Just because it’s possible to get the latest, greatest features doesn’t mean those features are always applicable or necessary. If a feature doesn’t aid the design, then it just adds cost. Engineers must continually weigh new features and capabilities against the true needs of a design and their mindsets, habits, and design comfort levels. Today with linear guide ways, also known as profile rails, square rails, linear-motion guides, and ball-rail systems, it’s easier then ever to over specify.
Common mindsets
Two of the most common parameters of rails that engineers tend to over specify are size and tolerance class. Many engineers will call for a bearing that can handle a load larger than the application needs. Part of this oversizing is due to comfort levels developed from years of working with other linear motion systems. When it comes to handling a 3,000- lb load, for example, engineers used to specify a round rail bearing with at least a 2-in. diameter shaft, or larger, a pillow block, and support rail. Such a system would be 5-in. high. To go from this system to a profile-rail type design that measures 1½-in. high can be discomforting — even though both systems are rated to handle the same load.
It is not uncommon to see profile rail systems with the capacity to handle 10,000 lb handling 500 to 1,000 lb. To some engineers, “it just looks right.”
A few engineers will admit that they over specify because “it’s cheap insurance.” The logic is that the system will last longer and be less likely to be the cause of a system shutdown. However, engineers should follow the selection formula, the life-load curves and so on, found in the manufacturer’s catalog to choose the correct size bearing. Overspecifying the size can cost up to 50% more than selecting the right size bearing and it won’t necessarily provide “insurance” against downtime. Regardless of a linear bearing’s capacity, if a rail system is not properly installed it can still fail before its rated life.
Tolerance
As with radial bearings, rails come in tolerance classes: “N”ormal, “H”igh, “P”recision, “S”uper “P”recision, and “U”ltra “P”recision. Normal grade assemblies, for example, will have height tolerances (H) of 60.004 in. and running parallelism of 0.0017 in. in 10 ft, Figure 1 and Figure 2. The ultra precision grade will have height tolerances of 60.0002 in. and a running parallelism of 0.0003 in. in 10 ft, Table 1.
Engineers tend to over specify tolerance class too, but manufacturers’ debate whether this is a problem. For machine- tool applications, no one debates the need for the upper precision grades of rails. The debate begins for other applications. One side says it’s better to always specify a precision grade two or three times higher than what is needed by the application, because you can save cost and time for preparing the mounting. This side claims the mounting need not be as precisely ground in average applications that use one of the higher precision rails.
The other side says you’ll save money by choosing the proper size rail (not oversize) and quality level for the right application and installing it properly. They say the mounting should be carefully prepared.
Several factors effect the debate:
• The more accurate the product, the better the load sharing on all rolling elements. Therefore, theoretically, the actual life of the system will closely match the calculated life.
• A more precise system can minimize a stackup of tolerances.
• If conditions aren’t near perfect for linear-guide way rails, then the rails are subject to a decrease in life. The roller or ball goes through the hole created by the groove in the carriage and the groove in the rail, Figure 3. Anything that causes misalignment, may cause degradation because it forces the ball or roller into an elliptical rather than spherical shape. Round rails, by contrast, are more forgiving.
• Right out of the box, rails may appear to have a bow. (See the box, “Straight and narrow.”) This bow disappears after proper installation, yet engineers may specify a higher tolerance in order to avoid that initial appearance.
Above precision grade, if the application is not in a climate controlled environment, any benefit of high tolerance is lost. Summer temperatures in a shop easily reach 90 F. Winter temperatures may drop to 50 F. Any precision grade rail will flex in either condition because it is ground for temperatures that do not vary from 72 F. In the ultra precision grades, even body heat can cause rails to flex. So why buy precision that can’t be used?
Installation affects a rails features
Today’s rails require different methods of installation than previous linear motion systems. Round rail linear bearings used to be simply bolted into place. Their self-aligning features and ability to “roll” into co-planer alignment allowed some imperfections in installation.
As mentioned earlier, linear guide way rails are less forgiving of installation conditions. Engineers cannot similarly “bolt on” a precision rail and expect to receive the benefits of precision tolerances. If the machined quality of the rails, or tolerance, is important in an application, then engineers must pay attention to rail mounting fixtures.
A radial bearing will prematurely fail if put on a poorly machined shaft. Similarly, a linear profile rail will also fail if put on a poorly prepared surface. The reference edges that will hold the rails should be at least as precisely machined as the rails. Otherwise, both the life of the bearings and linear accuracies will suffer. For example, instead of carrying a rated load, the system will now be carrying an induced load, which may result in premature failure. Other problems may be brinneling or spalling on parts of the rail. It is not uncommon for customers to call the manufacturer complaining that the rails were not hardened sufficiently, when the problem was not poor heat treatment but improper installation.
After installation, rails and carriages will follow and reflect any deflection of the installed rail. Nothing evens out the bumps and twists of an improperly prepared surface or improperly installed system. Although, excessive variances on the mounting surface may be milled, scraped, or hand stoned into specification.
To properly install, first, follow the manufacturer’s recommendations for selecting the correct size and tolerance in a system, even if it doesn’t look right. Then, for moderate precision systems, use a reference edge. Push one rail up to that reference edge and let the other rail “float” into position. The procedure is not unlike locking and floating radial bearings. It will be the reference edge that determines the straightness accuracy. For moderate precision, a machinist straight edge and dial indicators to check and verify runout are sufficient, as long as you stay within recommended deviations.
If the second rail is not floated into position, or set into a machined channel of its own, it should not exceed 0.0005 to 0.001 in. out of parallelism with the first rail. (These dimensions are a general rule of thumb. They may differ depending on the size of the bearing, whether the bearing was preloaded, and which manufacturer made the rail.) Care must also be taken in the other plane. Rails should be held coplanar, with no more than 0.004 in./ft of spread between them. Keep in mind that as the preload value increases, the permissible deviation decreases.
For higher accuracies or long axes, the best tool is a laser alignment system. Three-axis systems are available for about $35,000, or engineers can purchase laser alignment services. If you choose to not buy one, often your local power transmission distributor will have access to such a system. Engineering consultants are another possible source for this equipment.
High-accuracy systems should receive final alignment on site, as parts of the profile rail system may have shifted or moved while in transit.
Rich Goldy is linear product specialist at Invetech, Southfield, Mich.