Yaw bearing
Encyclopedia
The yaw bearing is the most crucial and cost intensive component of a yaw system
found on modern horizontal axis wind turbine
s. The yaw bearing must cope with enormous static and dynamic loads
and moments
during the wind turbine operation, and provide smooth rotation characteristics for the orientation of the nacelle
under all weather conditions. It has also to be corrosion
and wear
resistant and extremely long lasting. It should last for the service life
of the wind turbine) while being cost effective.
s of these "primitive" windmills were surprisingly similar to the ones on modern wind turbines. The nacelles rotated by means of wind driven yaw drive
s known as fantails, or by animal power, and were mounted on the windmill towers by means of an axial gliding bearing.
These gliding bearings consisted of multiple gliding blocks fixed on the windmill tower structure. These blocks maintained sliding contact with a gliding ring on the nacelle. The gliding blocks were wooden cube-like pieces with convex gliding surface covered with animal fat
, or even lined with copper (or brass) sheet as a friction
reduction means. These wooden blocks were fixed in wooden slots, carved in the wooden bearing substructure, by means of nails or wedges and were carefully leveled to create a flat surface where the nacelle gliding ring could glide. The gliding blocks, despite the lubrication
would wear quite often and would have to be exchanged. This operation was relatively simple due to the wedge-based connection between substructure and gliding blocks. The gliding blocks were further locked via movable locking devices which, in a different form, remain as a technical solution in modern gliding yaw bearings.
The gliding ring of the windmill nacelle was made from multiple wooden parts and, despite the old construction techniques, was usually quite level, allowing the nacelle to rotate smoothly around the tower axis.
The hybrid yaw bearing system combines the solutions old windmills used. This system comprises multiple removable radial gliding pads in combination with an axial roller bearing.
Some manufacturers use a plurality of smaller yaw drives (usually six) to facilitate easy replacement. Such a configuration with plurality of yaw drives often offers the possibility of active yaw braking using differential torque from the yaw drives. In this case half of the yaw drives apply a small mount of torque for clockwise rotation and the other half apply torque in the opposite direction and then activate the internal magnetic brakes of the electric motor. In this way the pinion-gear rim backlash is eliminated and the nacelle is fixed in place.
and radial bearing, which serves as a rotatable connection of the wind turbine nacelle and the tower. Contrary to the old windmill concept, the modern yaw bearings support the nacelle also from the to thus restraining the nacelle from being rotated by the Y-axis due to the moments
induced by the upper half of the rotor sweep disk and the X-axis due to the torque of the drive train (i.e. rotor, shaft , generator, etc. ).
Principally, the simplest way to accomplish the yaw bearing tasks with gliding elements is with two gliding planes for the axial loads (top and bottom) and a radial gliding surface for the radial loads. Consequently, the gliding yaw bearing comprises three general surfaces covered with multiple gliding pads. These gliding pads come in sliding contact with a steel
disk, which is usually equipped with gear
teeth to form a gliding-disk/gear-rim. The teeth may be located at the inner or the outer cylindrical face of the disk, while the arrangement of the gliding pads and their exact number and location vary strongly among the existing designs. To assemble the gliding yaw bearings, their cages split in several segments that are assembled together during wind turbine installation or manufacturing
.
In its simplest form, the gliding yaw bearing uses pads (usually made out of polymers) distributed around the three contact surfaces to provide a proper guiding system for the radial and axial movement with relatively low friction coefficient. Such systems are economical and very robust but do not allow individual adjustment of the axial and radial gliding elements. This function importantly minimizes the axial and radial "play" of the gliding bearing due to manufacturing tolerances as well as due to wear of the gliding pads during operation.
To solve this problem, yaw systems incorporate pre-tensioned gliding bearings. These bearings have gliding pads that are pressed via pressure elements against the gliding disk to stabilize the nacelle against undesirable movement. The pressure elements can be simple steel springs
, pneumatic, or hydraulic pre-tension elements, etc. The use of pneumatic or hydraulic pre-tension elements allows active control of the yaw bearing pre-tension, which provides yaw brake function.
