Mean Time Between Failure in Slewing Drives

October 17, 2016- When designing equipment with reliable lifetimes it’s important to carefully analyze each and every component. In order to know the relevant Mean Time Before Failure (MTBF) of the drive, it’s also crucial to analyze the mesh as a complete system and which components are most likely the first to wear out. Once this is known, you can be secure that your equipment is built to last the required life time with regard to the slewing drive. You can design a plan for operations and maintenance around the components which are analyzed to likely break down first. Part stress method is used for components manufactured in house, part count method is used for generic hardware such as fasteners.

Reliability Definitions
The reliability terms that are widely used in industry are defined and briefly explained below:
Failure Rate – the number of product failures over time: λ = 1/MTBF
MTBF – Mean Time Between Failures is calculated in hours and is a prediction of reliability MTBF = 1/λ (failure rate)
MTTF – Mean Time To Failure may be substituted in some data sheets for units that will not be repaired
Reliability – the probability that given a certain failure rate, a certain number of units will pass (or fail) within a specified period

The relationship between failure rates, predicted reliability and MTBF can be summarized with the exponential formula R (t) = exp – (t/MTBF). It is worth mentioning that MTBF is not equal useful life of the product. Statistically, it simply represents a point of time when 63 % of the units have already failed, i.e. there is only 37% chance of a product to last until its MTBF.

System MTBF
The life and reliability of a mechanical system such as the gearbox depends on the life/reliability characteristics of its individual components at a design load. Since testing a large number of gearboxes to have statistically supported reliability values is often not practical economically, the ability to determine a system MTBF value using the reliability characteristics of the components is very instrumental.