Mechanical press safety standards and good practices require several things in the event of a failure within the valve that controls a pneumatic clutch and brake:
• Air must be exhausted quickly to ensure fast stopping time.
• A monitor must take action to prevent further operation of the press.
• A method to alert personnel should be incorporated.
Double valves, or dual valves, have all of these features and have understandably thus become the press industry’s safety standard for clutch and brake control. These valves incorporate two valve elements which are independently controlled by two solenoid pilots within one valve body. The two valve elements share common inlet, outlet and exhaust ports. When the pilots are simultaneously energized, the elements operate so that the valve functions as a 3/2 normally closed valve.
Double valves, such as the ROSS DM2 Series D products, have built-in self monitoring and can send a signal back to the press controls in case of a valve malfunction. Each independent valve element monitors and cross-checks the operation of the other on each cycle, so that if one side fails to operate properly the valve immediately exhausts the downstream volume to less than 1% of supply pressure. The built-in monitor places the internal elements into a “locked-out” condition which prevents further operation of the valve or the press. A momentary reset signal is required to clear the valve fault and continue operation. An optional status indicator switch can provide “go” or “faulted” signals.
Because double valves provide a level of safety for mechanical press/clutch brake circuits that single element valves, even when externally monitored, simply cannot provide, they are known as “control reliable.” Control reliability is defined as “the capability of a device or system to stop or prevent initiation of hazardous motion in the event of a single component failure within the device or system”. Control reliable devices can be applied anywhere there could be exposure to catastrophic injury. Standardizing on double valves such as the ROSS DM2 Series D will simplify circuits and add a significant measure of safety.
Control reliablility requires:
• Redundancy
• Monitoring
• Further operations to be inhibited
• A defined action to reset
• Failing to a “SAFE” condition
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