Two, direct-coupled frameless motors power die-cutters in a corrugated cardboard converting machine that operates at 1,000 fpm. The high-torque capability of the frameless motors eliminates the need for two ac vector-quality induction motors coupled through large gearboxes. Designed and built by Ward Machinery Co., Cockeysville, Md., this converting machine includes a servo-driven infeed section, four color-printing sections, this die-cutting section, and a stacker.

Frequently designed into machine tools to power high-speed spindles, frameless motors are now finding acceptance in other machines that have space constraints or unusual mounting requirements.

Recent technological advancements — new cooling techniques, more precise feedback devices, and expanding use of CAD — are extending the popularity of frameless motors into more registration-oriented operations such as paper converting and printing.

What are they?

Electrically, frameless motors are no different than conventional permanentmagnet and induction motors. The frameless designs can be controlled by the same drives as those used to control frame-type motors.

Mechanically, however, frameless motors are a horse of a different color. They are delivered as individual components — a rotor, stator and feedback assembly, Figure 1— which are installed as an integral part of the machine, Figure 2. Frameless motors typically have continuous torque ratings from 100 lb-in. to 10,000 lb-in.; speeds from 300 to 20,000 rpm.

Advantages

The basic approach of frameless motors offers advantages to the design engineer. Here is a summary:

Rigid load coupling. One of the frameless motor’s most important, yet least touted, advantages is the increased rigidity (or stiffness) that it delivers to the entire machine. In most designs, more stiffness produces improved product quality.

Overall machine stiffness is dependent on the cumulative stiffness of all mechanical elements between the motor rotor and load. These include belts, pulleys, gear sets or gearboxes, and couplings. In many applications, these can be eliminated by direct coupling.

Moreover, even the stiffness of the motor shaft affects stiffness. Shaft stiffness is a function of the cube of the shaft diameter. On frameless motors, the shaft diameter can be approximately three times greater than that of a conventional motor, thus giving 27 times more stiffness.

This higher level of stiffness is particularly beneficial for today’s most advanced converting machines with electronic line shafting (ELS). Such designs eliminate the long mechanical line shaft and the numerous mechanical attachments that have traditionally connected the machine sections together. Modern designs replace the line shaft with stand-alone machine sections, each with its own servomotor that is electronically synchronized to the others. These individual electronically “coupled” motors are stiffer than a line shaft, because the electronic coupling eliminates the wind-up encountered with most long shafts.

Creative motor cooling. Left to machine builder creativity, a number of innovative motor cooling methods have been developed. Among them, fluid cooling is highly efficient, enabling a compact motor to deliver high power. Some machines have achieved cost efficiencies by cooling frameless motors with the same fluids already being used for other portions of the machine.

Flexible shafting. Conventional motor designs include a rotor with an integral shaft that must be coupled to the load. Today’s frameless motors can incorporate a shaftless rotor with a bore through it or with a hollow shaft. This design offers machine builders the freedom to configure a variety of shafting options. For example, material or process fluids can pass directly through a hollow shaft. Plus, the machine can incorporate double-ended shafts or special mounting arrangements that suit the specific application.

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