Friction clutches and brakes remain vital links in the increasingly complex task of motion management.
Thanks to clutches and brakes, many machines can be cycled far more quickly, accurately, and safely than with a motor alone. Since the early days when crude setups used friction to resist or impart movement, these components have matured, able to keep high-inertia systems closely in hand.
Clutch and brake job
Clutches are used to engage or disengage a load from the prime mover. There are diverse actuation methods for clutches, but they all serve this basic purpose. While a clutch is typically used for parallel shafting with the output tied to a pulley or sprocket, a clutch coupling is used for connecting two in-line shafts.
Although found in various applications, clutches are often required where the load is cycled more rapidly than the motor alone will tolerate. Common ac and dc motors driving without a clutch are restricted to 8 or 10 cycles/min. Typically these motors work best near their base speed. An accelerating motor draws a higher current and undergoes more wear than when it runs steadily at that speed. A clutch regulates load engagement so high cycle rates can occur without having to start and stop the motor. This helps extend motor life. Clutches are also frequently used where a single motor drives several discrete machine functions. In such a case, clutches are an elegant way to disengage sections of machinery from the motor while it continues to move the remaining parts.
The overall use of brakes can be summed up in three separate functions: bring to a stop, regulate speed (such as slow down a load or maintain the surface velocity of a material roll in tension winding), and hold in place.
Brakes are commonly used together with clutches in standard electric motor applications. This combination ensures rapid and accurate starting and stopping of the load while letting the motor run at optimum speed. A clutch can disconnect the load from the motor, but the load will coast to rest over a length of time depending on speed and inertia. Adding a brake to the process promotes precise stopping. The clutch disconnects the load, and the brake stops it while the motor continues turning. It’s not uncommon to see 100 to 200 cycles/min in such applications.
Brakes are increasingly used sans clutches in situations where cycling is not an issue, but where load stopping and holding are. Nowadays brakes are frequently used with stepper and servo motors in applications where it’s necessary to hold the load stationary when the motor is off or at zero speed.
Along with progress in control technology, an increase in failsafe brake configurations designed for continuous cycling applications and for double-shafted motors has led to the common use of failsafe brakes with ac motors on conveyors. The motors themselves are more readily cycled thanks to increasingly sophisticated controls, but in the absence of motor power and, hence, control over the conveyor, the motion needs to be stopped. Failsafe brakes rely on electricity to remain disengaged. They can be wired to use the motor input as their power source, resulting in immediate braking in case of a motor shutdown.
Friction clutches and brakes can be actuated by several means: electromagnetism, hydraulic or pneumatic fluid pressure, and mechanical force. Mechanical units may use spring pressure or even brute strength to engage.
Electromagnetic clutches consist of three main components: a field, rotor and armature. The field and rotor are mounted close together so that when power is applied, the magnetic flux can jump the air gap and reach the face of the rotor. The rotor is normally mounted to the motor shaft or driving shaft, and it magnetically clamps to the armature, causing it (and the load) to turn.
Electromagnetic brakes have two main components, a magnet and an armature. Dc power is applied to the magnet, which is firmly mounted against a frame or machine wall, attracting the armature that’s connected to the rotating shaft. Upon engagement, shaft movement is hindered or stopped.
In pneumatic brakes a similar process occurs, although the actuating medium changes. A pneumatic unit consists of several primary components: an air tube or bladder, a pressure plate, friction disks, center plates, and back plate. When air pressure is applied, the air tube expands axially, pushing the pressure plate. This forces the friction plates and center plates together. A gear hub, which is typically bored and keyed to the shaft, provides the connection between the center plates and the shaft. Hydraulic systems have comparable operating principles.
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Electromagnetic clutches and brakes generally actuate the fastest. It’s possible for these units to achieve rates exceeding 200 cycles/min in packaging and indexing applications. Pneumatic and hydraulic systems simply cannot act that quickly. Furthermore, electromagnetic units have a wear-adjustment mechanism, so engagement times remain accurate and consistent throughout the unit’s life. Pneumatic and hydraulic units adjust through an increased pressure-tube expansion. This can lead to longer engagement times over the wear life.
Pneumatic and hydraulic units are suited for high-inertia systems as they can produce very large amounts of torque. Thus, in applications requiring more than 100 hp it’s uncommon to see electric clutches and brakes.
Like standard power-engaged units, the failsafe designs are available in electric, hydraulic, and pneumatic configurations. However, in these designs the brakes are released when electricity, hydraulic pressure, or air pressure is applied. The electrically controlled units may use springs or permanent magnets to actuate the brakes. Most of the hydraulic and pneumatic units are springset. The safety advantages are that they require no energy to hold the load fast and react immediately to power failures. Similarly, when a load is eccentric, or is otherwise compelled by gravity to shift, a failsafe unit holds it still without a constant power input.
Where it’s at
Clutches and brakes are most often located between the motor and the load. Many industrial plant machines, such as conveying, packaging, material handling, and processing equipment, typically use this configuration. An example is the in-feed conveyor for a packaging machine that uses a standard c-frame motor and reducer and cycles once each second. A similar clutch and brake application involves metering powders or similar dry products as they pass into containers through an auger feeder. By locating the clutch-brake between motor and reducer, the product feed can be cycled while the motor runs continuously at its most efficient level. The clutch-brake is made for the strain of constant, rapid start-stop cycling. Motors, on the other hand, can do without these variable loads.
Greg Cober is Technical Support and Training Manager with Warner Electric, South Beloit, Ill.