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more than 370 power transmission/motion control distribution and manufacturing companies, representing over $16 billion in product sales.

Resources | Product Training | PT/MC Tech Tips | Couplings & U-Joints

Tech Tip: Couplings & U-Joints

Enhance your employees’ product and industry knowledge with PTDA’s Tech Tips. This library of online tips, hints and techniques may be used to educate new and current employees on power transmission/motion control (PT/MC) products, technologies and concepts and serve as reference and reminder for more experienced staff. Tech Tips are based on the expertise of PTDA member companies, content from Motion Control Made Simple or PTDA’s Power Transmission Handbook®, the definitive resource and training tool on PT/MC products.

Visit the full library of Tech Tips for additional products.


What style couplings should be considered for applications with considerable misalignment?

Flexible couplings are the preferred choice for applications needing to compensate for a moderate amount of shaft misalignment and end movement. In general, the lower the angular misalignment the longer the coupling life.

What is the main difference between single and double U-joints?

 A double U-joint is capable of compensating for parallel misalignment.

Why would a user choose a U-joint instead of a flexible coupling?

U-Joints allow transmission of power at larger angles than is possible with a flexible coupling, generally used in applications where high misalignment must be accommodated (generally in excess of one degree and up to 30 degrees).

What is the primary advantage of using a grid coupling vs. a mechanically flexible chain coupling?

Grid couplings can dampen vibration and reduce peak or shock loads by 10-30%

Elastomeric Flexible Element Coupling

These couplings can be used in application to dampen vibration and noise.  They usually have two hubs and an elastomeric flexible element (sometimes referred to as a tire).  This coupling can usually take more misalignment than other couplings.  However, caution should be taken as some styles of this coupling require checking the torques of the bolts after a run-in period and after so many hours of operation.  Consult with the manufacturers' installation directions.

U-Joints Optimal Service Life

The most obvious answer is to use the manufacturer's engineering tables at time of primary design as well as proofing an existing application.  When proofing an application pay attention to the position of the coupled shafts, shaft speed, duty cycle and load transmitted.  If the existing U-Joint engineering specifications are below the actual operating parameters, then consider replacing the U-Joint with a proper size.  Then make sure the U-Joint gets proper lubrication maintenance.  This includes the manufacturers recommended type of lubrication, the amount to apply and the interval between applications.  A cover or "boot" is the recommended best way to keep the lubrication in and contaminants out.  Boots are available from most U-Joint manufacturers as well as specialty manufacturers.

Prevent Coupling Misalignment

In applications where coupled equipment is precisely aligned, the load is distributed among all of the coupling teeth. But, for misaligned equipment, the load is not equally distributed. Thus, misalignment reduces the load rating of a gear coupling and increases wear and reduces the coupling life. The best method of alignment is using laser alignment equipment. Couplings of different types and styles have different capabilities to accept misalignment. Select the coupling with the necessary misalignment capabilities for the application it will be used on.

Shaft Misalignment in a Flexible Coupling

Flexible couplings flex while rotating to accommodate misalignment between shafts.   There are two types of shaft misalignment:  angular and parallel.  Proper shaft alignment means that both types of misalignment must be reduced to within the equipment manufacturer’s requirements to assure satisfactory operation of the equipment.


The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered. All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA’s contributing members, but are made without guarantee or representation as to results. PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs. PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.