. To reduce friction, wear, and avoid stick-slip effects
(often present in such high friction slow moving systems), lubrication is often introduced. This solution generally solves the gliding issues, but introduces more components to the systems and increases the general complication (e.g., difficult maintenance procedures for removal of used lubricant). Some wind turbine manufacturers now use self lubricating gliding elements instead of a central lubrication system. These gliding elements are manufactured from low friction materials or composites
(e.t.g Teflon
) that allow reliable operation of dry (non-lubricated) gliding yaw systems.
of the wind turbine, it should be possible to replace worn out yaw bearing gliding elements or other components of the yaw system. To allow for replace-ability of worn out components, the yaw systems are designed in segments. Usually one or more gliding planes comprise several sub-elements that contain a number of gliding elements (radial or axial or a combination). These sub-elements can be individually removed and repaired, re-fit or replaced. In this way the yaw bearing can be serviced without the need of dis-assembly of the whole gliding yaw bearing (e.g., in case of a roller yaw bearing, dis-assembly of the whole wind turbine). This rep-arability offered by the segmented design of the gliding yaw bearing is one of the most important advantages of this system against the roller yaw bearing solution.
The only remaining issue is the replacement of the gliding elements of the gliding yaw bearing surface, which is not segmented. This is usually the top axial surface of the gliding bearing, which constantly supports the weight of the whole nacelle-rotor assembly. For the gliding elements of this gliding surface to be replaced, the nacelle-rotor assembly must be lifted by an external crane
. An alternative solution to this problem is the use of mechanical or hydraulic jacks able to partially or fully lift the nacelle-rotor assembly while the gliding yaw bearing is still in place. In this way and by providing a small clearance between the gliding elements and the gliding disk, it is possible to exchange the sliding elements without dismantling the gliding yaw bearing.
Yaw system
The yaw system of wind turbines is the component responsible for the orientation of the wind turbine rotor towards the wind.-History:The task of orientating the rotor into the wind was a complicated issue already for historical windmills. The first windmills able to rotate in order to "face" the...
found on modern horizontal axis wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
s. The yaw bearing must cope with enormous static and dynamic loads
Structural load
Structural loads or actions are forces, deformations or accelerations applied to a structure or its components.Loads cause stresses, deformations and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis...
and moments
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
during the wind turbine operation, and provide smooth rotation characteristics for the orientation of the nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
under all weather conditions. It has also to be corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
and wear
Wear
In materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface....
resistant and extremely long lasting. It should last for the service life
Service life
A product's service life is its expected lifetime, or the acceptable period of use in service. It is the time that any manufactured item can be expected to be 'serviceable' or supported by its manufacturer....
of the wind turbine) while being cost effective.
History
Windmills of the 18th century began implementing rotatable nacelles to capture wind coming from different directions. The yaw systemYaw system
The yaw system of wind turbines is the component responsible for the orientation of the wind turbine rotor towards the wind.-History:The task of orientating the rotor into the wind was a complicated issue already for historical windmills. The first windmills able to rotate in order to "face" the...
s of these "primitive" windmills were surprisingly similar to the ones on modern wind turbines. The nacelles rotated by means of wind driven yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
s known as fantails, or by animal power, and were mounted on the windmill towers by means of an axial gliding bearing.
These gliding bearings consisted of multiple gliding blocks fixed on the windmill tower structure. These blocks maintained sliding contact with a gliding ring on the nacelle. The gliding blocks were wooden cube-like pieces with convex gliding surface covered with animal fat
Animal fat
Animal fats are rendered tissue fats that can be obtained from a variety of animals.- Pet nutrition :In pet nutrition, the source of animal fat concerns food manufacturers. AAFCO states that animal fat is "obtained from the tissues of mammals and/or poultry in the commercial processes of rendering...
