Table of Contents
A slideway fails quietly. Pull the axis down to 0.05 m/min and the oil film falls out of hydrodynamic contact. The friction coefficient rises from 0.08 to 0.15, so the slide sticks and releases in short steps. No alarm sounds. The metal wears.
An air oil lubrication system answers from the metering side. A pump pushes a measured volume into a divider, the divider splits it between outlets, and low-pressure air carries each slug down a small tube to one point. The oil arrives once per cycle, in the quantity that point was designed to take.
Flood lubrication does a different job, moving litres per minute to cool and flush. An air-oil system moves millilitres per hour. This guide works the metering arithmetic behind that difference.
The Snapshot
- 0.01 to 0.05 mL per outlet per cycle is the slug band of a single-line progressive divider. A 3 mm piston with a 4 mm stroke gives 0.028 mL.
- 6.3 mL/h is what 14 points consume at 0.03 mL per outlet on a 4 minute cycle, which is 25.2 L over 4,000 machine hours.
- 3 to 10 m/s is the working air velocity band in the tube. A 0.03 mL dose fills 2.4 mm of 4 mm bore as a plug.
- 704,000 DN is where a 32 mm bore spindle at 22,000 rpm sits. Grease holds to roughly 300,000 to 500,000 DN, so that duty needs air-oil.
Why More Oil Does Not Fix a Worn Way
Doubling the oil flow to a slideway does not double the film. The carrying film is dragged into the converging wedge at the entry of the moving way. Past a certain supply rate, more oil leaves the contact than enters it, and the surplus runs into the coolant tank.
Drag is the first cost. A slide carrying 2,000 N on a 10 micron film at a friction coefficient of 0.05 needs 100 N of feed force. Raise that coefficient to 0.10 with a thicker oil and the same slide needs 200 N. At 0.5 m/s the extra 100 N becomes 50 W of heat.
Thermal growth is the second. An extra 50 W in a 100 kg cast-iron slide adds 50 / (100 x 460) = 0.0011 K per second, or 3.9 K an hour. Over a 4 K rise a 500 mm steel way grows 500 x 12 x 10^-6 x 4 = 0.024 mm, so the tool point moves 24 microns.
Boundary wear is the third, and it is why under-lubrication is the worse error. A way starved into boundary contact loses 2 to 10 microns per 1,000 hours of sliding. Ten microns of local wear on a 600 mm guideway is 10 microns of lost straightness, and the ball screw inherits it as pitch error.
What the System Delivers, Point by Point
An installation has five parts: a pump unit, a metering divider, tubing, an air preparation set and a nozzle at each point.
The pump unit holds 1 to 4 L and delivers to the divider at 15 to 60 bar, depending on line length and outlet count. The divider splits that volume into fixed slugs, one per outlet, and a pressure switch or cycle counter watches it. Standard pistons cover 0.01 to 0.05 mL per outlet per cycle, and micro pistons reach 0.005 mL for high-speed bearings.
The tubing runs from the divider to the point, usually 4 mm, 6 mm or 8 mm outside diameter nylon or copper with compression fittings. Bore matters more than outside diameter, because bore sets the air velocity. The air set holds 2 to 6 bar and cleans it with a 5 micron coalescing filter and a dryer.
The nozzle at the point is a tube end or a shaped tip that lays the slug onto the surface. That layout separates the two families buyers confuse. An oil mist system makes a fog at a generator, and the mist travels to the point as a suspension. In an air-oil system the oil stays a slug on the tube wall, and the air is only the conveyor. Oil air lubrication and air-oil lubrication name the same hardware, and a mist system also carries oil out with the exhaust air.
The Slug: Where 0.01 to 0.05 mL Comes From
A slug is not a setting on a dial. It is piston displacement, so it follows V = pi/4 x d squared x s, where d is piston diameter and s is stroke.
