Spray Lubricant on Stamping and Rolling Lines: Cut Oil per Part

A stamping press can run 2,400 strokes an hour, and one dry patch on a blank decides the scrap bin. A film measuring 0.4 g/m2 where the draw needs 1.2 g/m2 pushes the friction coefficient from about 0.08 to 0.15. The punch then tears the flange wall on the next stroke. A 2 percent rejection rate at that cadence is 48 scrapped parts an hour, each carrying its own coil and its own press time.

That gap between the film a die needs and the film a line delivers is where spray lubricant loses money. Oil laid on too thin leaves the die dry and the panel scored. Oil laid on too thick runs off the strip, floods the pit and returns later as a cleaning problem. This guide sizes a spray lubricant from the film target downward, through viscosity, orifice, fan angle and standoff. It closes on transfer efficiency, overspray and the oil per part audit that holds the setting.

The Snapshot

• A stamping or coil line runs a film of 0.5 to 3 g/m2, which at a lubricant density near 0.9 g/cm3 is 0.6 to 3.3 microns of oil on the steel.
• A 1,250 mm strip at 60 m/min presents 75 m2/min of surface, so a 1.2 g/m2 target needs 90 g/min of oil landed on the sheet.
• Air-assisted points hold a Sauter mean diameter of 20 to 80 microns at an air-to-liquid mass ratio of 0.1 to 1.0 and draw 0.2 to 0.6 Nm3/h of air each.
• Published lubricant transfer efficiency runs 60 to 85 percent with correct aim and 40 to 55 percent with poor aim, so overspray alone can add 40 percent to the oil bill.

Where Spray Lubricant Goes Wrong on a Line

Oil per part on a stamping or rolling line fails in four ways, and only one of them is a pump problem.

A dry band is the visible fault. It comes from insufficient fan overlap or from a standoff that has drifted. Scoring and tearing follow within a few hundred strokes, because the film is the only thing separating the die from the steel.

Overspray is the silent fault. Oil that misses the strip reaches the pit, the floor and the extraction duct. It costs the same as oil that lands and does no work.

Residue is the delayed fault. A heavy film oxidises on a hot die or roll surface and builds a carbon layer. The layer then marks the part and needs a manual clean.

Wear is the slow fault. An orifice grows over a few thousand hours, so the film creeps above target and the oil bill follows it upward unnoticed.

A buyer who raises flow to fix a dry band pays twice. The band stays or moves, and the extra oil leaves the line as overspray or as cleaning cost.

The Film Target: 0.5 to 3 g/m2

Published guidance for sheet forming puts the working film on a blank at 0.5 to 3 g/m2 of sheet surface. The low end suits light forming on a clean blank at moderate speed. The high end suits a severe draw with a long flange path and a high-strength alloy.

Film mass converts to film thickness through density. A representative drawing lubricant sits near 0.9 g/cm3, so 1 g/m2 is about 1.1 microns. The whole band therefore spans roughly 0.6 to 3.3 microns of oil on the steel.

That thickness is thin, and it changes how the spray has to behave. The boundary film that actually carries the load in a draw is on the order of 0.1 to 1 micron. Oil above that figure does not work in the contact. It sits on the surface, and surface oil is the oil that drips, runs and reaches the pit.

Demand then follows from line geometry alone. Take a 1,250 mm coil run at 60 m/min. The surface passing one header is 1.25 m multiplied by 60 m/min, which is 75 m2/min. At a 1.2 g/m2 target the line needs 90 g/min of oil on the steel. That is 5.4 kg an hour, or about 6.0 litres an hour at a density of 0.9 g/cm3.

One figure matters in that arithmetic, the mass on the steel. What the nozzles spray is always larger, and the ratio between sprayed mass and landed mass is the lubricant transfer efficiency.

Viscosity, Orifice Size and Atomizing Pressure

A hydraulic nozzle is defined by two numbers, the orifice and the pressure. Flow through a fixed orifice follows the square root of the supply pressure, so doubling pressure raises flow by 41 percent. Trimming flow is therefore an orifice job, and pressure is the atomizing control.

On a thin oil, pressure atomizes weakly. A single-fluid pressure nozzle on a 20 cSt oil at 5 bar produces a spray whose Sauter mean diameter typically runs 100 to 250 microns. That is coarse enough to land as a heavy sheet rather than as a fine film.

Viscosity changes the outcome more than pressure does. Published airblast correlations scale droplet diameter with liquid viscosity to a power of about 0.2 to 0.35. Take 0.3 and an oil that doubles in viscosity on a cold morning grows its mean droplet by roughly 23 percent.

