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All About Bellows Couplings

Bellows couplings are single-piece , flexible shaft connectors, which are used to connect drive and driving shafts within mechanical power transmission units. Flexible shaft couplings can be used to stop the inevitable misalignment between shafts connected, and, in some instances they are used to absorb shock.

Coupling design

Bellows couplings comprise of thin pleated tubes made of steel connected by shaft connectors on their ends. It is rotationally stiff and is a great device for demanding instruments and motion control. It is also utilized in certain industrial processes, in which its capacity to handle an axial temperature increase can be useful in installation with huge temperature swings. The designs can be heavy-duty with shaft diameters ranging from many inches. This design has a very low energy, which means it places a little stress to the operation of a combination of drive and driven equipment.

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For the production of bellows, nickel is electrodeposited on an amandrel. The mandrel will dissolve leaving behind a thin, flexible component. This process produces bellows which are extremely thin, but with extremely controlled thicknesses, which makes an extremely responsive and sensitive coupling, even though they have a smaller torque capacity than those of bellows created through other methods. The more rigid versions are produced using stainless steel, as well as compatible versions of bronze generally through hydroforming. Aluminum is typically used for the ends in order to reduce inertia. Other metals are employed in this process too. The whole assembly can be welded or joined using adhesives. The majority of bellows couplings are connected to shafts using integrated clamps, but other connecting methods like taper-lock bushings are available. Couplers can often include keys, however using a keyway is usually not advised over the more popular shaft clamping methods accessible.

In smaller motor control, coupling inertia could constitute a significant portion of the system’s inertia. this hollow bellows coupling can make efforts to reduce it, especially in comparison to other servo-system couplings, like Oldham, beam, and jaws with zero-backlash.

Failure of bellows couplings result mostly due to fatigue of the metal resulting from the deformation and relaxation of the convolutions in the metal to adjust for shaft misalignment, particularly when the bellows are located closest to connectors. A peak torque applied will usually break the material. If a problem does develop it is when the torque transfer to the machine that is being driven usually ceases, and the coupling needs to be replaced in full. If the misalignment is kept to a minimum and the bellows couplings are able to be able to endure many, many times without fatigue. Some manufacturers boast that they’ll last for years.

Applications

Bellows couplings typically allow for angular misalignment as low as 1-2 degrees and parallel misalignment ranging from 0.01 up to 0.02 in. The design is flexible enough to allow an axial expansion too. It is a little less accommodating to alignment issues than other designs of couplings bellows couplings provide very little force to restrain the equipment that is coupled, while remaining solid in the axis of rotation and flexible in the angular, parallel and the axial planes.

In determining the bellows coupling that is suitable for applications using servos, the best method is to calculate the torque at the highest that the motor produces, calculate the amplification of any gearing, then apply an appropriate safety factor of 1.5. The coupling you choose must have at least that capacity as the value calculated. For frequent torque reversals as well as stop/starts the acceleration torques are taken into consideration along with moments of inertia as well as the impact of shock on to calculate a suggested torque for couplings. The couplings could also be measured by torsional deflection or in rare instances it is based on resonance frequency. Many manufacturers offer the sizing application to help users through the selection process.

A majority of bellows couplings that are standard can run at speeds as high as 10,000 rpm. And if the hubs are balanced dynamically more than that, the speed can be much higher. Some manufacturers provide double-bellow models to be used in industrial applications that require heavy force.

Specifications and Attributes

Bellows specifications for couplings include these parameters as well as attributes

Bore diameter or size The diameter of the bore that connects to the shaft. The sizes of the bore may be identical or differ on either part of the coupling according to the model.
Overall length is the entire distance of the coupling measured from the two ends sides on the coupling.
Hub width is the size of the hub, measured from the face that ends at the inside face which joins with the bellow.
Hub material – the substance that the hub is built.
Bellow material is the material that the bellow is made.
Hub diameter – the total dimension of hubs in the section that makes up the coupling.
Bellow diameter is the total dimension of the bellow section in the coupling.
Maximum torque rated – the maximum torque rating of the coupling that can handle maximum torque load.
Lateral offset, also referred to as parallel misalignment, is the maximum distance tolerable for shaft misalignment on the axial dimensions that can be taken care of through the coupling.
An angular offset, sometimes referred to as an angular misalignment is the maximum angle deviation of shaft misalignment on the radial dimensions that can be taken care of via the coupling.
Fastening of the shaft coupling The method through which the coupling is fastened to the shaft that drives it generally clamped or with an adjustable screw.

Things to Consider

Because many bellows couplings can be utilized for precision-controlled motion that are zero-backlash devices, they’re not very flexible to misalignment contrasted with other designs of couplings. When installing such couplings it is essential to align the driving and driven equipment as precisely as is possible to ensure the longevity that the coupling.

Another thing to consider is that the coupling could be the least expensive component in the system and could be the source of failure in overload. There are designs that limit torque to fulfill this function as well.

Bellows couplings provide high stiffness and, as the bellows is symmetrical throughout its design They are immune to the extreme maximum forces that non-symmetrical couplings may produce which makes them a good choice for motion control applications that require sensitiveness. Their torsional stiffness lets them handle rapid velocity and dynamic movements that have become characteristic of various motion systems. They are able to withstand high speeds and thermal expansion. The only downside is a slightly less tolerance to alignment issues.

As with all couplings that is exposed, bellows couplings must be protected from injuries.

Most shaft couplings are equipped to take care of shaft misalignment, but to some degree or even more dependent on the coupling’s type. The most common causes of misalignment are an angular and parallel form. Parallel misalignment refers to shafts with axes that are parallel, but not collinear. Angle misalignment can be defined as shafts that intersect but with an angle that is less than 180 degrees. Manufacturers of couplings will announce each of these limitations in their respective products. Methods to align machine trains differ from the manual rule-and-feeler-gage method to the use of dial indicators mounted on shafts and laser-based methods. Installation of rotating equipment is a common practice. shaft-alignment solutions that utilize different methods.

Summary

The article provided a short review of bellows couplings comprising applications, attributes, specifications, as well as considerations for selection.