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Job Sheet 3 – Slewing Bearing

Slewing Bearing

A primary component in an active pitch control system is the bearing that secures the blade to

the hub and allows for variations in pitch angle. This component is called a slewing bearing,

and it is a large rolling bearing that can accommodate axial, radial and moment loads in any

direction (Figure 3-1).

Figure 3-1. Slewing Bearing.

Slewing bearings can perform slewing, oscillating, and rotational movements. Their designs

vary; however, a typical slewing bearing consists of an inner ring, which incorporates a

toothed gear; an outer ring; and rolling elements that are made up of balls or cylindrical rollers,

and separated by polyamide spacers (Figure 3-2). Each ring of the bearing is provided with

threaded holes to accommodate attachment bolts to the turbine hub and turbine blade. Some

slewing bearings have gear teeth on the inside or outside of one of the rings. These gear teeth

interact with a pinion gear to rotate the attached components.

A B

Figure 3-2. Ball (A) and Roller (B) Slewing Bearings.

Slewing bearings are bolted directly to the hub with the rotating ring bolted to the blade base

(Figure 3-3).

Slewing Bearing

34 © Festo Didactic Inc. 88393-20

Figure 3-3. Slewing Bearings Securing Blades to the Hub.

A slewing bearing and its drive components are included in the hub trainer to assist in the

understanding of the functional characteristics of a slewing bearing in operation on an

actual wind turbine. To facilitate classroom usage, the blade itself is not present; rather, it is

represented by an outline sketch on the front of the trainer. If the blade were present, it would

extend off the back of the trainer, as shown in Figure 3-4.

Figure 3-4. Hydraulic Pitch Hub Trainer with Simulated Blade.

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Slewing Bearing

Tightening Bolts

Maintenance of a slewing bearing begins with an inspection of the mounting bolts. If any bolts

documents provided by the manufacturer of the bearing. Because of its relatively small cross

section, the slewing bearing cannot bear much direct loading and the mount to which it is

free of any protective oils or coatings that could compromise the integrity of the mating of the

bearing to its mount.

All bolts and nuts should be tightened with an accurate torque wrench (Figure 3-5) in a

Figure 3-5. Common Torque Wrench.

For the torque measurement to be valid, the threads of all bolts and nuts must be coated with a

thin layer of light oil prior to being tightened. To provide uniformly distributed stress across the

slew bearing, the process of bolt tightening should follow a star pattern (Figure 3-6).

Figure 3-6. Typical Star Pattern for Tightening Bolts.

Slewing Bearing

36 © Festo Didactic Inc. 88393-20

A standard four-step tightening process follows:

Step 1: Spanner tight ensuring that 2–3 threads extend above nut.

Figure 3-6.

Step 3: Increase the torque to full torque following the pattern shown in Figure 3-6.

torque.

When loosening bolts, a torque value that is higher than the tightening torque is often required.

This is referred to as breakout torque, and it is necessary due to corrosion and deformations

in the bolt and nut threads. Breakout torque can take up to 2.5 times the input torque to

breakout. The use of penetrating oils or anti-seize products is always recommended when

performing breakout operations.

When bolts become too large and torque stress is too high for common wrenches, a torque

multiplier may be used. A torque multiplier is a small gearbox that increases the torque wrench

force commonly at a ratio of 5 to 1, or a hydraulic bolt tensioner (Figure 3-7). A hydraulically

operated bolt tensioner enables bolts to be tightened accurately without applying torque directly

to the nut and mating threads. To accomplish this, a hydraulic pump is coupled to the tensioner.

to the bolt. This applies a large stretching force on the bolt. The pre-threaded nut can then be

hand tightened until it contacts the face of the slewing bearing. When hydraulic pressure is

released from the tensioner and, therefore, the bolt, an exact tightening force is applied to the

nut-and-bolt combination. When multiple tensioners are used simultaneously, they enable all

bolts to be uniformly preloaded and tightened. Paint pin lines are then applied to each nut, bolt,

and frame to allow for quick inspection on future visits. Loose nuts are given paint pin lines that

Tensioner

Brace

Slew Bearing

Pressurized

Hydraulic Fluid

Hydraulic Body

Skirt

Socket to Hand

Tighten the Nut

Figure 3-7. Hydraulic Tensioner.

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Slewing Bearing

Seals

The next inspection point is the bearing seals. Slewing bearings commonly have integral seals

strips of a non-reinforced rubber. They seal axially against the side face of the inner or outer

ring, or radially against the cylindrical surface of the inner or outer ring (Figure 3-8).

