6 Most Common Causes of U-Joint Failure

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 in!

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. 

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.

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: NiuShang Auto Parts 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: NiuShang Auto Parts' 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 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.