There is a lock former in a shop outside Jakarta that I visit twice a year. The first time I saw it, there was a dark smudge on the floor under the folding cylinder, roughly the size of a hand. The operator told me it had been there "a while" and that the machine was fine. Eighteen months later the same machine lost its folding pressure mid-cycle, bent a 1.2 mm panel out of square, and stopped a delivery. The repair was a seal kit and two hours of labour. The cost was the ruined panel, the truck that came back empty, and a customer who now checks every duct diagonally.
Hydraulics and pneumatics on a duct production line are unglamorous. Nobody specs a machine because of its oil filter. But these two systems account for a large share of the unplanned stops I get called about, and almost all of those stops announce themselves weeks in advance, in ways that cost nothing to notice and a lot to ignore.
On a typical line, hydraulics drive the guillotine shear, the folding beam, and on some machines the notching station and the decoiler brake. Working pressure sits between 14 and 20 MPa on most of the equipment we build. A compact lock former might carry a 40 to 60 litre tank with a gear pump; a full line with several stations runs 100 to 160 litres and often two pumps. The folding circuit is the one that matters most for quality, because a folding beam that hesitates mid-stroke does not just slow you down — it leaves a radius that varies along the length of the panel, and your flange width tolerance goes with it.
Everything downstream of the pump depends on the oil being the right viscosity, clean, and not too hot. That is the whole game.
We ship most lines with ISO VG46 mineral oil, which behaves well between roughly 10 and 35 °C ambient. In a Gulf or Southeast Asian shop where summer ambient hits 40 °C and the power unit sits in a corner with no airflow, VG46 thins out more than it should and you get more internal leakage, slower cycles and more heat. VG68 is the better choice there. In an unheated warehouse in northern China in January, VG32 gets the pump through the cold start without cavitating.
Oil temperature is the number to watch. Normal running range is 35 to 55 °C at the tank. Above 65 °C, oxidation accelerates sharply — as a rough rule, every additional 10 °C above that point roughly halves the remaining oil life, and it also cooks the nitrile seals. If your tank is running hot, the cheapest fix is usually airflow and a bigger cooler, not a different oil.
Changing oil by the calendar wastes money on machines that run one shift and risks damage on machines that run three. Sample instead. Take the first sample at around 500 hours on a new machine, because that is when break-in wear metals peak, then every 1,000 to 2,000 hours depending on duty. A basic lab report should give you four numbers worth reading:
One sample costs less than a drum of oil. Two samples a year on a machine that runs 4,000 hours will tell you more about its condition than any amount of listening to it.
Return-line filtration at 10 micron absolute is our standard, with a 100 micron suction strainer. The return filter has a bypass valve, and that valve is the quiet saboteur of the whole system: when the element clogs, the oil keeps flowing, unfiltered, and nothing on the machine tells you. Fit a differential indicator, or at minimum change the element on hours rather than on symptoms. First change at 500 hours, then every 1,000 to 2,000 hours, and immediately after any seal failure or hose burst.
The breather is the part nobody orders. A 3 micron breather on the tank cap stops airborne grit from being pulled in every time the oil level rises and falls. In a shop with a plasma table running nearby, that breather loads up fast. Quarterly replacement in a dusty environment is not excessive; it is cheaper than a pump.
Rod seals are the wear item. Polyurethane is the usual choice for the rod because it resists abrasion; nitrile handles the static seals fine. The rod itself is hard chrome, and the wiper is what keeps cutting grit off the rod so it never reaches the seal. When the wiper is torn and nobody notices, the seal follows within weeks.
There is a simple test worth doing monthly on the folding and shear cylinders. Extend the cylinder under working pressure, hold it for five minutes with the directional valve centred, and measure movement at the rod. A healthy cylinder drifts less than about half a millimetre per minute. More than that means internal bypass, and internal bypass on a folding circuit turns into inconsistent bend angles long before it turns into an obvious failure.
Also look at the rod surface with a light at a low angle. Scoring shows up as fine longitudinal lines before it becomes a visible groove. A polished rod with a new wiper will outlast three seal kits on a scored one.
Pneumatics on a duct line usually handle the beading station, clamping, the corner punch, and sometimes the TDF lock forming assist. Supply pressure is normally 0.6 to 0.8 MPa. What kills air systems is not pressure, it is water and dirt.
The filter-regulator-lubricator assembly needs its bowl drained daily in humid climates — and I mean daily, not when someone remembers. A 5 micron element is adequate; change it when the differential gets noticeable or every six months. If your valves are specified for oil-free air, do not fill the lubricator just because it has a bowl; adding oil to a system designed dry washes out the factory grease in the spool and causes sticking.
