The best approach is to define the most critical parameters of the application first, narrow the choices based on these requirements, and then apply specific, critical variables to the given linear guide system.
When the system requires motion in the three basic X-, Y-, and Z-axes, consider each axis separately.
Linear guide-rails, guide-ways, and slides are mechanical systems composed of rails and bearings that support and move physical loads along a linear path with a low coefficient of friction. They are usually classified as rolling element or plane bushing types. Because many shapes and sizes are available from various manufacturers designed to satisfy specific engineering needs, your unique application determines the list of critical parameters that you should consider as well as their order of importance.
The most common types of guide-ways and bearings include profiled (square) rails with recirculating ball bearing blocks, guide-ways for roller bearings, and round rails with recirculating ball bushings or plane bushings. Profiled rails suit applications that require exceptional rigidity and precision, such as in machine tooling heads and precision circuit board movements. Roller bearing systems are intended for a wider range of applications such as lifting and transferring parts, or pick-and-place applications.
To select which of the rails work best for an application, first, analyze the system’s specific needs. Next, understand the customer’s requirement or program guidelines, which include the number of axes, repeatability, tolerance, and accuracy required to achieve the end result. Finally, consider environmental contamination, such as dust, water, fibers, and other substances.
For any system, the operating environment determines the type of bearings that need to be selected. For instance, dirty environments can contaminate the assembly and interfere with the proper functioning of re-circulating ball paths. The contamination is more manageable in roller systems because the rolling elements are generally larger. Plane bearings suit applications where surface-contact lubrication is not recommended or cannot be exposed to the environment such as in certain research laboratories or silicon chip manufacturing facilities.
After selecting a system, assemble the parameters to properly size it. For each movement in a linear guide-way system, consider the following parameters: stroke, load, speed, duty cycle, mounting area, and mounting orientation.
Size the system
The static load consists of the weight of the saddle, nest fixture, payload, and bearings. If 40.0 lb is centered horizontally fore/aft and left to right in a typical dual rail and four carriage set, each of the bearing blocks would be statically loaded at 10.0 lb.
Slides come in two basic types: saddle and cantilever. The standard, horizontally based saddle slide uses a saddle or block that moves between two fixed end blocks. With the cantilever slide, the main body and cylinder remain static, while the tool plate extends and retracts. A second cantilevered application exists when moving loads vertically. With one rail and two carriages, both the bearing carriages can be loaded equally in a radial direction. For sizing the bearing or carriage, the total load for the most statically stressed slider is typically set as the worst-case scenario.
When sizing the bearings, organize the load parameter and its distance to the center of gravity (C.G.) or center of mass. Load refers to the weight or force applied to the system, which includes both the static and dynamic loading. The static load comprises the weight of the saddle, nest fixture, payload, and bearings. The dynamic (or kinetic) loading must account for the applied loads as they interact with the bearing-laden saddle. Normally this load would place a torsional requirement on the bearings. The C.G. for the saddle provides a single load value at some distance from the bearing centers.
These dynamic values as well as 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). The variables can then be used in most any bearing sizing application to select to the appropriate size of carriage. Loading values are normally presented in lb or Newtons (N) statically and in.-lb or Newton Meters (Nm) for dynamic loading.
The center of the individual loads are a relative distance to the center of the guide-way system or bearing centers, the total mass has a C.G. distance to the guide-rails of 1.5 in. (60 in.-lb/40 lb). The bearings would have to manage a torque load of 60 in.-lb, especially as the saddle is accelerated or decelerated quickly.
Speed—Speed is critical to consider because applied loads affect the system differently during acceleration and deceleration versus motion at a constant rate. Speed is typically provided as in./s or the metric equivalent in m/s. Factors such as the type of move profile, determines the acceleration needed to reach the desired speed or cycle time. The load accelerates quickly in a trapezoidal move profile, and then moves at a constant speed before slowing. A triangular move profile, however, accelerates and decelerates quickly. What’s more, when calculating the speed of the application, consider the top rate of movement, as well as the acceleration and deceleration needed to achieve the overall timing for a movement.
Duty cycle—The duty cycle parameter must consider the full motion of the saddle through a complete cycle, which is most often two times the stroke plus idle operations in a desired amount of time. The stroke of the application is the length of complete overall movement in one direction along a linear path. Typically, the duty cycle parameter is organized as the number of cycles required per minute.
Mounting area—The mounting area for the guide-rail and saddle bearings helps to determine the overall length (O.A.L.) and rail separation of the guidance system. In most applications, it is best to consider the widest possible footprint for bearings to operate. Unless you use telescopic linear bearings, which act in a manner similar to simple drawer slides, the O.A.L. of the guide-rail has to include the stroke of the linear movement as well as the bearing footprint.
In addition to the standard horizontally balanced saddle, two other forms of common loading for linear applications include cantilevered conditions. Here, P1 = P2= (a/b)F + F. The total load for the most statically stressed slider is set as the worst-case scenario for sizing the bearing or carriage.
The other cantilevered application typically occurs when moving loads vertically. With one rail and two carriages, both the bearing carriages can be loaded equally in a radial direction because P1 = P2 = F(a/b).
The mounting area also needs to take into account the substrate or framing system for holding the guide-way. The bearing footprint is the distance from the front of one carriage to the rear of the farthest carriage along one linear guide-way. Many profiled shafts have to be mounted to completely machined and ground surfaces to properly meet the program requirements for precision. Other designs can be applied directly to structural aluminum or tubular framing without losing capacity or rigidity.
In this application, the direction running along the guide-way and the stroke is 118 in. — the exact length required; however, sometimes designers allow an inch or two more at each end of the stroke for limit switches, shock absorbers, or sensors.
Orientation—The mounting orientation of the ways is critical for setting up the loading parameter because the saddle could be moving horizontally, vertically, along a wall-mount, or even in an inverted position. For best performance, manage the loading of the application with the strongest part of the bearing system. For example, the radial ball-bearing slider should be oriented to carry the load radially, not axially.
Now make a selection
This is an example of an application containing a standard light dust-contaminated environment that requires medium repeatability. Because of these two factors, a preloaded roller-based bearing system running on hardened steel raceways is selected. The speed is quick and longer life can be achieved without having to push the maximum capacity levels.
Generally, for a 1-in. guide-way, plane bearings should not exceed 20-in./s, re-circulating ball systems 80 in., and rollers about 200 in./s. To achieve the full 118 in. stroke in 3 s, we will accelerate and decelerate 6 in. in 0.5 s each. This would allow 106 in. of stroke and 2 s to reach the target timing. Each of the guide-ways must be at least 162 in. long, because the stroke is 118 in. and the saddle length is 44 in. in the dimension running along the guide-way. Sometimes it is useful to allow an inch or two extra at each end of the stroke for limit switches, shock absorbers, or sensors.
Each of the bearings will be equally loaded at 100 lb., because the bearings are mounted at each corner of the saddle, and the center-of-gravity of the mass is centered fore-aft and left to right. Each of the bearing carriages can handle 500 lb of maximum radial loading, so an adequate life is calculated here because the bearings are loaded within the 20 to 50% range of total capacity. DW
Filed Under: Linear motion • slides, Mechanical, Motion control • motor controls