Productivity Forum: Linear actuators and slides

Jan. 1, 2002
We asked several people familiar with linear slides and actuators how to keep machine productivity up. Here they explain the most important things to consider to make sure those components are not the weak link

How do actuators increase productivity? Slides and screws indeed reduce friction, making movement easier. Where else can their designs increase — or decrease — performance? The ultimate goal is quick precision despite heavy loads.

Charles: Conventional actuators like hydraulics, pneumatics, and electromechanical systems like ball screws have numerous parts ... like external bearing systems [and] gearing ... these characteristics can cause friction, backlash, and binding.

Ron: Slides that use mechanical reduction devices limit throughput by reducing the effective peak speed and acceleration of the actuator. These mechanical reduction devices also induce error over time due to wear in their contacting elements. In very high resolution applications, static friction in mechanical bearings can limit settling times, and adversely affect velocity regulation performance.

Jim: Increased speed capacity allows for more work to be completed in the same amount of time. This allows machinery to exhibit higher throughput, which is beneficial to just about any operation ... As a result of the retained, circular ball paths in its dual raceway design, [our] system can be run at linear speeds in excess of 10 m/sec, more than three times the speed that competitive technologies (ball bushing or recirculating ball square rail) can achieve. Competitive recirculating ball linear guide designs ... have oval-shaped ball circuits, which include sharp 180 hairpin turns at each end of the circuit. These sharp turns are what limit the speed capacity of these alternative recirculating ball technologies.

Ron: Direct-drive offers the best performance in terms of peak speed and acceleration capabilities, and has the added advantage of consistent performance throughout the life of the actuator. Our strengths are based on our vertical integration ... For example, our ... linear motors deliver up to 60% more continuous force for the same volume versus competitive products ...

There is a tradeoff between winding complexity and efficiency. Most vendors use the simplest winding design, therefore opting for ease of manufacture over efficiency. [We use] a more complex winding design that maximizes efficiency. It all comes down to an issue of the “resolution” of the control of the current flow in the forcer; the higher the resolution of the flow of current in the winding, the higher the efficiency and force output of the motor.

Chris: The duty cycle and system design are the two most important characteristics to consider ... Screw jacks are most appropriate for low duty cycles, but high linear force requirements. As duty cycles increase, more service factor is required. Therefore, even though the same load needs to be moved, jack size will be determined by how often it has to be moved and the life required.

Gary: [Our] recirculating ball bushing bearings [are] engineered with HRC 65 hardened steel bearing plates, which compensates for deflection or misalignment of the raceway. These precision bearings have benefits of large load capacities ... the lowest rolling friction (dynamic coefficient as low as 0.001), smooth operation, and self-aligning capabilities.

Ron: Throughput on linear motor-based actuators is primarily limited by bearing technology. Improvements in the speed and acceleration capabilities of mechanical bearings will be required to fully utilize the potential of these systems.

Applications determine performance parameters, which in turn determine the most appropriate actuator.

Ron: Overall productivity in fiber automation requires the ability to make small, accurate steps for power optimization, coupled with high-speed motion to find first light. Semiconductor inspection applications require high acceleration capabilities to minimize the turnaround times on the edges of the wafer, coupled with superior straightness, flatness, and velocity regulation capabilities while over the wafer. Electronic manufacturing and testing require high acceleration and velocity to minimize pick-and-place and post-process inspection times.

Gary: Each linear guide or actuator will have an envelope where it is best suited. For instance, you may have a heavy payload that requires careful, precise positioning and high speed is not a critical factor. Another application may have a light payload, positioning accuracy is not critical, but speed is required ... The first would likely require a large bearing slide with a preloaded ballscrew as the driver. The latter would require a more moderate sized bearing slide with a belt drive.

Chris: Machine screw jacks have typically been less expensive, less accurate, and allow lower duty cycles and actuation speeds than ball screw jacks, which are more appropriate for high duty cycles and higher actuation speeds. Through more accurate screw machining and efficiency enhancements, the performance gap [is] no longer as wide. Therefore, machine screws can be considered for more applications.

