Electromagnetic clutches

These clutches use a single plate friction surface to engage the input and output members of the clutch. This style of clutch is used in applications ranging from copy machines to conveyor drives. They are the most common type of electromechanical clutches. Other applications for these clutches could include packaging machinery, printing machinery, food processing machinery and factory automation.

Engagement: Electromechanical clutches operate via an electric actuation, but transmit torque mechanically. When the clutch is required to actuate, voltage/current is applied to the clutch coil. The coil becomes an electromagnet and produces magnetic lines of flux. This flux is then transferred through the small air gap between the field and the rotor. The rotor portion of the clutch becomes magnetized and sets up a magnetic loop that attracts the armature. The armature is pulled against the rotor and a frictional force is applied at contact. Within a relatively short time the load is accelerated to match the speed of the rotor, thereby engaging the armature and the output hub of the clutch. In most instances, the rotor is constantly rotating with the input all the time.

Disengagement: When current/voltage is removed from the clutch, the armature is free to turn with the shaft. In most designs, springs hold the armature away from the rotor surface when power is released, creating a small air gap.

Cycling: Cycling is achieved by turning the voltage/current to the coil on and off. Slippage should occur only during acceleration. When the clutch is fully engaged, there is no relative slip (if the clutch is sized properly). Torque transfer is 100% efficient.

There are two engagement times to consider in initial electromagnetic clucth action. The first is the time it takes for a coil to develop a magnetic field strong enough to pull in and attract an armature. Within this scenario there are two factors affecting this. The first one is the amount of turns in a coil which will determine how quickly a magnetic field is generated. The second one is the air gap which is the space between the armature and the face of the clutch. This is because the magnetic lines of flux diminish quickly in air. The further away the attractive piece is from the coil the longer it will take for that piece to actually develop enough magnetic force to be attracted and pull in to overcome the air gap. For very high cycle applications floating armatures can be used that rest against the brake face. In this case the air gap is zero but more importantly the response time is very consistent since there is no air gap to overcome. Air gap is an important consideration especially with a fixed armature design because as the unit wears over many cycles of engagement the armature and the clutch face will wear creating a larger air gap which will change the engagement time of the brake. In high cycle applications where registration is important even the difference of 10 to 15 milliseconds can make a difference in registration of the driven material. Even In a normal cycle application this is important because a machine that was at one time good, can eventually see a “drift” in its’ registration.

Second factor in figuring out response time of a clutch is actually much more important than the magnet wire or the air gap. It involves calculating the amount of inertia that the clutch needs to accelerate. Many customers refer to this as time to start. In reality this is what the end customer is most concerned with. Once it is known how much inertia is required for the clutch to start then the appropriate size of clutch can be chosen. Remember to make sure that the torque chosen for the clutch should be after the clutch has been burnished.