Choosing Linear Guides for Smooth Moves

May 22, 1998
Although many engineers choose the linear guides they are most familiar with, taking time to study new and different types can lead to a superior fit and a better product, often at a lower price.

Scott Schuler
Product Manager
Thomson Industries Inc.,
Port Washington, N.Y.

While the 90s has been the decade of record sales for sport-utility vehicles, it has also been the decade of record applications for profile-rail linear guides designed into machinery. Although the comparison may seem unusual, car buyers and machine designers have something in common. For instance, drive around any city and you’re sure to see a suit-wearing professional climbing into a spotless truck of monster proportions that will never leave pavement. Machine designers similarly buy linear guides that don’t perfectly fit their needs, such as profile-rail linear guides. Although these guides revolutionized the industry by increasing rigidity and load capacity, they’re not the only game in town.

More recent advancements in linear guides give engineers the opportunity to cut costs and improve system performance. Yet, take a look at an application that uses a linear guide and its likely to contain a traditional profiled rail, whether the system needs the rigidity or not. But engineers who choose guides aren’t entirely to blame. New technologies have improved old designs but they haven’t replaced them, leaving engineers with many more linear guide choices today than in the past.

Original linear guides consisted of a pillow block with bearings that moved along a round rail. Initially there were few choices for linear guides. The procedure included checking the load requirements of the guide and choosing a size that could handle the load. As load and corresponding guide size increased, cost and overall system weight increased substantially.

One solution to load handling came from profile-rail linear guides. Profiled rails are flat on the top and bottom, while the sides have a concave shape with ballconforming races where the bearings roll. This shape not only stiffens the rail and the entire guide, the contoured races increase load capacity due to a greater contact area between the races and balls.

Another development in bearing design is self-aligning bearings. These bearings have separate internal tracks of circulating balls. Each group of balls moves independently of the others and each track moves slightly within the housing. This lets the entire bearing compensate for slight misalignments in pitch, roll, and yaw and distributes loads equally to all balls, ensuring ultrasmooth travel.

Increasing the load-carrying capacity of linear guides means smaller guides can carry the same loads as earlier guides, which lowers the overall guide height. While low-profile guides are necessary when build envelopes are limited, sometimes they impose other design constraints. When using a drive mechanism like a ball screw, for example, some low-profile linear guides require shims to reach the height of the ball screw. Guide height is one of many factors to check before choosing a linear guide.

The key to choosing the correct linear guide is accurately defining the application. Common design parameters are load, accuracy, rigidity, smoothness of travel, and speed and acceleration. They also include envelope size, environment, installation requirements, and the cost of the product, its installation, and its replacement. Many applications have one or two parameters that narrow the range of options for the optimal linear guide.

When profiled rails were introduced they solved many application problems that required rigidity and high load-carrying capacity. Machine tools, for example, often use boxways to guide workpieces. High friction in boxways produces resistance as motion begins. When performing multiaxis operations, which require smooth direction changes to produce curved shapes, friction can show up in the machining. Profile-rail linear guides offer a solution to this problem because their low-friction motion produces smooth direction changes and their stiffness helps produce the accuracy required from multiaxis machine tools.

Profile-rail linear guides offer higher rigidity than round-rail guides. Because of their rigidity, they must mount to dimensionally accurate surfaces to operate properly. Applications that require high-precision guides often have surfaces machined accurately enough for mounting profiled rails. However, when less precise applications use high-precision guides, installation expenses increase because surfaces often require a grinding operation before mounting the guides.

Due to the motion-control problems that profiled rails solved, some designers inadvertently think they are the only choice for linear motion. But sometimes their rigidity is a liability. Most nonmachine tool applications instead require compliance. Compliant linear guides are not as sensitive to imperfections on mounting surfaces and are easier and less expensive to install than profile-rail guides. Packaging applications are an example of where compliant linear guides are preferred. In packaging equipment the driving design factors include smooth travel, low installation cost, and low overall cost. Profile-rail linear guides are often the wrong choice for these applications. Round rails are a better choice because their compliance lowers installation and overall costs.

Deciding between rigidity and compliance, however, isn’t the only choice. One example is semiconductor manufacturing, where smooth travel is paramount. In this process a tray mounted to a linear guide holds a batch of silicon wafers while a ball screw drives the tray. A sensor in the drive measures the torque required to drive the system. Any variations in the torque cause the ball screw to shut down, preventing crashes that can damage the wafers. This application requires a linear guide that carries a low load with extremely smooth travel. While many semiconductor manufacturers use a profile-rail guide, this is not the best choice. The best guide for this application is a small round-rail guide, which produces the smooth travel required to carry the wafers.

