Brake and clutch basics

One of the unsung heroes of mechanical power transmission are brakes and clutches. Often hidden away, these humble components are indispensable in many machines and systems.

Fundamentally, they’re used to stop, hold, or index a load. More specifically, brakes are used to stop a load, usually a rotating one. Brakes are used where accurate stopping of the load is a must, and typically the motor needs to stop as well.

Clutches, on the other hand, are typically used to transfer torque. So a clutch is used where it’s required to engage or disengage a load and motor while leaving the motor running. With a clutch, the load coasts to a stop.

Other application needs may call for a brake and clutch combination; for example, if the load must be stopped and started while the motor continues to rotate. Brakes and clutches can mount to a motor shaft or can even be basemounted with input from a belt drive, chain drive, or coupling.

A new line of micro brakes from Ogura is designed specifically for servo motor applications, especially in robotics and medical equipment. Ogura micro holding brakes come in both square and round designs with the newest round design being 10 mm in diameter with length as short as 9 mm. Micro brakes can weigh 20 g or less allowing for reduced inertia on fast moving robotic arm applications.

Sizing and selecting the right brake or clutch for the job requires knowing a few parameters, specifically the horsepower and frame size of the motor. Manufacturers typically provide selection tools making it easy to pick the right component size by finding the intersection of the motor horsepower and speed at the clutch shaft. These selection charts are constructed using the dynamic torque capacity for the specific clutch and the motor torque capacity plus an overload value. This method works well in most uses; for example, where the motor is sized correctly. However, other instances may not be suitable. For instance, where cycle rates are considered aggressive for the inertia of the load, it’s better to consult with others including the manufacturer, especially to get more information about heat dissipation capacity.

Another consideration in selecting brakes and clutches is the coil voltage. Common options include 6, 24, and 90 Vdc, where 90 V are more common in North America, whereas 24 V supplies are more common in Europe.

The main brake and clutch designs are mechanical, electrical, and fluid power models.

Mechanically actuated brakes and clutches use a lever for mechanical advantage to trigger engagement and disengagement. The lever works by squeezing friction disks together to transmit torque. Movement of the mechanism engages or disengages the brake or clutch, with some designs featuring a locking mechanism to maintain the state until repowering. These brakes and clutches typically don’t have bearings, are available with optional one-position setups, and offer automatic overload release during over-torqueing or locking. On the downside, these types of brakes and clutches need adjustments to compensate for mechanical wear and they don’t automatically disengage in the event of a loss of power.

Miki Pulley’s BXW springactuated electromagnetic brakes use internal compression springs to provide power-off, fail-safe braking. The primary moving part is the armature plate.

Electromechanical brakes and clutches, on the other hand, incorporate a magnetic coil to generate a magnetic flux, which is used to move an armature. Different designs mean they can be electrically activated or what are known as spring-applied electrically deactivated types, which also fail-safe during a loss of power.

Electromagnetic brakes and clutches are generally easy to control, are tolerant of high speeds, and have a long life cycle. Most also come equipped with a selfadjustment mechanism to compensate for friction-disk wear. However, they do require a bearing to support the stationary coil and the engagement time may be a bit longer compared with other designs due to the need to generate the magnetic field for the coil.

Lastly, fluid-power based designs work by fluid shear action between friction disks and drive plates. The fluid in shear transmits torque as parts come together, eliminating direct friction-disk contact during highspeed slip. Oil-shear based technologies are suitable for applications that must rapidly stop, start, reverse or change speed.

Design tips for dynamic clutch or brake applications

The motor driving the load partially dictatesthe needed clutch or brake size. The required torque for an axis needing dynamic clutching or braking is defined by the motor horsepower (hp) and operational speed. So the required dynamic torque in lb-ft is:

5,250 x (hp/rpm) x (safety factor)

where rpm is the rotational shaft speed that the brake must stop (or the speed difference between clutch output and input). The safety factor adjusts for the motor type and the typical torsional output during operation. Most electric motors have a safety factor near unity as their output is fairly consistent.

A key consideration is the time needed to change the axis speed. Another parameter is the duty cycle – if there are more than a few instances of engagement per hour, duty cycle becomes an influencing design factor. For instance, in high-cycle applications running to 300 cycles/min as in indexing applications using a clutch-brake, each clutch engagement transfers a torque spike to the connection between the motor shaft and the input shaft of the clutchbrake. Such high-cycle applications can cause severe hammering on the shaft connection of the motor to the clutch-brake.

C-face motor connections are the most common as they offer convenience and easy assembly. However, their loose fit can cause torque to transfer through the key and keyway, which may cause failure. Alternate connection options include clutches and brakes with shrink-fit or clamping couplings for a 360° connection.

A note about the dynamic torque calculation above; it’s based on the time to stop the load or bring it to speed and doesn’t account for the time needed for the clutch or brake to actuate. For electromagnetically engaged clutch-brakes, a coil generates an electromagnetic field for armature activation and friction-disk engagement. This process can take from 10 to 500 msec depending on size.