A 3 mm piston with a 4 mm stroke displaces 28.3 cubic millimetres, or 0.028 mL. A 2 mm piston with the same stroke gives 0.013 mL, and a 4 mm piston gives 0.050 mL. Three piston sizes cover the published band, which is why that band stops near 0.05 mL.
Ratio matters as much as size. Mixing a 2 mm and a 4 mm piston in one block gives outlets of 0.013 mL and 0.050 mL, a ratio of 1 to 4, so a large way and a small probe bearing receive doses that match their areas.
Repeatability is a band, not a number. Piston clearance is a few microns, so oil weeps past the land when the oil is warm. Measured dose typically varies 10 to 20 percent across a 20 to 50 degree C window.
The demand side sets the size. Charts for machine tool points commonly call for 0.2 to 1 mL/h at a bearing and 0.5 to 2 mL/h at a way. At 15 cycles an hour, 0.03 mL per outlet delivers 0.45 mL/h.
Progressive Dividers and the Other Metering Options
Pistons sit in a bore in series. Pump pressure moves the first piston to the end of its stroke, its discharge feeds the next piston, and each piston drives its neighbour. Oil leaves outlet A, then B, then C in fixed sequence until the cycle closes.
That sequence gives the block a monitoring feature no other metering device matches. If a piston fails to complete its stroke, the whole block stalls and the next cycle cannot begin. A proximity switch or a pressure switch sees the missing signal, so a blocked point or a collapsed tube is found at the divider rather than at the damaged surface. A primary block carries 4 to 24 outlets, and a secondary block after a primary outlet reaches 40 or more points from one pump unit.
The two-line, or dual-line, divider uses a reversing pump that applies pressure to line one, then line two. Each reversal strokes the dividers with a larger dose, so two-line systems suit large beds and presses at 0.1 to 2 mL per point.
Metering cartridges and injectors divide a common oil flow at the point. They retrofit easily on short runs, but they meter by pressure and orifice, so the dose moves with oil temperature and back pressure. Micro pistons at 0.005 mL will also stall on particles that a coarse filter passes, so the oil line takes a 25 to 40 micron element and the air a 5 micron coalescing filter.
Metering Arithmetic: Cycle, Shift and Year
The figure that matters in a budget is the annual oil volume. It follows from four inputs: slug size, outlet count, cycle interval and running hours.
| Step | Value | Arithmetic |
|---|---|---|
| Outlets on the divider | 14 | 8 way points, 4 spindle bearings, 2 ball screw nuts |
| Slug per outlet per cycle | 0.03 mL | a 3 mm piston with a 4 mm stroke |
| Oil per cycle | 0.42 mL | 14 x 0.03 |
| Cycle interval | 4 min | set by load and point size |
| Cycles per hour | 15 | 60 / 4 |
| Oil per hour | 6.3 mL/h | 0.42 x 15 |
| Oil per year | 25.2 L | 6.3 mL x 4,000 h |
| Reservoir interval | 238 h | 1.5 L / 0.0063 L per hour |
The reservoir interval is missed most often at design. A 1.5 L reservoir on a 6.3 mL/h draw empties in 238 hours, about six working weeks on two shifts, and a 20 percent level trip leaves 48 hours of warning.
A 4 L/min spindle oil supply moves 240 L/h, about 38,000 times the metered flow, and most of that returns to the sump. Make-up plus an annual sump change still lands at 185 to 310 L a year on a 60 L system.
The air that carries the oil has a price. At 0.3 Nm³/h per point, 14 points draw 4.2 Nm³/h, which over 4,000 hours is 16,800 Nm³, about $504 at $0.03 per Nm³, against $202 of oil at $8 per litre. The compressed air cost guide works the same arithmetic for atomising nozzles, and a 20 second air purge every 4 minutes cuts the draw to an 8.3 percent duty and the air bill to about $42.
