected onto the u-joint. The multiplication is nominal up to 15 degrees and then the curve progresses from there. This means that a u-joint that is working near its yield point can deform if you’re crawling or bogging and putting a max load on the driveline via angularity.

If a u-joint is being pushed to failure there are ways to increase its survivability envelope or at the very least prolong its life momentarily. It’s all about rings and clips. Using full circle snap rings in place of the traditional c-clips will resist distortion of the joint as the yoke ears begin to thrust the caps out of place. The c-clips usually used to secure the caps cannot resist the side forces very well but the full circle clips will do a better job and therefore help delay u-joint failure. The full circle clips must be installed at assembly and OE axles may require some clearancing in the yoke to allow fitment.

Historical Context

The concept of the universal joint is based on the design of gimbals, which have been in use since antiquity. One anticipation of the universal joint was its use by the Ancient Greeks on ballistae. In Europe, the device is often called the Cardan joint or Cardan shaft.

The mechanism was later described in Technica curiosa sive mirabilia artis (1664) by Gaspar Schott, who mistakenly claimed that it was a constant-velocity joint. Shortly afterward, between 1667 and 1675, Robert Hooke analysed the joint and found that its speed of rotation was nonuniform, but that property could be used to track the motion of the shadow on the face of a sundial.

The first recorded use of the term 'universal joint' for this device was by Hooke in 1676, in his book Helioscopes. He published a description in 1678, resulting in the use of the term Hooke's joint in the English-speaking world. The term universal joint was used in the 18th century and was in common use in the 19th century. The term 'Cardan joint' appears to be a latecomer to the English language.

Advantages and Disadvantages of Universal Joints

Advantages:

Flexibility: Universal joints can accommodate changes in the relative orientation of the two coupled shafts, allowing them to move and rotate freely in different directions.
They allow for the transmission of rotational motion between shafts that are not in a straight line.

Disadvantages:

Limited range of motion: Universal joints can only accommodate a limited range of changes in the relative orientation of the two coupled shafts.
Loss of power: Due to how universal joints are designed, some power is lost as the torque is transmitted through the joint.
Maintenance: Universal joints require regular maintenance and lubrication to function properly and to maintain their durability and performance.
Single joints are subject to non-uniform speeds at input and output shafts. (Z Configuration).

Applications of Universal Joints

Universal joints are commonly used in various applications, including:

Automotive: Drive shafts for vehicles. Drive shafts and universal joints are present on most rear-wheel drive and four-wheel drive vehicles. A drive shaft and U-joints connect the transmission to the rear drive axle on most rear-wheel drive vehicles. Many four-wheel drive vehicles also use drive shafts with U-joints, with one drive shaft between the transfer case and rear drive axle, and a second drive shaft between the transfer case and the front drive axle.
Industrial: Machinery for industrial processes.
Other mechanical systems: Any system where flexible and versatile coupling is needed.

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