The
trend in servo motor and servo system designs requires manufacturers
of couplings, as well as other components to focus their design and
performance attributes. Servo motors have become smaller, with lower
inertia, greater torque and greater performance and duty cycle capabilities.
These
changes and others require different balance of coupling attributes.
How well each attribute is balanced among other design attributes
of the servo coupling is important to system performance and reliability.
Some
of the design attributes considered in the design of the new ServoClass
couplings are:
•
Zero backlash
• Torsional stiffness
•
Inertia
• Torque
• RPM
• Misalignment
• Coupling reaction force
• Shaft mounting arrangement
• Assembly and construction
• Dimensions and weight
• Total cost, performance & reliability attributes
Zero Backlash - the Benchmark
Zero
backlash is a "benchmark" attribute in the selection of
a coupling for a servo drive system. Next are "torsional stiffness"
and "inertia". A coupling with too much inertia can detrimentally
affect the performance of the servo motor system.
The coupling must have an integral clamp-type hub mounting in order
to provide zero backlash mounting. The clamping design should be as
compact as possible so that it does not increase the inertia of the
coupling. Additionally, the design of the clamp hub can also affect
the torsional stiffness of the coupling.
Balanced Coupling Attributes
Very high coupling torsional stiffness is often interpreted
as providing "better" mechanical position response. However,
VERY high torsional stiffness is often unnecessary and may sacrifice
other coupling attributes needed for long-term system performance
and reliability.
Likewise, a coupling selected for its "soft" torsional stiffness
and "vibration" damping can be a poor choice for a servo
drive system. "Ringing" may occur while the system attempts
to compensate or hunt for the true position. And "electronic
damping" may decrease system performance. The torsional stiffness
of the driven equipment must be considered.
Excessive torsional stiffness often means misalignment capabilities
will be reduced and misalignment reaction forces increased. (see illustration)
There is no practical escape. The important coupling design attributes
must be balanced against the value of the others.
The design of ServoClass® couplings also minimizes
coupling reaction load while transmitting torque.
ServoClass® Couplings - a "Balanced Design"
By design, ServoClass couplings substantially reduce reaction
loads, even those generated by torque transmission.
ServoClass® couplings utilize two flexible, stainless
steel disc mounted to a lightweight, high-strength aluminum center member
and precision mounting hubs. The lightweight, small diameter ServoClass
coupling provides very low inertia, another important attribute, along
with torsional stiffness in avoiding system resonance and increasing
system performance.
As an example, a ServoClass® coupling is compared to
another coupling with similar torque, misalignment capacity and dimensions.
The same servo motor and linear ball screw are used in the application
comparison.
Data concerning the motor inertia, coupling inertia, torsional stiffness,
actuator load, lead screw diameter, pitch, system gain, etc. are entered
into a 2 Mass Natural Frequency calculation to determine whether there
may be a system resonance problem.
Comparison results showed the older design alternative coupling produced
a natural frequency of 229 Hz. The system natural frequency was 880
Hz. The design attributes of the ServoClass coupling placed the system
natural frequency far beyond the typical resonance range of 150-350
Hz.
The new ServoClass® coupling is a valued component for
many servo drive systems, SMT (Surface Mount Technology) and positioning
control applications.