Flood, Mist and Air-Oil: The Consumption Comparison
| Metric | Circulating oil (flood) | Oil mist | Air-oil, single line |
|---|---|---|---|
| Oil consumed, 14 points, 4,000 h | 185 to 310 L | 40 to 120 L | 25 L |
| Oil per point per hour | not measurable per point | 0.5 to 2 mL | 0.2 to 1 mL |
| Reservoir volume | 40 to 100 L | 1 to 4 L | 1 to 4 L |
| Return lines | required | none | none |
| Oil to the coolant sump | high, through carryover and leaks | low, but the exhaust carries oil | low |
| Air draw per point | none | 0.2 to 0.5 Nm³/h | 0.2 to 0.5 Nm³/h |
| Annual air cost, 14 points | none | $336 to $840 | $336 to $840 |
| Heat removal | kW scale | under 1 W | under 1 W |
| Blocked point detection | pump pressure and level | none by default | divider stall signal |
The heat row is the one buyers misread. At 6.3 mL/h and a 40 K rise the oil removes 0.0057 kg/h x 1,900 J/kgK x 40 K = 433 J/h, about 0.12 W. A circulating supply at 240 L/h over a 10 K rise removes 0.06 kg/s x 1,900 x 10 = 1,140 W. The two figures sit four orders of magnitude apart.
A spindle with a 5 kW loss therefore needs its own cooling path. At an 8 K water rise that path is 5,000 / (4,180 x 8) = about 9 L/min through a jacket or an oil cooler.
On a flood system a blocked point shows only as a small change in pump pressure or sump level, and on a mist system it may never show at all.
Viscosity, ISO VG 68 and the Working Temperature
Viscosity grades are quoted at 40 degrees C. A machine rarely runs there, so the relevant number is the viscosity at the point, at the temperature of the surface.
| Grade | At 40 C | At 100 C | Approximate at 20 C | Typical duty |
|---|---|---|---|---|
| ISO VG 32 | 32 cSt | 5.4 cSt | 110 to 150 cSt | spindle bearings above 700,000 DN |
| ISO VG 46 | 46 cSt | 6.8 cSt | 160 to 220 cSt | general spindle and machine oil |
| ISO VG 68 | 68 cSt | 8.7 to 9.5 cSt | 300 to 450 cSt | slideways with a tackifier |
Film thickness scales with viscosity, and the exponent differs by contact type. A hydrodynamic film on a slideway scales with the square root of viscosity, so doubling viscosity raises the film 41 percent. An elastohydrodynamic film in a rolling bearing scales with viscosity to the power of about 0.7, so a 53 percent viscosity drop cuts the film about 41 percent. VG 68 to VG 32 is exactly that drop.
Between 40 C and 100 C a mineral oil in this grade range falls about eight times in viscosity.
That drives the rule for ways oil. A tackified ISO VG 68 ways oil suits guideways, because the film has to survive low sliding speeds and the tackifier holds it in place. The same tackifier adds churn and heat in a high-speed spindle bearing, so machines that run both grades carry two pump units. A shared unit takes VG 46 as the compromise, and the hydrodynamic film on the way is then about 31 percent thinner than on VG 68.
Air Pressure Bands and Slug Transport Velocity
Air does two jobs in the tube. It holds positive pressure at the point, which keeps coolant and dirt out of a bearing, and it carries the slug from the divider.
The working band is 2 to 6 bar. Below 2 bar the velocity in a normal tube is often too low to move a slug on a vertical rise. Above 6 bar the air cost rises with the flow and the film at the nozzle starts to strip into a mist.
| Tube bore | 3 L/min | 5 L/min | 10 L/min |
|---|---|---|---|
| 2.5 mm | 10.2 m/s | 17.0 m/s | 34.0 m/s |
| 4 mm | 4.0 m/s | 6.6 m/s | 13.3 m/s |
| 6 mm | 1.8 m/s | 3.0 m/s | 5.9 m/s |
| 8 mm | 1.0 m/s | 1.7 m/s | 3.3 m/s |
A 4 mm bore at 5 L/min runs at 6.6 m/s, inside the 3 to 10 m/s band. An 8 mm bore needs 10 L/min to reach 3.3 m/s, which costs twice the air, and a 2.5 mm bore at 5 L/min is at 17 m/s, where the oil film is stripped.
