by Tony Kliber, Lead Design Engineer, Nexen Group
Reviewing the basics about the most popular linear positioning devices will help users properly specify them in their applications.
Linear drive systems have been used for thousands of years, as far back as the ancient Egyptians who used tree trunks to move heavy blocks of stones, providing the precursor to linear motion solutions such as ballscrews and roller pinion systems. Today, linear motion technology is used to support, locate, guide and move machinery components in a range of applications in the aerospace, machine tool, medical, factory automation and packaging industries, among others. While the basic requirements of linear motion technology are to provide load-bearing capability and accurate motion, other requirements are also important in many cases.
The linear drive technology used in a product plays an important role in providing the functionality, performance, durability, energy consumption and other attributes that enable the product to outperform competition. Engineers can choose from a range of linear drive technology choices such as leadscrews, ballscrews, rack and pinion systems, belt drives, linear motors and roller pinion systems. It’s critical to carefully consider all of your options to avoid common problems such as low accuracy, backlash/vibrations, high cost, dirty operations, high maintenance, low load capacity, excessive noise, low speed and so on. A systematic selection process can ensure that the linear drive technology is selected to match the requirements of the application. Selecting the right technology can also reduce design complexity, improve performance and reduce the overall cost of assembly.
Leadscrews use the helix angle of a thread to convert turning motion into linear motion. The large area of sliding contact between the male and female members of the thread generate relatively high frictional losses. The result is that leadscrews are less efficient and not as accurate as ballscrews. Leadscrews provide relatively high load-carrying capacity, although typically less than ballscrews. A key advantage of leadscrews is low cost—they are typically only one-tenth the cost of equivalent ballscrews. Leadscrews are available in a range of lead sizes, meaning that the lead can be selected to provide the right balance of positioning accuracy and speed. Leadscrews generally produce minimal noise as long as sufficient lubrication is maintained. However, they can be difficult to use in long-distance moves as the screw is typically unsupported between the two ends.
The ballscrew drive consists of a ball screw and ball nut with recirculating bearings that roll in the grooves formed by the screw and nut. The ballscrew distributes the load over a large number of rolling elements, resulting in a high load carrying capability. The use of a large number of precision rolling elements also provides high levels of accuracy. In addition, ballscrews provide low levels of friction. This translates into high mechanical efficiency and reduced power requirements. The duty cycle of ballscrews is quite high because they generate relatively low levels of frictional heat. However, compared to other linear drive technologies, ballscrews tend to be expensive and they require that lubrication be maintained to provide reasonable life. Finally, ballscrews are prone to noise, caused primarily by ball recirculation.
Belt drive systems
Belt-drive linear motion systems with ball guides are typically selected when high speeds, high rates of acceleration and long stroke lengths are the most important criteria. Belt-driven, slide guided linear actuators offer somewhat lower speed and acceleration capabilities at a corresponding lower cost. Linear positioning accuracy of belt drives, as might be expected, is not as great as either leadscrews or ballscrews. The load capacity of both ball-guided and slide-guided belt drives is somewhat lower than ballscrews. Belt stretch and stiffness are other drawbacks. Belt-driven linear motion systems are generally capable of high duty cycles because they avoid concerns about frictional heat buildup in the bearing of a leadscrew or ballscrew. Additional advantages of belt-driven systems include the fact that they generate relatively little noise and that they require relatively little maintenance.
Chain drives convert rotary motion to linear motion through a series of chain links that mesh with a toothed sprocket and with a linear slide. Because they are metal, chain drives hold an advantage over belt drives. They also take up less space than belt drives and are not prone to damage by oil, grease, sunlight or age. Unlike belt drives, chain drives are also capable of operating in wet conditions. On the other hand, chain drives typically generate more noise and have a greater tendency to vibrate than belt drives and also provide a lower load capacity and service life than gear drives.
