Machinedesign 1976 Direct Coupled1195 0 0

Frameless motors offer new solutions to precision motion systems

Nov. 1, 2000
As the volume available for motors decreases, the need to optimize the machine-motor fit increases. Hereís one path to improve motion systems with less space.

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.


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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Tailored bearing structure. Frameless motors enable machine builders to tailor bearing structures to the precise needs of each machine design and application. High-speed applications, such as spindles, can use a structure that minimizes bearing heat. Low-speed applications, such as rotary tables, can be designed to handle large radial or axial loads.

Compact and quiet. With no pre-defined frame, shaft, bearing structure or cooling package to get in the way, frameless motors can be one-seventh the volume of conventional framed motors with the same power rating, Figure 3. Thus, frameless motors can be squeezed into tight spots, increasing design flexibility.

Space at the motor location — and cost — are saved by eliminating the geartrain, couplings, belts and pulleys. However, space, remote from the motor, is required for a cooling heat-exchanger that frequently exists on the machine to cool other components.

Avoiding geartrain and motor blower reduces audible noise. One converting machine designer, running a direct-drive die cutter for the first time, commented: “It’s so quiet that I need to install an alarm just to let people know it’s running.”

Are they for you?

Although frameless motors offer many benefits, they are not the right solution for every application. For example, if you are building one or two machines, it will probably be a waste of your time — and the supplier’s time as well — to design a frameless motor into a machine. Generally, doing so takes 50 to 100 machines to pay for the initial design investment.

Plus, maximizing the benefits of these units requires investing in thorough machine engineering, rather than accepting quick solutions.

In short, if your company is willing to invest in quality engineering, spend the time to develop a working team relationship with the motor supplier, and the motor quantity is sufficient, then frameless motors may give you a competitive advantage.

Machine designer-supplier partnership is critical

Today, the traditional relationship between machine designer and motor supplier has changed from the frameless motor’s early days. Now, working as partners, they jointly develop an approach to integrate frameless motors into a machine. Such a relationship is essential to maximize performance — speed, torque, power, vibration limits, and operational life.

In exchange for more flexible designs, machine builders take on additional burdens. They include determining motor requirements and capability, plus making allowances for field replacement should repair become necessary.

Key to high performance

In precision motion systems, feedback resolution is a limiting factor for all applications. Traditionally, motor feedback devices have provided a resolution in the range of 1/10,000th of a revolution, which could be improved only through mechanical gearing.

New feedback devices provide resolutions greater than l/2,000,000th of a revolution, and are suitable for direct mounting on a frameless motor. This newer capability opens many application doors for all types of motors.

William M. Erickson is applications manager for Indramat Div., The Rexroth Corp., Hoffman Estates, Ill.

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