Shafts and rails keep linear motion tasks on track.
Much time and attention is focused on choosing the best bearings for linear motion tasks. But what about the components that guide the application and keep everything in line? Shafting and rails are often an afterthought or — worse yet — assumed to be all the same. Nothing could be further from the truth. Following are some tips, tools, and state-of-the-art linear motion products to keep your designs on the straight and narrow.
Specifying linear motion components
Tips from Rollon Corp., Sparta, N.J.
Understand the application
When applying linear motion components, many considerations must be taken into account. Guide rails and guideways are produced globally in differing forms to suit very specific engineering tasks. The most common forms of guides and their relative bearings are profiled rails with recirculating ball bearing blocks, guideways for roller bearings, and round rails with recirculating ball bushings as well as plane bushings. Choosing the best combination for the job is where things can get tricky.
Initially, each project should be broken down into its individual parts. If the project requires the use of an X, Y, and Z axis to meet the application's needs, then each axis must be considered separately. Each axis should then be further reduced to its core elements because the direction of motion for each axis will have its own set of unique parameters. These parameters are used to define the linear motion solution that is best for the application.
As the linear motion guidance world is generally made up of the previously mentioned engineered rail systems, the first step in the selection process is to understand the end user's requirements. For example, what level of repeatability, tolerance, or accuracy is required to do the job? Are there environmental factors that must be considered, such as dust, water, or fibers? As a designer gains understanding of where the motion must exist and how closely it must hit the target, linear guidance choices can be narrowed down:
For example, square (profiled) rails are a suitable choice when the application requires both rigidity and precision, such as in machine-tool heads or circuit board processing. That said, the dirtier the environment, the more opportunity there is for contaminants to wreak havoc on recirculating ball paths of profiled square rail blocks and round rail ball bushings, so these are not good choices for dirty applications.
Roller systems are suited for larger automation tasks, such as lifting, pick-and-place, and parts transfer applications; because these systems generally use larger rolling elements, contamination is manageable.
Finally, plane bearings are useful for applications where lubrication cannot be used, such as lab environments.
Know the terminology
To properly size a linear guidance system, designers must keep the following parameters in order:
The stroke of the application is the length of complete overall movement in one direction along a linear path.
The load of the application involves both static and dynamic factors. The static load includes the weight of the saddle, nest fixture, payload, and bearings. Thus, if 40 lb (as shown in Table 1) is centered horizontally fore/aft and left to right in a typical dual-rail and four-carriage set, each bearing block would be statically loaded at 10 lb.
The dynamic (or kinetic) loading must take into account the applied loads as they interact with the bearing-laden saddle. Normally, this load places a torsional requirement on the bearings. It is best to have the load parameter organized as the center of gravity (C.G.) or center of mass (as shown in Table 2), which is very important when sizing bearings.
The saddle's C.G. provides one single load value at a distance from the bearing centers. As shown in Table 2, if the center of an individual load is specified at a relative distance from the guideway system or bearing centers, then the total mass has a C.G. distance to the guide rails of 1.5 in. (60 in.-lb/40 lb). Here, the bearings must manage a torque load of 60 in.-lb, especially when the saddle is accelerated or decelerated quickly.
These dynamic values along with static loading values can then be organized as radial (corad), axial (coax), torque about X axis (Mx), torque about Y axis (My), and torque about Z axis (Mz). These variables for each individual bearing carriage can then be used in almost any bearing sizing application to choose the right carriage size. Values are usually supplied as lb or Newtons (N) for static loading, and in lb or Newton meters (Nm) for dynamic loading.
The speed of the application must not only involve the top rate of movement, but also the acceleration and deceleration required to achieve the overall timing for a movement. Speed is typically provided as inches per second (ips) or meters per second (mps). The duty cycle parameter must account for full saddle motion through a complete cycle (usually two times the stroke) plus idle operations in a desired amount of time. Typically, this parameter is organized as the number of cycles required per minute.
The mounting area for the guide rail and saddle bearings is important for determining the overall length (O.A.L.) and rail separation of the guidance system. A bit of advice: Specify the widest possible operating footprint for bearings, because wider stances improve bearing loading characteristics. Unless the application calls for telescopic linear bearings, which act in a manner similar to simple drawer slides, the O.A.L. of the guide rail must include the linear movement stroke as well as the bearing footprint.
The bearing footprint is the dimension from the front of one carriage to the rear of the furthest carriage along one linear guideway. The mounting area also needs to take into account the substrate or framing system for holding the guideway. Many profiled shafts must be mounted to machined and ground surfaces to meet the application's requirements for precision; other designs are more innovative and can be applied directly to structural aluminum or tubular framing without causing a loss of capacity or rigidity.
The mounting orientation of the guideways is important for setting up the loading parameter, as the saddle could be moving horizontally, vertically, wall-mounted, or even inverted. For optimum performance, it is best to manage the application loading with the strongest part of the bearing system. To illustrate, a radial ball-bearing slider should be oriented to carry the load radially, not axially.