Keep one bore for the whole run, because a step from 4 mm to 6 mm drops the velocity by a third at the joint and a slug can stall there. The flow and diameter relationship follows the square law that the flow rate calculation guide sets out.
A tip at the nozzle adds under 0.5 bar of back pressure, so wait one cycle plus the transit before judging a point after a change. Tip choice is covered in the air nozzle selection guide, and our air-driven hardware sits under air atomizing nozzles.
Spindle Bearing Lubrication and the DN Ceiling
The DN factor is bore in millimetres multiplied by speed in rpm. Grease holds to roughly 300,000 to 500,000 DN, depending on seal type and ball material. Oil-air covers roughly 700,000 to 1,500,000 DN, and oil jet takes over above that.
A 70 mm bore spindle at 20,000 rpm sits at 1,400,000 DN, in the air-oil band. A 32 mm bore spindle at 22,000 rpm sits at 704,000 DN, above the grease ceiling.
The dose at a bearing is small. Published practice runs 0.01 to 0.03 mL per bearing per cycle, with the interval set by speed: 1 to 3 minutes at 20,000 rpm, 10 to 20 minutes on a slow spindle.
The film that matters is elastohydrodynamic, and it is thin. A well-lubricated bearing runs on a film of 0.2 to 0.5 micron. With counterface roughness of 0.05 to 0.1 micron, the lambda ratio sits at 3 or better. Drop one viscosity grade and the film falls about 41 percent, which can take 0.35 micron to 0.21 micron and the lambda ratio toward 2, where asperity contact begins.
Holding 0.2 to 0.5 bar of positive pressure in the housing keeps coolant and chips out, which is why the air often stays on between lubrication cycles on a spindle.
Guideway Film Thickness, Stick-Slip and Wear
A moving slide generates its own film as long as oil reaches the entry. The film on a machine way at normal feed runs 5 to 20 microns, and metered oil only has to keep the wedge supplied.
The deposit from one slug is far thinner. A 0.03 mL dose spread over 0.1 square metres of way face is a mean film of 0.3 micron. The carrying film comes from way speed, not from the divider, which is why a small dose works and why the dose must repeat.
Hydrodynamic thickness scales with the square root of sliding speed, so a 600 mm way running at 5 m/min on a 10 micron film drops to about 1 micron at 0.05 m/min, which is boundary contact. Low feed rates, not high loads, are where a starved way shows itself.
Stick-slip is the visible symptom. A starved way with a static friction coefficient of 0.15 and a dynamic value of 0.08 on a 2,000 N load releases 140 N when it breaks away. In a drive loop with 10 N per micron of stiffness, that arrives as a 14 micron step, which is the whole tolerance band on a finishing cut.
A way starved into boundary contact loses 2 to 10 microns of material per 1,000 hours of sliding.
Fault Diagnosis from the Symptoms
| Symptom | Likely cause | Check | Corrective action |
|---|---|---|---|
| Stick-slip at low feed, dry ways | interval too long, or slug too small for the way area | catch 20 cycles at the point, expect 0.6 mL at 0.03 mL per outlet | shorten the interval or fit larger pistons |
| Oil pooling under the nozzle, spots on the work | air velocity too low, or a bore step in the run | measure flow at the tube end against the velocity table | raise the air flow or rework the run to one bore |
| Divider stalls, no cycle signal | one outlet blocked by debris or a deformed tube | isolate each outlet in turn and watch the piston indicator | clear the point, filter the oil at 25 to 40 micron |
| Bearing 10 to 15 K hotter than baseline | too much oil, or a grade one step too thick | compare the dose and the grade against the machine record | cut the slug or step down a viscosity grade |
| Oil use rising slowly over months | pump wear, or a leaking piston land | measure the reservoir drop per shift against the record | service the pump and verify the oil grade |
| Milky reservoir, oil use drifting | wet air past a check valve | check the dew point at the filter outlet | fit a dryer and a coalescing filter |
Log the reservoir drop per shift, because a slow change in oil use is the earliest signal of a worn piston or a wrong grade. Log the divider cycle count too, because a change in cycle rate at a fixed load points at a pressure switch or a pump losing output.
