Conveyor as well as stand-alone automated equipment, Numatics AC/DC solenoid valves are used to feed air cylinders to produce motion in lifts, tooling, and slide mechanisms.
After 1-2 days of downtime, manual actuation of many of these valves is required to "loosen up" the spool enough for electrical operation.
Lubrication helps with the valve operation and a small role in the solenoid, but you need to check if it is the Valve spool or the core of the solenoid that is sticking.
Remember - if you are using a lubricator on valves, then you MUST continue to use lubrication, as the oil spray will knock out the grease on valve spools. If your lubricator has run out of oil (Shell Tellus 32 works well) you may be running dry valve spools.
If your system is plc controlled - I'm sure you can edit the logic to run a test cycle every couple of hours while production is down. (just a suggestion!)
Solenoids do not vary too much, but valves do, Glandless (no rubber seals between the spool and the body) last the longest (millions of cycles), however most pneumatics manufacturers use the same solenoid assemblys to fire them. (ie. the solenoid is usually expected to last as long as the valve)

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