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Holding Heavy Loads
Gantry manufacturer uses servo motor brake for fail-safe performance
Al Presher, Contributing Editor -- Design News, April 28, 2008
In emergency stop situations, to achieve fail-safe operation, heavy loads need to be stopped quickly in a controlled manner. A major designer and builder of robotic gantry systems faced these issues when general purpose brakes repeatedly failed in the field.
The gantry builder manufactures overhead pick-and-place robots and conveyor systems to move products throughout the factory. A robotic arm rides along the gantry system, picking up and stacking a variety of products onto pallets which are then moved along by a series of conveyors. The “pick” device end effector can be either vacuum or mechanical in nature and operates in the X, Y and Z axes.
To solve the problem, the machinery OEM specified a servo motor brake from Nexen Group because it is designed to fit the shape of a servo motor and offers higher torques and stopping and holding capabilities. The solution reduced the customer's installation time, simplified machine design by eliminating special modifications and increased gantry performance.
There are dozens of brakes on the gantry system, which is used in a fully automated distribution center to collect product, put individual boxes into large shipping containers and move the containers to the shipping area. There are long runs of gantry systems that move products in all X, Y and Z axes and provide a contained system with robots for material handling.
The application previously used electric brakes mounted between an Indramat motor and a NEMA-flanged worm gear reducer. Prior to the original installation, the customer needed to modify each brake by designing special flanges to accept Indramat MKD112, MKD090 or MKD071 motors and mount onto a NEMA- flanged gear reducer. Additionally, the original modified brakes could not absorb the energy produced by each stop and failed to perform adequately on subsequent stops.
To solve this problem, the customer looked to servo motor brakes. These products are spring-engaged, air-released brakes that offer a very high torque output with rugged durability for dynamic stopping applications — even strong enough to stall a servo motor. Designed for horizontal or vertical applications and bi-directional braking, the servo motor brakes gave the customer the ability to use the brakes throughout the gantry system with the added benefit of zero backlash for precise holding.
The brakes utilize a flange-mounted design that can fit virtually any servo motor. Out of the box, the servo motor brake was mounted between the servo motor and NEMA-flanged, worm gear reducer on the lift and rail systems of the gantry robot. The output end of the size 4 and 5 brakes are supplied with a NEMA shaft, pilot flange and bolt circle to mount directly to 56 or 145TC worm gear reducer. Both input and output flanges have slotted bolt holes that allow easy bolt installation and provide a range of bolt circle diameters.
“We've been very flexible with this design in terms of working with customers to design for their needs,” says Edd Brooks, senior technical specialist for Nexen. “We've got a very extensive line where the amount of customization we have today is a lot less than what we did five or 10 years ago when we were just starting with this product line.”
Brooks says the customizations usually addressed mounting features. To do an installation, all that is needed is the shaft size and shaft length of the servomotor, the pilot diameter of the servomotor face and the bolt circle for mounting the brake. With that information, a brake can be sized for the application and, if the brake properly fits the motor, there is torque available to stop or hold the load.
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