Traditional motion still having a ball

By Jason Christopher, Field EditorTECHNOLOGY advances in motion and drives seem to be everywhere. Every facet of our industry reveals an innovation that promises never-been-seen-before performance compared to what it seeks to replace. Fortunately, manufacturers of the more traditional linear motion technologies haven’t been resting on their laurels. In fact, they’ve been busy improving their and our processes.“We work with a lot of small manufacturing companies that have small budgets, but that doesn’t mean they have small expectations,” says Rick Lamb, president of SpecPlus Automation Consultants, an Indianapolis-based system integrator, specializing in turnkey automated control and information services, mostly for discrete parts manufacturing. “They come to us with performance targets and cost targets, and we need to meet both of them.”Lamb believes the technology used doesn’t matter as much as its ability to meet the system criteria. “It doesn’t matter if it’s linear motors, servo-driven slides or a servo-pneumatic cylinder,” he adds. “We select the technology that can best help us get to where we need to be. This is true, both for the machine’s performance measures, as well as its cost. If a pneumatic cylinder or ball screw-driven slide can give customers the results their processes need, then there is no reason to even bother looking at a linear motor.”Lamb isn’t the only person with these sentiments. While linear motors have some advantages, many machine builders and integrators report that traditional ball screws and fluid-powered servo systems still have much to offer.Still the One
Linear motors have been around for many years, but this technology became readily available only in the past decade or so. It’s now gaining popularity, but not at the lightning pace some predicted. Linear motor manufacturers boast about phenomenal acceleration rates and final traverse speeds, but, in reality, traditional competition isn’t far behind.For example, performance and cost factors both have improved. “Lead screw-based slides, 2 meters long, running 2 m/sec velocities, were an expensive dream just a few years ago,” says Tom Solon, PE, applications engineer at Kerk Motion Products. “Now, they’re available for $500. The linear motor has higher ultimate speed and accuracy, and might generate more force. But why spend five to 10 times more money, accommodate the much larger footprint, and use the necessary power if the lead screw solution is good enough?”The questions Solon raises are good ones. Many of us have a “Tinker Toy” mentality and a desire to experiment with new technologies. We’re engineers, after all, but we also understand how our decisions affect the bottom line. It can be difficult to justify experimenting with newer technologies when the lower-risk choice might still exceed targeted performance requirements at a lower installed cost.In short, they do work well. “The traditional rail and screw technologies are proven over time,” reminds Sheldon Prom, chief engineer at Komo Machine of Sauk Rapids, Minn. “These solutions have been fine-tuned and they consistently work well. Traditional methods, even when combined with new innovations in seals and coatings, often represent a low-risk/low-cost solution.”     FIGURE 1: HONE IN ON PERFORMANCE

Ball screw motion on the horizontal axis of this honing machine is allowing it to achieve speeds of 150 m/min while prolonging machine life, even in a harsh environment. Source: Sunnen Products

Likewise, for equivalent applications, linear motors still require a much higher initial investment. “They require far more energy for the similar motion, and, as a result, have secondary cooling requirements,” says Russ Jacobsmeyer, product design and development manager at Sunnen Products, a St. Louis-based manufacturer of bore sizing and finishing equipment. Jacobsmeyer recently helped Sunnen transform its product line using ball screw-driven motion. Its finishing equipment has axes reaching speeds of 150 m/min, while enabling machine life in excess of 30,000 hours at worst case loading conditions (See Figure 1).For many manufacturing companies, where margins generally are slim and competition surely is fierce, not meeting performance requirements is a very serious issue. Capital equipment costs are significant, but lost production and failure to achieve payback targets often is a bigger issue. Risk aversion is a big reason why traditional technologies haven’t lost significant market share to linear motors.Mature Advances Possible?
Despite their advantages, how do mature technologies, such as ball screw-driven slides, keep making performance enhancements? The answer has several parts.“First is the availability of high-precision thread rolling (See Figure 2 below), which enables very accurate positioning and actuation at low costs,” says Solon. “With micron and submicron positioning accuracy and repeatability, new products can be brought to market that were previously not saleable.” Kevin Gingerich, director of marketing services for Bosch Rexroth, echoes this sentiment. “Technology has advanced so substantially that a rolled screw can now rival ground screws in smoothness and precision” he says.FIGURE 2: NO LONGER A GRIND

Precise-rolled ball screws can be a key factor in keeping per-axis costs low, while still meeting performance requirements. Rolled ball screws now approach the smoothness and precision of ground screws, but at much lower cost. Source: Bosch Rexroth

