Establishing what is a machine safety deficiency
Nov. 15, 2016 - A food manufacturing client required my assistance regarding a “perceived” machine safeguarding issue. “Perceived” is used because up to the point of my involvement, the issue was considered adequately mitigated by the employer. The issue surrounded an already provided access door on a piece of packaging equipment. The door itself was a ubiquitous metal framed clear plastic interlocked door. The minimum starting distance off the floor and overall height of the door complied with CSA Z432 requirements. The provided safety interlock and safety relay was installed and operated as required. So the question remained — what would the “perceived” safety concern be?
Upon operating the interlocked door of interest, a mechanical arm that indexed boxes from one area of the machine to the other would randomly fall from any position and continue to try to moving for any variable amount of time after the interlocked door was opened. The client remained concerned because they felt they had done what was required by providing a CSA Z432 compliant interlocked door and by doing do, establishing due diligence. They had used the proper interlocking switch and safety monitoring relay. A Pre-Start Health and Safety Review was also completed, satisfying O.Reg 851 S (7), but still the issue of this randomly falling arm persisted. The question remained — was there still a machine safety deficiency present?
What complicated the situation was that this employer was also a unionized facility. The continuance of the randomly falling arm after the guarding and PSHSR completion was looked upon as an unresolved machinery safety deficiency, and employees servicing the machine in this area were still at risk of injury. While no injury had ever occurred, the union felt the risk was still unresolved regardless of the guarding and safety system being implemented. Given this, the union was in a legal position to file a grievance against the manufacturing and potentially seek a work refusal with the Ontario Ministry of Labour.
My approach to this problem was simple — I needed to conduct a Failure Modes and Effect Analysis (FMEA) as my starting point. For readers familiar with this technique, you will recall FMEA as a systematic, proactive method for evaluating a process to identify where and how it might fail and to assess the relative impact of different failures in order to identify the parts of the process that are most in need of change or present a risk. Firstly, I needed to establish my FMEA framework within the context of the machine itself, specifically at the arm in question and start documenting what potential direct and indirect sources of energy/other variables could contribute to this random motion. As is the case with many older/existing machines, my client did not have the requisite mechanical, electrical, pneumatic or hydraulic drawings. Given this reality, much time was spent at and within the machine.
As part of the investigative process, sometimes what a potential contributory factor is, is exactly what it is not. By removing the variables we are certain do not contribute to the problem, we are left with those can only contribute by default. This technique not only effectively reduces the number of potential causes, but also removes any potential false positives as well. A thorough inspection of the arm and associated mechanism itself indicated the system was mechanically sound and did not display any defective components or loose/failed fasteners/welds that could contribute to the random motion.
An inspection of the actuating system revealed the pneumatic valve that controlled the arms motion was only single acting. This could be a contributor to the problem. Upon closer examination, I discovered the valve arrangement was a two-position five-ported valve. Air was positively provided via the solenoid on only one side of the valve and a simple spring return was present on the other position; hence air pressure was only positively provided to keep the arm moving in one direction, the indexing direction, and not the other (the return stroke). When the air pressure was removed (for instance via the opening of the interlocked door), the associated valve would lose electric power, and the solenoid would de-energize, allowing the arms weight to counteract what remaining air was in the valve/supply line. As tested, the physical mass of the arm would overcome the exhausted valve and the falling motion would then drive the valves spool back — causing the arm to fall time and time again. I was confident this was the source of the continued plight for this employer.
What I concluded and recommended to my client was the following: replace the existing two-position five-way valve with a three-position centre blocking valve. It was fitted with double acting solenoids, piloted and spring return on both sides. By replacing the original single acting valve with this new one, we introduced control on both sides of the arms travel via the two solenoids. The spring assist on both sides of the valve ensured the valve would always go to the centre blocking position and keep the arm in its last position prior to having its air supply blocked. Having the centre block capability allowed the opportunity for air trying to leave the valve was mitigate. This centre blocking also allowed discernible control and stopping ability over the arms positions, regardless of where the arm was in its stroke when the interlocked door was opened. Given the valve was now double acting, some PLC reprogramming was required as well. The revised system also has local flow valves installed, allowing control over any residual “springiness” the arm-stopping motion may cause.
To the savvy reader, one can discern the problem described and solution implemented was not actually a machine safety related issue. The issue had to do with the design and control of a pneumatic arm. My client originally wanted to replace the non-locking switch with a timer-based solenoid locking one. While this arrangement would have addressed the falling arm, it would not have addressed why the arm was falling. Every entry would require an operator to watch the arm fall, and wait for timer to allow access — not productive at all.
Once we discovered what the true problem was, implementing the valve replacement yielded the arm to stop immediately in every position with no residual motion. As the changes implemented did not affect the existing safety system (interlock or relay) nor the existing PSHSR report provided, no additional safety review was required. As a result, the “perceived” machine safety deficiency grievance was withdrawn by the union and no further issues regarding the arm have been reported.
This column was originally published in the October 2016 issue of Manufacturing AUTOMATION.
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