Tissue machine lubricants: requirements and critical points by machine section

In the machine that makes toilet paper and paper towels, the start is always wet, the end is always hot, and the gears and pressurized oil systems each need the type of oil that their standard defines. That is why the lubricant is chosen point by point, according to what attacks it in each place. Below, the machine from the inside and what each point asks for.

How does a tissue paper machine work?

It turns a wet mass of fibers into dry paper in a single pass. Tissue is the thin, soft paper: toilet paper, towels, napkins and facial tissue.

The most common machine, the one in this guide, receives at one end a pulp of cellulose fibers that is almost all water and delivers a reel of paper at the other. It is like wringing out a wet cloth and then ironing it:

1. Forming: the headbox spreads the pulp over the wire, a fabric sieve that lets the water drain away.

2. Press: a felt (a blanket) carries the sheet to the press, which squeezes it against a huge cylinder heated inside by steam, the Yankee cylinder.

3. Drying: the sheet sticks to the Yankee, and a cover, the hood, blows hot air from outside.

4. Creping: a blade peels off the dry sheet and wrinkles it, which gives it softness; the reel winds it into a roll.

In the mill, the machine is split in two: the wet end (forming and press), where the water is squeezed out, and the dry end (from the Yankee onward), where it leaves by heat.

Figure 1. In the wet end the water is squeezed out; in the dry end, it leaves by heat.

For those who want the number: the pulp reaches the forming section with about 99% water (Machinery Lubrication, 2004).

Where does a paper machine need lubricant?

At three types of point: bearings, gearboxes and hydraulic systems.

Every roll and every cylinder turns supported on the ends of its shaft, and that support is the bearing. In a paper machine it is almost always a rolling bearing: balls or rollers turning between two rings, as in the hub of a bicycle wheel.

The gearbox is a case of gears that lowers the motor speed and raises the turning force, like the low gear of a bicycle on a climb. It drives rolls and the Yankee itself.

The hydraulic system uses pressurized oil to move pistons, like a garage jack.

There are usually two, each with its own oil. One moves pistons that position and press rolls, such as the press roll against the Yankee. The other works inside certain long rolls and pushes them from within, so that they do not sag in the middle and press the sheet evenly across the whole width.

In a paper machine, the lubricant reaches these points in four main ways:

• Grease: a paste that stays in the bearing, as in most forming-section bearings.

• Oil bath: the gears of the smaller gearboxes dip into the oil at the bottom of the case and spread it as they turn; large gearboxes also receive pumped oil.

• Circulating oil: a pump takes the oil from the tank to the bearing, and it comes back.

• Hydraulic: the oil circulates in its own system, and its job is to move pistons; along the way, it lubricates the pump and the valves.

In the press, grease and circulating oil coexist: the large, fast bearings use circulating oil; the smaller ones use grease. This guide deals with oil; grease follows each equipment manual.

Figure 2. Grease in the forming section, oil bath in the smaller gearboxes, circulating oil in the press and Yankee bearings, and hydraulic oil in the pistons.

For those who want the number: 80% to 90% of paper machines use grease in the wet end, where it also works as a barrier against water (Machinery Lubrication, 2004).

Of the four, circulating oil is the one that looks after the large, hot bearings.

How does circulating oil reach the bearings?

Through a closed circuit, like an aquarium filter. The pump pulls the oil from the tank, the filter cleans it and a cooler (a heat exchanger) removes part of the heat. Then the oil passes through the bearings and returns to the tank by gravity, already hot from the heat it carried.

In the tank, the oil rests. The heavy dirt settles, and the water that is already in droplets sinks to the bottom, from where it leaves through a valve, the drain. Water dissolved in the oil does not sink: removing it takes equipment, such as the vacuum dehydrator.

So the oil lubricates the bearing and carries its heat away. The Yankee bearings usually have a circuit of their own, because they run hotter and turn more slowly. The Yankee is much larger than the other rolls, so it makes fewer turns per minute for the same sheet speed (Machinery Lubrication, 2004).