Commissioning, First Fill and Metering Checks
Confirm the oil grade and the fill volume against the drawing. Grade errors are the commonest cause of a spindle overheating after a service.
Prime the lines and run five cycles by hand, watching the piston indicator. Every piston should complete a stroke on every cycle.
Measure the dose. Catch or weigh 20 cycles: at 0.03 mL per outlet that is 0.6 mL, or 0.54 g at a density of 0.90.
Set the alarm window to 1.5 times the normal cycle time, so a 4 minute cycle alarms at 6 minutes without a piston signal. Record the reservoir level per shift, since a 10 hour shift draws 63 mL at 6.3 mL/h and a 20 percent level trip leaves about 48 hours of warning.
Check the runs: one bore per run, no step down at a junction, and air dried below the lowest ambient the shop sees. Label fittings with standard and size, because way blocks and housings mix BSP and NPT ports, and the thread size guide carries the conversion table.
Frequently Asked Questions
What is the difference between an air oil lubrication system and an oil mist system? An air oil lubrication system meters a discrete slug with a positive displacement divider and carries it along the tube wall, so the exhaust is almost oil-free. A mist system makes a fog at a generator, so consumption is set there and a blocked point can go unnoticed.
How much oil does one lubrication point need per cycle? Standard practice sits at 0.01 to 0.05 mL per outlet per cycle, and a 3 mm piston with a 4 mm stroke gives 0.028 mL. Charts commonly ask for 0.2 to 1 mL/h at a bearing and 0.5 to 2 mL/h at a way.
Which viscosity grade should an air-oil system use? ISO VG 32 for spindle bearings above 700,000 DN, and ISO VG 68 with a tackifier for guideways, checked at the working temperature. A mineral oil in this range falls about eight times in viscosity between 40 C and 100 C.
How often should the pump cycle? A single-line progressive system typically cycles every 1 to 30 minutes, from 1 to 3 minutes at 20,000 rpm down to 10 to 20 minutes on a slow spindle.
Does an air-oil system cool a spindle? No. At 6.3 mL/h and a 40 K rise the oil removes about 0.12 W, while a circulating supply at 240 L/h removes about 1,140 W. A spindle with a 5 kW loss needs about 9 L/min of cooling water.
Can one pump unit serve the ways and the spindle bearings? Yes, if both accept the same grade, but a tackified VG 68 ways oil is a poor fit for a high-speed bearing. Two pump units are normal, and a shared unit takes VG 46 as the compromise.
What air pressure does an air-oil point need? Run 2 to 6 bar and set the flow from the bore, since a 4 mm bore at 5 L/min gives 6.6 m/s, inside the 3 to 10 m/s band. Below 2 bar a slug can stall on a vertical rise.
Sizing Your Duty
An air-oil system is a metering system, not a cooling system and not a flood. Set the slug from the piston, the interval from the load, and the grade from the working temperature.
Four figures carry the specification: 0.03 mL per outlet per cycle, a 4 minute cycle for a 6.3 mL/h draw across 14 points, 25.2 L of oil a year, and 3 to 10 m/s in a single-bore run.
If your machine floods its ways to get lubrication, send the point list, the duty and the grade to our application team. We size the air side of the circuit, and the air-driven hardware sits under air atomizing nozzles.
Next Step
Send the Duty. Get Sized Nozzles Back.
Send your flow, pressure, fluid and target coverage. We come back with nozzle options and figures, not a catalogue number.
Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.