Ballscrews or Linear Motors?

 

BALLSCREWS OR LINEAR MOTORS? ’APPS NOT SPECS,’ DETERMINE WHICH IS BEST

by Greg Hyatt, Makino manager of process and product development

It seems the entire machine tool industry has been debating the superiority of either ballscrew or linear motor technology. And, while it is a popular discussion, there is a key point being overlooked. Simply put one technology does not necessarily fit all applications. Based on extensive research and development, Makino recommends examining technologies based on each individual application instead of a cursory comparison of specifications.

Approaches To The Axis Drive

The strengths of ballscrews, or rotary motor systems, are clear after years of performance in machining applications. Ballscrews provide robust capabilities: high levels of accuracy and thrust, as well as optimal deceleration during loss of power. Additionally, ballscrews provide extremely beneficial damping which minimizes chatter. High-performance ballscrews used in advanced machining centers are ideal for applications requiring less than 13 feet of travel in ferrous material. Low-speed ballscrews are optimal for applications with low feed rates or low levels of spindle utilization.

Typically, linear motors are compared against lower performance ballscrews which does not adequately define ballscrew capabilities. However, in older or less advanced machining centers, ballscrews can provide limitations. The maximum rpm of the ballscrew depends on its diameter and length which can be problematic in applications with longer workpieces. As the mass of the ballscrew increases with its length, acceleration is limited and stiffness decreases.

Linear motors, a more recent entrant to the market, provide high levels of velocity and acceleration. Since there is no correlation between its performance and the length of travel, linear motors can provide fast drive speeds regardless of size. This makes linear motors desirable for a variety of applications like extremely large structural airframe components. But while linear motors have earned the accolades they receive, there are also some limitations to this technology.

Linear Limits

The attractive force of a linear motor’s magnets is typically three times greater than the thrust force. A motor with 2,000 lb. of thrust, for instance, would subject the linear guideways to an additional 6,000 lb. load. This thrust can crush the linear guideways. This problem has been addressed by mounting motors in pairs, back to back. This arrangement must be done carefully as it can result in a loss of bed strength. Metal must be removed to create troughs in the machining center bed to accommodate the motors, impacting bed rigidity and dynamic stiffness. In more recent designs, these structural problems are being addressed by using a large number of small motors.

Controlling deceleration during a power outage is also a challenge with linear motors. When ballscrews lose power, the mechanical system back drives the ballscrew and the motor so it decelerates rapidly. Inductive linear motors merely have the friction of the ways to decelerate the moving axis. It continues moving at the velocity it achieved prior to the power outage until it hits something spindle, workpiece and tooling can be damaged. Permanent magnet linear motors are more beneficial in this regard as they will decelerate without power at about one third the rate achieved under power. To combat this problem, some manufacturers are installing shock absorbers and stretching the axes travels to provide controlled deceleration while the spindle is moving away from the workpiece. But this still does not address deceleration if the spindle is approaching the workpiece.

In ferrous applications the magnets can be problematic as magnetized chips could be drawn in to the motor. And the electromagnetic fields created by linear motors can prove to be a safety issue for operators with pacemakers.

Common Ground

When analyzing the two technologies, some industry opinions can be misleading. For example, some note peak acceleration specifications when determining which technology is superior. This is misleading as there are a variety of performance levels within each technology.

Overlap in Performance

There is also an overlap in performance between the two technologies (see figure one). Acceleration rates and achievable thrust of high-performance ballscrew systems match linear motor rates in some cases. Ballscrew systems are available with specs from .1 g to 1.4 g specs and linear motors typically accelerate at rates of 1 g to 2 g.

Regardless of whether ballscrews or linear motors are used in an application, core cooling is essential. Without linear motor core cooling or chilled ballscrews, thermal growth can be problematic. If linear motors are not core cooled effectively, the bed and column are impacted, affecting static alignments and volumetric accuracies. Without proper ballscrew cooling, thermal displacement causes linear growth of the screw.

"A Well-Balanced App"

Keep in mind that no single solution is optimal for every application. In evaluating a high-performance machining center, and the technology supporting it, compare the requirements of a particular application to the strengths and limitations of the available technologies.

For example, are high-thrust levels required in an application? Large diameter drilling and heavy hogging in milling applications require higher thrust levels than linear motors might be able to provide. The highest possible acceleration rates would not only be unnecessary, they would compromise the thrust and rigidity needed.

Rather than getting into "spec wars," Makino recommends working with the suppliers under consideration. Conduct test cuts and make sure the supplier not only provides the right technology, but also supports the application after installation.

Ballscrew and linear motor technology has improved over time and, based on current industry research and development, this evolution will continue. The debate over which technology is superior may also continue, but one thing is certain the application approach will help you select the right technologies for your application.