Electromagnetic clutches and brakes

This article is about the working principles of single face friction plate clutches and brakes.

This field (flux) overcomes an air gap between the clutch rotor (A2 yellow) and the armature (A2 red).

The frictional contact, which is being controlled by the strength of the magnetic field, is what causes the rotational motion to start.

Conversely, if a coupling is required to have a slightly higher torque (mostly for low rpm applications), a high coefficient friction material can be used.

[1] The electromagnetic lines of flux have to attract and pull the armature in contact with it to complete engagement.

Copper (sometimes aluminum) magnet wire, is used to create the coil, which is held in shell either by a bobbin or by some type of epoxy/adhesive.

Clutches used in most mobile applications, (automotive, agriculture, construction equipment) do not use friction material.

(A-3) The frictional contact, which is being controlled by the strength of the magnetic field, is what causes the rotational motion to stop.

When voltage is applied the stationary magnetic field generates the lines of flux that pass into the rotor.

If a piece of copper wire was wound, around the nail and then connected to a battery, it would create an electro magnet.

EM couplings are similar; they use a copper wire coil (sometimes aluminum) to create a magnetic field.

A constant power supply is ideal if accurate or maximum torque is required from a coupling.

The first one is the time it takes for a coil to develop a magnetic field, strong enough to pull in an armature.

For very high cycle applications, floating armatures can be used that rest lightly against the coil shell or rotor.

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 rotor will create a larger air gap which will change the engagement time of the clutch.

The second factor in figuring out response time of a coupling is actually much more important than the magnet wire or the air gap.

To do this, engineers use the formula: T = (J × ΔΩ) / t, where T = required braking torque (in N m), J = rotational inertia of system to be braked (in kg m2), ΔΩ = required change in rotational speed (in rad/s), and t = time during which the acceleration or deceleration must take place (in s).

There are also online sites that can help confirm how much torque is required to decelerate or accelerate a given amount of inertia over a specific time.

When a new "out of the box" coupling is initially engaged most peaks on both mating surfaces touch which means that the potential contact area can be significantly reduced.

Burnishing is the process of cycling the coupling to wear down those initial peaks, so that there is more surface contact between the mating faces.

If a clutch has a separate armature and rotor (two piece unit) burnishing is done as a matched set, to make sure proper torque is achieved.

In the voltage/current section, it was shown why a constant current supply is important to get full torque out of a coupling.

If a specific response time is needed, the dynamic torque rating for a particular coupling at a given speed is required.

The theory is, for the coil to generate as much of a magnetic field as quickly as possible to attract the armature and start the process of acceleration or deceleration.

Once the over excitation is no longer required the power supply to the clutch or brake would return to its normal operating voltage.

Like the coils, unless bearings are stressed beyond their physical limitations or become contaminated, they tend to have a long life and they are usually the second item to wear out.

Based upon the size of the clutch or brake, the speed and the inertia, wear rates will differ.

Designers can estimate life from the energy transferred each time the brake or clutch engages.

Obviously oil or grease should be kept away from the contact surface because they would significantly reduce the coefficient of friction which could drastically decrease the torque potentially causing failure.

But in general, this is normally not a major concern since the rust is worn off within a few cycles and there is no lasting impact on the torque.