In the work of bearing, due to various complex reasons, such as unreasonable structure, poor material quality, low performance, defects on the working surface, impact, vibration, improper installation and poor lubrication, it can cause the early failure of bearing. However, in some cases, some bearings are not put into use (operation) at all, but the original integrity of the user is damaged and lost usability during installation, resulting in premature failure of the bearing. What this paper wants to reveal is a practical example of this failure form.

1、 Failure background

When assembling S195 diesel engine gearbox in a power engine factory, the inner ring of 6205 deep groove ball bearing cracked intermittently or continuously, accounting for a certain proportion in quantity (four sets for each gearbox). According to the on-site understanding, the total supply of this batch of bearings is 3000 sets, 1000 sets have actually been installed, and the remaining 2000 sets dare not be installed due to the "lingering fear" of users.

It is verified that the size of the gearbox shaft meets the requirements of the drawing, and the bearing installation method is correct, which is completed by fixed skilled workers. The user has unpacked and sampled 40 sets to measure the hardness of the inner and outer rings of the bearing. Through flaw detection by dz-2000 large magnetic flaw detector, the hardness is in the range of 61 ~ 65hrc, which meets the provisions of jb1255 standard. No cracks are found by visual inspection. In view of this, in order to find out the cause of bearing ring cracking, the damaged bearing was returned to the site for routine physical and chemical tests.

These rings are heated and quenched in a box type resistance furnace at 840 ~ 860 ℃ and 160 ℃ × 2 ~ h low temperature oil tempering treatment.

2、 Test method and results

1. Macro observation

Figure 1 is a physical photo of the bearing damaged during installation.

After careful observation, it can be seen that:

(1) On all damaged bearings, there are no damage marks left due to installation knocking, and the steel ball, cage and other parts are intact.

(2) The cracking of the inner ring starts from one side of the center of the raceway, accounting for about 1 / 2 or 1 / 4 of the end face diameter.

(3) There is a secondary longitudinal straight crack extending into the inner diameter surface at the fracture, which looks like a knife cut and penetrates the wall.

(4) The appearance of the whole collapse is uneven, but the fracture surface is smooth and flat. There are no signs of plastic deformation and only a few tear marks.

(5) The fracture is gray fine grain porcelain brittle fracture.