Let’s face it. We are all human and we all make mistakes. Mistakes degrade the quality of our work and introduce defects. Not noticing that a mistake is made or a machine is not functioning does not make a person stupid or foolish. It just means that we don’t have a system in place for preventing or detecting these mistakes in the first place. The challenge is how to detect and eliminate these mistakes so they don’t result in unwanted consequences like safety or unscheduled equipment downtime.
What is Mistake-Proofing?
Mistake-proofing is therefore; -
“an action you take to remove or significantly lower the opportunity for an error or to make the error so obvious that allowing it to reach the customer is almost impossible.”
Mistake proofing has been used ever since man started using tools. It really started to become recognized in the industrial revolution and came into prominence through LEAN Manufacturing in the quest to eliminate defects.
Some common examples of mistake-proofing are shown below.
Fig.1 Fuel filler cap on a lead Fig. 2 Drier door interlock Fig 3. Colour coded filler caps Fig 4. Loose wheel nut indicators
The lessons and techniques learned in developing LEAN Manufacturing can be applied just as well from the manufacturing process to the mining maintenance and repair processes. Some of the types of Human Error are shown below and there may be others.
- Forgetfulness - Lack of concentration and standards from one part to the next
- Misunderstanding - Misinterpreted instructions, both verbal and written.
- Wrong identification – Incorrect parts, tools used or the wrong component worked on.
- Lack of experience - For example a new worker who is not familiar with operation
- Operator or maintainer thinks their way of doing things is best
- Lack of attention to detail - This can be inadvertent or sloppiness
- Slowness - This can be caused by a lack of setup for the job or the wrong tools for the job. Standardized tools, jigs, access, drain and fill equipment all significantly impact the duration of the task.
- Surprise (unexpected machine operation, etc.) – Equipment presented with unknown failures, jobs poorly planned or wrong setup for the anticipated job
- Lack of standards - Standards not produced in a quick, easy to read and follow format. Typically these should be One-Pagers supporting the task.
- Willfully ignoring rules or procedures or intentionally sabotaging the work like discarding the parts or doing a defective (bodgey) job.
To eliminate these human errors in maintenance Mistake-Proofing should focus on prevention, as well as detection. This is shown in Figure 5 below.
Figure 5. Prevention and Detection to prevent mistakes becoming defects
For Prevention use Point of Origin Inspections
- Set up for maintenance to reduce mistakes.
Check for optimum maintenance conditions before it is done and errors can be made. Check and see that you are prepared for the tasks to be carried out. This includes reading the work instructions, cleaning and preparing the work area, laying out the service parts, tools and equipment to be used in the task. Setup work includes placing the parts to be used for each task step in the matching parts trolley pigeon hole so work progress can be easily seen by all.
Figure 6 - Parts Trolley Organizer with Pigeon Holed matched to the Task steps
- Provide instant feedback. Anybody else should be able to come to your task and see where you are in the execution of it. The mistake proofing systems should be obvious to all persons. There can be reasons that a job is not completed that are beyond your control. It could be an unplanned for event, shift change or an emergency however the mistake proofing systems in place should prevent anybody else introducing defects in the maintenance task.
- Corrections should be made before they become defects. Using visual cues as reminders helps to identify where corrections are required. Correct the error as soon as possible. Having a go-to person to call up for immediate help is just part of the defect elimination process. Examples are seized or broken components that require deviation from the planned task and can introduce defects into the task.
- Mistake-proofing your systems. Have simple to read one-page instructions to support the tasks. These should reference the tools and parts used in the step and include simple graphics of the steps such as picture process maps. More detailed instructions belong in the OEM manuals. Having tools for the task only on a mini shadow board with service parts organized to match the task steps help to mistake proof your systems.
- Do not rely on the maintainer or operator alone detecting the mistakes. The mistake proofing systems should be robust enough that anybody else can detect a mistake.
- Use Point of Origin inspections. These are the easiest to do and the most inexpensive.
- Give quick feedback. Do it 100% of the time when a mistake is suspected. It is easier to ask a “dumb” question of the person doing the task then to let a mistake go through to the keeper and become a defect. Simply asking if the oil level has been checked in an engine (because the mistake proofing flag indicate that it is not yet done) is better than starting the engine with no oil.
