Nearly all discrete manufacturing processes incorporate linear motion, and as manufacturers become familiar with the flexibility and simplicity of modular linear motion, these systemswhether one, two or complete three-axis Cartesian robotics systemsare finding their way into more and more areas of production.
Component selection among ball screws, linear guides, and actuators begins to make sense when some basic application considerations are applied.
How to Handle Stress
Machines known as laners organize packaging line products for the next downstream operation by rapidly dividing the line and diverting products left and right as needed, creating multiple lanes for case packing, shrink wrapping, or further processing.
Hartness International, Greenville, S.C., recently introduced a laner that pushes the limits of line speed, while protecting product packaging along the path (See Figure 1 below). The GlobalLaner 2260s ambitious design called for a linear belt drive module with an acceleration rate of 1 m/s2 to keep pace with the line speed of 200 ft/min. To give the laner the ability to relocate a section of bottles to any point in the grid, Hartness incorporated three linear modules on each machine. Two X-axis linear modules are connected to a common shaft and driven by one servo motor. The third linear module also is servo-driven and acts as the Y axis.
|FIGURE 1: KEEP PRODUCT TRAFFIC MOVING|
The GlobalLaner 2260 uses a linear belt drive module with an acceleration rate of 1 m/s2 to keep pace with the laners line speed of 200 ft/min.
The MKR 2080 module supplied by Bosch Rexroth features an AC servo motor and a pre-tensioned toothed belt drive for demanding speed and load requirements from orange juice jugs running at 550 bottles per minute. The modules are designed with anodized aluminum frames and carriages with low-maintenance, one-point lubrication. Each modules cover is constructed using a corrosion-resistant steel strip that performs to the DIN EN 10088 standard.
The GlobalLaner 2260 prototype included linear modules that couldnt handle the speed, says Coulomb. With the high acceleration and deceleration rates of 1 m/s2, the previous modules began losing position.
Bosch Rexroth has experience helping users make the right selection. A common mistake that engineers and designers make when sizing and selecting linear motion systems is to overlook critical application requirements in the final system, believes Danielle Collins, systems product manager, Bosch Rexroth Linear Motion and Assembly Technologies. This can lead to costly redesigns and re-works in the worst case, but also might result in an over-engineered system that is more costly and less effective than desired.
How much load will the system need to handle? How fast will it need to move? What is the most cost-effective design? We considered all of these questions when our group developed a guide to specify the appropriate linear motion components or modules in any given application, says Collins. LOSTPED is an acronym that stands for load, orientation, speed, travel, precision, environment and duty cycle. Each is a factor to be considered when sizing and selecting a linear motion system.
Rexroth engineers determined that each factor must be considered individually as well as in conjunction with the others to ensure the best overall system performance. For example, the load imposes different demands on the bearing system during acceleration and deceleration than during constant speed movements, says Collins. As more linear motion solutions move from individual components to complete linear module systems, the interactions between linear bearing guides and ball screw, belt, or linear motor drives become more complex. The system can help designers avoid mistakes by simply reminding them to consider all of the interrelated factors during system development and specification.
We wanted to upgrade our Classic SL-18 Micro shrink bundler, says Devandra Shenge, product development specialist at Omega Design Corp., Exton, Pa. The pneumatically-driven, PLC-controlled machine required frequent maintenance and was expensive to operate.
Speed was a critical factor in designing the new machine. Omega needed a machine that could process multiple quantities of varying sized and shaped personal care items such as shampoo and deodorant bottles, says Shenge. With the pneumatic system, products had a tendency to get turned in the wrong direction and jam up the machine, resulting in downtime and a loss of profits.
Omega ultimately used a new linear slide in a new machine design. As a result, the company has been able to reduce maintenance, the number of sensors needed, air leaks and labor costs.
Almost every piece of industrial machinery today is designed for long life with minimum maintenance. A big contributor to premature end of life is lubrication failure, says Tom Solon, applications and sales engineer at Kerk Motion Products, which supplied the rapid guide screw RGS10000 to Omega. Now many manufacturers have jumped on the PTFE bandwagon. When used with high performance polymers, PTFE coatings can make slides and lead screws truly maintenance free. The challenge is protecting the comparatively soft plastic parts from premature wear. Solon says Kerk has spent many years refining its Kerkote TFE coating for this task.
The new machine now can complete products at a rate of 60 in./sec, exceeding the needed rate of 50 in./sec, says Shenge. Before it was just a continuous 0-50 in./sec motion. Now, the customer can accelerate or decelerate the machine. This is critical, because products with unusual and heavier shapes cant be thrust through the machine at top speed, because both the machine and product could be damaged.
A manufacturer of personal care products saw the machine at PackExpo, says Shenge. We had quoted them with the original actuator, but they liked it more with the new guides.
Hone in on the Solution
Honing is the machining step that removes small amounts of material after boring out high-precision parts. The honing process often is considered a mystery because many people dont understand how it works, notes Jose Martin, senior mechanical engineer at Sunnen Products Co.. Sunnen builds industrial bore sizing and finishing machines (See Figure 2 below), catering to high profile U.S. customers, including U.S. automakers, Caterpillar, and Cummins. What started out as an internal project to improve machine productivity led to a significant change to ball screws and rails for the tool-stroking process in its new machines.