Figure 3. The oil goes to the bearing, carries the heat and comes back; in the tank, the water in droplets sinks and leaves through the drain.

Even with a tank, a filter and a cooler, the tissue machine is one of the toughest on oil.

Why is the tissue machine an extreme case of lubrication?

Because it brings together the three enemies of oil: water, heat and dirt, like a kitchen with the sink, the oven and flour in the air, all side by side. And it runs day and night, so every oil change becomes a production decision.

The water comes mostly from the wet end: splash, mist and washdown get in through the shaft seals and the tank breathers. The humid air that enters through the breather also leaves water behind when it cools, especially with the machine stopped.

In the dry end, water arrives too: steam enters the Yankee through a rotating part at the end of the shaft, the rotary joint, and what escapes from there can reach the bearing.

The heat comes from the Yankee, and its bearings are among the hottest points in the mill. The dirt (paper fiber, wear particles, washing products) clogs the filter and wears the parts.

Water is the one that does its damage most out of sight, in stages:

1. It gets in and dissolves in the oil, still invisible.

2. Past the limit that the oil can dissolve (the saturation point), it forms tiny droplets trapped in the oil, the emulsion, and the oil turns milky, like mayonnaise.

3. With water in the middle, the oil film that separates the parts weakens, which leads to metal-to-metal contact and wear.

4. Water also rusts the bearing raceways, the surfaces where balls and rollers turn.

5. The wear ends in an unplanned stop, on the machine that can least afford to stop.

How this shows up in your mill: cloudy or milky oil in the tank sight glass, water coming out of the drain, a filter clogging ahead of schedule.

Against water, the oil has a defense, and it has a name.

What is oil demulsibility and why does it decide in the wet end?

Demulsibility is the oil's ability to release the water after being mixed with it, like a vinaigrette, which separates by itself in the jar. Mayonnaise does not separate.

In the tank, the oil that releases the water lets it sink to the bottom; the oil that holds it carries it back to the bearing, even if it is just as thick. That is why, in the wet end, water separation enters the choice along with thickness.

The oil receives additives, substances that give it protections it does not have on its own. One of them, the detergent, keeps water dispersed in the oil and appears in some hydraulic oils: it does the opposite of what the wet end asks for.

Releasing the water is not enough: the additives must also not break down with water.

Many paper machine oils do without zinc-based additives, which with water and heat can form deposits and clog the filter (STLE, 2005). When buying, it is worth asking whether the oil contains zinc.

The Kelpen Oil laboratory, in Arujá (SP), measures demulsibility.

Figure 4. On the left, the oil releases the water; on the right, the mixture stays in emulsion.

For those who want the number: the test follows NBR 14172, corresponding to ASTM D1401. Oil and water are shaken in a cylinder at 54 °C, and the time is measured until no more than 3 mL of emulsion remains. Because ISO VG 220 (the thickness grade, explained in the next section), common in the paper machine, is above 90 cSt at 40 °C, ASTM D1401 recommends running the test at 82 °C, closer to the bearing temperature.

In the dry end, a different enemy rules.

What is viscosity and why does a hot bearing call for a thicker oil?

Viscosity is the oil's resistance to flowing: honey flows slowly, water flows fast. And oil thins when it heats up, like warmed honey.

The thickness is written as a number, the ISO VG grade, always measured at 40 °C: the higher it is, the thicker the oil. Together with the turning speed, it is what decides the thickness of the film between the parts.

The Yankee bearing runs well above 40 °C, and there the same oil becomes much thinner. That is why, in the hot bearing, the grade goes up, so that the hot film has the thickness the design asks for.

The Yankee adds another reason: it is much larger than the other rolls and makes fewer turns per minute, and at slow speed the film forms with more difficulty.

Figure 5. In the hot bearing, the same oil forms a thinner film; a thicker oil restores the thickness.

For those who want the number: ISO VG is the viscosity at 40 °C, in centistokes (cSt), per ISO 3448, with a 10% tolerance. Typical grades in the paper machine: 220 for machine oil in general and 220 to 320 in the dry end (STLE, 2005).

