Sluggish, Jerking Reject Pusher on Packaging Line 3
The reject pusher on packaging line 3 is a double-acting cylinder (32 mm bore, 400 mm stroke) driven by a 5/2 single-solenoid valve, with speed set by two meter-out flow control valves. Standard extend time is 1.2 s; it now takes 2.4 s and moves in jerks, and the operator reports a hiss near the valve exhaust. The FRL unit gauge reads 5.8 bar against a 6.0 bar setpoint, and the filter bowl was drained this morning after water was found. The element has not been changed in 14 months. Ambient humidity is high and the compressor is a reciprocating type with no dryer. A silencer on the valve exhaust was removed three weeks ago to make it faster and was never refitted. The cylinder rod shows a light polish mark along one side. Production wants the line running again within one hour, and the nearest spare filter element is in the storeroom.
1. List, in the order you would perform them, the safety steps required before the technician removes the silencer, adjusts the flow controls, or opens any fitting. Explain why each step applies to this specific circuit.
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Isolation sequence
- Notify and isolate. Tell production the line is going down, stop the machine at a safe position, and isolate at the electrical panel and at the air supply ball valve. Apply personal lock and tag at both points. A 5/2 single-solenoid valve returns on power loss and can be driven by a manual override or a stray signal from the PLC, so locking the electrical supply is not optional.
- Vent the stored air. Open the downstream bleed, confirm 0 bar on a gauge, then leave the drain open. Air trapped between the valve and the piston can hold the rod extended; the hiss reported by the operator means energy is present even at rest.
- Support the load. The reject pusher and any product on it must be mechanically supported or moved to a stable position before the rod is disconnected.
- Verify zero energy. Gauge on 0, attempt a start, check for movement, and confirm the rod cannot be pushed by hand.
- PPE. Safety glasses with side shields (air at 5.8 bar drives debris and oil mist), gloves, and a face shield if the fitting will be opened under residual pressure.
- Only then remove the silencer or touch the flow controls.
Skipping step 2 is the classic error: the pressure gauge at the FRL can read zero while the cylinder side still holds pressure.
2. Describe your fault-finding sequence, from the air source to the cylinder, and state what each check tells you. Include what the water in the bowl and the missing silencer suggest.
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Work from source outward, cheapest first
- Flowing pressure, not static. Tee a gauge at the valve inlet and watch it while the cylinder strokes. If it drops more than about 0.5 bar below the static 5.8 bar, the restriction is upstream: a 14-month-old element in a wet, dryer-less system is saturated, and a saturated element gives exactly this slow, surging behaviour. Replace it and record the date on the bowl.
- Exhaust path. With the silencer removed, dust and swarf can enter the exhaust port and choke the restrictor or its check valve. A partially blocked meter-out restrictor starves the exhaust, so the cylinder creeps and jerks. Clean or replace the silencer, but refit it: without it, the exhaust noise hides leaks and the exhaust restrictor no longer controls speed predictably.
- Speed control setting. Count the turns out from fully closed on both flow controls and compare them. If one was turned in during the earlier attempt to speed up the line, restore a matched setting and adjust in quarter turns.
- Mechanics. The polish mark on the rod means side load, a bent rod, or a worn rod bearing. Disconnect the clevis and stroke the cylinder by hand; it must move freely. A binding cylinder is slow, jerky, and loads the seals.
- Internal leakage. Cap the ports at end of stroke and listen at the valve exhaust; continuous flow means piston-seal bypass. Confirm with a soap test at the fittings to separate a leaking seal from a leaking push-in fitting.
- Moisture damage. Water in the bowl means free water reaches the valve and cylinder, corroding bores and swelling seals. Drain daily, and raise the air quality with a dryer or coalescing filter rather than accepting repeated moisture.
3. State the corrective actions you would complete before release to production, the settings you would leave behind, and how you would verify the repair.
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Corrective actions
- Drain the FRL, fit a new filter element, refit a clean silencer, and replace any push-in fitting that failed the soap test.
- Free the rod: correct the pusher guide alignment or replace the bent rod, then re-grease.
- If the piston seal leaks, replace the seal kit; do not compensate for internal leakage by opening the flow controls.
- If the valve exhaust hisses with the ports capped, service or replace the 5/2 valve.
- Regulator at 6.0 bar flowing, element date-tagged.
- Both meter-out restrictors matched, giving 1.2 s ± 0.1 s extend and a smooth, non-impacting end of stroke.
- Silencer fitted; no audible leak at rest.
- Cycle 20 times and time each stroke; the spread should be small and repeatable.
- Check for drift by cycling with the conveyor running, which is when the pressure dip occurs.
- Record flowing pressure and cycle time on the PM sheet, then hand over with the isolation removed and the line guard refitted.
- Add the filter element and silencer to the PM checklist so the same failure is not repeated next quarter.