hot misalignment
Here we deal with the issue of hot misalignment..
The hot misalignment
In machines that work at high temperatures there are always phenomena of expansion that can lead to the occurrence of misalignment the hot. when this happens, if in alignment, no cold compensation is given, for the expansions that will occur when hot, cold machine does not vibrate, but when it warms up, high amplitude vibration phenomena appear. Thus, in machines that operate at high temperatures, it is always necessary to check:
- Existence of heating of the bearing support structures
- Existence of cold compensation of hot deformations, if the bearing support structure is heated.
Example of thermal expansion
For example:
A bearing is supported by a steel frame with 1,5 meters high, which is exposed to thermal radiation originating in a duct of hot exhaust gases from a cement kiln and is heated to 200°C, when in operation.
Its dilation is calculated as follows:
Expansion = height x temperature variation x linear expansion coefficient of steel =
= 1500 mm x 200 ºC x 0,000012 /C = 3,6 mm
That is, if no cold compensation is given, the coupling, the hot, will have a misalignment of 3,6 mm.
Hot misalignment symptoms
Symptoms of hot misalignment are the same as any other misalignment, easily identifiable with a vibration analyzer, being able, However, do some additional tests:
- Monitoring the vibrations during the machine's warm-up phase in order to determine whether the vibrations increase as the temperature rises.
- Temporary isolation of the heat source or cooling of the bearing support structure, monitoring the evolution of vibrations.
Example – Hot misalignment reduction with hot source isolation
In an oven exhaust fan, high levels of vibrations occurred when the machine was running hot.. To confirm that this was due to expansion causing misalignment in the coupling, a blanket of thermal insulation material was placed, between the hot source and the bearing pedestal and the evolution of the vibrations was monitored.
The following figure shows the temporal evolution of the vibration frequency spectrum in the axial direction of the motor. (bearing 3) after installing the insulation.
Figure – vibrations to 1 X and 2 X RPM, in the axial direction of the bearing of a fan operating with hot gases, within two hours after thermal insulation has been installed, in the bearing support structure.
Note how the component at 2x rpm was reduced and became smaller than the component at 1 x rpm, after some time. In other words, misalignment symptoms have been greatly reduced.
