Brakes and clutches in SCARAs and more

BXW spring-applied brakes from Miki Pulley excel in large SCARA applications.

Brakes, clutches and torque limiters work in medical equipment,packaging machinery, industrial hoists, andinstallations. Recall that brakes stop andhold loads. Clutches and torque limitersdisengage and engage power-transmissionoutputs to transfer torque from aninput. Torque limiters modulate powertransmission to protect mechanicalequipment from overload.

When selecting clutches, brakes, and torque limiters for an application, options include mechanical, electric, fluidic or self-actuation. Electric brakes are easy to control and cycle, even to a couple thousand cycles per minute. In contrast, common air-actuated brakes and clutches run coolly and maintain hold with minimal power. Friction brakes with disc, drum and cone morphologies work well as emergency brakes because of their failsafe holding — essential on conveyors, escalators, airport-baggage handlers, elevators and other safety-critical designs.

The size and type of brake most suitable for a given design depend on whether the machine needs emergency stops or softer stops that sacrifice brakes to protect loads from shock. Then, other application criteria (including the machine envelope, thermal capacity, cycle rates, and inspection and repair schedules) dictate the final selection. In other machines, it’s more important that brakes prevent shifting loads and misalignment.

In most cases, it’s best to size brakes and clutches to the motor torque of the machine axis. But especially for axes where the brake must stop a vertical load, engineers should account for the fact that motors can temporarily draw added current to output more than their rated torque. Helpful resources include published performance curves from brake and clutch manufacturers. These list dynamic torque ratings for various operating speeds to help engineers match brakes and clutches to peak motor-output torque.

Beyond these general guidelines, fast, hot and other extreme applications require extra evaluation. Some tips: Spring-set brakes benefit motion designs that slow loads with the motor before the brake engages; they’re also suitable as holding mechanisms.

Machinery that runs beyond a few hundred cycles per minute may require brakes and clutches that are larger than performance curves indicate.

Industrial designs that operate in hot environments may benefit from friction brakes with specialty frictionsurface geometries and materials (such as phenolic resin or ceramic). Motion applications that don’t get a lot of maintenance also benefit from the longer life these friction materials get. Brakes that use permanent magnets for operation automatically adjust for wear.

Consider one technology for high-cycle robotic applications. BXW spring-applied brakes from Miki Pulley for larger robotic arm applications can brake heavy loads as well as hold them in position. These functions let the brakes work even during power failures — to protect the robot from system collisions and mechanical system collapse that can cause costly equipment damage and manufacturing downtime. An optional release lever lets operators release the brake from a hold position when restarting the servo motor and the robot’s operation. The friction brakes handle speeds to 5,000 rpm and torques to 5.2 Nm with compression springs that engage and capture the brake rotor hub to immediately stop arm movement. A heavy-duty design reduces idling wear in articulating joints; rmature engagement is smooth, quiet, and sans chatter.