December 21, 2011
Overlooking a small component on a tube or pipe mill—even something as seemingly insignificant as the roller bearings inside the inboard and outboard stands—can lead to excessive downtime. Learning the proper way to install and maintain these bearings can extend their service life and improve the mill’s uptime.
When considering how much attention to allocate to installing and maintaining tube and pipe mill components, you probably focus on some more than others. Simple, out-of-sight parts such as bearings probably don’t warrant as much attention as complicated, expensive equipment items, such as the welding unit. However, downtime is downtime, and it all costs the same regardless of the root cause. If you’re tearing apart your tooling frequently to replace the tapered roller bearings inside, a little extra time spent on preparing and installing the bearings might extend their service life and reduce the mill’s downtime.
It is critical to preload the bearings properly. Preload is the amount of rotational force in inch-pounds required to rotate a shaft continuously when the tapered roller bearings are properly tightened. Preload is most commonly measured by a dial-type torque wrench, not the clicker-type torque wrench. A proper preload is critical. Excessive preload results in premature bearing failure. Insufficient preload results in loose bearings, which affect the mill’s alignment.
Before getting started, verify that you have the necessary hardware and tools.
Determine the preload value. If you don’t have this information, ask the bearing supplier for it. Let your bearing supplier know whether you’ll perform the preload with or without the seals installed. Preloading without seals is preferable because the seals add to the preload value. However, in some applications, this is not possible.
Bear in mind two factors that affect the preload value—the lubricant type and bearing condition (new or used). Shafts that run at high speed in oil have a lower preload value than shafts that run slow in grease-packed bearing assemblies. Used bearings have a lower preload value than new bearings. This is because the break-in period of new bearings absorbs some of the preload. If the initial preload is not high enough to compensate for this, it could be lost after the bearing breaks in.
To make the job easier, make assembly fixtures to hold shafts or bearing blocks for performing the preload. Also, make an adapter to connect the torque wrench to the bearing shaft. For example, if the shaft has a large threaded end for a large nut, fit a piece of hose over the threaded end and fit the other end of the hose to a socket of the same OD as the threaded end of the shaft.
After disassembling the roll tooling, clean all of the components. Whether the bearings are new or used, apply a light coating of grease to the outside races before checking the preload. Never preload a dry bearing.
Assemble the shaft and bearing assemblies, following the manufacturer’s guidelines or prints. When using applications that require double locknuts, be sure the second nut is tight against the first nut before checking the preload.
With the torque wrench attached to the end of the shaft, rotate the shaft and note the force required to rotate the shaft continuously. Ignore the initial peak force, which is higher than the force needed to keep the shaft rotating.
After you have achieved the final preload value, bend the lock washer tab down on the locknut. Next, while rotating the shaft assembly, grease the bearing block assembly. After it is lubricated, it is ready to be put back into service.
If a bearing continues to work itself loose, look for a mechanical problem. A loose fit to the bearing journals or bore of bearing blocks often is the primary cause.
With proper lubrication, a properly preloaded bearing assembly will last for years. Establish a good preventive maintenance program for lubricating the machine and verify it is not overloaded, potentially causing bearing failure. A central lube system is one of the most effective applications because the bearings are lubricated as they are rotating and deliver only measured amounts of grease to the assembly,saving on grease costs.
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