6 Ways Your U Joint Will Fail

Wouldn’t it be wonderful to live in an ideal world where mechanical components never failed? A bad u joint can cause all sorts of problems for you application.

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Top 6 Signs of a Bad U Joint

For those of us who live in the manufacturing real world, we all know that components do break, especially when selected or applied incorrectly.  Our goal in this article is to help you understand the main failure modes of universal joints, and how to correct for them. Let’s dive into the most common signs of a bad u joint.

1. Too Much Torque 

If your system is applying too much load to a universal joint, it is likely to fail.  A complete failure will most likely appear as a shearing of the ears of the yoke or a pin failure. On a joint made from unhardened stainless steel, a deformation failure from high torque is likely, which will be noticeable as poor articulation or binding of components that limits future operation. Even if the loads are not enough to break or deform the joint, excessive loads lead to increased wear.  Increased wear can increase play and backlash and will remove material, which will lead to weakened components.

Solution: if you are confident that the rest of your system is designed correctly, it is possible that you need a larger universal joint size or a different style u-joint – such as our High Strength or Leveler Strength designs – to handle the elevated loads. 

2. Too High of an Angle

Each universal joint design has a maximum operating angle.  That maximum angle decreases as loads and speeds on the u joint increase.  An excessive angle failure will appear as ‘notchiness’, or a lack of smooth operation, in the u joint. 

Solution: If reducing the operating angle is not an option, it may be worth considering a double u-joint to decrease the operating angle of the coupling “per joint.” In some cases, it is the combination of angle, speed and torque that overloads the joint. A reduction in torque or RPM leads to a higher allowance for the operational angle.

3. Too High of Speed

It is possible for a universal joint to fail if the RPM applied to the coupling is too elevated.  The recommended max operating speed of a u-joint depends on several factors:

• the bearing configuration (needle bearing designs can generally handle higher speeds than friction bearings at significant angles)

• the torque that is being applied (higher torque requires lower speeds, all else equal)

• the operating angle (higher angles require lower speeds)

• the length of a Double or Telescoping U-Joint assembly

• the type of lubricant used

Friction-bearing joints at elevated speeds and angles will wear out much faster than properly sized needle-bearing joints.

High torques in connection with high RPM and especially at continuous operation will create a lot of friction heat that will lead to lubrication failure, excessive wear, and joint damage.

High angles increase the movement on the bearing surfaces (at 00 angle the is actually no movement in the bearings of the joint), so angle is a major contributor to bearing speed – even more than the actual RPM.

The length of a double joint or telescoping drive shaft limits the maximum speed due to the natural frequency of the assembly.

At very high speeds, a friction bearing joint with an oil filled boot will provide better lubrication and thus better lifetime than a grease filled boot and will be significantly better than a joint running dry.

Solution: if your u-joint is struggling with an excessive RPM failure, work with your universal joint application engineer to determine whether the design of the joint is correct for the application.  If you are using a pin & block joint, perhaps a needle bearing or composite bearing design would be more suitable for high speed, continuous operation.  If the angle is too great to accommodate the speeds required, a double universal joint may be the best solution to accommodate high RPM at a high angle. 

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4. Not Enough Lubrication 

Any bearing surface must be properly lubricated to operate effectively and efficiently.  If your u-joint is binding or you notice unusually high wear in operation, a lubrication failure might be occurring.

Solution: there are several ways to add lubrication to a universal joint. Regular relubrication is one option – most easily done when the joint is equipped with lubrication fittings. If a u-joint features a protective boot, this boot cannot only keep dirt and debris away and protect the u-joint components, it can also be filled with lubricant that forms a grease reservoir close to the bearing surfaces and ensures constant relubrication.  If a protective boot is not feasible with your design, there are several dry film coatings and platings that can provide long-term lubrication to bearing surfaces.  An external oil drip or oil spray system can also be used on larger friction bearing u-joints. If your application allows you to consider a needle bearing design, the enclosed bearings are pre-lubricated to allow for long term operation without the need to add or replace grease over the lifetime of the u joint.

For more information, please visit Differences between single, double and telescopic joints.

5. Poorly Installed Universal Joint

Any universal joint must be installed properly to work as intended within your machine.  A poorly connected driving or driven shaft, a missing support bearing, or an incorrect mating shaft dimension may result in signs of play or sloppiness and can make the joint fail in a number of ways due to the improper application of stress.

Solution: Belden Universal offers many standard and custom machined hub configurations to make installation simple and straightforward for operators and maintenance technicians.  Our design engineers can come up with a solution to perfectly mate with the input and driven shafts within your system. If you can engage Belden early in the design process, we would be happy to walk you through which bore configurations are standard and most economical for your needs.