For an example of choosing a linear guidance system based on these parameters, read the complete whitepaper, Specifying Linear Motion Products, in the news section of www.rolloncorp.com.
Shafts should not be an afterthought
Tips from PBC Linear, a Pacific Bearing Co., Roscoe, Ill.
Thinking of giving your shafting short shrift? Think again. Contrary to what many design engineers believe, not all linear shafting is the same, and knowing what to look for is key to choosing the best shaft for the job.
Packaging equipment, assembly lines, pick-and-place robots, automotive plants, paper processing, and every other application that requires linear movement also requires linear shafting. While many designers consider it a commodity or “part of the package,” shafting is just as important as the bearing itself. Not even all hardened steel shafting is created equal, and three critical factors differentiate the choices — dimensional tolerances, surface finish, and hardness, including depth of hardness.
Most importantly, shafting must be compatible with selected bearings and made of high-quality material processed to exacting tolerances. Here's what else to request:
Dimensional data — Consistent dimensional tolerances along the entire length of a shaft are critical for consistent bearing operation. Any variance is transferred through the bearing, producing inconsistent results in the entire piece of equipment or assembly. Look for a shaft that supplies uniformity of load distribution through the bearing, high system accuracy, and long bearing life. Ask the supplier for data and testing to back up product claims.
The proper shaft extends rolling-element bearing life expectancy due to greater normalization of load distribution; plane bearings will also have a lower coefficient of friction and decreased binding occurrences due to a more consistent contact interface with the shaft. Roundness is another important factor. It's particularly important that shafting maintain roundness at any given cross section along the shaft. Not only does this result in true roundness, but it also builds high accuracy (for cylindricity and straightness) into processes.
Surface finish data — Surface finish is a key factor in bearing performance, affecting areas such as cleanliness, load, life, and friction coefficient. Look for shafting that is ground and polished within a consistent surface-finish range, as well-polished shafting boosts predictable performance: Loading is more evenly distributed because the balls or bearing surfaces have increased contact area with the shaft. In addition, a polished finish generates less wear particulate at the bearing-shaft interface during operation. Less particulate generation makes shafting better suited to cleanrooms and other hygienic applications. The polishing process also removes microscopic peaks from the shaft surface, resulting in a consistently smooth surface reducing drag (friction) on the bearing components.
Hardness data — Steel linear shafting is surface hardened to increase its resistance to deflection and damage due to point loading or line contact from rolling element bearings. Hardening also increases the material wear capabilities, thus extending the life of any bearing-shaft assembly by holding acceptable operational tolerances for a longer period of time.
Two factors come into play when analyzing hardness: the hardness measurement itself and depth of hardness. As the surface of the shafting becomes harder, performance in all areas is increased. But to maximize the effects of hardness, it must evenly penetrate into the material. Sudden drop-offs at shallow depths can cause the surface to become brittle and result in premature wear. Drop-offs in hardness deeper into the material diminish hardening's benefits, with the shaft often not performing to specifications.
For more information, visit www.pbclinear.com.
Optimum Guideway System
Shafting urban legends
Al Ng of Thomson Industries Inc. hopes to dispel some myths involving linear shafts.
Myth: All steel shafts are the same.
Fact: The most commonly used material for linear motion bearing shafts is moderately high carbon steel. Engineers should work with suppliers to verify that the carbon content of the steel used plus straightness, roundness, surface finish, hardness, and case depth are adequate for the bearing application. Contaminants in the steel can result in premature failures due to the potentially high Hertzian contact stresses of bearing applications. Chemistry and lack of uniformity in the material can degrade shafting machineability, particularly the ability to minimize asperities or high spots. (It is much better for the surface to generally consist of plateaus with some valleys, rather than peaks and valleys.) Hardness and case depth also must be adequate to support the Hertzian stresses under high bearing loads to ensure that no premature sub-surface failure occurs.
Myth: When a shaft grooves, it is failed.
Fact: Counter to intuitive belief, shaft grooving is not necessarily bad. Linear bearings running at high loads sometimes score the shaft after the first few passes, a phenomenon called “shakedown” where Hertzian contact stresses are quite intense — high enough to yield even hardened high carbon bearing steel. Because this is a compressive stress condition, the yielded material does not move or shift to cause a subsurface dislocation sufficient for material failure or fracture. Instead, it stabilizes so that actual contact stress goes below the equivalent stress due to the increased contact area from the grooving. The load is supported without further material yield. If this phenomenon occurs, wherein the shaft initially grooves and then stabilizes, the engineer should not rotate or replace the shaft, as the bearing would then cause another shakedown cycle — and possibly exceed the limits of the balls if not sufficiently hard.
For more information, visit www.thomsonlinear.com.