Have you ever asked yourself, “Why are we replacing bearings at an alarming rate?”
It is very important to have the proper preload of the tapered roller bearings typically found in all tube, pipe and roll form mills.
Preload is defined in this text as how many inch-pounds of rotational force are required to continually rotate a shaft when the tapered roller bearings are properly tightened.
Preload is most commonly measured by a dial type, inch-pound torque wrench. Not the “clicker” type torque wrench.
An excessive preload will result in premature bearing failure. An insufficient preload will result in loose bearings, which in turn will affect the alignment issues of any mill.
Tightening of tapered roller bearings is primarily done one of two ways:
Single or double nut, tightening on the inside races of tapered roller bearings.
End caps with shims which tighten on the outside races of the tapered roller bearings.
Shafts with tapered roller bearings that run at high speed in oil will 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 due to the fact that as the new bearings break in, some of the preload is absorbed. If the initial preload is not high enough to compensate for this, the preload could be lost after the bearing breaks in.
A clean assembly is a must before attempting to check the preload of a bearing.
Inside and outside races of the bearings must have a slip fit to the shaft or bearing blocks. Bearings that require a press fit are next to impossible to preload properly. If necessary, correct this condition by changing from a press fit to a slip fit.
New or used bearings must have a light coating of grease applied to the outside races before preload can be checked. Never preload a dry bearing.
Make assembly fixtures to hold shafts or bearing blocks so the preload can be performed.
Obtain the preload value for the application you are using from your bearing supplier. If possible, the preload should be performed without seals installed, for they will add to the preload value. In some applications, this is not possible. Be sure to inform your bearing supplier how you will be checking the preload.
Assemble the shafts and bearings assemblies, following the manufactures guidelines or prints. When using applications that require double lock nuts, be sure the double nut is tight against the first nut before checking preload. Always use a spanner wrench (never a chisel or a punch) to tighten or loosen lock nuts. Also, always use new lock nuts and lock washers. Never finish bending the lockwasher tab down on the locknut until the final preload reading is taken.
Adapt the end of the shaft to be checked for preload for the dial type inch-pound torque wrench. For example, shafts with a large threaded end for a large nut can simply be adapted to the torque wrench by using a piece of hose that will fit over the threaded end. From that you can fit the opposite end of the hose to a socket of the same O.D. as the threaded end of the shaft.
With the inch pound torque wrench attached to the end of the shaft, rotate the shaft and note the reading in inch-pounds it takes to continually rotate the shaft. Note that it will take higher inch pound reading to begin rotating the shaft. This is not the preload reading you want to take, but rather what it takes to continually rotate the shaft.
An inadequate preload requires you to tighten the assembly components. With locknuts, it simply means tightening up the lock nut(s). With outside race preload design, it will require the removal or addition of shims for a tighter assembly, depending on the design of the bearing block and the bearing block retainers.
An excessive preload requires you to loosen the assembly components. With single lock nuts, you simply back off the single unit. With double lock nuts, first try to back off the inside nut against the outside nut then recheck preload. If it’s still too high, you will need to first back off the outside nut just a small amount, then again adjust the inside nut to the outside nut and recheck your preload. Lock the lockwasher tab when final preload is obtained. Again, with the outside race preload design, you will need to add or subtract the shims to obtain the proper preload.
While rotating the shaft assembly, grease the bearing block assembly before putting unit back in service.
With proper lubrication, a properly preloaded bearing assembly will last for years. Insure a good preventive maintenance program is in place for the lubrication of the machine; verify it is not overloaded causing bearing failure. Central lube systems are 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.
If a bearing continues to work itself loose, first look for a mechanical condition. Loose fit to the bearing journals or bore of bearing blocks are often the primary causes.
Written by Robert Sladky, VP of Tube Mill Engineering
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