Sea air in Westphalia: how Rothe Erde makes its bearings weatherproof

Stormy sky with wind turbines on the sea

Even though the sea is over 200 kilometers away from Lippstadt, in the German region of Westphalia, it plays a role in Reinhard Jürgens' (see photo on the right) workplace every day. Jürgens works in the production engineering department at the Rothe Erde plant in Lippstadt. As a coating technician, he and his colleagues ensure that the slewing bearings manufactured here win the battle against the elements. “Our main focus is corrosion protection,” says Jürgens. His most difficult clients: pitch bearings for offshore wind turbines and swivel bearings for oil production ships.

“The goal is to avoid having to pick up a paintbrush for the lifetime of a bearing until it is reconditioned,” says Jürgens. In the wind power sector, this typically spans 20 to 25 years or more. “Main or gearbox bearings in wind turbines are located inside the nacelle and are well protected,” explains Jürgens. “The pitch bearings, on the other hand, are directly exposed to the salty atmosphere, high humidity and harsh environment.”

The same applies to the swivel bearings, which are used when transferring oil from ship to ship. These bearings prevent the pipes from twisting when the ships move. Jürgens: “The guidelines and documentation requirements are particularly strict in this area.”

In order to make a pitch bearing from Rothe Erde weatherproof with the right coating, Jürgens needs seven work steps, which are divided into many smaller processes. Many of rothe erde^® bearings are custom-made. In these cases, the coaters carry out most of the work manually. “We can also apply coatings automatically in series production. However, this only makes sense if a certain number of identical bearings have the same requirements. The work steps are basically the same, except that they are carried out by a robot

In a class of its own: corrosion protection

The requirements are set by the customer and defined through various corrosion protection classes: From C1 to C5 - and more recently CX. “Class C1 applies to an office environment: it's heated, dry air, a roof over your head,” explains Jürgens. “C4 could then be a wind turbine on flat land, like those in our region. And CX is the offshore turbine on the sea.”

Jürgens and his colleagues cover the entire spectrum. For low requirements, regularly applying grease or a layer of oil can be sufficient to prevent corrosion.

But for the CX class, Jürgens has to take a more sophisticated approach. This is how the coating specialists prepare a pitch bearing for use at sea:

1. Cleaning

“We coat fully assembled components. They come directly from production, where oil and lubricants are used. It is not permitted for these to accumulate in the blasting material,” explains Jürgens. “We clean the bearings with cloths and special cleaning agents.” In addition, the blasting material used later also contains an additive that removes oily impurities.

2. Masking

Depending on the application and specifications, parts of the bearing remain uncoated. Jürgens: “In certain cases, they must not be affected by paints, the thermal spray coating and blasting. This is due to the technical requirements.”

The bearings are therefore masked before blasting. This means that drill holes, sometimes hundreds per bearing, are sealed with silicone plugs, and parts such as the centering or the entire toothing are masked off.

“In the case of the blade bearings, however, the holes are also coated,” says Jürgens. “The corrosion protection requirements are very high in these cases.”

The coaters insert running samples into some of the drill holes. These are similar to steel plugs that are removed after treatment and used to measure how well the coating adheres. This ensures the measuring process does not damage the freshly applied coating on the slewing bearing.

3. Blasting in the blasting chamber

Bearing being blasted in the blasting chamber

During blasting, an employee blasts the bearing with blasting material that is shot out of a blasting nozzle using compressed air. Blasting roughens the bearing and thus increases its surface area. This enables mechanical bonding with the thermal spray agent that is applied later.

“We use what is known as a sharp-edged abrasive,” says Jürgens. “We use steel grains that are broken again during the manufacturing process. We use an operating mixture of larger and smaller grains for this. The larger ones round off over time as we reuse the abrasive. The smaller ones don't, which is why the effect lasts longer.”

The employee blasting the surface in the closed blasting cabin looks like a fireman. The blasting nozzle hangs from a hose as thick as the ones used by the fire department. The blasting material is transported via this hose. The coater wears a helmet with fresh air supply so that he does not inhale any dust.

