Reliability is a quality control outcome. Achieving precision and accuracy in equipment construction, operation and maintenance is vital for outstanding equipment reliability. Simple, practical methods for achievement of high reliability precision can be used by every in manufacturing, operation and maintenance team.
The Precision Principle for Producing Maintenance and Operations Mastery. The need for precision and accuracy dominate those industries that use machinery and equipment. It is the single most critical requirement for successful operating performance in manufacturing, processing, transport, shipping, in fact all industries that use machinery. These industries require that their equipment run reliably (no failures or unplanned stoppages) with high availabilities (ready for immediate use) and high utilization (continuously in use) all their working life. The Precision Principle will deliver those results.You achieve outstanding reliability, availability and utilization by being precise and accurate with your equipment’s assembly and use. Recognising the importance of precision and accuracy in equipment construction, operation and maintenance will let you move one step closer to world class performance.
The Importance of Precision
Man-made equipment and machinery only work well for a long time, when they are built and run precisely. Precision, as used it in this article, means meeting specified standards to within allowed tolerances. Precision requires that the specific standards needed for high reliability over a long operating lifetime are set and continually achieved during design, manufacture, assembly, operation and maintenance.
Precision is only achieved by controlling accuracy. Accuracy is closeness to a specified value. Man-made machinery must be built and operated accurately.
An example of precision is the alignment between two rotating shafts. If two shafts are meant to be in-line, but they are actually off-set to each other, then they will run out-of-true. When these shafts turn they will tear each other apart and cause massive forces to be loaded onto their bearings and coupling. Eventually the bearings, coupling or shafts will be destroyed because of the inaccuracy in their alignment.
If two shafts are meant to be in-line then they must be aligned with sufficient accuracy that will insure they run without unwanted destructive forces being created. This principle applies to everything made by man-kind.
If you introduce into your workplace methods to ensure accuracy with everything man-made you will have a highly reliable operation with outstanding equipment availability and performance.
The precision principle even applies to your operating and business processes. If you want to be in control of your processes then introduce standards and tolerances and have the people using the process measure how close the process meets those standards. Develop standards for every stage of your process and measure how accurately the operation meets the standards at that point.
If you control to appropriate standards throughout the process you must automatically deliver the necessary product quality and throughput you specified in the standard. By being everywhere precise you will always get the end result that you want. This principle is a certain way to guarantee successful performance.
Simply develop appropriate standards with specific targets and tolerances and make sure the real results are regularly measured and compared to the standard. Let the people in charge of the equipment and operations adjust their own performance, and that of the equipment and process, to meet the required accuracy.
Measure Accuracy if You Want to Control of Reliability
When a reliability standard is set (another name for it is a quality standard), for example our two shafts must be aligned to within 0.05 mm (0.002″) from the start of one to the end of the other, you introduce something that can be measured. The measurement tells you how accurate, how close to the standard, you are and what you need to do to adjust the situation so you do achieve the required precision.
With a means to measure accuracy you will always get the precision you require. If you have wisely chosen the right standards to guarantee precision, then measurement creates a way to control your process, your equipment and your workforce’s efforts that automatically produce a quality product consistently, reliably and repeatedly.
If you do this with every aspect of your operation you will remove any guess-work, you will give people targets to work to and a means to test accuracy. You will provide guidance for the operation. You will find people take ownership for meeting the targets. Humans are a goal-driven species. When you give your people goals they will work hard to meet them. Using the precision principle means that you have a vehicle that in time lets you reach the goals.
Once the accuracy is continually met set more precise standards, and in time they too will be met.
How to Set or Determine the Limits to Work Within
Where do you go to find the appropriate standards to apply in your operation? Most times the standards were set at the design stage and you only need to get the design standards and use them to measure your current performance.
In the case of machinery and equipment the standards are set by its engineering design. You only need to insure that the same standards are being used and met during its operating life. Are the shafts aligned to sufficient accuracy? Are the bearings lubricated correctly, with the right grease and the right amount, at the right frequency? Is the frame distorting so badly that the internal components no longer run within their designed tolerance? Engineering standards are easy to apply, to check and to prove they are done accurately during operation.
It is harder to set standards for a process. But it can be done. In an industrial process there are tell-tale signs of when it is performing correctly and there are tell-tale signs of when it is not. You must determine what causes the tell-tale signs and then set standards and tolerances to measure the influencing factors and keep them under control before they affect the process.
Take an example of a process liquid filter. When there is too much material build-up on the filter screen it will reduce the flow through the filter. A pressure difference arises between the upstream-pressure and the down-stream pressure as the material thickens on the screen. The difference in pressure across the filter is a tell-tale sign of material build-up.
You can use the pressure difference to control the accuracy of the flow through the filter. By setting an upper and lower limit, or tolerance range, in which to operate the filter you can determine the appropriate time to clean the screen with minimal disruption to the process.
You can even go one step further and use the filter cake thickness on the screen to act as a tell-tale sign of other process problems. Here is a real example.
The flow through a horizontal leaf filter mysteriously fell to 25% of full flow with no observable cause. When the filter cake was examined it was noted that it was not the usual thickness that historically caused that much pressure difference. Investigations into the operation found that slimes produced during production, which were usually removed from the process, had not been remove because there was no place to put them. The presence of the slimes acted to blind the filter cake and so a thinner cake was able to cause the same pressure loss as the normally thicker cake without the slimes present.
In this case the filter cake thickness was a tell-tale of a process operating normally or one out of control. The cake thickness became another standard and measure to be used to control the process. If the cake was not thick enough after a certain time in service, or throughput, it meant that the ratio of slimes in the process was too great and they had to be removed to start fresh.
