How to size and select bellows couplings for motion applications

Electrodeposited bellows couplings are commonly used in motion-control applications requiring precision but also having shaft misalignment. If your application fits this description, it’s key to understand the parameters that dictate the optimum bellows coupling for a given axis. Here are a few of the key considerations for selecting a bellows coupling.

1. Torque

Determine the maximum instantaneous torque the coupling will need to endure. If there is a clutch start or brake stop, measure the maximum instantaneous torque value. If a low inertia motor with gear reduction drives a massive load through the coupling, the maximum torque will be the starting torque of the motor, multiplied by the gear speed reduction ratio. Acceleration of rotational inertias generates resisting torques, so these must be computed and taken into account.

Determination of true maximum torque can prevent premature coupling failure. When a coupling is used in a compressed state, use 75% of rated torque as maximum. If the coupling is to be extended during its operation, apply the normal torque rating.

2. Misalignment factors

Determine the maximum parallel misalignment to be encountered. Also determine the maximum angular misalignment expected to occur between the two shafts.

Bellows couplings can flex laterally and axially within their design limitations, so when considering the selection of a bellows coupling, the alignment of a coupling is a great starting point. Because bellows couplings are commonly selected for their misalignment capability, understanding the types of misalignment and how they can impact coupling performance and lifecycle is critical.

Misalignment of shafts, with axes parallel but offset, is the severest load condition that can be applied to a coupling. The bellows portion of the coupling is compensating for the difference between the shafts and needs to be calculated. Angular misalignment, or arc bend, is also a critical factor which can change the performance capability when combined with a parallel offset. Finally, understanding the extension or compression of the bellows is also important. If the bellows operates in compression, only 75% of the rated torque value is possible.

In the bellows coupling specification charts, the parallel offset, angular misalignment, and the axial extension and compression capabilities are specified as mutually exclusive. When a value is set for each performance specification, an equal percentage value can be applied to it being used on its own. For example, if a coupling angular offset is specified at 31° of angular misalignment and the user needs 31° of misalignment, there is no room for any additional parallel offset or extension or compression deflections. On the other hand, if the user only needs half of the angular misalignment (15.5°) the user can use an equal percentage of the available performance.

3. Windup

Specify maximum allowable windup of the coupling. Shorter lengths or larger diameters may be necessary where low windup values are required.

4. Axial compression

Determine whether the axial compression or extension of your application will require a special length of bellows.  Do so by comparing your values to the charts detailing standard couplings from various manufacturer. Also consider the environment and any special end-piece configuration.

Some final considerations when selecting a bellows coupling

Knowing the range of temperatures that the bellows coupling will be subjected to in the application is also important to ensure survivability of the coupling. Electrodeposited bellows are typically specified as having a maximum temperature capability of 350° F. On the low temperature end, there is no limit. The maximum and minimum temperatures of a bellows coupling will be a factor of the method of assembly of the electrodeposited bellows to the end pieces. Electrodeposited bellows couplings are offered as solder assemblies, with a maximum operating temperature of 300° F, or as adhesive assemblies, with a maximum operating temperature of 240° F.

The end pieces or hubs pair the same size or different size shafts in a motion control device. More often, the shafts are combining imperial and metric dimensions since component sourcing is occurring in a global marketplace. Couplings can be manufactured with bore sizes to match any shaft size. Also, custom end piece configurations can me manufactured to match specific geometry of the shaft to help installers recognize the side in which the end piece matches and to increase performance within a system.

In order to secure the shaft to the end piece, set screws or integral clamps are the most common methods. Set screws are tightened with Allen keys and positioned 120°. Integral clamps will actually reduce the diameter of the end piece around the shaft due to a split in the component and clamp. Each style is easy to install and comes down to a matter of preference. Aluminum and stainless steel are the two most common materials for end pieces for their strength, price and operating condition capability.

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