Date Published:Oct. 25. 2016

Choosing the most suitable coupling

There are lots of ways to improve the productivity in the Servo System.
For example, there is the possibility that you can shorten the production cycle time if you use a high performance servo motor and an actuator.

There are many different kinds of couplings.
Today, I would like to explain the basic points on how to select the most suitable couplings for your needs.

In general, the word “coupling” means a component which connects two rotating shafts, including large “shaft couplings” made of cast iron.
Today, I will explain about smaller miniature precision couplings which are mostly used for factory automation, technologies.

How to select couplings

A simple and generally used method to select the most suitable coupling is the four step approach.
That is,

1. Select the coupling type suitable to the intended usage
2. Select the material of the coupling in line with the environment of usage
3. Select the coupling size fitted to the torque, space and bore size required
4. Select the fastening method between the shafts and the coupling

In this video, I will explain the first step, “how to select the coupling type to the intended usage”.

Dual Purpose of Miniature Couplings:Motion Control and Power Transmission

First, there usage of miniature couplings for FA is divided into two groups.
The first usage is “Motion Control”, which means to control the movement of systems by the combination of servo-motors or stepping-motors with couplings.
The other usage is “Power Transmission”, which means to transmit the power to the next component.
Usually combination of general-purpose motors with couplings is used for this purpose.
For the “Motion Control”, the most important requirement for couplings is “accuracy”, achieved by high torsional stiffness.
It is a mandatory condition to select from “zero-backlash” coupling types, which do not have any “play” in rotation direction.
On the other hand, for “Power Transmission”, the key requirement for couplings is “strength”, which can accept and transmit high torque.

Motion Control

Which type of miniature coupling should be used for “Motion Control” purpose?
Over a decade ago, “XH” Disk-type coupling, with all metal structure, was highly recommended because of its high torsional stiffness.
However, because of dramatic improvements in motor performances, now the best coupling to use is “XG” High-Gain Rubber type coupling.
By the combination of high vibration absorption capability and the optimum torsional stiffness, Rubber type couplings avoid the co-vibration of the system, and realize the highest performance of the motor.
Having said that, there are customers who want to continue to use the traditional types of couplings they are familiar with.
For such customers, we have “MBB” Bellows couplings, which have high torsional stiffness next to the Disk type.
And, we also have “MS” Slit couplings with simple monolithic structure.
Of course, each type of coupling I am introducing has wide range of performances based on the different size or materials.
Therefore, please note that the chart you are looking at explains the comparisons of general characteristics of coupling types, as an “image chart”.
There is another type, as well. If you do not want to allow any misalignment and need to connect two shafts with very high coaxiality, then “XRP” Rigid coupling is the most suitable thanks to its superior accuracy.

Power Transmission

Next, for “Power Transmission” usage, “MJ” Jaw coupling is the most suitable. It can transmit high torque, accepting reasonable levels of misalignment.
As it also has vibration absorption capability, Jaw type is widely used in many applications.
If you need to allow more misalignment, we recommend “MOR” Oldham-type coupling.

As I explained so far, you can select the most suitable coupling type for your needs, from the candidates suitable to the usage, and then by judging the importance of key features such as vibration absorption, torsional stiffness, transmitted torque or acceptable misalignment.

Once you select of the type of coupling you need in the first step, then the following three steps to select “material”, “size” and “fastening methods” are relatively simple and straight-forward.
NBK website, and the “Product Finder” there, will help you a lot to complete these steps and decide the final part number you need.

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