Choosing the right servo system coupling is a matter of balance

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.

Other attributes are important and must be considered.

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.

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.

SC Series 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.

For more detailed information please download the ServoClass PDF catalog