Thickness is only half of what the Yankee bearing asks for.

Which oil to use in the Yankee dryer bearing?

A circulating oil in the grade that the real bearing temperature calls for, that withstands the heat without breaking down and arrives in sufficient quantity and free of water. The most common grade is ISO VG 220; in the dry end, STLE cites 220 to 320 (STLE, 2005).

The steam enters through the hollow end of the shaft, through the rotary joint, and that end passes through the inside of the rolling bearing. It is usually insulated, and even so the bearing heats up. The oil carries the heat to the cooler; that is why the flow rate (how much oil arrives per minute) matters as much as the grade.

Figure 6. The steam passes through the hollow end of the shaft, inside the rolling bearing, and the oil carries the heat away.

Heat also shortens the life of the oil. The oil reacts with oxygen in the air, and this reaction, oxidation, goes faster the hotter the oil is. It is what happens with reused frying oil, which darkens and thickens:

1. Hot oil oxidizes, which leads to higher acidity and the formation of sludge (a pasty residue) and varnish (a hard deposit that sticks to surfaces).

2. Sludge and varnish clog filters and passages, which leads to less oil reaching the bearing.

3. With less oil, the bearing loses lubrication and cooling, which leads to wear and a stop.

That is why the Yankee oil needs to:

• resist oxidation, even with water;

• release the water when hot as well;

• pass through the filter and not form deposits;

• protect steel from rust and brass from corrosion (some rolling-bearing cages, the part that keeps balls and rollers apart, are made of brass);

• release the air that enters in the return, because an air bubble that reaches the pump and the bearing weakens the film.

With the grade and these requirements, the product is chosen with measurements on the machine: the Kelpen Oil Technical Team recommends the oil and the grade for each dryer bearing based on the temperature and flow measured with the machine running.

For those who want the number: dryer bearings can reach 125 °C even with an insulated shaft; with insulation and enough water-free oil flow, modern machines usually stay below 90 °C (Machinery Lubrication, 2004). Oxidation resistance with water is measured by the TOST test (ASTM D943).

Gearboxes and hydraulics follow a different rule.

Which oil to use in the gearboxes and hydraulic systems of the machine?

Each follows its own standard, and the standard says the type of oil.

In the gearbox, under high load, the film between the gear teeth can break. The oil then carries an extreme pressure (EP) additive, which protects the tooth when the load tightens. DIN 51517 part 3 calls this oil CLP (gear oil with rust and oxidation protection, plus the EP additive) and requires EP load-carrying capacity proven by the FZG test. In this test, test gears turn under increasing load.

At Kelpen Oil, the Turan EP line, mineral-based (oil refined from petroleum), meets DIN 51517 part 3 from ISO VG 68 to 1000 and protects even with high humidity.

The Turan S line, with synthetic bases (made by chemical synthesis, more stable in heat), is also EP and meets the same standard; it pays off in a gearbox that runs hot, or when the stop to change the oil is costly, because it allows longer intervals between changes, and the paper industry is among its applications. The grade comes from the gearbox nameplate.

In the hydraulic system, the oil carries an anti-wear additive, which protects the pump under pressure. This is the HLP class (hydraulic oil with rust and oxidation protection, plus anti-wear), with anti-wear additive treatment per DIN 51524 part 2, a standard that also tests water separation. At Kelpen Oil, it is the Supra AW line. The grade comes from the system manual; if it calls for a grade above ISO VG 150, the highest the standard covers, the Technical Team recommends the oil.

For those who want the number: DIN 51524 part 2 (2017 edition) covers ISO VG 10 to 150 and tests water separation per DIN ISO 6614. In the hydraulics that move pistons, typical grades run from ISO VG 46 to 68; in the roll hydraulics, from 100 to 220 (STLE, 2005).

Now it all comes together.

Which lubricant to use in each section of the tissue paper machine?

At each point, an oil that withstands what attacks it there. The same oil can serve more than one point, if it meets the requirements of all of them.