Above about 100 cSt a single-fluid pressure nozzle stops atomizing at ordinary shop pressure. The liquid leaves the tip as a jet or a heavy sheet. Air has to supply the break-up energy at that point, which is why a viscous drawing lubricant is normally air-assisted.

Flow also falls as viscosity rises. The discharge coefficient of a small orifice drops with thick, cold oil. A point set in summer then delivers less in winter at the same gauge pressure. A pump curve checked at the coldest shop temperature is the safe way to size the supply. The square-law behaviour on the liquid side is worked through in the spray nozzle flow rate calculation guide.

Table 1 sets out four representative combinations.

Viscosity at working temperature	Nozzle type	Liquid pressure	Air pressure	Typical droplet SMD	Where it fits
10 to 20 cSt	hydraulic flat-fan, 65 to 95 degree	3 to 8 bar	none	120 to 250 micron	light drawing lubricant, thin sheet, high line speed
20 to 40 cSt	hydraulic flat-fan, or air-assisted	3 to 10 bar	none to 2 bar	80 to 180 micron	general stamping, blank preparation, coil lubrication
40 to 70 cSt	air-assisted flat-fan	2 to 6 bar	2 to 4 bar	40 to 110 micron	heavy drawing lubricant, thick gauge, moderate line speed
70 to 100 cSt	air atomizing, external mix	1 to 3 bar	2 to 4 bar	25 to 70 micron	tacky drawing lubricant, low film target, edge trim

Those bands describe published practice rather than a single product line. A point is selected at the working temperature of the oil in the tank, not at the 40 C grade reference point on the drum.

Orifice wear sits on the same list as viscosity. A 10 percent growth in orifice diameter raises the flow area 21 percent, and flow rises with it. An orifice checked on a 2,000 hour interval keeps the film inside its band.

Air-Assisted Against Hydraulic Atomizing

Two families of point can place a lubricant film on a moving strip, and the choice follows from viscosity and from the film target.

A hydraulic point uses the oil itself as the atomizing medium. It needs no compressed air, which keeps the running cost low and keeps the air out of the extraction load. Drop size is coarse, so the pattern is durable across a wide fan. Published transfer efficiency for a coarse spray on a strip runs 60 to 75 percent when the aim is correct. Heavy drops carry momentum all the way to the surface.

An air-assisted point mixes air into the oil at the tip. It breaks a viscous oil into 20 to 80 micron drops, which spreads a thinner and more even film at a lower oil rate. The price is compressed air. A representative point draws 0.2 to 0.6 Nm3/h at 2 to 4 bar of atomizing air.

Air is the cheap half of that trade. Six points at 0.4 Nm3/h draw 2.4 Nm3/h, which over 4,000 hours is 9,600 Nm3. At $0.03 per Nm3 that is about $288 a year. The same line spends tens of thousands of litres of oil at $3 a litre. The compressed air cost guide works the same arithmetic in more detail.

Air-driven hardware for this duty sits under air atomizing nozzles, and the bodies accept a range of tips and retaining caps for different fan widths.

A line that sprays lubricant onto a strip is not the same duty as a burner that atomizes fuel, yet both rely on the same relation between viscosity, pressure drop and ligament breakup, worked through in the oil atomization guide. Keep the two duty classes separate when you size pumps and filters: lubricant circuits tolerate finer filtration and lower velocities than fuel service, and confusing them buys wear in the wrong place.

Fan Angle, Overlap and Line Speed

A fan nozzle lays an elliptical pattern on the strip, and the pattern width follows from the fan angle and the standoff. Width is close to twice the standoff multiplied by the tangent of half the fan angle.

The arithmetic is short. A 110 degree fan at 200 mm standoff gives about 571 mm of pattern. The same fan at 300 mm standoff gives about 857 mm. A 95 degree fan at 200 mm gives about 436 mm, and an 80 degree fan at 200 mm gives about 336 mm.

Overlap then sets the spacing. Published practice for a coating or lubrication header runs 25 to 30 percent edge overlap. Spacing is therefore about 70 to 75 percent of the pattern width.

Apply that to a 1,250 mm strip. A 110 degree fan at 200 mm gives 571 mm of pattern. Multiplying 571 mm by 0.75 gives 428 mm of spacing, so 1,250 mm needs three points. An 80 degree fan at the same standoff gives 336 mm of pattern. Multiplying by 0.75 gives 252 mm of spacing, so the same strip needs five points.

The spray angle guide covers the geometry for the other patterns, and fan tips for this duty sit under flat fan nozzles.