, or even lined with copper (or brass) sheet as a friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
reduction means. These wooden blocks were fixed in wooden slots, carved in the wooden bearing substructure, by means of nails or wedges and were carefully leveled to create a flat surface where the nacelle gliding ring could glide. The gliding blocks, despite the lubrication
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...
would wear quite often and would have to be exchanged. This operation was relatively simple due to the wedge-based connection between substructure and gliding blocks. The gliding blocks were further locked via movable locking devices which, in a different form, remain as a technical solution in modern gliding yaw bearings.
The gliding ring of the windmill nacelle was made from multiple wooden parts and, despite the old construction techniques, was usually quite level, allowing the nacelle to rotate smoothly around the tower axis.
The hybrid yaw bearing system combines the solutions old windmills used. This system comprises multiple removable radial gliding pads in combination with an axial roller bearing.
Types
The main categories of yaw bearings are:- Roller Yaw Bearing: Large diameterDiameterIn geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
bearing (usually four-point bearing) - Gliding Yaw Bearing: Dry or lubricated gliding bearing with plurality of axial and radial gliding pads being in friction contact with a large diameter steel disk, usually combined with the gear-rim as a single element
Roller yaw bearing
The roller yaw bearing is a common technical yaw bearing solution followed by many wind turbine manufacturers as it offers low turning friction and smooth rotation of the nacelle. The low turning friction permits the implementation of slightly smaller yaw drives (compared to the gliding bearing solution), but on the other hand requires a yaw braking system.Some manufacturers use a plurality of smaller yaw drives (usually six) to facilitate easy replacement. Such a configuration with plurality of yaw drives often offers the possibility of active yaw braking using differential torque from the yaw drives. In this case half of the yaw drives apply a small mount of torque for clockwise rotation and the other half apply torque in the opposite direction and then activate the internal magnetic brakes of the electric motor. In this way the pinion-gear rim backlash is eliminated and the nacelle is fixed in place.
Gliding yaw bearing
The gliding yaw bearing is a combined axialAxial
Axial may mean:* Along the same line as an axis of rotation in geometry* A type of modal frame in music* One of several anatomical directions in an animal body* Axial age, the period from 800 to 200 BC in China, India and the western world...
and radial bearing, which serves as a rotatable connection of the wind turbine nacelle and the tower. Contrary to the old windmill concept, the modern yaw bearings support the nacelle also from the to thus restraining the nacelle from being rotated by the Y-axis due to the moments
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
induced by the upper half of the rotor sweep disk and the X-axis due to the torque of the drive train (i.e. rotor, shaft , generator, etc. ).
Principally, the simplest way to accomplish the yaw bearing tasks with gliding elements is with two gliding planes for the axial loads (top and bottom) and a radial gliding surface for the radial loads. Consequently, the gliding yaw bearing comprises three general surfaces covered with multiple gliding pads. These gliding pads come in sliding contact with a steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
disk, which is usually equipped with gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
teeth to form a gliding-disk/gear-rim. The teeth may be located at the inner or the outer cylindrical face of the disk, while the arrangement of the gliding pads and their exact number and location vary strongly among the existing designs. To assemble the gliding yaw bearings, their cages split in several segments that are assembled together during wind turbine installation or manufacturing
Manufacturing
Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...
.
In its simplest form, the gliding yaw bearing uses pads (usually made out of polymers) distributed around the three contact surfaces to provide a proper guiding system for the radial and axial movement with relatively low friction coefficient. Such systems are economical and very robust but do not allow individual adjustment of the axial and radial gliding elements. This function importantly minimizes the axial and radial "play" of the gliding bearing due to manufacturing tolerances as well as due to wear of the gliding pads during operation.
To solve this problem, yaw systems incorporate pre-tensioned gliding bearings. These bearings have gliding pads that are pressed via pressure elements against the gliding disk to stabilize the nacelle against undesirable movement. The pressure elements can be simple steel springs
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
, pneumatic, or hydraulic pre-tension elements, etc. The use of pneumatic or hydraulic pre-tension elements allows active control of the yaw bearing pre-tension, which provides yaw brake function.