Linear motors are based on the concept of unwrapping a conventional rotary servomotor with the stator becoming a forcer and the rotor transforming into a coil or magnet rail. Linear motors make it possible to achieve direct linear motion without any rotary to linear transmission devices. Brushed linear motors use coils in the linear rail and magnets in the forcer. Brushes in the forcer contact a bar running the length of the motor to provide commutation. Both the windings and forcer are contained within the forcer of a linear step motor. The advantages of linear motors include high speeds, high levels of accuracy and fast response. The limitations of this approach include high cost, large package size and generation of considerable amounts of heat. Linear motors also have a low capacity, similar to the comparison of direct-drive motors versus motors with a gearbox—the direct-drive motor must be significantly larger to get the same capacity.
Rack and pinion sets
Rack and pinion gear sets consist of a circular gear called a pinion that engages the teeth of a linear gear called a rack to convert rotational motion to linear motion. Rack and pinion gears are commonly used as linear actuators in a wide range of machinery. For example, they are used to move the axes of CNC machine tools such as machining centers. Helical racks offer quieter running at high speeds and a higher load carrying capacity due to the higher tooth contact ratio. Lubrication is important to ensure long life for rack and pinion sets. Because rack and pinion sets have relatively few components, they help save time during installation and increase reliability. They also offer high levels of accuracy even over long travel lengths. The disadvantages of rack and pinion sets include their relative high levels of friction, which reduces their efficiency, increases power consumption and limits their life. Plus, rack and pinion drives need to run with clearance so backlash can be a major disadvantage.
Roller pinion systems
At first glance, roller pinion systems look similar to rack and pinion sets, but instead of spur gear teeth, bearing supported rollers engage the rack teeth. The rollers engage a tooth profile designed to match the pinion’s path, providing friction-free meshing that allows the pinion to be pre-loaded into the rack, eliminating mechanical clearance. The rollers approach the tooth face on a tangent path and then smoothly roll down the face. Each tooth is precisely measured relative to the first, eradicating cumulative error and maintaining high positional accuracy. The resulting smooth rolling friction provides 99% efficient rotary to linear motion conversion. Due to the smooth way the rollers engage the rack teeth, the new approach generates minimal noise and vibration. The system is whisper quiet at low speeds and less than 75 dB at full speed.
Positioning accuracy of roller pinion systems ranges from 30 to 80 µm. For instance, a given size product in a premium model may deliver positional accuracy of ± 30 µm, a life expectancy of 30 million cycles/tooth and a maximum dynamic load of 14,000 N. For medium loads, a universal model can deliver accuracy of ± 50 µm for 5 million cycles/tooth and carry a maximum dynamic load of 750 N. The RPS system is capable of speeds up to 11 m/sec (36.1 ft) making it the second fastest mechanical linear drive system second only to linear motors. The roller pinion system requires little maintenance. The pinion consists of 10 or 12 needle-bearing supported rollers that are sealed and lubricated for life. The rack is lubricated with high performance light grease at installation, then again every six months or 2 mil pinion revolutions. In special applications the roller pinion system can be run lubrication free as long as the speed is less than 30 m/min.
Roller pinion systems are offered with both metal and plastic racks. Thermoplastic racks offer an alternative that delivers high corrosion resistance, high durability, low maintenance requirements, medium accuracy, and load-carrying capacity at a relatively low cost point. Thermoplastic racks are impervious to corrosion. They are made of a self-lubricating polymer so they can withstand dirty environments and outdoor operation without concern over failure due to the loss of lubricant. Their ability to run at full speed without lubrication reduces maintenance requirements and means that these systems can easily withstand washdown, outdoors operations and operation in coastal climates with salty air.
All of these linear drive systems offer their own unique mix of advantages and disadvantages. To apply the correct type of linear motion technology in a particular application, the design engineer should carefully consider the specific capabilities of each alternative. Selecting the right technology can improve performance, ensure long life and reduce the overall cost of the assembly.