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# What is the difference between rigid and elastic coupling in master-slave architecture and why direct speed control available only in elastic coupling?

Published date: 23 May 2018

Rigid and elastic are 2 often used types of mechanical connection between 2 motors.
As their name say, the rigid connection is the type of connection where motor shafts are firmly connected together or connected via gearbox (e.g with cogged wheels). In other word saying, in rigid connection the motors have to have the same speed. It is not possible at any time that one motor rotates faster than the other.
On the orther hand elastic connection is the type of connection where motors are not directly linked together, but they are linked via elastic belt or load material. For example belt conveyor that has one motor on one side and other motor on the other side are linked by the converyor belt. In this case it is possible that one motor has greater speed than the other, either temporarily (belt can prolong or shrink) or continuously (if the belt slipping at least on one side possible).
In some application it is required to synchronize speed of 2 motors even if the motors do not have any mechanical linkage (or the linkage is very flexible). For example 2 independent conveyors. Here we speak about function "speed follower" but the setting will be done in the same way as elastic connection and with direct speed control.

The reason why direct speed control in rigid connection is not availble is the fact that both motor in rigid connection must have the same speed which is done naturaly by rigid mechanics. So it would be useless to regulate the speed of each motor individually. On the contrary the target is to balance the load (torque) between 2 motors with rigid coupling.
In elastic connection it is possible to choose speed or torque control. The torque control make sense if the mechanical connection between 2 motors can naturaly balance the same speed. But if the mechanical conenction is very weak or too elastic, you have to use speed follower (sped control).
Few pictures in attachment.