Riyadh, August, a Line 5 in a shed with no evaporative cooling. The customer called because the shear had started to lose force after about three hours of running, and by mid-afternoon the hydraulic unit was too hot to put a hand on. The gauge read 9 bar when it should have read 140. Nothing was broken. The oil had thinned until the pump was bypassing internally, and the reason it had thinned is that the tank was 90 litres, the cooler was air-blast and undersized, and nobody had specced either of them for a 45 degree ambient.
We added a 60 litre reservoir and a bigger cooler, changed to ISO VG 46 instead of 32, and the machine has run three summers since. But the underlying problem was in the quotation stage. Somebody picked a hydraulic unit on price and never asked what the duty cycle was, what the ambient was, or what the shear was actually being asked to cut.
Duct line power systems get treated as a black box. They are not. There are three of them on a typical automatic line — electric drive, hydraulic, pneumatic — and each has a sizing logic that takes about twenty minutes to work through. Here is how I do it.
Electric drive moves material: the decoiler, the straightener, the feed rolls, the main line drive, the beader, the lock former. It needs torque at speed and it needs to be variable, because a line that runs at one speed is a line that cannot handle a change of material.
Hydraulic does the high-force, short-stroke work: the shear, the notching unit, the folding beams on some configurations, the flanging heads. It gets its force from pressure rather than from mechanical advantage, which is why a 5.5 kW hydraulic unit can shear 1.5 mm stainless that would stall a 15 kW mechanical drive.
Pneumatic does clamping, positioning, and the small fast actions: hold-downs, side gauges, the sheet pusher on a transfer, the ejection on a corner punch. It is cheap, fast and clean, and it is the system most often starved because somebody teed it off the shop's existing 6 bar line.
The number on the datasheet — 5.5 kW, 7.5 kW — is the motor rating, not the power the machine uses in normal running. A Line 5 with a 5.5 kW main drive typically draws 3.2 to 4.1 kW at 12 m/min on 0.8 mm galvanized. The margin exists for the acceleration and for heavy material.
What actually determines the requirement is the pull force at the feed rolls. For a line with a straightener, the drive has to overcome:
For 1.25 m wide, 1.0 mm galvanized at 12 m/min, the steady-state pull is around 2.8 kN, which at 0.15 m/s is 420 W of actual work. The 5.5 kW motor is not there for the work. It is there for the 0.8 second acceleration ramp from zero to line speed with a 4 m sheet already in the machine, and for the day somebody loads 1.5 mm stainless.
Nine out of ten drive faults I get called about are acceleration ramps set too short. A factory default of 0.3 seconds on a 5.5 kW drive accelerating a loaded line will trip on overcurrent every time the operator hits start with material in the rolls. Set the ramp to 1.2 to 1.8 seconds, set the current limit to 130 percent of nameplate, and use sensorless vector mode rather than V/Hz if the drive supports it. That combination alone has fixed more "weak motor" complaints than any motor swap.
Second most common: the drive is set for 50 Hz and the site is 60 Hz, or the reverse. A motor rated 50 Hz run at 60 Hz will spin 20 percent faster and lose 17 percent of its torque, which on a fixed-torque load means it stalls under heavy sheet. Either specify a dual-rated motor (50/60 Hz, 380-440 V) or set the drive's maximum frequency to match the motor nameplate and accept the lower line speed.
Hydraulics is where the real specification work sits, because it is the system that fails in hot climates and the one where undersizing is invisible until the machine is on site.
Start from the cylinder. Shear force required for 1.5 mm stainless at 1.5 m width: shear strength around 400 MPa, cut length 1500 mm, thickness 1.5 mm, so force is roughly 0.7 x 400 x 1500 x 1.5 = 630 kN, call it 63 tonnes. A cylinder with a 125 mm bore at 160 bar gives 196 kN of thrust — not enough. You need either two cylinders or a 160 mm bore. Two 125 mm cylinders at 160 bar give 392 kN, still short, so the real answer is a 200 mm bore single cylinder at 160 bar (502 kN) plus a shear blade geometry that reduces the instantaneous cut length. This is why the shear on a duct line is a 200 mm bore unit and why the pump is 5.5 to 7.5 kW.
Once you know bore and pressure, flow follows from cycle time. A 200 mm bore cylinder with a 100 mm stroke displaces 3.14 litres per stroke. If the shear must complete in 1.2 seconds, you need 157 litres per minute. That is a big pump. Most lines use a two-stage or a pressure-compensated pump that gives high flow at low pressure for the approach and low flow at high pressure for the cut. The datasheet number you want is not "flow" but "flow at 160 bar".
| Item | Small line (Line 3) | Standard (Line 5) | Heavy duty (Line 5, 1.5 mm) |
|---|---|---|---|
| Motor | 3.7 kW | 5.5 kW | 7.5 kW |
| Max pressure | 140 bar | 160 bar | 180 bar |
| Pump flow at rated pressure | 18 L/min | 25 L/min | 32 L/min |
| Reservoir | 60 L | 90 L | 140 L |
| Oil grade, 10-30 C ambient | ISO VG 32 | ISO VG 32 | ISO VG 46 |
| Oil grade, 35-45 C ambient | ISO VG 46 | ISO VG 46 | ISO VG 68 |
| Cooler | Air blast, 60 L/min | Air blast, 100 L/min | Air blast + 200 L tank or water |
| Target oil temperature | 40-55 C | 40-55 C | 40-50 C |
| Filtration | 10 micron return | 10 micron return | 10 micron return + 5 micron pressure |
Hydraulic oil heats by roughly the pressure drop times the flow, and the rule of thumb is that 20 to 30 percent of installed power becomes heat in a system with a pressure-compensated pump sitting at relief. A 5.5 kW unit therefore needs to shed about 1.2 to 1.6 kW continuously. In a 25 degree ambient, a 100 L/min air-blast cooler handles that comfortably. At 45 degrees ambient it handles about 60 percent of it, which is exactly the Riyadh failure.
