Vertical motion creates a challenge when using linear motors, because the load will fall under the force of gravity in the absence of power. While a counterbalance is the typical solution to the problem, it can as much as double the weight of the system, thereby doubling the required force to maintain the same acceleration. "That means a larger, more costly linear motor is needed for the application," says Jeff Maher, a design engineer at Nexen Group.
Relying on their expertise in brake technology, engineers at Nexen Group set out to develop a compact, lightweight brake for linear motor applications involving power-off holding and stabilization for all axes. Called the Eclipse Linear Motion Brake, it consists of a track and a brake head designed for easy incorporation into many different machine configurations. The track is a custom aluminum extrusion with T-slots on two adjacent sides for mounting to the machine, a slotted section in which the brake head rides, and opposing friction surfaces that are engaged by the brake head's friction pads. The brake head itself consists of an aluminum body which houses a piston, wedge, friction pads, calipers, and return spring. Mounting holes, machined into the body provide a means for attaching the brake head to the linear motor.
The general operating principle of the brake, which is normally engaged, involves equilibrating the forces of air pressure and a spring. In normal use, standard shop air is supplied to an electrically-controlled solenoid valve that forces the piston against a wedge, moving it out of contact with the calipers and friction pads and compressing the return spring. In this mode, the friction pads float freely in the aluminum body so that the brake head is free to move along the stationary track.
When the solenoid valve is de-energized, either deliberately to park and hold the linear motor or accidentally when a power failure occurs, the solenoid automatically shifts to cut off airflow to the brake head. The air chamber that houses the piston is depressurized, allowing the return spring to move the wedge against the calipers and friction pads. The pads are forced outward against the sides of the stationary track to stop movement of the brake head and linear motor. The entire process occurs in approximately 50 ms.
The wedging action, which Nexen engineers were able to patent, is the key achieving 100 lbs of the brake force in a compact size. "Our goal was that the stationary channel cross section fit into a 2.5 x 3.0 inch envelope and that the moving element have a mass of less than 1.1 kg. To meet these constraints, we realized that obtaining an output force that is at a right angle to the input force would be critical. The angled sides of the wedge push the calipers in a perpendicular direction to provide braking in a small cross section," said Maher. The current version of the brake provides a holding force of 50 lbs with no backlash, say engineers. Additionally, there is zero drag when the brake is disengaged.
To keep weight down, engineers decided to use two types of plastic for the wedge and caliper. The two materials run against each other with a low amount of drag and are strong enough to transfer the force needed to brake. Moreover, the plastics stood up well against aluminum in a rigorous durability test.
|Brake product||Eclipse Linear Motion Brake|
|Brake type||Spring-applied, air-released, inside|
|Maximum holding force||Up to 100 lbs|
|Maximum stroke length||1,600 mm|
|Weight (brake head)||1.1 kg|
|Track weight||4.15 kg/m|
|Track cross section||61.5 X 74.9 mm|
|Contact Jeffrey Maher, Design Engineer, Nexen, 560 Oak Grove Pkwy., Vadnais Heights, MN; Tel: (651) 286-1000; Fax: (651) 286-1097; E-mail: [email protected]; or Enter 501|