Durability is the most important linear guide quality for the automotive industry. Some manufacturers of machinery for automotive assembly must guarantee their equipment for up to five years. To make matters worse, linear guides on these machines are often difficult to access. Shutting down an automotive assembly line due to bearing failure on one of these linear guides is a tremendous expense. For this reason robust design and product life are first priority for automotive assembly equipment. Steel profile-rail linear guides are used on automotive assembly equipment. Although these guides are primarily intended for machine tool applications because of their rigidity, the automotive industry uses them because their steel construction makes them the most rugged.

One new technology useful to the automotive industry is self-lubricating linear guides. These have an oil-impregnated polymer plate mounted to each end of the carriage. The plate maintains an oil film between the balls and races. Many industries, such as automotive assembly, have extensive lubrication systems permanently connected to numerous linear guides. Other applications have guides with grease fittings for intermittent lubrication. Self-lubricating guides reduce system cost by eliminating the need for such lubrication systems. They are also useful in applications where oil-related environmental pollution is a concern.

Linear guides with self-lubricating plain bearings are good for carrying low loads in applications where lubricant is undesirable. Since these bearings are insensitive to washdowns, they are well suited for foodprocessing applications. They also are frequently used in medical applications because they run quietly. Because they contain sliding contact bearings, however, they require about 100 times more force to move than ball bearings. When carrying low loads, systems with plain polymer bearings can probably use the same motor and drive system as those using rolling-element bearings. But as loads increase, a guide with plain bearings will need a larger motor and drive system than a guide with ball bearings.

When a compact package is the driving design factor, dual-shaft linear guides are a common choice. These guides have a carriage that rides on a pair of parallel round rails, producing controlled motion from one simple unit. To maximize space savings, many dual-shaft guides can accommodate a drive such as a ball screw between the rails. Dual-shaft linear guides are preassembled and ready to bolt to the mounting surface without requiring adjustments for parallelism as with a pair of single-rail guides. They are considered a “complete axis solution” because they can stand alone and guide motion in one direction. Although dual-shaft linear guides often provide a space-saving solution, they are not always a wise choice. When guiding large overhung loads, for example, it may be advisable to use two single-rail guides and spread them apart.

When choosing linear guides, a common mistake is using the same type in a machine with several different linear-motion requirements. Medical imagers for example have patients lying on a scanning bed, which slides into a “tunnel” that holds detector heads. When the bed stops in the proper position, detector heads slide into place around the patient. Each sliding motion works independently of the other and has different requirements. The scanning bed only requires coarse adjustments. The detector heads, on the other hand, require finer adjustments. Some engineers might use the same high-precision linear guides for both the bed and detector heads. However, a better choice is using high-precision guides for the detector heads, where accuracy is important, and using lower cost, more compliant linear guides for the bed.

Design errors also crop up when choosing a system to carry heavy loads. Because two parallel single-rail guides spread apart is the proper approach to carry large loads, many engineers assume it’s acceptable to use more than two rails, or more than two carriages per rail, to carry even heavier loads. One reason they’re tempted to do this is to avoid the cost of a larger rail. While increasing rail size may be more expensive, using three or more rails or carriages produces a statically indeterminate system and can lead to rough operation. Unless the installation is perfect, excessive rails and carriages will inadvertently be slightly out of line. The expense of installing such a system accurately enough to produce smooth motion will likely exceed the cost of using a larger rail.

The earliest ball-bushing bearings provided low-friction motion by using recirculating balls at the interface between the bushing and the rail. These bearings, however, are sensitive to proper alignment and bind easily when the load misaligns. The next-generation bearings compensated for variations in pitch, making them self-aligning in this direction, therefore allowing smoother travel. The next improvement produced bearings that selfalign for pitch, roll, and yaw, further ensuring smooth travel. Each advancement produced a smoother-running product.

Smaller sizes resulted from the internal bearing construction. The first size reduction came from designing curved paths where the balls travel. This increased contact area and, therefore, load capacity. The next size reduction resulted from using smaller balls and increasing the number of ball tracks. This increased load capacity because more balls carry the load.

© 2010 Penton Media, Inc.

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