In a honing step, a fuel injector, for example, is presented to the cutting tool on a spindle. Either the part or the spindle reciprocates back and forth, so motion occurs relative to the part. As the part is stroked, the tool expands, removing small amounts of material. Because part geometries are becoming increasingly complex, bore non-uniformities along the axis of the bore need special motion control to generate the required precision. The part must be allowed to float within three axes.
|FIGURE 2: BALL SCREWS BUMP CAMS|
Sunnen replaced cam-based technology in which a cam profile was generated mechanically via four bar linkages in its vertical honing machines. The company found, contrary to original thinking, that ball screws provided the need improvements.
Initially ball screws were considered unsuitable for a honing application, with its short cycling motion and high stroke rate.
The key advantages provided by ball screws include higher speed and more rigidity. In this case, the ball screw drives the stroking mechanism of the honing axis and is connected directly to a servomotor. Ball screws are ideal because they can achieve multiple Gs of acceleration with very-high-linear-cycle speeds to meet the rapid reversal move profiles in the new Sunnen design.
The Bosch Rexroth ball screws we chose reduced part quantity by a factor of ten, said Martin. This, plus the increased control, increased accuracy, decreased vibration and increased velocity, was a huge benefit according to Russell Jacobsmeyer, Sunnen manager of product design and development, who says assembly time is reduced due to fewer components and field repairs are simplified with only the ball screw assembly, coupler, and servo system to troubleshoot. Machine capacity is much more flexible with a simple replacement of the ball screw or the motion profile, added Martin.
Ball screw actuators can have many distinct advantages over other technologies. Because of the compressibility of the balls and resultant rigidity on the system, as well as tight manufacturing tolerances, ball screws tend to be highly accurate and extremely repeatable, says Steven Buffamonte, product management specialist, Festo Corp. Also, as a result of the low rolling friction of the recirculating balls, the power out is nearly equivalent to the power in, translating to a drive mechanism that generally is more than 90% efficient. This means low energy consumption and low wear.
Ball screws also are capable of producing high thrust forces. To calculate the total linear force of the screw, all the torques required in an application, such as the torques to overcome friction and gravity, drive torque, and acceleration torque, need to be calculated. Here we see the significance of the lead to the thrust force, says Buffamonte. The lower the lead, the lower the inertia and torque required to move the object, and subsequently the greater the force generated by the screw. Therefore, while a high screw lead causes the ball nut to travel transversely farther per revolution, the smaller the lead of the screw, the higher the force that is generated.
Some Basics to Help Sort It Out
Here are a few advantages and disadvantages related to performance, duty cycle, speed, and load capacity for industrial and precision actuators.
Industrial actuators come in different varieties, and are intended for intermittent duty with lower accuracy requirements. They can be AC or DC motor controlled. Component reduction is of prime concern in many instances for reliability and cost reasons. Typically used for actuation of doors, louvers or other low-duty cycle applications, there is little to be concerned with beyond peak and continuous force. The screw-type of actuators is leadscrew or Acme screw and the rodless version usually is belt driven. High-force applications commonly use the screw and perhaps gear or belt-couple the motor for torque advantage, says Lee Stephens, systems engineer, Danaher Motion. Belt-driven systems are for high speed.
Precision actuators can have precision ground ballscrews, Acme screws, or belt drives. The advantage of using an actuator is that a great deal of the mechanics are taken care of in the package. Linear bearings, slides, motor mounting, and coupling difficulties are pre-packaged for users who can live with the existing standard lengths, adds Stephens. Custom lengths sometimes can be accommodated, but the general price advantages can be lost quickly. Users still have the advantage of a pre-packaged motor driven solution with a relatively easy mounting method.
Belt actuators use a synchronous toothed timing belt as the drive mechanism. The belt runs over geared pulleys mounted at each end of the actuator profile. One pulley acts as the driving gear, the second acts as the driven, or slave gear. A drive gear attached to a motor via a coupling converts the rotary torque of the motor to a tangential force. The diameter of the pulley directly influences the thrust, says Buffamonte. Additionally, because belt material is available in exceptionally long lengths, the limiting factor for travel would be the actuator profile itself. This differs greatly from a ball screw actuator, where the ball screw is limited in length by its diameter, and consequently, its critical speed. As a result of the belts low mass, a toothed belt actuator is capable of high linear speeds and accelerations, whereby the limiting factor for speed is usually the guiding mechanism, not the belts capability. Belt drive actuators usually employ either a recirculating ball bearing guide or a roller guide.
Use a belt drive when you want faster speeds, lower cost, long strokes, and medium precision seems to be sound advice. Use a ball screw for higher precision applications, smoother motion, and high thrust forces, Buffamonte says. Finally, think of lead screws typically for slow speed, low-duty cycle applications with medium precision. Theyre lower cost than ball screws and non-backdriveable, ideal for vertical applications.
Some linear versions have modular design and open architecture. This provides an opportunity to customize with interchangeable internal and external components such as drive screws, motors, front and rear attachments, controls, limit switches and others.
Among various guiding systems, profile rail guides serve where application requirements demand precise positioning, smooth operation, high load-carrying capacity, high stiffness, unlimited stroke, and low-noise operation, agrees Wayne Greer, engineering manager and project sales for SKF Linear Motion & Precision Technologies. A square configuration of raceways will result in a guidance system with good rigidity able to accommodate equal load capacity in all directions. Guides with pre-lubricated reservoirs will realize optimized lubrication of rails. Precision rail guides generally suit linear guidance and positioning applications requiring limited strokes, high stiffness, smooth operation, and positioning accuracy.