Linear-motion devices abound in automated assembly equipment such as robots, conveyors, and pick-and-place systems. Applications include handling, positioning, joining, stamping, and sorting parts, as well as coordinating motion between two or more axes.
As manufacturers continue to demand higher speeds, faster throughput, and greater precision, suppliers have responded with linear-motion systems that feature high acceleration and deceleration rates, rapid traverses, good accuracy, and the ruggedness to handle high loads with little downtime.
That means engineers have numerous options when it comes to linear-motion devices, including electric linear motors, electromechanical ball-screw or belt drives, and hydraulic cylinders. But designers of automated assembly machines increasingly turn to pneumatic linear guides because they provide a combination of features that enhance overall machine performance.
Pneumatic systems require a source of pressurized air. By using a piston and rod assembly to create linear motion, air cylinders accurately position light loads and produce inherently smooth movements at fast speeds.
In contrast, hydraulic systems use a piston and rod assembly that converts pressurized flow of hydraulic fluid into linear motion. Such systems are best suited for heavy loads at high speeds.
According to Pneumatic Product Specialist Gary Cooper from Parker Hannifin Corp., Wadsworth, Ohio, many benefits arise from pneumatic automation systems.
"To begin with, the cost of pneumatics is considerably less than those involving hydraulics or electromechanics," Cooper says. "Most plants have air lines that are compatible with pneumatic systems, while hydraulic and electromechanical systems are more expensive to install.
"Pneumatic technology is currently making a big push toward developing solutions for smaller, faster, and more precise applications," Cooper continues. "These applications would mainly include clean-room environments dealing with the semiconductor industry and computer-chip handling."
Pneumatic systems tend to offer low maintenance, long service life, and reliability. In addition, they withstand dust, shock, vibration, and extreme temperatures. Pneumatic circuits are relatively easy to build, and standard modular components produce complex motions.
According to Tom Kane, product manager at Bimba Manufacturing Co., Monee, Ill., pneumatic products such as cylinders and linear guides offer quick, clean, reliable, and inexpensive linear motion for situations that require sterile surroundings.
"Pneumatic systems are a lot cleaner than hydraulic systems," he states. "They are environmentally friendly because if something leaks, it's leaking air instead of oil."
Linear guides differ from conventional pneumatic cylinders in that they provide precise, low-friction, linear motion through an assortment of rails, bearings, contact elements, and mounting configurations. Many consist of tubular channels or round rails that serve as load-bearing axes. While many designs and sizes are available, common linear-guide configurations have four bearings and two rails. Contact type, rolling elements, inner race geometry, and self-aligning capabilities all affect guide performance.
Originally, linear guides possessed pillow blocks with bearings that moved along round rails. Until recently, few choices existed for linear guides, and design procedures involved checking load requirements and choosing sizes that could carry those loads. As load and corresponding guide size increased, so did cost and system weight.
Recent developments in linear guides have given engineers the chance to cut expenses and improve system performance. Here's a look at some newer innovations.
Self-aligning bearings: One notable development in linear guides is self-aligning bearings. These bearings have separate internal tracks of circulating balls that move independently of one another while each track moves within the housing. This lets the entire bearing compensate for misalignments in pitch, roll, and yaw. Loads are distributed equally to all balls to ensure smooth travel.
Profile-rail linear guides: Profile-rail linear guides offer a solution to load handling. Flat on the top and bottom with concave-shaped sides that have ballconforming races where the bearings roll, profiled rails stiffen the rail and the entire guide, while at the same time increasing load capacity. Increasing load-carrying capacities of the guides allows smaller linear guides to carry the same loads as earlier versions, thereby reducing overall guide heights.
"While cylinders are great for power and motion, they aren't typically good at carrying heavy loads," says Walt Hessler, vice president of sales and marketing at PHD Inc., Fort Wayne, Ind. "The advantage of a linear guide is that you get the best of two worlds: the cylinder provides power and motion while the bearings carry the loads."
Because profile-rail linear guides have higher rigidity than round-rail guides, they must mount to dimensionally accurate surfaces to function properly. When less-accurate treatments use high-precision guides, installation expenses rise because surfaces generally require grinding operations before mounting the guide.
Many nonmachinetool applications use compliance instead of rigidity. Compliant linear guides are less sensitive to imperfections on mounting surfaces. As a result, they are easier and less expensive to install than profile-rail guides.
Self-lubricating guides: Self-lubricating guides eliminate the need for oil. Today, polymers are used as bearings in carriages and as coatings bonded to linear rails and slides, ensuring constant lubrication with fewer parts.
Self-lubricating systems come in two basic versions. One uses plain bearings or carriages that ride on sliding contact bearings made from special polymers. The second version has no bearings because mating parts are coated with a polymer that serves as both a bearing surface and lubricant. Self-lubricating guides are suited for carrying low loads in situations where external lubrication is undesirable, such as in the automotive industry.
Pneumatic linear guides are available from a wide variety of sources, with new designs often customized to perform specialized tasks. Here is just a sampling of the latest designs.
Linear thrusters from Bimba Manufacturing Co. are said to ensure smooth, guided motion. A durable housing with guide shafts keeps thrusters from rotating, allowing them to carry higher loads.
"The thruster's source of power is its original stainless-steel body cylinder," Tom Kane, product manager of Bimba, explains. "The thruster is a package solution to traditional guiding applications because it is essentially a cylinder contained in housing that provides motion and guidance. It incorporates force to move loads and guidance to carry those loads."
The thrusters come in two different models: ball-bearing T Series and composite-bearing TE Series. The former series offers case-hardened steel shafts and ball bearings that present less friction and higher precision, while the composite-bearing TE series offers high load capacities. A composite bearing/303 stainlesssteel shaft combination makes it suitable for corrosive environments.
