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Quill Shaft - Soft Couplings |
Quill Shaft Designs
Background
A quill shaft, by definition, is a
thin, solid shaft which is strategically designed and carefully
machined so that it carries the same torque that a larger shaft
would handle by operating at higher stress levels. In carrying
torque the quill shaft acts like a torsional spring, twisting
along its length.
Quill shafts have been used for decades in refrigeration
compressors with outstanding success rates. There are several
reasons for the success of the quill shaft in these applications:
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- First, the small diameter of the quill shaft allows
for the use of much smaller seals which can work at lower
rubbing velocities than their larger counterparts.
- Second, the quill shaft greatly reduces the amount of overhung weight, which, in turn, reduces the vibration
in the system.
- Third, the quill shaft is generally used in conjunction with a single hinge coupling. Because the quill shaft is capable of bending in a cantilever fashion, only a
single hinge is needed to provide complete misalignment
capability.
- Fourth, the thin shaft torsionally isolates the high
speed compressor from forcing frequencies such as gear
mesh frequencies or driver pulsations.
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There are some other subtleties with the quill shaft that add to the success of these applications.
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Quill shaft with a flywheel
and flexible elements.
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Double Hinge Applications
While the quill shaft has seen much success in compressor applications as a single hinge and cantilever design, there are other applications
which have benefited from a quill shaft used between two flexible elements. The term double hinge means that the coupling accepts offset misalignment of the coupled shafts by angulations of each flexible element at the
ends of the spacer.
In a torsionally soft application, the quill shaft would typically be used in a coupling with a flexible element at both ends of the quill
shaft. In simple terms, the quill shaft would be the spacer in a spacer
coupling. With a flexible element at both ends, the quill shaft does
not have to bend in response to misalignment because the flexible elements
accept the misalignment. The quill shaft is left with the sole responsibility
of reducing torsional vibrations and pulsations.
The quill shaft makes it possible to minimize transmission of torsional
pulsations from one machine to another. It can be designed with a low
enough spring rate to smooth out vibrations such as those produced by
reciprocating engines or compressors.
Some manufacturers make torsionally soft couplings using a
rubber element instead of a quill shaft. Using rubber causes several disadvantages: |
- Rubber torsional springs are heavier and much larger in diameter
- Rubber has a finite life, and it fails even quicker with
hard use
- It is difficult to have a rubber spring with a high torque
rating and a very low spring rate.
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Example Application
An example of a quill shaft application is a diesel engine driving
a compressor through a gear box. The pulsating diesel explosions can
be isolated from the gear box and compressor with a properly designed
low torsional spring rate quill shaft coupling.
Another example is a synchronous motor which produces high torsional
pulsations during startup. A low torsional spring rate coupling can
greatly reduce torsional stresses in the shafting during this critical
period.
Coupling Corporation of America • 250 N.
Main St. • Jacobus, PA 17407-1011
800.394.3466 • 717.428.0570 • Fax 717.428.2865 • Contact
Us
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