R+W: Basics of flexible Shaft Couplings: Bellows, Jaw and Servo Couplings
Most mechanical systems at some point require that a flexible coupling
be used to couple two or more steel shafts. Motors, gears, ball screws,
and many other mechanical components typically involve shafting which
is supported by multiple bearings. Bearings as their name would indicate
are intended to keep the shaft rigid and straight while in rotation in
order to avoid balancing and lateral support problems. Consequently shafting
is typically restricted from lateral movement. As components interface
with each other mechanically (e.g. an electric motor frame and gearbox
housing), manufacturing tolerances will normally cause the two coupled
shafts to be offset from one another if only very slightly. The result
can often be that a high lateral restoring force be placed on the shafts
as the two compete to maintain their original position. This is commonly
referred to as lateral misalignment. A very slight angular misalignment
also exists in most cases as the two surfaces being mounted together are
not always precisely perpendicular. Furthermore an axial load can also
be created as elements heat up and expand, potentially placing a thrust
load on rigidly supported shafting as well. All three forms of shaft misalignment
have the potential to cause shafting to break, bearings to fail, and excessive
vibration to exist as a machine operates.
The purpose of a flexible shaft coupling therefore is to absorb the shaft restoring loads while maintaining some amount of rigidity in rotation and transmission of torque. The amount of torsional stiffness (resistance to twist) required varies from one application to another, and coupling stiffness, as well as backlash, can also affect the amount by which a coupling is able to flex. Two major categories of flexible couplings exist for this reason. Torsionally flexible couplings, which typically allow for backlash, can also compensate for relatively large shaft misalignments, while zero backlash couplings, which offer a greater degree of precision in their rotation, often come at a compromise to flexibility.
The most commonly used flexible coupling type
In uni-directional applications, and other applications which do not required that precise position accuracy be maintained throughout the mechanical system, a coupling with backlash is normally used as they tend to be less
expensive and can compensate for greater misalignment. The most commonly
used flexible coupling type is a jaw or “spider” coupling.
These couplings consist of two hubs, each with a set of interlocking teeth,
and a flexible rubber or plastic piece in the shape of a star which fits
between them. The hubs each fit onto their respective shafts via a round
bore with a square key, locking the hub onto the shaft in the direction
of rotation. A set screw is also used to keep the hub from sliding axially
up and down the shaft. Since some amount of space is left between the
jaws and the insert, freedom of motion exists, not only in the directions
of misalignment, but also in the direction of rotation. This gap between
the jaws and the spider leads to what is commonly referred to as backlash – a
delay between the rotation of one hub and the rotation of the other hub.
Some mechanical systems, such as those driven by servo motors, require a very quick and precise response from the driven components. A computer controller converts position readings from a shaft encoder at the end of the mechanics into velocity and position commands for the motor. When an error exists between what the motor is attempting to accomplish and what the shaft encoder reads as actual position, it compromises the performance of the machine. Since flexible couplings are the easiest component to twist and the most likely to exhibit backlash, they are also one of the most important mechanical components to properly select in these applications. In precision applications, couplings need to compensate for all three axes of shaft misalignment, while precisely transmitting position, velocity and torque. In many servo applications, servo couplings must not only be backlash free, but also exhibit minimal torsional deflection (twist).
The ideal flexible servo coupling
The ideal flexible servo coupling is a bellows type. Bellows couplings
consist of two clamping hubs assembled to a round bellows, normally hydroformed
from stainless steel. Their round accordion-like shape allows for lateral
and angular bending as well as axial compression while applying only a
very light restoring load to shaft bearings. The most important feature
of the bellows is that it has a very high torsional stiffness, as it is
formed form one complete tube of stainless steel. This allows for precise
velocity and position accuracy, and also increases the mechanical frequency
of the machine so that high frequency servo loop gains will not lead to
excessive vibration. A further advantage of bellows couplings is that
they typically possess a very low mass and moment of inertia, so they
will not consume as much power as the motor attempts to quickly accelerate
them.
The above two flexible coupling designs are appropriate for many different types of coupling applications, but there exists a wide variety of other flexible coupling designs intended for more specific uses. A spiral cut in a cylindrical bar of metal, a stack of thin metal discs riveted together, loose fitting gear teeth, and other means of achieving flexibility between shafts are employed for such purposes as compensating for very high misalignment, rotating at very high speed, transmitting very high torque, etc. Specialty couplings are available from a number of different manufacturers, each of which designed them with specific applications in mind.
Flexible Couplings Facts
Flexible couplings protect machinery from wear and premature breakdown.
They can play a key role in extending the service life of the components
they connect, and often without compromising machine performance. Proper
coupling selection is often overlooked but can actually be critical to
quality machine design.