Figure 3-8. Common Seal Arrangement.

Some bearings may have multiple seals per side. The rubber sealing material is resistant

to most mineral oils and greases, and can operate over a wide range of temperatures. Any

damage should be recorded and the seal should be replaced as soon as possible.

Lubrication

of nine numerical grades based on consistency. As shown in Table 3-1, grades range from

NLGI

Grade

Worked Penetration

After 60 Strokes

at 25 °C (0.1 mm)

Appearance Food Consistency

Analog

000 445-475 cooking oil

00 400-430 applesauce

0 355-385 very soft brown mustard

1 310-340 soft tomato paste

2 265-295 “normal” grease peanut butter

3 220-250 vegetable shortening

4 175-205 frozen yogurt

5 130-160 hard smooth pate

6 85-115 very hard cheddar cheese

Table 3-1. Table of NLGI Grades and Worked Penetration Ranges.

Slewing Bearing

38 © Festo Didactic Inc. 88393-20

through 4 are often used in rolling contact bearings, where grade 2 is the most common.

rating, mixed with a lithium soap thickener and extreme pressure additives. Re-lubrication of a

customized slewing bearing depends on the needs of the application.

3-9). Auto lubrication can be accomplished via a pressurized can or a more complex piped

pumping system.

Figure 3-9. Grease Port with Fitting and Auto Lubricator.

Wind turbines also employ automatic lubrication systems to grease slewing bearings. These

automatic systems can simply be an attached bottle of grease and they provide repeated

greasing between regular maintenance visits.

The hydraulic pitch hub trainer incorporates a system that allows the use of either a manual

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Slewing Bearing

A

B

C

Figure 3-10. Hub Lubrication System.

to purge the grease contained in the lubricator (C). For training purposes, this cap should be

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Slewing Bearing

OBJECTIVE

In this job, you will become familiar with the basic principles of slewing bearing

lubrication.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment

required for this job.

SAFETY PROCEDURES

Before proceeding with this job, complete the following checklist.

You are wearing safety glasses.

You are wearing safety shoes.

You are not wearing anything that might get caught such as a tie, jewelry, or

loose clothes.

If your hair is long, tie it out of the way.

The working area is clean and free of oil.

Your sleeves are rolled up.

Instructor initials:

PROCEDURE

Basic Setup

Lockout/Tagout

Verify that the system has been locked out according to the Lockout/

Begin

Remove the grease plug (C) as shown in Figure 3-11.

Slewing Bearing

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C

Figure 3-11. Hub Lubrication System.

3-12.

B

Figure 3-12. Grease Fitting B.

Slowly apply a small amount of grease until grease comes out of the

grease plug (C) shown in Figure 3-13.

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Slewing Bearing

C

Figure 3-13. Plug C.

NOTE :

could result.

Replace the grease plug (C).

Figure 3-14.

A

Figure 3-14. Grease Fitting A.

Slewing Bearing

44 © Festo Didactic Inc. 88393-20

NOTE:

the pump unit. with the piston o-rings.

Remove the grease plug (C).

Remove the top cap of the lubricator as shown in Figure 3-15.

Cap

Figure 3-15. Lubricator Cap.

Ensure that all dip switches are set to the off position as shown in

Figure 3-16.

Figure 3-16. Auto Lubricator with Cap Removed.

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Slewing Bearing

With a rag or wipe at the ready, set the dip switch 7 (purge) to the On

position.

Once the lubricator starts to dispense grease (this takes approximately

1 minute or less), set dip switch 7 to the Off position.

The lubricator dispenses grease for approximately 1 minute and then

stops.

NOTE:

shut down due to a low ambient temperature of 15° F (-9.44° C). Switches

1–5 control the number of pump stokes and the time period over which they

are delivered (15 days, 1 month, 2 months, etc).

Wipe any grease away and replace plug C.

Lockout/Tagout

Review Questions

1. How is a slewing bearing used in a pitch control system?

Slewing Bearing

46 © Festo Didactic Inc. 88393-20

Name: _________________________________ Date: ______________________

Instructor approval: ___________________________________________________

2. Which item carries the bulk of the force loading on the blade: the slewing bearing

or its mount? Explain.

3. Why are slewing bearing bolts tightened in a star pattern?

4. Where is automated lubrication commonly used in a pitch control system?

5.

table contain and what is the basis for the grading?