Second, advances in composite polymer technology are occurring in conjunction with precise thread rolling. “New plastics allow low-cost, mass-production with incredible material properties,” adds Solon. “High strength, self-lubricating materials allow motion control components to outlast the application, while offering ‘tunable’ electrical properties, extreme environment compatibility, light weight, and the flexibility of injection molding to provide economical custom components.”Third, the design of ball screw nuts has progressed with improvements to the internal ball deflector. “These improvements have increased the maximum speed rating of the screws, allowing for significantly greater speeds,” says Prom. “Changes in seal technology also have had a significant impact on the life of components. These changes, in turn, extended the life of our machines, reduced warranty costs to us as a manufacturer, and aided end users over the life of the machine.”While actual application of ball screw technologies has changed little, the precision of the process used to create these products has evolved. “Manufacturing efficiency has been optimized, materials have been developed, and a large user base spreads costs,” states Solon. “The majority of capital and development investments have been paid many times over, so the incremental cost of advancement is lower.” Unlike pioneers of new technologies, who have tremendous pressure to recoup new capital equipment investments, manufacturers of traditional ball screw technologies actually benefit from the maturity of the product line.In essence, developers say these very technologies are being used to make better, higher precision, products of the same sort. The net result is a cycle that continuously leads to better quality products at lower prices.Benefits for Users
Industry observers say price-performance ratios are still frequently in favor of traditional technologies in many applications, which might be one reason that linear motors have not yet seen widespread acceptance. “Ball screws provide better price-performance and size-performance ratios in applications where high thrust forces are required,” says Gingerich.Referring to traditional ball screw driven slides, Solon adds the technology has advanced so much that “it helps equipment designers get what they need in an affordable package, which includes parts consolidation, motor optimization, environmental compatibility and zero maintenance. In the past, it might have been necessary to overbuy to meet a single performance parameter. With today's technology, we can tailor the product to the application and keep cost manageable.”     FIGURE 3: A WELD EVERY SECOND

This multi-axis welder achieves 30 welds in 33 seconds, using the fast, precise motion of ball screw-driven axes. Ease of implementation and environmental compatibility are two reasons why ball screw driven slides remain popular. Source: Centerline/Bosch Rexroth

The advantages provided by traditional linear motion systems are apparent in several ways. “The ease of implementation reduces build costs and eases manufacturing constraints,” says Lee Stephens, system engineer for Danaher Motion. “Using multiple actuators in a system can create a simplified, "Cartesian Axis" robotic manipulator (Figure 3) without requiring an entire design team of CAD experts and solid modeling examples. Though some useful functions might require some added complexity, a simple system shouldn't require more complications than absolutely necessary. The [traditional technologies] can reduce the complications of a very accurate and repeatable system.”The net result is that we can have our cake and eat it too. Technological advances and process improvements mean that the traditional components that our mechanics and electrical technicians are used to working with are now delivering more performance. In fact, they have enough performance that they can still satisfy most applications, while fitting within our specified budgets. Drawing the Line?
It’s fair to say that newer technologies typically are developed to solve new problems. “In other words, a motion system designer might find that he can’t achieve his desired solution with so-called traditional technologies,” explains Gingerich. “He might need something smaller, faster, quieter, or more precise. Or, he might need something larger, less expensive, or easier to install.” It is here that newer technologies are best applied, and where they will shine brightest.“In an application requiring high speed, relatively high force and high accuracy, the linear motor solution is likely to be the only way to ensure performance,” states Stephens. “The stiff coupling of a linear motor can’t be duplicated with ball screws, belt drives or standard actuators. The backlash and directional repeatability will suffer.”However, Stephens adds, “Linear motors typically are sold as frameless devices requiring a significant amount of engineering. They offer some advantages over any other form of control, but the added complexity might not be justifiable in all cases. For cost-sensitive applications requiring nominal accuracy in the 0.001 in. range, a precision ball screw-based actuator might be the correct solution (See Figure 4 below).”FIGURE 4: MANIPULATED MILLING

This five-axis milling machine derives its performance for the precision and stiffness from ball screws. Source: Metrom/Bosch RexrothProm adds the application of new technology generally is performance driven. “We typically search for new technologies where greater accelerations and machine performance is required,” he states. “Quite often, our customer’s will demand an improvement in performance that is unachievable with current hardware. Higher equipment speeds and greater precision requirements beyond what we’re currently capable of often requires us to search out a better way to do things. This results in expanding the materials and hardware solutions to new technologies. Traditional methods often are used to achieve linear movements and in simple pick-and-place applications, where the primary design goal is lower cost.” There clearly is a place for all kinds of technology. As traditional technologies improve, and as performance indices of the ball screw-driven technologies are enhanced, the rift separating the two closes. The pressure to keep costs low and uptime high, found in virtually all manufacturing sectors today, determines which technology to deploy. Because traditional ball screw technologies generally have less risk and lower costs, they retain their status as important players in the servo-driven linear motion arena.

Simplify, Simplify, Improve

SOMETIMES, there are innovations with benefits that can be applied universally to any technology. That seems to be the case with Monorail AMS from Schneeberger, a manufacturer of linear guide technology. Monorail AMS is an integrated rail system that includes a magneto-resistant feedback encoder directly in the linear rail. “We’ve been focusing on the integration of technologies,” states Glenn Bythrow, U.S. systems sales manager for Schneeberger. “We’ve integrated encoder technology directly into the bearing, thereby simplifying the assembly and increasing the overall MTFB.”

The rail system is industrially hardened, and reportedly can withstand the rigors of 24/7 operation. “Also, it clearly offers a lower cost of ownership to the end customer by eliminating the need to purchase and install a separate secondary encoder,” adds Bythrow.

This type of innovation, though not directly part of the ball screw/ball nut assembly itself, aims to raise the value of whatever type of linear motion is being used, whether linear motor, ball screw, or fluid-powered. By reducing the implementation costs for all technologies equally, it become even apparent that the lower the original system cost, then the greater the proportional cost advantage provided.

Bythrow adds, “This newer, integrated technology is based on tried and true older technology. It therefore provides no additional risk to the customer, so there’s no reason not to use it.”