Compressed air dew point matters more than most people expect. In a coastal plant, a dryer that is undersized or has a saturated desiccant puts water into the lines, and the first symptom is a solenoid coil that fails in a month, then another, then another. If you are replacing coils more than once a year, look upstream at the dryer before you blame the valve brand.
| Interval | Hydraulic checks | Pneumatic checks | What to record |
|---|---|---|---|
| Every shift | Oil level at the sight glass, look for leaks under the unit, listen for pump noise | Drain FRL bowl, confirm supply pressure at the gauge | Level, pressure, any new leak location |
| Weekly | Tank temperature at end of shift, tank surface and cooler fins cleaned | Check for air leaks at fittings with soapy water; check cylinder rod condition | Temperature, leaks found and fixed |
| Monthly | Cylinder drift test at working pressure, hose and fitting inspection | Valve manual override function, exhaust silencer condition | Drift in mm per minute, parts replaced |
| 500 hours (new machine) | First oil sample, filter element change, tighten suction line clamps | First filter element change | Lab report filed against serial number |
| 1,000 to 2,000 hours | Oil sample, return filter change, breather change, accumulator precharge check | Filter change, lubricator refill if used | Sample trend, precharge pressure |
| Annually | Full oil change if sampling calls for it, pump coupling and motor alignment, hose replacement on age | Seal kit on high-cycle cylinders, full leak-down test | Machine downtime hours, cost per hour |
The folding cycle slows down through the shift. Almost always heat. Oil thins, internal leakage rises, and the pump is working harder for less. Check tank temperature before you touch the relief valve; someone has usually already raised it, which is the wrong fix.
The pump is noisy and the sound changes with load. Cavitation. Either the suction strainer is blocked, the oil is too thick for the ambient, or air is being drawn in at a loose suction fitting. Running it like this for a week will damage the pump internals.
The cylinder creeps when it should hold. Internal bypass past the piston seal, or a directional valve not fully centred. If drift appears on one cylinder only, it is the seal. If it appears on every function at once, look at the valve block or the relief setting.
Oil turns milky. Water. Could be a failed cooler, condensate from humid air entering through a missing breather, or wash-down water getting past a damaged rod wiper. Fix the source, then change the oil — changing oil without fixing the source is a subscription, not a repair.
A solenoid valve sticks intermittently. Usually contamination or water in the air, occasionally a coil that is failing thermally. Swap the coil with a known good one to isolate it in ten minutes rather than an afternoon.
For a single line running two shifts, I would stock: one seal kit per cylinder bore size, two return filter elements, two breathers, one 10 micron air element, one solenoid coil matching the most-used valve, one pressure gauge, one spare hose assembly for the highest-flexing run, and a litre of the correct oil grade for top-ups. That is maybe three hundred dollars of parts and it covers the failures that stop production. The pump, the valve block and the motor are better handled through the supplier, because stocking them ties up money in parts that fail once a decade — the same logic applies to the dies on a TDF flange former, where a spare roller set costs more than it saves.
One more thing, and it is not a part: write the oil grade and the filter part number on a label inside the cabinet door. The number of machines I have seen filled with the wrong viscosity because the manual went missing is not small, and it is entirely avoidable.
How often should hydraulic oil be changed on a duct line?
On condition, not on a fixed date. Sample at 500 hours on a new machine, then every 1,000 to 2,000 hours. With clean oil, a good breather and tank temperatures under 55 °C, many units go three to five years between changes. Hot, dusty shops with no breather maintenance can need a change in twelve months.
Can I use engine oil if the correct hydraulic oil is not available locally?
As a short-term top-up in an emergency, a comparable viscosity engine oil will keep you running. It is not a good long-term choice: the detergent package holds contaminants in suspension instead of letting the filter catch them, and it can foam. Order the right grade before you need it.
Why do my pneumatic solenoid coils keep failing?
Moisture and heat, in that order. Water in the air supply shorts or corrodes the coil, and a coil that runs hot because the valve is stuck will fail early. Check the dryer, drain the FRL, and confirm the coil voltage matches the supply rather than the label on the old coil.
Is a bigger hydraulic tank worth the money?
Usually yes on a line that runs three shifts in a hot climate. A larger tank gives more dwell time for heat to dissipate and for contamination to settle, which extends oil and seal life. It costs far less than a cooler retrofit later.