You can find and then specify standards and tolerances in the same way for all your process.
The Tolerance Range
Once a standard is set you must also set acceptable limits either side of it that allow you some range for management. The tolerance limits are set tight enough to insure sufficient accuracy so that precision operation is maintained. The tolerances are determined from the engineering design or from the difference between acceptable normal and unacceptable abnormal performance.
For example to say that our two shafts must always be perfectly inline is nonsense. Perfect alignment is not humanly achievable with current technologies. Even if the alignment was perfect before start-up it would not be perfect after reaching running temperature. Hence we set a tolerance of 0.05 mm (0.002″) from end to end at operating temperature and accept the accuracy as adequate if the final measurement is within tolerance.
To align the shafts to that accuracy requires laser alignment equipment, the right procedure and a control chart on which the measurements are recorded. The orientation of the shafts are adjusted until the measurements are within the control chart limits.
In the case of our filter screen we set a maximum pressure difference of say 200 kPa (2 bar, 30 psi) and a minimum of 100 kPa within which the filter screen must be cleaned. So the screen is cleaned after the pressure difference measures 100 kPa but before it gets to 200 kPa. This allows the operators some flexibility to do the cleaning at a time that minimises production disruption. When they clean the filter they also take a cake thickness sample and check it against thickness tolerance to determine if slimes are starting to become a problem.
Can you see how being precise keeps you in control of situations and stops problems happening?
Control Chart the Performance
It is best to be able to view how progress is going. This requires developing an image of the situation so you know if you are on track or not. Most simply this is done with either comparison tables, graphs, quality control charts or the like. The simpler the charting device the better, as long as it also is sufficiently accurate to give you the precision you need to keep control.
On the master chart you put down the standard required and the tolerance limits either side of it which are acceptable. You then take measurements from the actual operation, process or action and plot those against the master chart. So long as the actual results are within tolerance you are in control. When they show a trend toward loss of control, or are outside the tolerance limits, you have accurate information to make the decision to alter, change or stop the process or operation.
You can apply a Precision and Accuracy Measurement System (PAMS) to machinery, industrial processes and business processes. The precision principle applies in any man-made situation.
By measuring and comparing the current accuracy of the observation against the required accuracy you can control your performance. If you want higher performance then set more demanding standards and\or more tighter tolerances. The one proviso is that your operation and equipment has the capacity to achieve those higher standards.
If the current operation, business process, people or equipment do not have the capability to meet more demanding standards you must first invest education and financial capital into the operation so that you develop the capacity to meet the higher standards. Do not try to make a silk purse from a sow’s ear. Do not expect high standards if your current systems, processes and people cannot meet them. First introduce the necessary capacity and capability to achieve higher targets.
Measure Where You Are at the Moment
By setting the standard, and the allowable range, you have introduced a goal to be met. It then becomes necessary to measure your current performance against the goal. You need to find ways to measure present conditions and compare them against the required standard sufficiently often to allow you time to analyse, correct and maintain control.
It will require charting the measurements and comparing them against the standard and its tolerance limits. As you get close to the tolerance limits, or past them, you make adjustments and changes to bring you back toward the standard. This approach lets people see the process is under control and what they need to do to correct it if it is not yet in control.
All this sounds incredibly like a quality management system. It is!
Error Proof the Work by Checking, Testing and Confirming the Way Forward
By adopting a method that permits you to compare present conditions against the ideal conditions you want you proactively establish a means to control and foretell the future performance of the process or equipment to a very great extent. You introduce the error proofing concept into the way people work.
By regularly checking, testing and comparing against the goal you put yourself and your people in a position of controlling and managing the plant and equipment with great accuracy and purpose. You now know what should be happening, when it should be happening and you know when things are no longer as they should be. Most importantly you know what has changed and what to do about it to bring it back into control.
Accuracy Keeps You in Control
When you set-up a PAMS you establish check-points that if achieved will guarantee control of the results. Can you imagine having a process or machine where each step in its use provides feedback to insure the next step is being started correctly?
Such a situation means that before the next action or process is performed you already know that all the right parameters and specifications have been met to insure the next step will happen in a controlled and exact fashion.
With this quality system approach your PAMS will give you the greatest chance of getting things running right every time an action is undertaken.
Accuracy Lets You Detect Waste and Loss
Once you have established standards and can measure them accurately you have a means to recognise wasted and loss. When a machine is not performing right you can gauge the cost of the poor performance to your operation against what it would be had the machine been operating to standard.
When you know the discrepancy between good performance and unacceptable performance you can introduce changes to correct it back to where it should be.
Achieving Accuracy Empowers Your People
Once the standards are set and the limits are clear you can ask your people to find better and newer ways to achiever them. People love a challenge. The work they put into improving what they do will make them happy and get them to grow in spirit and character. In the process their knowledge and skills will also grow.
PAMS will set your people to thinking how to do it better, faster, cheaper and they will discover new ways and continuous improvements that they will create, then test and fine-tune to consistently hit those standards.
Where to Start Applying the Precision Principle?
The recommended place to start applying this principle is with your rotating equipment. These machines rely on near-perfection of assembly, installation, operation and care to run properly. By keeping your rotating equipment highly reliable, highly available and in constant use you will maximise their worth to the organisation and to the process in which they operate.
After that start looking at your static equipment that undergoes severe operating conditions and how the process in which they are used affects them. Set benchmark performance requirements and tolerances and trend how the equipment goes against the standard. When they do not meet the target then investigate and solve the problem.
Author: Mike Sondalini