Question to Journal Bearing Experts

Question to Journal Bearing Experts

Plain Bearings are usually made from bronze, babbitt, some other softer-than-steel material so that wear will be on the bearing rather than on the steel part rubbing against it.

I checked the shaft steel materials hardness range:

1- Mild Steel (such as AISI 1018): Approximately 120-180 BHN.

2- Carbon Steel (such as AISI 1045): Typically around 170-210 BHN, but hardness can be increased through heat treatment.

3- Alloy Steel (such as AISI 4140): Typically around 200-250 BHN, but can be higher depending on the heat treatment.

Here is also sorted list of bearing materials along with their respective hardness in BHN (Brinell Hardness Number), from highest to lowest:

1- Alkali-hardened steel: Typically ranges from 600-800 BHN.

2- Phosphor Bronze: Generally around 180-220 BHN.

3- Tin Bronze: Usually falls between 60-120 BHN.

4- Tin-base babbitt: Typically has a hardness of around 30-50 BHN.

5- Lead Bronze: Typically around 20-40 BHN.

6- Lead-Base Babbitt: Generally has a hardness of approximately 15-25 BHN.

7- Copper-lead: Usually falls in the range of 8-18 BHN.

8- Aluminum Alloy: Hardness can vary depending on the specific alloy, but it typically ranges from 30-150 BHN.

9- Cadmium base: Usually has a low hardness, typically around 5-15 BHN.

I have seen below two combinations:

A)Shaft Material: Alloy Steel (AISI 4140)

Hardness: Typically around 200-250 BHN, depending on the heat treatment.

Bearing Material: Alkali-Hardened Steel

Hardness: Typically ranges from 600-800 BHN.

In the above combination (Alloy Steel shaft and Alkali-Hardened Steel bearing), both materials have relatively high hardness values. While the Alkali-Hardened Steel bearing is harder than the Alloy Steel shaft, it is true that the difference in hardness between these two materials is not as pronounced as in some other bearing/shaft combinations.

The selection of these materials is based on their specific properties and requirements for gas turbine applications. Alloy steel, such as AISI 4140, is often chosen for the shaft material due to its high strength, good fatigue resistance, and ability to handle high temperatures and loads. The hardness range of 200-250 BHN provides a balance between strength and machinability.

Alkali-hardened steel is commonly used for the bearing material in gas turbine applications due to its excellent wear resistance and high load-carrying capacity. The hardness range of 600-800 BHN ensures that the bearing can withstand the demanding operating conditions and provide reliable performance over an extended service life.

B)Traditional and common bearing material that is softer than Alloy Steel, you may consider using a bearing material such as Bronze or Babbitt. These materials are softer and have lower hardness values compared to steel, allowing them to sacrificially wear while protecting the shaft.

Here's an alternative combination:Shaft Material: Alloy Steel (AISI 4140)

Hardness: Typically around 200-250 BHN, depending on the heat treatment.

Bearing Material: Bronze or Babbitt

Hardness: Bronze typically ranges from 60-120 BHN, while Babbitt typically falls in the range of 30-50 BHN.

With this combination, the softer bearing material (Bronze or Babbitt) would provide sacrificial wear and help protect the harder Alloy Steel shaft.

Question to Journal Bearing Experts

Which of above selections is preferred for Turbo Machinery applications like Gas Turbines, Steam Turbines and Turbo Compressors?