For Detection use final inspections and detection devices
Types of Inspection
- Self-checking - Before handoff
- Joint-inspection - During handoff
- Successive check - After handoff
- Source inspection - Immediately after mistake, but before it results in a defect
Use Control and Warning Systems to highlight errors
A control system is a method that uses sensors or other devices for catching errors that may pass by maintainers or operators. It takes human element out of the equation and does not depend on maintainer or operator. It has the benefit of automatically stopping a machine when an error is detected. A Warning System is used when an automatic shut off system is not an option. In this case devices are linked to interlock and warning indicators such as lights and sirens have their place in maintenance activities as well as production processes detecting mistakes. Jig devices for correct fitting and alignment assure 100% compliance all the time! These can include: Switches that detect miss-fitted parts, guides that prevent mis-feeding, and warning lights and sound signals when parts or tools are not inserted or removed correctly.
Visual cues of size shape and color coding should also be used as an effective non automatic option. Sensory prompts can be used to help the maintainer know that a job has not been completed or the equipment is not ready to be operated or run. Examples are where final operating levels have not been achieved after draining and replacing a fluid.
Figure 7 Operating range visual cue example
Figure 8 Visual cues using Shadow Boards - These should be organized with tools only required for the tasks and mounted beside the job. Other tools can be stored away in larger tool chests.
Tagging, is a visual process that is used in the airline industry to indicate if a part is serviceable and airworthy. It indicates quickly and easily if the aircraft is ready to fly. In the airline industry three coloured tags are commonly used.
- Red Tag: component is scrap or unusable
- Yellow Tag: component is serviceable and airworthy
- Green Tag: component is not airworthy but is repairable
For mining equipment maintenance, this process can be borrowed and enhanced as follows.
- Red ribbons- Task is in progress. Unit is not ready to run.
- Yellow ribbon - job complete. Unit is ready to run
- Green tag- removed component is repairable
Using red ribbons 30 to 60 cm long on holder magnets and numbering the tags to match the steps on Task list.
Figure 9 Example of Visual Tag indicating unit is not ready to operate
“Remove Before Flying” tag
There are six mistake-proofing principles or methods. These are listed in order of preference or precedence in fundamentally addressing mistakes:
- Elimination seeks to eliminate the possibility of error by redesigning the product or process so that the task or part is no longer necessary. Example: product simplification or parts standardisation that avoids a part defect or assembly error in the first place.
- Replacement substitutes a more reliable process to improve consistency. Examples: use of automatic dispensers or applicators to insure the correct amount of a material such as oil is applied.
- Prevention engineers the product or process so that it is impossible to make a mistake at all. Examples : Limit switches to assure a part correctly placed or fixed before process is performed; part features that only allow assembly the correct way, unique connectors to avoid misconnecting wire harnesses or cables, part symmetry that avoids incorrect insertion.
- Facilitation employs techniques and combining steps to make work easier to perform. Examples: visual controls including colour coding, marking or labelling parts to facilitate correct assembly; exaggerated asymmetry to facilitate correct orientation of parts; a parts staging tray that provides a visual control that all parts were assembled, locating features on parts.
- Detection involves identifying an error before further processing occurs so that the user can quickly correct the problem. Examples: sensors in the assembly process to identify when parts are incorrectly assembled, built-in self-test (BIST) capabilities in products.
- Mitigation seeks to minimize the effects of errors. Examples: fuses to prevent overloading circuits resulting from shorts; products designed with low-cost, simple rework procedures when an error is discovered; extra design margin or redundancy in products to compensate for the effects of errors.
Mistake proofing will not complicate your jobs further. It will make your job easier and safer. There should be no instances of having to do the job over a second time to get it right. Starting a process of mistake process opens up a whole world of ideas to limit mistakes before they become defects. The ideas are only limited by the imagination of the team.
- Use the process of mistake proofing as a catalyst for improvement
- Use mistake proofing as a tool for reaching measurable targets- set a goal. The goals can be numbers of defects, time taken to do a task, amount of rework, missing tools and other measures only limited by the inventiveness of the team.
- Use it as a learning device to tap into people’s creativity in developing mistake proofing solutions. Quick, Simple, Cheap and Effective
Bluefield are specialist providers of practical asset management and reliability services. Our work over many years has identified work quality as the most significant contributor to equipment unscheduled downtime. We have developed processes to enable clients to get in control initially and then continue to improve through innovative methods which are proven to deliver results.