A sliding bearing's lifetime is a complex story, which results from the sum of the many parameters' effect. In our section: Parameters for bearing selection, we discuss the most important. These are:
• Opposing rotational partner
• Bearing housing
• Operating environment
• Bearing play
There are many more factors that affect a slide bearing's lifetime, however, with the help of those mentioned above, it is possible to determine approximately how long a slide bearing will function as it ought.
The reason for using the phrase "as it ought" is that the slide bearing's lifetime is wholly dependent on how much wear or abrasion the design can allow. The slide bearing and its opposing rotational partner function quite simple as a closed tribological system. When the sliding surface of the bearing has worn down to the level that can be allowed, before the bearing play is adventured, the bearing is worn out. Below, the wear factor k is shown as a function of some normal lubrication conditions.
The wear factor is dependent on the lubricant condition and the material combination. It is normally determined by laboratory testing and is based on Archard's wear equation.
where V is the volume of material worn away.
S the total sliding length.
F the normal force i.e. the load,
and H is the hardness of the worn material.
Below is presented a summary of the factors influencing the rate of wear and in consequence to be considered during design.
The values should be viewed as a rule of thumb and not exact data.
In contrast to ball bearings, lifetime calculations for slide bearings are very approximate. The exception is fully lubricated slide bearings, where a numerical calculation can be performed, as there is good theory as a base. However, since for the most slide bearings interface lubrication prevails, the lifetime calculations are difficult to perform based on just theoretical formulae. For interface lubricated slide bearings, laboratory experiment often forms the base.
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