Quality 101: Considerations for Measuring Large Precision Bearings
January 6, 2012
Mechanical bearings, including ball, roller, taper roller, needle, spherical and cylindrical, to mention a few, have been in use for hundreds of years, with countless designs, applications and sizes, and just as many measurement requirements that need to be defined and verified.
With today’s emphasis on miniaturization, one would think the measurement of tiny bearings is the biggest challenge-just think of having to handle and measure all the components of a 0.10 inch diameter bearing. But the measurement of extremely large bearings-such as those used in power turbines, wind generators or large earth moving equipment- also are a challenge. Here we’re looking at bearings that measure anywhere from 12 inches to 20 feet in diameter. Despite their large size, these bearings still need to be inspected to meet design and manufacturing requirements, and in many cases, the tolerances involved are not that different from small bearings.
The key specifications for bearings include numerous linear dimensions, as well as form and surface finish characteristics. The challenges are many when doing such measurements on extremely large bearings, not only in the gaging process itself but many times in just handling the parts. Often, gages must be used on the shop floor because the parts are too large to be carried to a measurement lab in the far corner of the building. This brings environmental considerations into play. For example, thermal expansion is much greater on large parts than small, and vibration can be more problematic on a tall gage column than on a smaller one.
Indeed, all the elements of good gaging-defined by the acronym SWIPE: standard, workpiece, instrument, people and environment-come into play with large precision bearings. Let’s look at the particulars by specification.
Dimensional MeasurementsThe first series of checks usually done are to make sure the part is correct dimensionally. And since bearings tend to be round, diameters need to be measured. Inside diameters (IDs) and outside diameters (ODs) are probably the most common dimensions measured on the shop floor and there are endless methods of doing so. But few of these methods reach the level of precision required for bearing tolerances, let alone work on some of the sizes required.
Building a dimensional gage for checking a very tight tolerance on a large part includes many of the same concepts required for any precision measurement. Bearing tolerances are much too tight for hand gages such as an ID micrometer or a large caliper, and many coordinate measuring machines (CMMs) have neither the capacity nor the accuracy. In many cases, the only option is to build a custom gage for the family of parts to be measured.
For the tolerances required, long-range scales are generally not accurate enough, and a comparative gage tends to offer the best performance. With a comparative gage you have to have masters (or qualified parts) to set the gage. Just as you would have for a bench gage measuring a 3-inch bearing component, you also need a master for a 36-inch part. Having a good reference is critical to making the measurement.
Sometimes, though, there may not be masters or qualified parts for each particular set up. For such cases, a gage design that incorporates precision measuring heads and a reference ball can use gage blocks to set the radius of the part-and thus the gage-to measure the diameter.
Also important is having the ability to easily manipulate the part to allow measuring at different heights and diameter locations. Manipulating a 36-inch part is a lot different from a 3-inch part. What’s needed is to have the gage manipulate the part to the commanded position. Designing a gage that can position the part reduces the time an operator handles it, helping in two ways. It relieves the operator of the need to handle heavy parts, preventing injury, and it also reduces one of the biggest sources of error in the shop: parts that are not the same temperature as the gage. Since the amount of thermal expansion on a large part is very significant, the less an operator touches it the better.
Then there are large bearings that have very thin walls; so thin that holding their shape is very difficult, particularly when gaging force is applied. In these cases special fixturing consisting of six measuring heads can be used to help “round up” the part. These fixtures also can provide additional valuable information to the manufacturer. Because each measuring head works independently, there is increased capability for part analysis. For example, these six heads provide:
Three 2-point diameters
Two 3-point diameters
Total indicator runout (TIR) values.
Because these six heads do a lot of averaging, they will produce very repeatable readings despite rotating the part to multiple points on the part diameter. In addition, special equations have been developed to compensate for the six points of gaging pressure being used.
Form MeasurementWhen bearings get to the 20-foot diameter range, they can get pretty heavy. And since the typical form machine is designed around parts that are less than 12 inches in diameter, form machines for large bearings need to be “beefed up.”
The first issue to consider is the base of the machine. A large base adds stability, absorbs vibration and provides a foundation to stage the part. Any vibration that passes through the machine gets amplified as it moves up the structure. The more you can eliminate vibration in the base, the less error will show up in the measurement.
Then there is the rotary table for large bearings. For large capacity, air bearings are the only practical choice. Air bearings have all the characteristics required: heavy load capacity (thousands of pounds), excellent axial stiffness, and an air cushion that absorbs vibration and reduces the potential for damage if a part is accidentally placed a little too hard on the table.
Another area to look at is the vertical stand and horizontal arm which holds the sensing probe. The mass of these components is a key factor: they need to be tall and have enough reach to get at all the parts within the range of the table. But without sufficient mass, they will magnify any vibration coming up through the base and shake the arm like a string of spaghetti.
The last area to consider is the number of counts that can be obtained from the rotary encoder. To collect form analysis data for the part, a digital encoder is built into the table. On large diameter parts, circumferences can become enormous. It is important to have a rotary encoder that has as high a resolution as possible. The higher the resolution, the more data points that can be collected, which means better analysis of the whole surface and more accurate results.
Surface Texture and ContourWith some of today’s portable handheld surface gages having 30 parameters available, along with the ability to store and retrieve data, most measurements are easy and can be taken in virtually any orientation, regardless of the size or location of the part. Portable gages offer a wide range of available probes to allow for measuring just about anywhere on the part and provide the parameter required.
But while measuring surface finish is relatively easy, there are usually more parameters than just finish requirements called out on large bearings. Many bearings have curved races in which the balls run. Surface finish is important to reduce friction on these tracks, but also critical is the form or curvature of the track. If the curvature is not correct, the ball will not ride in the correct location, or too much pressure will be applied to the balls which will produce heat and result in early failure of the bearing.
For this measurement a contouring gage needs to be used to trace the races and other areas on the bearing. A contour gage has a long arm, capable of very long range and high resolution, allowing for curvature analysis of the races. Staging and mass are important for this measurement-as they are for other measurements-but special vibration isolation also is necessary, since the contour probe is so sensitive and extremely susceptible to vibration. For this application large air vibration isolation pads are a must. The gage holds the part at a slight angle and keeps it in place with adjustable guides to allow the contour system to be positioned at the proper location.
Precision Nature of TaskIn measuring extremely large bearings, size matters. Not only must gages be robust enough to accommodate extremely heavy parts, but they must also be able to manipulate parts. They must allow measurements of high precision, be able to operate in shop floor environments, and do so with enough operator ease and efficiency to keep pace with the manufacturing process. Consideration must be given to issues such as gage mastering, thermal expansion and vibration, and the ability to accommodate and analyze increased data from longer part traces.
Finally, operators must approach large bearing measurement with the appropriate mindset. It is intuitively hard to grasp the fact that a bearing many feet in diameter and weighing perhaps several thousand pounds can have the same tolerance requirements as a 3-inch roller bearing. But they do, and unless care is taken and operators are fully sensitive to the precision nature of the task, measurement results will be less than adequate.