MTBF – Mean Time Between Failures

What is detectable – controllable?

Statistical Distribution, Maintainability,
response to Condition Based Maintenance, inspections and monitoring

What is acceptable

This Standard is defining which defects to tolerate and which ones you should stop from developing.

It lists “Defect Symptoms“, describing practically all defects occurring at the equipment of any production plant.

When certain attributes had been added (following list column (b) in parentheses), you can also standardize capacities regarding where they are tolerable and how they can be controlled.

This standard represents the result of analyzing several hundred thousands of defects occurring in industrial plants of nearly any kind.

 

Nr.Description of DefectAlpha of Wei- bull Func- tion

Min

TBF

Min

TBF

Cat. to tolerateEffect serviceControlabi- lity
(a)(b)(c)(d)(e)(f)(g)(h)
01.1aging (electronic element)1,5300,3Snono
01.2aging (mechanical or control element, membrane, flexible tube)3,0101Snonno
01.3aging/brittleness (rubber, plastic, leather)3,0101Syesyes
01.4aging (chemical/physical of lining)2,0301Snoyes
01.5aging (metal, stressed by heat)3,031Mnoyes
01,6storage of liquid (hydraulic, lubrication, cooling/heating)3,03,01Mnoyes
02.1triggering (mechanical safety device)1,01000Nnono
02.2triggering (electrical fuse, relays)1,01000Nnono
03.1blocking/seizing1,01000Nyesno
04.1fracture (except of lining)1,01.0000Nnono
05.1erosion from gas/fluid3,0103Snoyes
06.1short circuit (electric insulation of cable)1,01.0000Nnono
06.2short circuit (electric insulation of winding)1,03000Nyesno
07.1deviation of characteristics (control element)2,0100,3Mnoyes
08.1contact defect (monitor, touching product)1,530,03Mnoyes
08.2contact defect (sliding contact, coal brush)3,031Mnoyes
08.3contact defect (< 500V, actuated < 100 operations/day)1,5301Snono
08.4contact defect (<500V, actuated > 100 operations/day)2,0101Mnono
08.5contact defect (>500V, operating switch)3,0103Mnoyes
08.6contact defect (>500V, non-operating switch)1,51001Nnoyes
09.1corrosion (inside, material not corrosion resistant)2,0301Snoyes
09.2corrosion  (outside, with corrosion protection)2,0303Syesyes
09.3corrosion (inside, material corrosion resistant)3,010030Nnoyes
10.1lamp defect3,031Mnono
11.1conductor fracture/interruption (electric, not moved/bound)1,51.0001Nnono
11.2conductor fracture/interruption (electric, moved/bound)1,51001Nnono
12.1loosening (electr. connection)1,51.0001Nnoyes
12.2loosening (mech. fastening)1,51.0001Nnoyes
13.1leakage (seal moving, relatively; valve/piston; shaft)2,0100,3Syesyes
13.2leakage (seal not moving relatively, flange, pipe)1,51001Nnoyes
14.1deformation of solid element (including springs)1,01.0000Nnoyes
15.1displacing of element (fastening device had still been fast)1,01.0000Nnoyes
16.1loss of auxiliary gas/liquid (intentional consumption of cooling/hydraulic/lubricating liquid)3,00,030,01Mnoyes
16.2loss of auxiliary gas/liquid (no intentional consumption of cooling/hydraulic/lubricating) liquid)3,010,3Mnoyes
17.1wear (gliding bearing, links, gear wheel, worm spindle)2,0301Syesyes
17.2wear (rolling bearing, ball/roller/needle)1,53001Nyesno
17.3wear (slide, transport element, through touching material, product ); wear of running wheel and partner2,030,1Mneinyes
17.4wear (element not or hardly lubricated; cable, chain, belt)3,031Mnoyes
17.5wear (friction element, brake, clutch)3,031Mnoyes
18.1clogging (moving product in pipe, tube, vessel, transporting device)1,0100Snoyes
18.2clogging (auxiliary liquid, moving product in pipe, tube, vessel)1,01.0000Nnoyes
18.3clogging (material not moving, measuring device)1,0100Snono
19.1contamination of surface, liquidfilter, sieve, strainer)3,00,30,1Mnoyes

Legend:

“Defect Symptom” describes what you see when looking at a defective element.

Column (a): Digits in front of the dot summarize similar appearance; digits following the dot identify differences inside one appearance, resulting from differences of the following columns.

Column (b): Verbal description of Defect Symptom.

Column (c): Factor in the Weibull Equation indicating the form of the statistical distribution: “1,0” indicates random distribution; “1,5” indicates logarithmic (non-symmetric) normal distribution; “2” and higher indicate symmetric (Gauss) normal distribution (the larger alpha, the narrower the distribution).

Column (d): Statistical value. Resulting from dividing the total number of components in a plant, that actually develop this Defect Symptom, by the number of defects at the same entity actually occurring per year.

Column (e): Statistical value: Period of time, until which 10% of all components, developing this Defect Symptom, actually fail.

Column (f): Resulting from column (d): “M” (multiple), when MTBD is 9 years and less (you have to tolerate); “S” (single) when MTBD  is from 9 to 30 years (within your own experience you should not meet them more frequently than just once); “N” (non) when MTBD is larger than 30 years (you should stop them from re-occurring at all).

Column (g): “yes” indicates that with more than 50% of all elements developing this Defect Symptom, this defect would develop more frequently, when the element will not be cleaned, lubricated, impregnated or conserved.

Column (h): “yes”  indicates that with more than 50% of all elements developing this Defect Symptom, this defect could be by periodic inspection economically detected, before the defect would trigger a break down of the peace of equipment affected (i. e. you should control and not let occur any loss event).

Is this defect standard universally applicable?

Some initially doubt that such a standard would apply equally to all plants and processes.

The only exception, however, are plants that are designed from the outset for a useful life of considerably less than ten years and whose elements are subject to correspondingly higher specific stresses.