Tech Tip: Couplings & U-Joints

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What are the two categories of special purpose couplings?

Lubricated and non-lubricated.


What is the primary disadvantage of magnet to magnet coupling?

Vibration induced in the drive train when the coupling is overloaded.


What misalignment range are universal joints typically used in? 
 4-90 degrees.


What are the three ways a flexible coupling can flex?
1. Mechanical motion - sliding or rolling.
2. Flexing motion of elastomers, thin metallic elements or composite material.
3. Compressing or stretching of resilient or composite materials.

What are the three types of shaft alignment?

Angular, parallel, and axial motion.

What are the advantages of using a clamp rigid coupling as opposed to a sleeve (set screw) rigid coupling?

They eliminate problems inherent with sleeve type couplings such as requirements of soft shaft material, shaft marring, and difficulty in assembly.

What could result from using a rigid coupling in application with misalignment between shafts?

Shaft misalignment can cause excessive wear or failure of the shafts, coupling, or bearings.

For what applications would a universal joint, or U-joint, be an appropriate selection?

U-joints allow transmission of power at larger angles than what is possible with a flexible coupling. U-joints are generally used in applications where high misalignment must be accommodated (generally in excess of 1 degree and up to 30 degrees).

What parameters should be considered before choosing a coupling?

1. Maximum acceleration/deceleration torque the coupling will see. Include all motor/load inertia and gearing effects.
2. Maximum parallel and angular misalignment. Use worse case scenarios.
3. Maximum allowable windup. A shorter length or larger diameter may be required to achieve lower values.
4. Expected life expectancy.
5. Input/output shaft bore sizes.
6. Peak or shock loads present.
7. Environmental conditions including vibration and temperature.
8. Maximum motor and shaft speed (RPM).
9. What method of attachment is reuired (i.e. set screw or clamp)

What are corded tire couplings and what kind of misalignment are they designed to handle?

 

A corded tire coupling is a shear-type coupling that is basically a flexible rubber tire with tension-member chords that carry the load. The tire is secured to the flanges, which are secured to the shafts. Internal reinforcement and external clamping of the tire enhances the torque capacity and stiffness. Rubber-tire couplings are often used to reduce the transmission of shock loads to the surrounding equipment in drives subject to jamming. These types of couplings accommodate up to 4 degrees of angular misalignment, 1/8 in. parallel misalignment, and 5/16 in. end float.

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.


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