Wear and lubrication
In all gliding bearings wear is an issue of concern, as well as lubrication. Conventional gliding yaw bearings incorporate gliding elements manufactured out of polymer plastics such as POM or PAPolyamide
A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...
. To reduce friction, wear, and avoid stick-slip effects
Stick-slip phenomenon
The stick-slip phenomenon, also known as the slip-stick phenomenon or simply stick-slip, is the spontaneous jerking motion that can occur while two objects are sliding over each other.- Cause :...
(often present in such high friction slow moving systems), lubrication is often introduced. This solution generally solves the gliding issues, but introduces more components to the systems and increases the general complication (e.g., difficult maintenance procedures for removal of used lubricant). Some wind turbine manufacturers now use self lubricating gliding elements instead of a central lubrication system. These gliding elements are manufactured from low friction materials or composites
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
(e.t.g Teflon
Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
) that allow reliable operation of dry (non-lubricated) gliding yaw systems.
Maintenance and repair
Despite the fact that the gliding yaw bearings and their components are designed and constructed to last the service lifeService life
A product's service life is its expected lifetime, or the acceptable period of use in service. It is the time that any manufactured item can be expected to be 'serviceable' or supported by its manufacturer....
of the wind turbine, it should be possible to replace worn out yaw bearing gliding elements or other components of the yaw system. To allow for replace-ability of worn out components, the yaw systems are designed in segments. Usually one or more gliding planes comprise several sub-elements that contain a number of gliding elements (radial or axial or a combination). These sub-elements can be individually removed and repaired, re-fit or replaced. In this way the yaw bearing can be serviced without the need of dis-assembly of the whole gliding yaw bearing (e.g., in case of a roller yaw bearing, dis-assembly of the whole wind turbine). This rep-arability offered by the segmented design of the gliding yaw bearing is one of the most important advantages of this system against the roller yaw bearing solution.
The only remaining issue is the replacement of the gliding elements of the gliding yaw bearing surface, which is not segmented. This is usually the top axial surface of the gliding bearing, which constantly supports the weight of the whole nacelle-rotor assembly. For the gliding elements of this gliding surface to be replaced, the nacelle-rotor assembly must be lifted by an external crane
Crane (machine)
A crane is a type of machine, generally equipped with a hoist, wire ropes or chains, and sheaves, that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of...
. An alternative solution to this problem is the use of mechanical or hydraulic jacks able to partially or fully lift the nacelle-rotor assembly while the gliding yaw bearing is still in place. In this way and by providing a small clearance between the gliding elements and the gliding disk, it is possible to exchange the sliding elements without dismantling the gliding yaw bearing.
Bearing Adjustment
When the wind turbine nacelle is positioned on the tower and the yaw bearing assembly is completed it is necessary to adjust the pressure on the individual gliding pads of the bearing. This is necessary in order to avoid un-even wear of the gliding pads and excessive loading on some sectors of the yaw bearing. In order to achieve that, an adjustment mechanism is necessary, which enables the technicians to adjust the contact pressure of each individual gliding element in a controllable and secure way. The most common solution is the utilization of bottom bearing plates equipped with large opening, which accommodate the adjustable gliding bearing systems. These adjustable gliding bearings comprise a gliding unit (i.e. gliding pad) and an adjustable pressure distribution plate. In between the gliding pad and the pressure plate several spring (pre-tension) elements are located. The vertical position of the pressure plates is usually controlled by an adjustment screw. This adjustment screw presses against the pressure plate while being retained by a counter-pressure support plate, fixed on the bearing assembly with strong bolts. In this way it is possible to apply various levels of contact pressure among the different gliding pads and therefore to ensure that each gliding component of the yaw bearing arrangement is performing as anticipated.Further reading
- Wind Power Plants, R. Gasch and J. Twele, Solarpraxis, ISBN 3-934595-23-5
- Wind Energy Handbook, T. Burton [et al.], John Wiley & Sons, Ltd, ISBN 0-471-48997-2
- Molenbouw, A. Sipman, Zutphen, 2002, ISBN 90-5730-119-9