To select the proper shaft, determine
the following items:
● Torque requirement
● Shaft radius of curvature
● Length of path between driving and
driven systems
● Operating speed
● Acceptable torsional deflection or
backlash
Determining the torque requirement on a
shaft may not be easy. Remote-control shaft requirements can be
determined by attaching a lever of known length to the device to be
turned and pulling the end of the lever with a spring scale. Use the
highest torque as the required torque.
Measuring torque on a power shaft is
more difficult because the requirement is dynamic. The best way is to
instrument-load with a torque cell and measure the torque under
operating conditions. Component manufacturers usually have these data
available. A shaft efficiency of about 90 percent should be
attainable for most applications; therefore, increasing the output
torque by about 10 percent leads to the input torque.
The shaft path should then be
examined.A drawing should be made that shows the path in a true view.
The MOR should be determined from this drawing. A full-scale
prototype can be used. The length of the shaft can also be determined
at this time.
For power shafts, the operating speed
is generally given. For remote-control shafts, in which the speed is
essentially zero, the amount of torsional deflection acceptable
between the turning device and the turned device is the important
consideration.
Used intelligently, flexible shafts are
the product designer’s allies. They are more flexible than
universal joints; they are more versatile than gear systems because
they are totally unaffected by the exact angle or offset necessary;
finally, flexible shafts offer an inherent shock absorption
capability, ease of installation, and maintenance unmatched by other
forms of rotary-motion transmission.
However, flexible shafts do have to be
treated carefully if they are to provide the service life built into
them. They cannot be bent completely out of shape and must be used at
radii equal to or greater than the MOR specified by the manufacturer.
They should be secured approximately
every 18 in (46 cm) to prevent the possibility of helixing. They must
be lightly lubricated as preventive maintenance at regular intervals.
Replacement must match the original design because a remote-control
shaft is not interchangeable with a power shaft.
The type of service expected must be
clearly specified. Two differently built shafts, even of the same
diameter and length, are not interchangeable. Above all, flexible
shafts must be designed for the task they are to perform. For this
reason, early consultation between the user’s engineering
department and the shaft manufacturer is highly recommended.
Follow these simple, basic suggestions,
and the design flexibility of a rotary-motion flexible shaft will be
applied to utmost advantage.
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