Service life and maintenance intervals
In order to be able to make a service life prediction, the operating conditions to be expected must first be clarified
- Specified duration of use
- Operating temperatures
- Mechanical loads
- Media exposure
- Radiation exposure
Before reaching the service life, however, maintenance work is often necessary ( time between overhaul TBO), which is necessary due to the shorter downtimes of the machine, system or tool. The service life describes the time that this technical system or object can theoretically be used without interruption, i.e. around the clock (24 hours a day, 7 days a week).
Example: With a service life of 8 years, it is assumed that this machine can be used 24 hours a day for 8 years without the replacement of core components or complete failure.
The end of the service life can be determined by various events and criteria:
- Wear and tear or failure make further use impossible.
- The maintenance effort does not make further operation seem sensible.
- From the outset, the facility was only intended for a defined period of use (like a rocket for just one launch).
Modern systems, especially machines that have been manufactured in lightweight construction , are often only intended for a certain service life. With them, the components are designed in such a way that the probability of their failure increases very sharply after the end of the planned service life ( bathtub curve ). Scientific knowledge of materials and the loads that occur make it possible to predict the service life of some standard machine elements quite precisely.
Information on the service life is object-related, for technical systems usually in operating hours , for buildings in years . Non-temporal information is also possible: for linear guides , but also for car engines and other driving and aircraft components, the service life is sometimes also given in kilometers , and for pumps also as the delivery rate in liters or cubic meters .
Further dimensions in the product life cycle are:
- Mean Time To Failure (MTTF Mean time to failure) denotes the mean time to failure, and average life called
- Useful life
Special subject areas
Device and plant construction
The service life (in h) of components for electronics is the time from the start of a load to the failure of a defined percentage of a batch of components used. The failures can be total failures or change failures. In the case of switching loads (e.g. relays , supercapacitors ), the service life is also specified in a number of possible switching cycles .
In the case of light sources , such as fluorescent tubes, incandescent and halogen lamps, a distinction is made between "service life" and "useful life" or "lumen maintenance". The service life indicates the period of time in which the specified percentage of the lamps failed. Due to the long service life of an LED, “service life” is rather unsuitable. The more suitable terms "useful life" or "lumen maintenance" describe how much the [luminous flux] of an LED decreases in the specified period.
The following nomenclature is used here: The so-called L value indicates the relative luminous flux maintenance, followed by a letter (B, F, C) with another number that indicates the relative population that must be met for the first value.
The following mean: B is the maintenance of luminous flux without taking total failures into account, F also takes total failures into account, and C finally only considers total failures, whereby the L value is always 0 (e.g. L0 / C10 - here 10% would be broken)
Typical values are L70 / B50 30,000h, L80 / B50 50,000h or for premium LEDs also L80 / B10 50,000h. An exemplary service life specification from B10 states that 10% of all lamps emit a luminous flux below the L value in the specified time. By definition, this does not yet mean the failure of the LED; an (exemplary) useful life specification of L80 then indicates the period in which a lamp still has 80% of the specified luminous flux. The service life of LEDs today is usually at least 50,000 operating hours and more. What is meant is the period within which the LEDs still supply the residual luminous flux defined by the L value on average. The values are determined using a measurement using the LM-80 method over a certain period of time. The specified period, e.g. B. 50,000h, is then extrapolated using the TM-21 method. The specified period may not exceed 6 times the measured period.
In the case of structures - a subgroup of technical systems as structural systems - the total service life generally depends on the service life of the structure . The useful life is usually much shorter.
Dimensions of material fatigue:
Lifetime means the length of time satellites remain in orbit ; apart from their lifespan as a device, it depends on the fuel carried: On low orbits there is still so much atmospheric friction that satellites are regularly raised to their original orbit by accelerating (path correction).
Lifetime extension of welded constructions
- Christian Bonten: Plastics Technology Introduction and Basics , Hanser Verlag, 2014
- IEC / PAS 62717 .