Line speed does not change the drop size, and it does change the film. Deposition is mass per unit area, so the film scales directly with the oil rate and inversely with line speed. Raise a line from 60 to 90 m/min at a fixed flow and the film falls by a third, from 1.2 to 0.8 g/m2.

The standoff band for a lubricant header is usually 150 to 300 mm. A shorter standoff gives a narrower and more intense pattern with less wind loss. A longer standoff covers more width at a lower intensity and loses more to a cross draft.

A strip also drags the air next to it. At 120 m/min the boundary layer is measurable, and a spray aimed too close to the strip is deflected along the direction of travel. Keep the aim normal to the strip and the standoff inside the band.

Droplets, Overspray and Lubricant Transfer Efficiency

Lubricant transfer efficiency is the mass of oil that stays on the strip divided by the mass the spray lubricant nozzle delivers. That single ratio decides how much of the oil bill reaches the part and how much is thrown away.

Drop size sets the two loss paths. Drops above roughly 150 microns arrive with enough momentum to splash or bounce on contact. Drops below roughly 20 microns follow the air instead of the strip, and a fraction of them stays airborne as oil mist. The working window is 20 to 80 microns, which is where a mid-viscosity oil atomizes well. Droplet behaviour is worked through in the droplet size guide.

Published transfer efficiency for spray coating runs 30 to 60 percent for conventional air spray and 65 to 80 percent for high-volume low-pressure equipment. A lubricant header aimed at a flat strip does better than a paint gun aimed at a shaped part. A design band of 60 to 85 percent is therefore fair.

Aim is the leverage in that band. A header set at a 400 mm standoff instead of 200 mm spreads the same oil wider. The pattern is weaker, and more of it misses the strip. A fan angled 15 degrees from normal loses still more to one side.

The airborne fraction carries a second cost. Oil mist that escapes the header adds to the extraction load and to any exposure sampling at the line. Published exposure targets for oil mist sit near 0.5 mg/m3 in many plants. The NIOSH recommended exposure limit for mineral oil mist is 5 mg/m3 as a time-weighted average. Local limits govern, and they are sampled rather than assumed.

Stamping and Drawing: Galling, Scoring and Carbon

In a draw, the film only has to survive the contact for a short interval. The die wipes the blank across the bead and the wall in tens of milliseconds, so the film is renewed on every stroke. A press at 2,400 strokes an hour renews it 2,400 times.

The failure mode behind scoring is film breakdown. Friction at the flange rises as the film thins. Mild steel under a binder pressure in the 1 to 5 MPa range holds a friction coefficient near 0.08 while the film survives. Once the film breaks, the coefficient climbs toward 0.15 and metal transfers to the die.

Excess oil has its own fault. A film well above the target lowers friction at the binder as well as at the wall. A blank that slips at the binder wrinkles instead of drawing. A thicker film is not a safer film.

Extreme pressure additives buy film strength at high contact pressure. Chlorinated esters and sulfur-phosphorus packages are the two common families. Chlorinated products can leave chloride residue that interferes with a following weld or with paint adhesion. A line that feeds those processes often specifies a non-chlorinated drawing lubricant.

Carbon deposits come from oxidation. A lubricant with a high level of unsaturated components oxidises on a die or roll surface that runs 60 to 100 K above ambient. The result is a hard carbon film that marks the part and resists a normal wipe.

Coil Lubrication Along the Line

Coil lubrication is the same film problem applied to a moving strip rather than to a discrete blank. The targets are lower, and the width is fixed by the coil.

A rust-preventive film on a coil or a slit strip runs about 0.5 to 1.5 g/m2. It only has to keep moisture off the surface, so the film stays thin. The oil is often a light grade in the 10 to 30 cSt band.

A rolling line adds oil for two separate purposes. The rolling oil carries the friction control through the roll bite. Contact pressure is high there, and the sliding speed runs 200 to 1,000 m/min. The same header then wets the strip for the recoiler and the leveler.

Emulsion systems are the common answer on a cold mill. A rolling oil is emulsified at 3 to 10 percent concentration and sprayed at 3 to 8 bar through headers along the mill. The water carries the heat and the oil carries the film. Concentration is the master variable, and a 1 percent drift in concentration changes the film more than a 10 percent drift in flow.

A neat oil line is simpler and stays where it lands. It suits a stamping press or a slitting line that cannot tolerate water on the strip. Filtration then matters, because a neat oil is recirculated and a 25 to 40 micron element keeps the orifice clear.

Oil Consumption: A Worked Comparison

Take a representative line. Strip width 1,250 mm, line speed 60 m/min, film target 1.2 g/m2. The surface passing the header is 75 m2/min, so 90 g/min of oil must land on the steel. That is 5.4 kg/h, or 6.0 litres an hour at a density of 0.9 g/cm3.