“Manual blasting is physically demanding work,” emphasizes Jürgens. “You work up a sweat.” In the automated process, the bearing is fixed to a rotating table and the blasting nozzle is held by a robot arm that follows the contours.

Once one side has been roughened, the coaters turn the slewing bearing over. In the case of the blade bearing, they also blast the drill holes.

Finally, the roughness is measured. Jürgens: “The roughness measuring device has a diamond tip and travels a defined distance on the surface. It measures the lowest point and the highest point in the microscopic area with the tip, which results in a deflection from which the so-called Rz score is calculated.”

4. Thermal spraying

Man in a protective suit and helmet coats a slewing bearing

“There are various thermal spraying processes,” explains Jürgens. “We use arc spraying here. We feed two spray wires, either made out of zinc or zinc-aluminum, in separate hoses. They come together at a nozzle. Since one wire is connected as the positive pole and the other as the negative pole, an electric arc is created that melts the wires.”

The device for this is called a coating torch and looks a bit like a futuristic cannon. Two thin hoses containing the wires lead to the nozzle. The coater points it at the surface to be coated and sprays the molten spray wire. You can see the bright arc and, if you look very closely, you can see that a protective layer is also being applied.

Jürgens: “We accelerate the molten material with compressed air and hurl these liquid particles onto the surface, where a gray, lamellar and slightly porous structure forms.”

And now the roughness of the surface also comes into play. “Because the surface is rough, a mechanical bond occurs between the surface and the spray agent. There are also galvanic processes that create a bond at an atomic level, but this process is purely mechanical.”

The porous structure is then sealed by the coating technicians, which also gives it better adhesion properties.

The fastening holes are not completely coated during this work step, but there is an overflow of the spray agent, called the overspray, so the transitions are also coated with zinc or zinc-aluminum. And there is a good reason for this: “When inserting the screws, the installers tend to scratch the surface. That can't always be prevented. With the zinc coating, however, the corrosion protection has a certain long-distance effect,” says Jürgens. “Even if the installers make a scratch down to the bare surface, if the zinc coating is two to three millimeters next to it, the protection also works on the scratch.”

So why aren't the drill holes also completely thermally coated? Jürgens: “Quite simple: the coating torch is quite large and needs a certain spray distance. However, the drill holes are too small for this, so precise coating is not possible.”

5. Unmasking and polishing the seal running surfaces

In this step Jürgens and his colleagues polish the seal running surfaces, then unmask the bearing, open all the holes and remove the adhesive tapes. This step completes the coating of the bearing, which, depending on the application, would be sufficient for corrosion protection. However, this is not the case for pitch bearings in the offshore sector. The drill holes still need a coat of paint.

6. Painting

Person in protective gear painting a slewing bearing

In case of the pitch bearings, the drill holes are now painted with a paint lance. A coater inserts the lance, where it paints the drill hole at a 360-degree angle. “This fulfills the highest corrosion protection class,” says Jürgens.

So why isn't the entire bearing painted? “That also happens depending on the customer's wishes; it's a question of corporate policy for some customers,” says Jürgens. “Some customers want to have all components in their brand color. Of course, we are happy to fulfill any request, but for cost and production reasons we take a more pragmatic approach. After painting, we need a certain amount of drying time and cannot continue to process the bearing, insert seals or carry out further work steps. This increases the processing time by up to an additional week for just one coat of paint. With several coats, this makes a big difference. With thermal spraying, on the other hand, we can continue immediately, it doesn't even get particularly warm.”

And the protective effect? Jürgens: “This is clearly defined according to a series of standards for coaters. And as corrosion protection, many things are possible: thermal spraying, completely without or in combination with painting; or only with painting. The required coating thickness changes depending on the choice of coating, but the protective effect is achieved in all cases.”

7. Quality checks and completion

At the end, the final quality checks and layer thickness measurements are carried out and everything is recorded. The Rothe Erde employees also replace the seals and grease fittings and measure the rotational resistance.

And now the pitch bearing is ready for the next 20 years at sea.