Much of what we do in engineering and maintenance we accept without question. People say, “It’s been done that way for decades,” implying that it must be correct. But for equally as many decades have come stories of failed and broken machinery, plant and businesses. How can we run our businesses on fact, rather than luck?
On one hand we continue to unquestioningly do what has been done for generations, yet on the other hand we cannot stop equipment failing. There is a subtle connection between the two of which we are only just becoming aware. The connection is obvious when you realise that we have been running our businesses by risk and luck, and not on facts and understanding.
A great misunderstanding exists throughout industry. It is that having a process in-place to do a thing guarantees control over its outcomes. Just because you can show me a documented procedure is not surety it will produce the required result.
Process vagaries introduce variability: the cause of most of our operating and business problems. Variability is ‘the range of possible outcomes’. A business does not want its operations producing out-of-specification merchandise. Out-of-specification results cause wasted money, time and effort.
A highly variable business process (a business process includes its people, the materials worked on, its documents, the selection process, the training performed, the work environment; everything that affects the outcome) allows results to range across good, mediocre and occasional disaster. This process is out-of-control—volatile—and if it is an engineering or maintenance process then failures and equipment breakdowns are built into the business.
When a process design is volatile the outcomes cannot be guaranteed, some will be right and some wrong; like playing a roulette wheel. Volatility is no accident: there are causes.
An example of a classic misunderstanding of variability that makes equipment break down is the tightening of fasteners. It is the root cause of many flange leaks, loose connections and machine vibration problems. There is a variation in the typical methods used to tighten fasteners.
The method with greatest variation, ranging ± 35 per cent, is ‘Feel-Operator Judgement’, where muscle tension is used to gauge fastener tension. Even using a torque wrench has a variation of ± 25 per cent, unless special practices are followed that can reduce it to ± 15 per cent.
The standard deviation for the ‘Feel’ method is ± 12 per cent. This means if fasteners tightened by ‘Feel’ are required to be within ± 10 per cent of correct tension (a figured arrived at by the Author on the realisation that those companies he knew that used load indicating washers no longer had fastener problems) then only about 60 per cent of them are within tolerance, with the other 40 per cent having great opportunity to cause problems.
It is impossible to guarantee accuracy when tightening fasteners by muscular feel. Using a process that ranges ± 35 per cent to get within ± 10 per cent of a required value is playing a game of chance. Every fastener in the world tightened by ‘Feel’ is at risk.
Those companies that approve the use of operator judgement when tensioning fasteners must also accept that there will many cases of loose fasteners and broken fasteners. It cannot be otherwise because processes that use muscle-induced torque to tension fasteners have a high amount of inherent variation.
It would be a very foolish manager or engineer who demanded that their people stop fastened joint failures, but only allowed them to use operator feel, or tension wrenches, to control the accuracy of their work. Such a manager or engineer might come to believe that they have poorly-skilled and error-prone people working for them, when in reality it is the process which they in ignorance specified and approved that is causing the failures.
They misunderstand totally that it is the process which is not accurate enough to ensure correct fastener tension. It is not the people with the spanners who are causing the failures.
Failure versus success
Joint failure is inherent in the muscular-feel process. Torque is a poor means for ensuring proper fastener tension. To stop fasteners failing needs a process that delivers a required shank extension. The fastening process must be changed to one that guarantees the necessary fastener stretch. Only after that, management decision is made and followed through by purchasing the necessary technology, quality controlling the new method to limit variation, and training the workforce in the correct practice until competent, that the intended outcome can always be expected.
The use of operator feel when tensioning fasteners is a management decision that automatically leads to breakdowns. Any operation using people’s muscles to control fastener tension has failure built into its design — it is the nature of the process.
The operating lives of roller bearings are another example where the effects of random chance and luck are not considered by managers and engineers when they select their maintenance strategies and engineering practices.
Another old custom used without concern is the process of replacing roller bearings on shafts and into housings. A work order is raised for a bearing replacement and the job gets done. Usually no one wonders how well the bearing was installed. The right fits and tolerances are critical to the correct clearance between roller and race for long, failure-free life.
Figure 1 (right) shows the effect that changes in clearance have on the life of a 50mm ball bearing. Clearly, an overload or under-load condition in a roller bearing, regardless of how it arises, will cause early failure. Any loss of design clearance is unforgiving to bearing life, especially when roller and race are forced together with greater than pre-load force.
