Etch Inspection Tests: Detection of Internal Flaws in Bearings – Part 3

The manufacturing process of the bearings is very intricate and complex in nature. This is required in order to make a perfect bearing. The manufacturing processes in certain stages due to the process complexity tend to induce some defects or deficiencies in the bearings. The majority of such defects created is normally surface bound defects and in certain rare cases tend to create an internal defect also.

Causes of Surface Bound Defects

· The surface bound defects in majority cases may go unnoticed due to their reflective or dull localized appearance in the surface of the bearings.

· One of the main cause of this surface based defects is abusive grinding of the bearing components be it the inner race, outer race or the rolling elements.

· The excessive heat that gets generated due to the excessive heating tends to alter the molecular structure of the steel beyond the specifications mentioned for the bearing.

· These defects are normally localized and appear only at specific spots on the bearings either as very bright spots or as a dark spot (indicating a burn mark).

One of the best method used to inspect such surface bound defects is Etch Inspection.

Etch Inspection

This is a chemical based inspection basically using acids to detect various types of flaws such as small cracks, stress based cracks, burns that are commonly found in improperly ground surfaces.

Methodology of Inspection

· The bearing or the area of the bearing to be inspected is acid pickled i.e. the area is bathed in a hot mild acid solution.

· The acid mainly attacks the weaknesses in the surface such as small cracks, burns, etc reacting with the particular localized area.

· Since the molecular structure in the area is altered in the localized area, the action of the acid makes these spots to be visible to the naked eye as dark or bright spots.

· The areas thus identified are magnified and visualized under a microscope to confirm the defect on the surface of the bearing.


· Good personal care of the inspecting personal is required with safety gloves, eye protection, etc since they are exposed to a harmful atmosphere of acids.

We will discuss on the other types of non destructive testing methods of bearings in the forthcoming articles.

This post is part of the series: Detection of Internal Flaws in Bearings

This series deals with the methodologies used for detecting the internal flaws in the bearings other than the dimensional inaccuracies present in the bearing.
  1. Detection of Internal defects in Bearings – Part 1
  2. Detection of Internal defects in Bearings – Part 2
  3. Detection of Internal Flaws in Bearings – Part 3
  4. Detection of Internal Defects in Bearings – Part 4
  5. Detection of Internal Defects in Bearings – Part 5