CNC Repair Tip #14: Troubleshooting Pneumatic Solenoid Valves (AC and DC)
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Recently I had a machine service call where an air cylinder was hesitating. This cylinder was actuated by two separate solenoid valves (SMC mod.#: EVP342-5Y0B-02FA-Q). One for advance of the cylinder and the other for retract. It was the retract that would occaisionally fail. This valve has a red LED indicator inside the electrical connector and, even though the LED was illuminated when the retract function was to execute, the cylinder would fail to retract. This led the tech to belive that electrically all was well, and that the valve was the cause of the problem. Later it was determined (after replacing the valve) that, due to a break in the wiring to the coil, there was insufficient current at the coil to fully actuate the valve. There was, however, sufficient current to illuminate the LED.
This CNC machine service experience had me thinking through some solenoid troubleshooting basics, so I thought this might be a good topic to write about. Here are some general tips and guidelines for troubleshooting pneumatic solenoid valves.
AC Solenoid Failures
Low Applied Voltage
Not enough magnetic force will be developed to allow the armature of the solenoid to seat. The unit will continuously draw high inrush current and burn out. Voltage should be checked at the coil with the solenoid energized. Possible causes of low voltage include: high resistance connections, too much load on the electrical circuit, and low voltage on the control transformer that powers the solenoid.
Valve Element Stuck
The armature may be held unseated because a valve element will not shift. The solenoid will draw high inrush current for an excessive time and burn out.
A metal-to-metal spool type valve may be varnished in place, or dirt may prevent the spool from shifting. Clean, lubricate and reassemble the valve. If varnish on the spool was the cause of sticking, placing an absorbative type filter in the circuit before the valve may resolve the issue.
A packed spool type valve may not shift due to swollen seals holding the spool in place, or because dirt prevents the spool from shifting completely. Clean the valve and repack the seals or replace the spool.
Solenoid Corroded
Use valves with adequate protection against moisture, coolants, etc. which may come in contact with them. Provide sealed electrical connections. Make sure that dirt and moisture covers are securely in place.
Solenoids Energized Simultaneously
On momentary contact, direct-actuated valves, check to make sure that both solenoids are never energized at the same time. This check is easily made by wiring a small indicator lamp temporarily across the coil of each solenoid. If both lamps are lit at the same time, the last solenoid to be energized will burn out. Correct the electrical circuit to prevent this. If the electrical circuit cannot be fool-proofed, consider the use of a pilot-operated, solenoid -controlled valve, where the two solenoids do not exert forces against each other.
High Transient Voltages
Solenoid burnout may be caused by high transient voltages that break down coil insulation, causing a short-circuit to ground. High transient voltages are most common where solenoids are connected to lines operating above 120VAC, which also control motors and other inductive-load type equipment. Swicting of such loads can create very high voltage peaks in the circuit. The remedy is to isolate the solenoid circuits from the main power circuits.
Temperature Too High or Too Low
Solenoid failures can be expected when a valve is operated above its rated temperature. Insulation may fail because it is not suitable for high temperature use. Specify solenoids designed for the ambient temperature, place the valve in a cooler location, or cosider the use of a pilot-actuated valve at the hot location, controlled by a remote pilot valve.
DC Solenoid Failures
Direct current solenoids should never burn out due to low voltage or failure of armatures to seat. This is due to the fact that no inrush current is experienced with a DC solenoid. Maximum current is the holding current, which is the same whether the armature is seated or not. Therefore, the coil is never subjected to an inrush current of three or four times the holding current as is the case with AC solenoids.
Valve failures due to shift can occur with DC solenoids as well as AC. Corrosion and temperature can also effect DC solenoid operation. The same troubleshooting guidelines for AC solenoids can be applied in these situations as well. Also, high transient voltage issues can be be handled the same as you would do for AC solenoids.
Solenoid Valve has Loud Hum
If the solenoid noise level is high and occurs each time the solenoid is energized, check to see that the armature is seating. Most direct actuated valves provide a manual override. If the solenoid noise decreases when the override is operated, this indicates incomplete motion of the spool. Check to determine whether or not moving parts are restricted and that the correct voltage is applied to the coil. Extremely loud humming noise may indicate a broken part within the valve and replacement will then be required.
Internally Piloted Valve Shifts Improperly
An internally piloted valve may shift partially and then stall. Air blows steadily through the exaust port. This is a sign that the pressure at the inlet port of the valve has fallen below the valve’s minimum operating pressure. Increase the supply pressure or provide a local air accumulator to maintain pressure at the valve during operation.
If the valve is provided with an internal pilot exhaust, change it to exhaust externally, if possible. Check for restrictions in the supply line. A gauge can be used to check the pressure available at the inlet port just before the valve fails to shift. Common causes of restricted supply lines include: clogged filter, restrictive lubricator, under-sized hoses or fittings.
Valve Emits Oil from the Exhaust Port
This is usually a sign of over-lubrication of the circuit. Adjust the oil feed-rate to the correct level.
Valve Occaisionally Malfunctions- Circuit Reaction is Slow
This type of problem can be caused by icing. Rapidly exhausting air can cool entrained moisture below its freezing point, causing ice particles to restrict air-flow. Mufflers can also freeze. Check the exhaust ports of the valve and/ or muffler for signs of icing. Dry the incoming air supply.
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