Friday, 09 October 2009 14:18
Properly installing large bore roller bearings is essential in achieving the maximum life of a mounted bearing. It's estimated that the second most common cause of failure of large bore bearings is improper mounting techniques. Failure can come in the form of shaft-attachment loss, excess vibration and elevated bearing temperatures. Following proper mounting techniques will ensure that the bearing is mounted correctly, and that the maximum life of the bearing will be realized.
There are many different methods of mounting large bore bearings. Common methods include manual assembly, heat shrink, jackscrews, oil injection and hydraulics. The most common is the manual method, which incorporates the use of a hammer and drift, or spanner wrench.
The manual approach involves a bearing, plus a sleeve, nut and washer. The adapter sleeve would be slid over the shaft, and the bearing over the adapter sleeve. The nut would be screwed onto the adapter sleeve, which then forces the bearing up the tapered OD of the sleeve. In turn, the movement of the bearing up the tapered OD of the sleeve would reduce the clearance in the bearing. This provides a press fit to the shaft.
Shim stock is used during this mounting process to measure the amount of radial clearance removed from the bearing. The shim stock is inserted between the roller and outer ring to determine the clearance reduction. When applying this method, the user tightens the nut, checks the clearance reduction, continues to tighten the nut, and measures the clearance reduction until the proper amount of clearance has been removed.
This method has always been suspect, as human error influences the measured values. It's also very time-consuming, requires extreme physical effort, involves potential safety hazards, and in the end, leads to a questionably mounted bearing.
A solution to this problem is to use hydraulics to mount the bearing. The use of hydraulics dramatically decreases mounting time, eliminates much of the manual effort required, reduces the potential for injuries and ensures a proper press fit between the shaft, adapter and bearing. When mounting a bearing using hydraulics, the standard nut is temporarily replaced with a hydraulic nut.
The hydraulic nut consists of two pieces: the nut and a piston. In this scenario the hydraulic nut is screwed onto the tapered adapter sleeve. Fluid is then pumped into the nut, which causes the piston to extend. The piston comes into contact with the bearing inner ring and pushes it up the tapered adapter sleeve to the starting position. The starting position is considered the point at which the clearance between the shaft, adapter sleeve and bearing bore has been reduced to zero, and the bearing is snug to the shaft.
The next step is to place a measuring device onto the bearing inner ring or face of the piston to measure axial displacement. The measuring device can be as simple as a magnetic-base indicator or a displacement gauge. Continuing to supply hydraulic pressure to the nut will further displace the piston until the final position is obtained. At this point, the hydraulic nut would be removed and replaced by the standard nut.
The hydraulic nut supplier and bearing manufacturer supply the starting position pressure and required final displacement values. Applying hydraulic pressure to set the bearing (and measure axial movement of the bearing to remove the required clearance) replaces the use of shim stock. This provides for a consistently and properly mounted bearing.
Dismounting the bearing can also be a very tedious and time-consuming process, especially if the bearing was mounted without a predetermined method to dismount the bearing. There are several methods of removing bearings, including a manual method, the use of oil injection or a hydraulic dismount nut.
Manually dismounting the bearing usually consists of torching the bearing from the shaft, which destroys the bearing and can damage the shaft. This is also a time-consuming and costly method, especially if the bearing had not failed, but just needed to be re-positioned.
The use of oil injection requires that the shaft, or a special adapter sleeve, contain a hydraulic port plus blind circumferential grooves, which allows hydraulic fluid to be pumped in between the bearing ID and the shaft or adapter OD. While this method will not cause damage to the bearing or the shaft, the hydraulic oil that filled the grooves spills out when the bearing suddenly becomes dismounted. The biggest disadvantage to this method, however, is the high cost of the specially machined adapter sleeve or shaft.
Recognizing the benefits of incorporating hydraulics to mount and dismount bearings, mounted-bearing manufacturers are now supplying products that include pre-assembled hydraulic mount and dismount nuts. Unlike conventional hydraulic nuts, which are separated from the bearings after assembly, the hydraulic nuts used in these new products are an integral part of the bearing assembly and stay with the bearing for the entirety of its life.
The use of the hydraulic dismount nut will remove the bearing from the shaft quickly and easily, without damage to either. Basically, a hydraulic dismount nut functions in the same manner as a hydraulic mount nut. Hydraulic fluid is supplied through the nut and to the piston, which causes the piston to extend and come into contact with the inner ring. Increasing the hydraulic pressure will force the bearing down the tapered adapter sleeve until the bearing is dismounted.
Correctly mounting large bore roller bearings is a critical step to achieving optimum bearing performance and maximum bearing life. By selecting a bearing with a hydraulic system already built in the product, you will reduce mounting and dismounting time, eliminate bearing and shaft damage, and most importantly, end up with a properly mounted bearing.
Greg Hewitt is a Dodge Roller Bearing Development Engineer with Baldor Electric Company. For more information, visit www.baldor.com.
Published in Features