Table 2 turns that landed figure into a sprayed figure through transfer efficiency.

Delivery method	Transfer efficiency	Oil sprayed, L/h	Oil lost off the strip, L/h	Annual oil at 4,000 h, L	Annual cost at $3/L
Air-assisted flat-fan, 150 mm standoff, aim verified	80 percent	7.5	1.5	30,000	$90,000
Air-assisted flat-fan, standoff drifted to 400 mm	55 percent	10.9	4.9	43,600	$131,000
Hydraulic flat-fan on a 25 cSt oil	65 percent	9.2	3.2	36,900	$111,000
Air atomizing with 20 micron drops, high mist loss	45 percent	13.3	7.3	53,300	$160,000

The spread between the top and bottom row is $70,000 a year on the same strip and the same film target. None of it comes from the oil price. It comes from how much of the sprayed oil reaches the steel.

The per-part view is smaller in scale and easier to audit. A blank 1,000 mm by 800 mm has 0.8 m2 of surface, so at 1.2 g/m2 it carries 0.96 g of oil. A press at 2,400 strokes an hour consumes 2.3 kg/h on the parts. Allow 80 percent transfer efficiency and the header must spray 2.9 kg/h, or 3.2 litres an hour.

That is the number to track on a weekly basis. Oil per part is a specification, and it is measurable in a way that a needle setting is not.

Cleaners, Emulsion Compatibility and Residue

The easy mistake on a line is to select the oil for the die and ignore the washer downstream. A lubricant that resists removal costs more to clean than it saves in oil.

Published practice for an alkaline cleaner runs 1 to 3 percent concentration at 55 to 65 C. Contact time is 60 to 120 seconds. A saponifiable or emulsifiable lubricant comes off inside that window. A neat oil that resists emulsification leaves a residue that a second stage has to remove.

Paint and weld processes add constraints. Silicone in a lubricant causes craters in a following paint film. A line that feeds a paint shop keeps silicone out of every lubricant, including a rust-preventive. Chlorinated additives leave chloride behind and can affect weld quality and paint adhesion.

Emulsion systems carry their own checks. Concentration, pH between 8.5 and 9.5 and tramp oil are the three to log. A tramp oil layer on a sump feeds a different lubricant to the roll bite than the one the header was sized for.

Residue on hot surfaces is a fourth item. A film that oxidises on a die or a roll leaves carbon, and carbon removal is a manual job. The cheaper answer is to hold the film at target and choose an oil with better oxidation stability.

Extraction keeps the air side under control. A hood at the header holds the oil mist load down. The capture velocity at the opening should run 0.3 to 0.5 m/s, and a coalescing element returns the captured oil.

Commissioning Checks and the Oil per Part Audit

A lubricant header is commissioned with a balance, a stopwatch and a marked panel. Each check takes minutes, and each one removes a class of fault.

Set the pattern on a marked panel at the working standoff. Spray for a fixed time, then read the wet pattern off the panel. Look for a dry band between fans, for a heavy stripe at a fan centre and for a fan that runs long on one side.

Set the flow with a measured mass. Carry a clean panel 300 mm by 300 mm through the header at line speed, then weigh it before and after. The panel is 0.09 m2, so at 1.2 g/m2 it should gain 0.108 g. A balance readable to 1 mg turns a film target into a measurement.

Log four numbers at the panel: supply pressure, oil temperature, standoff and fan angle. Those four explain most drift, and a handover sheet that records them catches a problem in hours rather than in weeks.

Audit on a schedule. A film weight once per shift on one panel is a production check. Oil per part weekly against the press counter is a cost check. A move of more than 15 percent on either reading is the trigger to inspect the header.

Check wear on the same route. Orifice growth of 10 percent raises flow about 21 percent. A header never checked at 2,000 hours runs a different film from the one it was commissioned on.

The Bottom Line

A spray lubricant is specified from the film target, not from a nozzle catalogue page. Start with the 0.5 to 3 g/m2 film target. Size the flow from line speed and strip width, then pick viscosity, orifice and pressure for the 20 to 80 micron droplet window. Fan angle and overlap keep the film even, and transfer efficiency decides what the oil costs.

Three checks hold the setting. A marked panel shows the pattern, and a weighed panel shows the film. A weekly oil per part figure against the press counter shows the cost.

Send the strip width, line speed, oil grade and film target and we will size the header: request a nozzle selection. Fan tips for lubricant headers sit under flat fan nozzles.