For other sectors, see the Kelpen Oil lubricants by industry.

Once the oil is chosen, the next question is whether it is still good.

How to monitor water in oil before it becomes a stop?

By periodic laboratory analysis, because dissolved water cannot be seen. When the oil turns milky, the water has already passed the saturation point.

The Karl Fischer test uses a reagent that combines with water and measures how much of it is in the oil, in parts per million (ppm), even when invisible. The same analysis also shows whether the viscosity is still in grade, the dirt, and how much the oil has oxidized, through acidity (TAN, total acid number).

The trend of these results allows changing the oil by condition, not by the calendar. To keep results comparable, the sample always comes from the same point of the circuit, with the machine running.

The in-house laboratory in Arujá (SP) measures, in the same report (the analysis document), how much water got in (Karl Fischer) and whether the oil still releases it (demulsibility). Read together:

• High water, oil still releasing the water: remove the water (drain, centrifuge or dehydrator, as the case may be) and find where it gets in.

• High water, oil holding the water: something changed in the oil, through oxidation or contamination (grease, another oil, a cleaning product), and the Technical Team investigates before deciding on a change.

It is a paid service, including for oil from another supplier, among the 25 tests of the laboratory.

The report also tracks three more things. The infrared spectrum (light that passes through the oil and traces its chemical fingerprint) shows oxidation and contaminants. The foaming characteristics show whether the oil foams in the return, which harms the pump. The particle count counts the grains of dirt and metal that got past the filter.

Viscosity, in turn, is measured at 40 °C and at 100 °C: the two together show whether the oil thins more than normal in the heat.

For those who want the number: the published reference is water below 500 ppm (0.05%) in the oil reaching the dryer bearings (Machinery Lubrication, 2004), and 350 ppm or less in new machines (STLE, 2005).

The right oil, well monitored, still depends on having oil available to replace it.

And availability, which nobody specifies?

A continuous machine needs oil for topping up and for the change, and the change cannot wait for the supplier. We at Kelpen Oil have formulated lubricants since 1999, with filling on demand from the pail to bulk and service across Brazil.

Frequently asked questions

How often should the circuit oil be analyzed? The sampling frequency is set together with the Technical Team: start shorter and widen it according to the water, acidity and particle results. The Yankee circuit calls for closer monitoring.

Can the dry-end oil be the same as the wet-end oil? It can: there are oils made for both parts, and some machines use a single one, in the grade that serves the hottest bearing. When the circuits are separate, what usually changes is the grade in the dry end (220 or 320, according to STLE).

Can the water be removed from the oil so it can keep being used? Water in droplets settles and leaves through the drain, or through the centrifuge (equipment that spins the oil at high speed and separates the water by weight). Dissolved water only leaves with the vacuum dehydrator, which evaporates it at low pressure without overheating the oil. If the oil turns milky and the water does not sink by itself (the emulsion does not break), the oil must be investigated: the demulsibility test shows whether it still releases the water.

Glossary

• Wet end and dry end: in the wet end the water is squeezed out; in the dry end, it leaves by heat.

• Yankee cylinder: a huge steam-heated cylinder on which the sheet sticks and dries.

• Bearing: the support for each roll's shaft; in a paper machine, almost always a rolling bearing.

• Circulating oil: oil pumped from the tank to the bearings, which returns to be cleaned and cooled.

• Emulsion: tiny water droplets trapped in the oil, which turn it milky.

• Demulsibility: the oil's ability to release water (NBR 14172).

• ISO VG: viscosity grade of the oil at 40 °C, in cSt.

• Karl Fischer: a test that measures water in oil in ppm, even when dissolved.

Talk to the Technical Team

Before you contact us, note how each point is lubricated, the temperature of the Yankee bearings, whether the tank oil is cloudy, the nameplate of the gearboxes and the latest report. With that, the Kelpen Oil Technical Team can recommend the oil for each point. If the oil is already in use, the Arujá (SP) laboratory performs used-oil analysis, as a paid service. Save this article or send it to whoever looks after the paper machine at your company.