The application of bottom steady bearings in mixers

The application of bottom steady bearings in mixers

A bottom steady bearing, is a type of thrust bearing that restrains radial forces.

Shaft of small agitators are usually restrained on the agitator drive side and free on the impeller side. If the shaft length or power consumption increases, these bearings can be used to optimize agitator design.
The bottom-mounted round bearing, which is installed inside the tank and at the end of the industrial mixer shaft, is used to improve the following items:

Bottom Steady Bearing
Bottom Steady Bearing-

1-Increasing the critical speed (resonance frequency)

According to standards, the first and second critical speeds of the agitator should be at least 40% higher than the agitator dynamic rotation speed. For example, if the critical speed is 100 RPM, the maximum rotation speed should be considered 60 RPM. By increasing the shaft length or impeller diameter, the critical speed tends toward lower RPMs. In this case, either the shaft diameter must be increased to maintain a 40% gap between the nominal and critical speeds, or a bottom-mounted bearing can be used.

2-Restraining vibration amplitude (Deflection)

When any equipment rotates, a certain amount of vibration is generated. This vibration is elastic in nature and does not damage the equipment itself, which in this case is the agitator shaft. According to standards, the permissible deflection at the outlet of the sealing or bearing housing part should not exceed 51 micrometers, and if the deflection exceeds this amount, there is a possibility of damage to the bearing housing part. By increasing the shaft length or power consumption, the amount of this Deflection increases. One of the best ways to restrain this vibration is to use a bottom-mounted round bearing.

Advantages of using a bottom-mounted bearing:

  1. Reduction of the agitator total cost price.
  2. Elimination of need for periodic maintenance.
  3. Reduction of shaft diameter and agitator weight.

Disadvantages of using a bottom-mounted bearing:

  1. Due to the thrust nature of this type of bearing, emptying and cleaning of the tank is required periodically for replacement. In many cases, simply emptying the tank or reactor is not enough and it needs to be cleaned internally so that personnel can work safely inside without inhalation of chemical gases.
  2. Installation of the bottom-mounted round bearing and alignment with the agitator shaft is very difficult.
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