The fix is either a bigger cooler (cheap, and it works), a larger tank (60 litres of extra oil at 15 degrees delta adds real thermal mass and buys you time), or a water-cooled exchanger if the site has cooling water. I now ask for the site's summer ambient on every quotation. It takes one line in the email and it saves a service trip.
Pneumatics are usually an afterthought, and small undersized air systems cause the most annoying faults because they are intermittent. The clamp holds, then it does not. The pusher fires, then it hesitates. The operator blames the PLC.
Start with cylinder volume. A 63 mm bore, 100 mm stroke clamp cylinder displaces 0.31 litres per extension at 6 bar. A line with eight clamps cycling once per section at 40 sections an hour uses 8 x 0.31 x 40 x 2 (extend and retract) = 198 litres per hour of free air. That is nothing. Add a transfer pusher at 80 mm bore and 400 mm stroke: 2.0 litres per stroke, 40 strokes an hour, 160 litres. Still nothing.
The real draw is leakage and the initial charge. A shop with 30 m of ageing 1/2 inch pipe and eight quick couplers will leak 200 to 400 litres per hour continuously, which is more than the machine uses. That is why the machine behaves badly on a shared ring main and perfectly when you put a dedicated receiver next to it.
My standard specification: 6 to 8 bar supply, 400 L/min free air at the machine, a dedicated 100 litre receiver within 5 m, a filter-regulator-lubricator unit at the inlet, and a refrigerated dryer on the branch if the site humidity is above 70 percent. Moisture in the air is what kills clamp seals and, on a vacuum transfer, what makes sheets drop.
Shear loses force after two hours. Oil temperature. Check the gauge cold and hot. A 20 bar drop between cold and hot means the oil is too thin or the pump is worn. Start with oil grade, then cooler, then pump.
Line trips on start with material loaded. Acceleration ramp or current limit. Not the motor. Log the drive fault code before you touch anything.
Clamps release slowly. Exhaust flow control set too tight, or a clogged silencer. Silencers are a 3 dollar part that gets ignored for years and then costs a shift.
Feed drifts on long sheets. Not electrical. Usually a worn feed roll or insufficient clamp pressure, and sometimes a slipping drive belt that only slips when the sheet is heavy.
Random E-stop trips. Moisture in a pneumatic limit switch, or a chafed cable at the drag chain. Check the drag chain first; it is the highest-wear electrical item on the machine.
Hydraulic oil: sample at 500 hours, change at 2000 hours or annually, whichever comes first. Change the return filter at 500 hours then every 1000. Do not skip the first change — that is when the assembly debris comes out.
Air system: drain the receiver daily, replace the filter element every 1000 hours, check the dryer dew point monthly in humid climates. Replace silencers annually whether they look blocked or not.
Drive: check the VFD heatsink every quarter. In a dusty shop the fins pack solid within a year and the drive derates, then trips on overheat in the afternoon. A brush and ten minutes solves it.
Belts and couplings: check tension at 250 hours, then every 500. A loose drive belt on the feed rolls is the single most common cause of dimensional drift on a line that was cutting square last month.
If you are specifying a new line, the Line 5 configuration page lists drive and hydraulic ratings per station, which is the fastest way to see whether a quoted machine is actually built for your material. For standalone machines the logic is the same: a hydraulic shearing machine is sized on cylinder force and cut length, while a pneumatic TDF folding machine is sized on air volume and clamp force, and the two should never be compared on motor kW alone.
Yes, with two conditions. The motor must be dual-rated or the VFD maximum frequency must be set to the motor nameplate, and the hydraulic pump must be checked for the higher speed — a pump rated 1500 rpm run at 1800 rpm delivers 20 percent more flow, which is usually fine but raises oil temperature. Set the pressure relief 5 percent lower and watch the temperature for the first week.
No. An oversized unit runs at low load, which means the oil never reaches working temperature and condensation accumulates in the tank. I have seen 7.5 kW units on machines that needed 5.5, running cold and emulsifying oil within a year. Size to the shear force and the cycle time, then add the cooler for the ambient.
400 litres per minute of free air for a typical Line 5 with eight clamps and a transfer, measured at the machine inlet. That is the number to give your compressor supplier, and it should include a 20 percent margin for leakage on the branch line.
ISO VG 46 for ambient up to 35 degrees, VG 68 above that, with a viscosity index of at least 95 and anti-wear additive. Do not use engine oil. Do not mix brands. And change it on hours, not on appearance — dark oil with a good viscosity is better than clear oil that has sheared down.