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Extended modified service life
The factor a 1 is merely for statistical use and may be selected from the table below according to the probability of
failure required. Please note that the service life end probability is 90% only if the rating life is determined according
to DIN ISO 281.
Mixed friction state
κ < 1
Factor a 1
Probability of failure[%] Fatigue service life Factor a 1
10 L 10 1
5 L 5 0,62
4 L 4 0,53
3 L 3 0,44
2 L 2 0,33
1 L 1 0,21
Limiting case of the contact of
the metal surfaces
κ = 1
What is decisive for using the possible extended service life of a roller bearing is the lubricant and its maintenance.
The lubricant located in the contact points of the rollers and races or the side rims, can, due to its properties, effect
the separation of the surfaces in the operational state, to significantly reduce the bearing friction and the wear as
a result.
Computationally, the lubrication state is expressed with the viscosity ratio κ = v/ v1 , where in principle the following
applies κ = ν/ ν 1 ausgedrückt, where in principle the following applies:
ν 1 : Reference viscosity [mm²/s]
The reference viscosity is a function of the average bearing diameter D pw ≈ (d+D)/2 and the speed n and is taken
from the diagram Refernce viscosity viscosit t . (Examplel ν1 = 21 mm²/s for a Bearing with D Dpw
= 140 mm, which runs
with a speed or n = 500 min -1 läuft.
ν: Operating viscosity [mm²/s]
The operating viscosity is determined by the operating temperature of the roller bearing. It depends on the selected
oil type (base oil in case of lubricating grease). The operating temperature is obtained from the thermal balance of
the bearing positions.
In the diagram Operating viscosity the operating viscosity of 20 mm²/s is obtained at an operating temperature of
70 °C, if an minaral oil with Reference viscosity ν0 = 100 mm²/s (kinematic viscosity at 40°C) is used.
In addition, the possible extended service life is determined by the degree of contamination of the lubricant. The
parameter is e C. The quantities for reference diameter d m ≈ D pw ≥ 100 mm aor the operational state to be estimated
are found in the table Degree of contamination
From the diagram Operating viscosity is taken at a operating temperature t = 70° C an operating v=iscosity belowr
27 mm 2/
From the diagram
2/
s
Contamination coefficient
Hard and ductile particles in the lubricating oil cause abrasion and indentation when the rollers press them onto the
races. Roller bearing service life may be strongly reduced this way.
This reduction can be expressed using the coefficient e C . The amount of this coefficient depends on:
• type. size, quantity and hardness of the particles
• the lubrication film thickness, depending on the viscosity ratio κ
• the bearing size, expressed by the reference diameter d m (approximately D pw )
Complete separation of
the metal surfaces
κ > 1
KRW 107