The bearings are made with extreme precision. Therefore, close attention must be paid to the details of the assembly and maintenance process so that the bearings achieve their best performance. Failure to do so will cause huge problems in the future, such as premature bearing failures and unplanned downtime, which are very costly to business operations.
An inadequate bearing is one of the common factors that can lead to premature failure; therefore, steps must be taken to select a bearing of the appropriate type and size. At this point, many would think this step is enough to ensure long bearing life and have ignored another important “tuning” detail: bearing adjustments. shaft and housing ; Even with a correctly selected bearing, it will continue to fail prematurely if the shaft and housing adjustments are incorrect.
Let's start with an introduction:
What is shaft alignment?
Shaft alignment is the process by which two or more machines are positioned in such a way that at the point of transferring power from one shaft to another, the axes of rotation of both shafts must be aligned when the machine runs normally.
The illustration above shows the ideal shaft alignment where the rotation axes of both shafts are parallel.
Misalignment
Misalignment is one of the most common problems that contribute to premature bearing seizure. Most rotating machines are often victims of misalignment and it can have a negative effect on the life cycle of the machine.
1. Shaft misalignment
Misalignment of the shaft relative to the housing causes the balls or rollers to become overloaded, resulting in failure.
2. Housing misalignment
Housing misalignment can be caused by the housing tilting relative to the shaft plane. It can also be caused by the installation of residential structures or foundations.
Diagnose misalignment
As bearings rotate over time, misalignment can often cause wear on the bearing's inner and outer races. It can be easily diagnosed by inspecting the path of the rolling elements inside the bearing.
A well-aligned bearing will show a bearing path along the center of the inner and outer rings, while a misaligned bearing will show uneven bearing paths.
Ways to Avoid Misalignment
One of the easiest ways to avoid misalignment is to have a proper installation process. Shaft or housing shoulders in contact with the face of a bearing must be at right angles to the centerline of the shaft.
How to recognize misalignment?
Vibration analysis can be used to diagnose misalignment along with other mechanical defects. Laser alignment is another accurate method for detecting misalignment.
In shape
In addition to alignment, another important factor when dealing with premature bearing failure is assembly.
Lose control
Although a loose fit makes the installation process easier, a loose fit poses a problem as the bearing ring can slip or “creep” on the shaft or housing. Slippage is where relative sliding occurs between the fitting surfaces and therefore creates a gap between them. As a result, it can cause excessive wear and, in the long term, lead to bearing failure.
Grip
A light press fit usually helps prevent warping, but too tight a fit will eliminate internal bearing clearance and cause an increase in operating temperature, which can lead to premature failure.
A proper fit
Press fits will naturally make bearing installation more difficult and susceptible to damage, but with proper care, most problems can be avoided.
The type of adjustment is also determined by factors such as load, operating temperature, and shaft or housing material.
Temperature conditions
In service, bearing rings typically reach a higher temperature than the components on which they are mounted. This may result in a loose fit of the inner ring in its seat, while expansion of the outer ring may prevent the desired axial displacement of the ring in its seat. Therefore, temperature differentials and the direction of heat flow must be carefully considered.
To load
The charge reduces inner ring interference. Thus, more interference would be required for heavier loads.
Housing and shaft materials
Different material strengths and thermal expansion rates make the correct fit different for each material. For example, aluminum expands more than steel. Therefore, an interference fit on an outer ring with an aluminum housing requires more interference than an interference fit on a steel housing. Furthermore, when making tuning recommendations, special allowances must be made for thin-walled casings, hollow shafts, split housings, and high vibration.
