Water-Jet Motion Tied to Precision Coupling

A machine built for precise, repeatable motion is only as good as its components. That idea was clear to Jet Edge Inc., St. Michael, Minn., when it designed a high-rail gantry water-jet.

The machine cuts, prepares surfaces, and removes coatings from metals, stone, composites, glass, rubber, and plastic up to 20-in. thick. Servomotors and ball-screw actuators in the cross and side beams control simultaneous motion in the X, Y, and Z axes, letting the nozzle trace cutting paths with bidirectional repeatability of 0.001 in. at speeds up to 1,000 ipm.

When the company was designing the drive for the water-jet, its engineers needed couplings to connect the Bosch Rexroth Model MSK60 servomotors, ball screws, and Rexroth-type GTE high-precision gearbox.

The couplings had to precisely transmit motion without adding bearing loads to the actuators. They also had to have high torsional stiffness and misalignment capacity, and the flexibility to cushion reaction loads as the machine cycled through complex part programs at top speed.

Software from Zero-Max, Plymouth, Minn., helped Jet Edge engineers size the servomotors and led them to use Zero-Max ServoClass SCO50R couplings.

The couplings have a disc design which gives them 221 lb-in. of operating torque at 10,000 rpm. They have a torsional stiffness of 2,471 lb-in./degree and axial stiffness of 137 lb/in. The couplings tolerate 0.011-in. misalignment in parallel, 1.0° angularly, and ±0.031 in. axially. In Jet Edge’s water-jet cutter the couplings operate at up to 1,000 rpm, yet must quickly reverse direction, making their relatively low 0.407 lb-in.2 moment of inertia especially important.

The couplings’ compliant, keyless, clamp-style mounting hubs can be aligned without special tools or lasers.

Burt Olson, a Jet Edge engineer, says couplings have been troublefree so far. “The only downtime for these water-jets comes when a part run is complete and a new one is being set up,” he says.