[Application] Unloading spool winders (winding rolls)
[Hazardous source] Pneumatic moving table
[Types of accidents] Accidents involving being caught
[Cause of the accident] Failure of the pneumatic closed center valve
[Solution] Introduction of safety cylinder control valve (duplex structure & failure detection function)
By the way, in this "Safety Application Note", we analyze in detail the occupational accidents that actually occurred at some manufacturing sites, and what kind of accidents occurred. What was the hazard? It is also a document that summarizes the causes of accidents and their solutions. We hope this will help prevent similar disasters from occurring.
Application
This is a machine that winds a sheet of material about 3.0m wide onto a spool like a carpet roll. Once the winding is complete and the material is cut, an operator goes near the roll and tapes the ends. The winding speed is slow and there is no obvious danger. There are usually two workers in this area, one on each end of the spool.
To unload the spool, move a large steel table about 1.2m to the winder and unload the spool by rotating it on the table. The table is then removed from the winder and returned . A forklift then removes the spool from the table.
This table movement is accomplished by the operator manually pressing the extend button to move the table up to the winder and then pressing the retract button away from the winder. The table travel function is accomplished by controlling two 4 inch bore pneumatic cylinders using a 5-way 3-position "closed center valve". The valve is energized in one direction to extend the table to the rewinder, blocks both sides when de-energized, and retracts the table to the rewinder when energized in the other direction. This action happens at least once an hour.
Hazard
During normal operation of the machine, the operator frequently winds the material for various tasks, such as starting a new reel of material and inspection functions to ensure material quality and proper winding. Stand between the picker and the table. If an operator stands between the hoist and the table when the table is extended, there is a risk of collapsing.
The risk is high, each cylinder develops a force of 272kgf to 499kgf in the typical factory pressure range of 0.55 to 0.69Mpa. In other words, if a worker is caught between the table and the winder, the combined force of the two cylinders can generate a crushing force of 544-907 kgf. Since moving tables is part of normal work, workers are regularly in hazardous areas.
The 1-1.2m movement of the table takes 3-5 seconds, so there is a possibility that the risk can be avoided. However, during normal operations, workers often faced the winder with their backs to the table and wore earplugs so they could not hear the movement of the table.
It should be noted that the operator controlling the movement of the table has a complete view of the area between the table and the winder.
Installation of Safety Devices
Recognizing the potential danger of table stretching when a person is standing between the table and the winder, the company considers the downtime of the table and places safety mats on the floor and sides between the machines. was installed. These are wired to a safety relay that can block the electrical signal from the telescopic button to the valve.
Any person on the mat will not allow the safety relay to send the extend/retract signal to the valve. The valve remains in the center position and the table cannot be moved.
When someone steps on the mat with the button pressed, the safety relay removes the electrical signal from the valve, resulting in the valve moving to the center position. This stops the movement of the table.
Accident Occurred
After extending the table, the winder was successfully lowered. After that, the operator pushed the retraction button to retract the table. However, when the operator released the button, the table began to extend toward the winder table instead of staying in the retracted position.
The root cause is that the spring of the 5-way valve 3-position "closed center valve" was damaged. Although the spring failed while the extend button was pressed, the valve remained in position to provide pressure for the cylinder to extend when the button was released.
This fault was not detected when the button was released because the table was at the end of the cylinder stroke.
When energized for retraction, the valve moved properly to the retracted position, but when the retraction signal was released, instead of being centered by two reacting springs, the valve spool moved to the extended position as if it had been given an extend signal. continued to move to
The most disastrous aspect of this failure mode is that the table retracts when the retract button is pressed, but when someone steps on the "safety" mat the retract signal is disabled and the table immediately begins to extend. to put away. Not only was the functioning of the valve unsafe, but a malfunction of the valve could have caused the safety to actually create an unsafe situation.
Solution
It was found that the original 5-way 3-position valve was not rated to a high enough category or performance level to maintain the control integrity of the overall safety system. The safety mat and safety relay used were category "3" , whereas the original valve was category "1" . Unfortunately, the system that included this valve had a cumulative category of “1” .
A minimum category “3” safety rated valve is required to maintain the integrity of the safety system control .
There are two solutions to this circuit: one with both sides closed to trap the pressure, like the original five-way three-position valve, and the other with the cylinder open on both sides to vent, removing pressure and force from both sides of the cylinder. I had a plan. Due to the weight of the table and the horizontal position of the cylinder, I decided to remove the pressure from both sides of the cylinder. Then you will be able to manually move the table if needed. Of course, any solution requires a valve with at least a Category 3 rating.
In this case, two 3-way 2-position normally closed valves or two 5-way 2-position valves using only 4 ports as normally closed function are used together to form a 5-way 3-position open center circuit. must be created. Due to flow requirements and manifold capabilities, a size 0 cross mirror 5-way 2-position spring return valve was selected.
Category 4 compatible (third-party certification)
Safety Cylinder Control Valve Cross Mirror (CM) Series Size 0