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Concept of the estimated wear amount

Describes the consideration of time-based progress of wear and the concept of the estimated wear amount.

Service life of bearings

In designing the bearings, it may be necessary to estimate their service life. We talk about their service life in general, but it is very difficult to accurately estimate it from an aspect of wear across various wear forms and service conditions.
Normally, the estimated wear amount is calculated using a specific wear amount actually obtained in an experiment. The service life is easily calculated by using this specific wear amount, but care should be taken in taking its numerical value.
Even with identical P and V values, the specific wear amount fluctuates greatly due to a difference in velocity, operation frequency (difference in calorific value between continuous and intermittent), type and amount of lubricant, size of clearance (cooling effect), material (hardness) and surface roughness of shaft, degree of mixed foreign matters, and many other factors. Also, it often changes depending of the lapse of operating time. Accordingly, the specific wear amount is used as a constant under the limited conditions. Generally, it should be considered a guide for comparison.

Calculation of the service life of bearings

Estimation of service life of the plain bearings has not been theoretically established. In actual design, accordingly, an emphasis is placed on an experimental method which defines the service conditions as much as possible, makes determination based on analogical reasoning by application examples and similar ones, and the results of similar experiments, model experiments or actual experiments. A very effective means is accumulation as experiential engineering such as organizing the results of maintenance and inspection during operation and feeding them back to next design.
As an experimental method concerning wear, the use of the above-mentioned numerical value of the specific wear amount allows you to calculate not only the estimated wear amount per wear time, but the wear time reaching a boundary wear amount (allowable value for each device, allowable value in terms of bearing performance).

A wear calculation formula obtained from the experiment is as follows.
W=K・P・V・T

W
Estimated wear dimension (mm)
K
Specific wear amount (mm/(N/mm2・m/s・hr) {mm/(kgf/cm2・m/min・hr)})
P
Bearing load surface pressure (N/mm2 {kgf/cm2})
V
Bearing sliding velocity (m/s {m/min})
T
Sliding hours (hr)

Time-based progress of wear

Time-based progress of wear cannot be discussed indiscriminately as well, but Fig. 1 shows typical examples.
Types A, B and C have been discussed differently from a mechanical aspect of wear. Type A is a typical example when initial running-in has not been properly done with no lubrication or when selection of the bearing material, etc. has not been properly made. It indicates that wear progresses at a high value, allowing a large amount of wear debris to fall off. It is inadequate as practical bearings.
Types B and C are widely seen in practical bearings. There is an initial wear portion of b1 or c1, and a steady wear portion of b2 or c2. The inclination of line and the position of inflection point change depending on the material and frictional condition. Most of initial wear of b1 or c1 is generated during a so-called running-in period. During this period, surface roughness and uneven contact are caused to change to b2 or c2 by running-in or adhesion of fine wear debris.
There is little wear or sometimes almost no wear in the state of b2 or c2. This condition can be maintained for a long time unless the atmosphere or frictional surface changes. In many cases, however, this condition progresses to the state of c3 subject to more wear because of higher bearing temperature by continuous friction, viscosity change or consumption of lubricant, mixture of foreign matter, behavior of wear debris, and material fatigue. A higher gradient of this progress tends to result in abrupt abnormal wear or seizure during practical use of bearings, leading to a major problem.
When estimating the service life of bearings, it is required to discuss with a steady wear value of b2 or c2 and carry out maintenance and inspection so as to prevent the occurrence of the state of c3.
From a viewpoint of steady wear, Type A may be identified with b2 and c2 except for higher wear rate. When a wear time is fully spared, a value of specific wear amount is obtained by connecting between the origin and the end point of b2 or c2, ignoring b1 and c1. Our calculation formula of the estimated wear amount is based on this concept.

Fig. 1 Time-Based Progress of Wear

Estimation of wear in actual design specifications

Estimation of wear in actual design specifications
As mentioned above, the wear (service life) of bearings changes depending on various factors. It is very difficult to estimate the wear (service life) by simple calculation at a desk.
A currently applied method defines the service conditions of actual equipment as much as possible and calculates with a wear calculation formula, using the specific wear amount of our test data closest to those service conditions.

Wear calculation formula

W=K・P・V・T

W
Estimated wear dimension (mm)
K
Specific wear amount (mm/(N/mm2・m/s・hr) {mm/(kgf/cm2・m/min・hr)})
P
Bearing load surface pressure (N/mm2 {kgf/cm2})
V
Bearing sliding velocity (m/s {m/min})
T
Sliding hours (hr)

Specific wear rate depending on lubrication conditions

Lubrication conditions mm/(N/mm2・m/s・hr) mm/(kgf/cm2・m/min・hr)
Dry 6×10-4~3×10-3 1~5×10-6
Periodic lubrication 6×10-5~3×10-4 1~5×10-7
Oil lubrication 6×10-6~3×10-5 1~5×10-8

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