6. Imprecise Machining

Remember, not all universal joints are created equal!  Inexpensive, offshore u-joints are often produced on antiquated, imprecise equipment. This will predictably produce sub-standard, wide tolerance bearing surfaces and mating features.  If you have a demanding application that requires tight tolerances to be held, an offshore product will not be right for you.  

Solution: Belden Universal’s products are precision machined on modern CNC equipment to ensure precise bearing surfaces and mating features. Our in-house design and engineering team can bring to life the universal joint that your specific application needs, all in a cost-efficient manner.

At the end of the day, if you’re having trouble with the universal joint or drive shaft in your highly demanding application, don’t hesitate to give Belden Universal a call at 708-578-6997 or Contact Us.  Our highly skilled application engineers are standing by to help you get exactly the right u joint or drive shaft at a competitive price.

When a u-joint fails, it can cause a vehicle to lose control and leave the driver stranded. There are potentially many other undesirable consequences. This is why understanding what causes u-joint failure is important for safe driving.

Let’s discuss the most common causes of u-joint failure:

1. Lack Of Grease

This applies to both greaseable and maintenance free u-joints alike.

Greaseable U-Joints

Greasing the grease-able style of u-joints regularly is the best way to prevent failure. It ensures that they stay lubricated and functional. Yet, it’s possible for a u-joint to fail even if it’s well greased. U-joints are exposed to the elements, which means moisture can make its way inside them. When that happens, the u-joint can seize and fail.

Maintenance Free U-Joints

Some maintenance free u-joints have a teflon coating inside in addition to grease. This helps to prevent corrosion. This should be enough to keep your joints well lubricated for their entire service life. But sometimes they still fail. If they do, it may be because of improper installation. Inspect u-joints during installation to make sure that the u-joint caps installed correctly. They also may fail due to excessive vibration.

Lack Of Grease Due To Vibration

U-joints don’t like vibration. A driveshaft that is out of balance can cause enough vibration that the life of the u-joints will be shortened. The vibration forces out the grease and causes the metal components to grind against each other. Eventually this leads to failure.

Lack Of Grease Due To Excessive Heat

U-joints don’t perform well under extremely high temperatures. The grease inside them supplies lubrication to the metal components. If too much heat builds up, it can cook the grease so that it fails to protect the metal. This can happen if a u-joint is located close to a source of heat, such as a muffler.

2. Wear And Tear

U-joints don’t always last forever. They wear out over time. This happens especially if you put a lot of strain on your u-joints on a regular basis. For example, towing or off-roading.

When a u-joint is subjected to high levels of strain for long periods of time, the metal components can become thin. They can also develop cracks. This reduces their ability to transfer torque properly, which is what causes u-joint failure.

Wear And Tear Due To High Loads

U-joints are designed to handle certain amounts of load. If you put more weight on your vehicle than it’s capable of carrying, your u-joints will have to work harder. This can lead to accelerated wear, which could result in failure after time.

Wear And Tear Due To Improper Installation

Proper installation is important for u-joint longevity. The caps and cups must be properly aligned to prevent damage. Also, make sure that your bolts are torqued down to their proper specifications.

3. Steeper Than Ideal Operating Angles

U-joints are designed to handle a certain amount of operating angle. If that limit is exceeded, the u-joints will wear out faster.

There are u-joints available for severe operating angles and there are standard u-joints for moderate angles. The more severe your operating angle is, the further you need to step up your u-joints. If you exceed the operating angles that these u-joints were designed for, they’ll struggle to handle their load. That will cause them to fail prematurely.

This is a common issue with lifted vehicles. The driveshaft operating angles become steeper if:

• The vehicle is lifted
• The pinion angles are incorrectly set

When operating angles are steep, excess vibration occurs. Vibration can damage the bearings. It can also damage the seals, which lets water and dirt in, and lets grease out.

Other Less Common Causes Of U-Joint Failure

There are a few other things that can cause u-joint failure, but they’re more rare. They include:

• Over-torquing
• Extreme use of the vehicle (racing, excessive towing, etc.)
• Aggressive off-road driving
• Heavily modified engines

Telltale Signs Of U-Joint Failure

Here are some telltale signs that the vehicle’s u-joints are failing:

• Clunking noise when the vehicle starts to move forward or backward
• Clunking noise when shifting from forward to reverse
• Vibration felt throughout the vehicle when it is moving
• Transmission or differential fluid leak

It’s Important To Find Out The Cause Of The U-Joint Failure

If you don’t fix the underlying issue that caused the u-joints to fail, the new u-joints will fail, too. Once you’ve determined the cause of the failure, the solution might be as simple as greasing your u-joints more often (if they’re not sealed). Otherwise, it may be necessary to address other issues like an out of balance driveshaft.

When u-joints fail, there’s a chance that the driver could lose control of the vehicle. If you suspect that they’ve failed, it’s important to replace them as soon as possible.

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