"The thruster's breadth of offering is unique because it comes in seven bore sizes," Kane continues. "The largest bore size can handle loads as heavy as 1,000 lb."
Bore sizes range from 9⁄16 to 3 in. Shock absorber options in the 9⁄16 to 2 in. bore sizes are available in light, standard, and heavy-duty models for absorbing high kinetic energies.
"The thrusters offer three distinct stopping positions in one selfcontained unit," Kane adds.
The thrusters can link to other thrusters or rodless cylinders for multiaxis movements, and rotary actuators can provide rotary motion.
Pneumatic guiding cylinders from Rexroth Bosch Group, Racine, Wis., are suitable for applications that require precise guidance in spite of high loads, including lateral forces. Main features include high torsional strength and absorption of high torques and lateral forces. Large guide-rod diameters and four plain-bearing bushes ensure stability.
The cylinder is offered in eight different diameters between 12 and 63 mm with stroke lengths to 200 mm. Depending on user requirements, two different versions are available. One contains a balltype push guide characterized by smooth operation while the other has a plain bearing guide that withstands high loads. Attachment threads, through holes, and longitudinal slots for sliding blocks provide easy mounting.
XLT guided cylinders from Parker Hannifin Corp. are designed for light loads. They provide precise, torque-resistant linear motion. Alignment couplers let piston rods self-center, extending cylinder life.
"A regular cylinder cannot withstand significant amounts of side or torsional loading," Cooper explains. "However, because of its linear support shafts and bearings, a guided cylinder can dissipate large amounts of energy from side or torsional loads away from the piston rod. This allows the piston rod to stroke more freely which, in turn, allows the cylinder to last longer."
The housing is manufactured from anodized extruded aluminum that incorporates T-slots for mounting flexibility. Four prelubricated recirculating ball bushings and two precision-ground support shafts support the main body. Outboard wiper seals protect the bearings from contamination while retaining lubrication and ensuring long life with less maintenance. Prelubricated stainless-steel air cylinders with stainless-steel piston rods provide thrust, while support shafts and bearings offer positive load support for repeated non-lube, trouble-free cycles.
Available options include reed, Hall-effect and proximity sensors, bumpers, adjustable stop collars, dowel pin holes, flow controls, fluorocarbon seals, and three-position cylinders.
The HBT Series linear slide, also from Parker, is suited for guided linear-motion applications. Four linear bushings guide two parallel precision-ground, casehardened support shafts and incorporate them into an H-Body assembly. The bearings are prelubricated, and the company can supply composite bushings with oversized shafting or linear ball bushings. The series features a machined, anodized, one-piece aluminum body with tapped and counterbored through holes for mounting flexibility. Dowel pin holes provide precision mounting for body and tool plates.
Cylinders align with couplers and wear bands provide internal guiding. This lets piston rod assemblies self-center and "float" in the bores, thereby eliminating metal-to-metal contact. This ensures long cylinder life with reduced maintenance and downtime. These pneumatic actuators are suited for applications that include material handling, packaging, product testing, assembly, parts transfer, machine loading and unloading, and clamping.
Available options include proximity sensors, Hall effect and reed switches, oversized shafting with composite bushings, shock absorbers, cylinder cushions, external bumpers, adjustable stop collars, flow controls, fluorocarbon seals, three-position cylinders, cylinders with rod lock mechanisms, and slides without cylinders.
SHP Rail Bearing Slides from PHD Inc. are pneumatically powered, rail bearing slides that provide smooth, precise movement within confined spaces. Available in three bore sizes (8, 12, and 16 mm) with five choices of travel length, the series is available in both imperial and metric units for worldwide applications.
A mechanism for adjusting travel for extension and retraction is located on the back of the slide. Integrated shock pads provide end-of-stroke cushioning, while multiple mounting options provide maximum flexibility. The bodies feature mounting holes on the sides, end, and bottom while tool plates offer mounting on top and end. Standard dowel holes on the body, end, and tool plate mounting positions ensure precise mounting and attachment of tooling.
"While some customers still like our traditional slides with two round shafts or ball bushings, they've discovered that rail bearings are more precise and have less deflection," states Walt Hessler, vice president of sales and marketing. "Our rail bearings carry heavier loads, and customers are receptive to the access and load performance they provide."
All Series SHP Sides have an operating range of 20 psi minimum to 100 psi maximum. The slides incorporate internal orifices to limit velocities. They are designed for use in temperatures between 20 and 180F. Using urethane and nitrile seals that are compatible with standard paraffin-based lubrication oils used for pneumatic cylinders, all units are prelubricated at the factory for service under normal operating conditions. The slide housing and tool plate are anodized aluminum alloy, while the linear rail and bearings are made from hardened and ground stainless steel.
The DFM Drive/Guide series from Festo, Hauppauge, N.Y., is suitable for handling, assembly, and transfer applications. The drives' centerline pneumatic cylinders, dual linear bushings, and recirculating ball bearing guides perform a wide range of pushing, stopping, and lifting functions.
According to Allan Poxon of Festo, the guides offer practical alternatives to component assemblies because of their torsional rigidity and capacity to absorb high torque and lateral forces.
Two guide rods provide stability with the use of sliding or ball-bearing guides. Because of large guide rod diameters and up to four plain bearing bushes, plain bearing guides can handle high lateral loads. Ball bearing guides offer extra precision, load bearing capacity, and low friction.
Diameter ranges from 12 to 100 mm with stroke lengths from 10 to 200 mm. The guides provide a wide variety of mounting options as well as choice of air connections. They are self-lubricating and maintenance-free. Applications include stopping, lifting, pushing, feeding, pressing in, and withdrawal of parts.