When and why should you use a bellows coupling?

Bellows couplings are single-piece , flexible shaft connectors utilized to join drive and driving shafts within mechanical power transmission units. Flexible shaft couplings can be used to prevent the inevitable misalignment between shafts connected, and, in certain cases they are used to absorb shock.

Coupling design

Bellows couplings are made of pleated, thin tubes made of metal that are joined by shaft connectors at 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 is a benefit for installation with high temperature swings. There are heavy-duty models available with shaft diameters ranging from many inches. The design is extremely low in Inertia, so it puts a minimal impact on the performance of a drive/driven combination.

For the production of bellows, nickel is electrodeposited on an amandrel. The mandrel will dissolve leaving behind a thin-walled flexible component. This method produces bellows that are extremely thin, but with precisely controlled thicknesses that allow for a responsive, sensitive coupling, even though they have a smaller torque capacity than those of bellows produced through other methods. These bellows are more rigid 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, but other metals can be employed in this process too. The whole assembly can be welded or joined using adhesives. The majority of bellows couplings are connected to shafts by using clamps that are integral, however other connection methods, such as taper-lock bushings can be found. Some couplings include keys, however using a keyway is usually not advised over the most popular shaft-clamping techniques that are readily accessible.

In smaller systems for motion controls, coupling inertia could constitute a significant portion of system inertia. this hollow bellows coupling can make steps towards lessening it, especially in comparison to other servo-system couplings, like Oldham, beam, and the zero-backlash jaw.

Failure of bellows couplings result mostly due to fatigue of the metal resulting from deformation and relaxation the convolutions in the metal to adjust for shaft misalignment, particularly when the bellows are located closest to connectors. The applied torque can damage the material. If a problem does develop and the machine is affected, the transfer of torque to the drive machine typically ceases, and the coupling needs to be replaced completely. If misalignment is not too severe 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 accept angular misalignments of 1-2 deg and parallel misalignment of 0.01 up to 0.02 in. The design is flexible enough to allow expansion in axial direction and expansion. A little less tolerant of alignment issues than other designs of couplings Bellows couplings exert only a small amount of restorative force on the coupling equipment, but remain solid in the rotational axis , and flexible in the angular, parallel and the axial planes.

In determining the bellows coupling to be used in applications using servos, the best procedure is to measure the maximum torque for the drive. calculate the amplification of any gearing, then apply the safety factor 1.5. The coupling you choose must be at or near its capacity to the value calculated. For frequent torque reversals, as well as start/stops 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. Couplers can also be sized according to torsional deflection or, in some cases the resonance frequency. The majority of manufacturers will provide Sizing software to guide specifiers through the selection procedure.

A majority of bellows couplings that are standard can operate at speeds as high as 10,000 rpm. If the hubs are balanced dynamically and balanced, they can run much faster. Some manufacturers provide double-bellow models to be used in industrial applications that require heavy force.

Specifications and Attributes

Bellows specifications for couplings comprise some of the attributes and parameters listed below:

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

Things to Consider

Because many trailer coupling bellows can be utilized for precision-controlled motion, such as zero-backlash units, they are not flexible to misalignment contrasted with other designs of couplings. When installing such couplings all efforts should be made to align the driven and driving equipment as precisely as you can to ensure the longevity that the coupling.

Another factor to consider is the fact that the coupling might be the least expensive part of the system, and is the most likely source of failure in overload. Torque-limiting designs can be found which can also serve this purpose.

Bellows couplings provide high stiffness and, since the bellows is symmetric throughout its construction and is symmetrical throughout its construction, they are able to avoid the high force caused by non-symmetrical couplings which makes them a good choice for motion control applications that require sensitiveness. Their torsional rigidity allows them to handle extreme velocity and dynamic movements that have become characteristic of various motion systems. They can withstand high speeds and thermal expansion. The only downside is that they have a less tolerance to alignment issues.

Like any coupling the bellows couplings that are exposed must be protected from injuries.

The majority of shaft couplings are able to take care of shaft misalignment, but to an extent or more according to the coupling’s type. It is common for misalignment to occur in the angular and parallel forms. Parallel misalignment can be defined as shafts with axes that are parallel, but not collinear. Angle misalignment can be defined as shafts whose axes cross but with an angle less than 180 degrees. Manufacturers of couplings will announce each of these limitations in their respective products. The methods for aligning machine train differ from the manual rule-and-feeler-gage method to the use of dial indicators mounted on shafts and laser-based strategies. Installers of rotating equipment frequently offer shaft-alignment solutions that utilize the various techniques.

Summary

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