So actuators must be physically capable of precise movement. To also move accurately, a system often must take commands from an operator. Or, the operator must be able to command the system ...

Richard: ... After all, a robot is only as powerful as its programming. Many extremely capable robots on the market today are actually limited in their use by their difficulty of programming and integration. Who wants to spend weeks learning an unfamiliar programming language? ... One notable advantage is the ability to assign meaningful names to each point and variable (such as “start point” and “wait point”).

Charles: ... as improved electronic packages become available (including some components we’re developing in-house), we will be able to use them to deliver even higher forces per volume and achieve easier programmability and control.

A big part of productivity is to get up and running — fast. So should a new system be easy to physically assemble? Or should it be easy to figure out? Here’s what our group had to say about assembly and integration.

Chris: To enhance the integration into machine designs, [we’ve developed] the widest range of connecting components, mounting options, and screw designs to give the design engineer maximum versatility in solving his applications problems.

Jim: Recirculating ball guide products require tighter tolerancing with respect to mounting surface preparation and their stringent alignment procedures often require multiple adjustments before they operate to expectations. The assembly technique involved with guide wheel-based designs is significantly less difficult. Adjustment of wheel-to-track fit up is as simple as rotating an eccentric bushing or stud, and re-adjustment after installation (i.e. to adjust the drag or to compensate for wear), is possible.

Larry: We have developed modular motor mounts so that we can adapt our actuators to NEMA, IEC, or non-standard motors more easily. Also, we have a design for our in-line cylinder style actuators which allows the load attachment point to be moved or modified without making major changes to the unit.

Ed: By making the slide an integral part of the equipment, we reduce tolerance buildup and assembly time. Our slides are among the simplest to install. Most require two fasteners ...

Richard: [With] turn-key positioning system solutions, the component selection process is all but eliminated.

Ed: Future improvements will be found in further integration of actuation methods, and customization of the slides into specific designs.

Durability, efficiency, and simplicity are three routes to continued machine health.

Al: As many of our dc units are used in outdoor applications, we find resistance to the elements and high pressure washdown to be a significant feature ... [Our] improvements to the cover tube, cover tube wiper, zinc housing, housing gasket and motor seal ... are internal to the actuator, and do not affect the physical envelope ... we added “guide” pins to the two housing to provide consistent alignment of the sealing surfaces of the components ... and switched to a [housing] gasket material with improved sealing characteristics in our application.

Ed: Our slides are among the smallest available, allowing less energy and space requirements ... The running characteristics are set at the factory. Preload, accuracy, and repeatability are inherent in the product. Richard: We’ve pioneered ... features such as: ballscrew and motor shaft integration, eliminating the need for a mechanical coupling — typically the “weakest link” ... mid-stroke support technology, allowing extremely long stroke lengths ... self-lubricating ballscrews, significantly reducing maintenance requirements ...[and] linear motion guide and actuator base integration, increasing rigidity and minimizing parts.

Gary: We use round rail and profile rail technology in conjunction with ball bearing bushings and fluoronyliner bushings. [A] proprietary polymer, [the] sliding contact bearing has benefits of a low sliding friction (maximum coefficient of 0.25 under load and speed), low wear rate characteristics, and a self-lubricating ability. These qualities are well-suited for applications operating in washdown or heavily- contaminated environments, such as the food processing or paper-pulp industry.

Charles: [Our] motor contains only one moving part, and is designed to provide direct drive, eliminating the need for the cumbersome accessory mechanisms mentioned ... Its smoother, more efficient generation of force offers a performance upgrade over traditional approaches. Our motors do not require messy fluids like hydraulic and pneumatic systems do.

About the Author

Elisabeth Eitel

Elisabeth Eitel was a Senior Editor at Machine Design magazine until 2014. She has a B.S. in Mechanical Engineering from Fenn College at Cleveland State University.

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