Superimposed over the roller bearing clearance life curve are thermal growth lines showing the change in clearance for each 10oC difference between inner and outer race. In normal operating conditions the differential temperature between inner and outer races varies from 5oC to 10oC. But greater temperature differentials are possible when a race is exposed to a large cooling effect or a large heat source, or if it is damaged or run in a way that generates excessive heat.
When the differential temperature between races is substantially hotter than the design intended, the added expansion forces the roller into the race, causing a rapid fall in bearing life. If the temperature differential allows the clearance to expand it also leads to early failure, but less rapidly. A necessary operating condition to get full roller bearing life is to ensure they run at design temperatures and see no unforeseen temperature differentials.
Bearing life is also fatally impacted when the clearance is wrongly set at installation. A race installed on a too-tight shaft, or into a too-tight housing, causes rapid loss of bearing life.
Figure 1 highlights the importance to roller bearing life of getting the correct interference fit on the shaft and in the housing. It warns us that any error in roller bearing fit means sure early bearing failure. A loose fit is not so severe, but maximum bearing life cannot be achieved. The right differential temperature must be developed across the bearing and the bearing must be fitted to a correctly sized shaft and a correctly sized housing.
Companies that allow roller bearings to be replaced without correctly measuring the shafts and housings with micrometers, and the result checked against the bearing manufacturer’s required fit and tolerance for the operating situation (not for the bearing, as it is common for bearings to be wrongly selected for the actual operating situation) are running by gosh and by golly.
Any bearing replacement process that does not ask for proof of correct bearing clearance selection, correct differential temperature control and correct fitting accuracy, by default allows bearing clearance errors to occur from human error, and people ought not to be surprised at the subsequent bearing failures that must happen.
The common maintenance practice of changing oil after it is black is another engineering and maintenance custom that designs failure into equipment. The risk of failure carried by a company’s plant and equipment from oil contamination is the direct result of the lubrication management processes applied (or not applied) to decide how much contamination will be sanctioned in their oil. When management decide to replace lubricant only when it is dirty they have unwittingly agreed to let their equipment fail.
Companies mistakenly allow their gearboxes, drives, bearing housings and hydraulic system oils to get dirty and blacken from wear particles before they change the oil. Often waiting for an oil analysis to indicate high contamination, or replacing dirty oil on time-based maintenance. Unfortunately, by the time lubricant becomes dirty from particle contamination, the probability of jamming a particle between two contact surfaces has markedly increased and failure sites may already have been initiated in roller bearings (or similar high elastohydrodynamic situations, such as gear teeth).
To significantly reduce bearing failures, gear failures and sticking hydraulic valve problems, the particle count must be kept at clear levels, or below, so the oil never has many contamination particles in it. Changing black oil is far too late to greatly reduce the probability of failure. The oil must never be darkened by particle contamination in the first place if you want to reduce the influence of luck and chance on your lubricated and hydraulic equipment breakdowns.
Many managers, supervisors and engineers are fervent that their company has the right maintenance practices and excellent preventive maintenance processes in place. If their processes include any of the ‘normal’ customs described above, they are of course wrong, because from time to time those processes naturally produce breakdowns and equipment failure.
This is why W. Edwards Deming said his famous warning to managers, “Your business is perfectly designed to give you the results that you get.” Poor equipment reliability is the result of choosing to use maintenance and engineering processes that have inherently wide variation.
These processes are statistically incapable of delivering the required performance with certainty, and so equipment failure is a normal outcome of their use and must be regularly expected. Failure is designed into these processes and luck plays a great part in keeping the equipment operating. The failure of all equipment is directly related to the volatility inherent in the processes selected to purchase, maintain and operate the plant and machinery.
Businesses still use processes long believed to be suitable, not comprehending that these processes naturally contain inherent volatility that make their equipment fail. Are you trying to achieve impossible results using engineering and maintenance processes with inherent variation outside the performance you need?
Trying to improve production equipment reliability using maintenance and engineering customs that naturally produce failure-causing outcomes, is an exercise in futility. It will cause great waste, produce distress for all concerned and lead to emotional burn-out for the managers, engineers and supervisors involved. The only approach that can work is to change the process to one where all its outcomes are what you want.