Electrodeposited bellows couplings are commonly used in motion control applications requiring precision control and where shaft misalignment is present. If your application requires precision, then its important to understand the performance factors that are critical for selecting the optimum bellows coupling for the task.
Here are a few of they key considerations for selecting a bellows coupling.
Determine the maximum instantaneous torque the coupling will receive. 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 degrees of angular misalignment and the user needs 31 degrees 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 degrees), the user can use an equal percentage of the available performance.
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.
Lastly, compare these values to the performance chart in the various manufacturer’s tables of standard couplings.
Also consider the environment and any special end-piece configuration.
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 350ºF. 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 300ºF, or as adhesive assemblies, with a maximum operating temperature of 240ºF.
The end pieces, or hubs, are used to 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 120º. 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.
Filed Under: Motion Control Tips