Carlisle Brake & Friction
www.carlislecbf.com

Windpower Engineering & Development

Low-speed-shaft braking is relatively straightforward in that a large disc brake, with a large friction lining area, is easy to accommodate. The drawback is that the brake must generate a high-braking torque.

Generally, the most cost-effective position is on the high-speed shaft between a gearbox and generator. The increase ratios of wind-turbine gearboxes produce a large reduction in output torque. In many cases, a major parameter regarding brake selection is choosing a friction-liner area of sufficient size to ensure adequate heat dissipation during emergency stops.

The energy to dissipate is the same regardless of brake location, so the total lining area must be the same. The brake-pad area must be sufficient to control the temperature rise.

These requirements are more difficult to meet on the high-speed shaft because speed and space are limiting factors with regard to the maximum disc diameter and brake selections. Nevertheless, braking on the high-speed shaft has been used on many turbines up to 750 kW. As the industry develops higher capacity turbines, the trend is leaning towards rotor-shaft braking.

A further consideration regarding brake position is the possibility of gear tooth damage. If brakes are installed on the gearbox-output shaft and the turbine is stationary, gusts are likely to cause the rotor to transmit a rocking motion within the backlash of the input and output gears. Without forced lubrication between the mating teeth, this effect could ultimately result in fretting and expensive gear damage

Pitch drive brakes: A series of high-torque, electrically released, spring engaged, static holding brakes can withstand the conditions on the pitch drive of large turbines. This brake is typically smaller in diameter than the motor assembly and adds minimal length to the overall package. One model, rated at 135 Nm, is 6.5-in. diameter and only slightly over 2-in. long. Typical design life calls for 500 to 1,000 stops. Some brakes exceed this range. Another plus for electric brakes: A short reaction time, 0.20 sec or less, making it a good choice for pitch drives.

A braking torque level for rotor brakes is one characteristic to calculate during initial stages of brake design. The maximum permissible braking torque on a rotor shaft is usually imposed by the blades, or their anchorage to the gearbox input shaft. On the other hand, braking on the high-speed shaft is usually related to the maximum permissible gear-tooth loading.

There is also a minimum level of braking torque, below which the variable nature of the frictional forces under different operating conditions could put a turbine rotor at risk.

It is therefore important to allow an adequate window of safety, or service factor, to ensure that the brakes will always operate effectively and under all climatic conditions. To achieve an adequate service factor it is helpful to consider how braking performance can vary with the same predetermined level of braking torque.

For example, suppose a turbine has a 1-MW rating, and aerodynamic (load) torque of 100,000 Nm.

Applying a brake during an emergency at 20% over-speed, the rise in disc temperature and stopping time will vary depending on the chosen service factors. A commonly applied factor of 2.00 suggests:

Tb/TL = 2

where T_b = brake torque and T_L = load torque.

Hence, the calculated braking torque Tb = 200,000 Nm.

Windpower Engineering & Development

For hydraulic pitch adjustment of wind turbines, Liebherr supplies not only cylinders, but also the matching large diameter bearings.

German company Liebherr is offering their hydrau-lic cylinder for hydraulic pitch adjustment in wind turbines to the American market. The company offerso both electromechanical pitch adjustment with gearboxes as well as hydraulic pitch adjustment with the corresponding blade bearings as a full system. Cylinders are supplied in accordance with a customer’s particular requirements, in-cluding piston rod heads and integrated stroke measuring systems.

The cylinder fits perfectly to a large diameter bearing of 2,100 mm in diame-ter, typical for a 2-MW wind turbine. It has a piston diameter of 125 mm and stroke length of 780 mm, weighing 172 kg including piston rod head.

The company says their hydraulic cylinders are significant because of careful selection of material,  high standard of technical precision of the components, an exactly matched sealing system, and particularly low wear in operation. The manufacturing range for hydraulic cylinders extends to up to 5,000 mm stroke length and 500 mm piston diameter.

Liebherr, though a sys-tem supplier, is able to offer bearings and drive systems, as well as hydraulic cylinders and control technology, from its own development and production resources. In the sector of large bearings for blade and yaw adjustment in wind power systems, Liebherr manufactures four-point bearings in single-row and two-row versions. All these bearings are characterized by top quality and long service life. The range available among drive systems includes multi-stage coaxial planetary gearboxes, which can be flexibly adapted to every customer’s particular requirements. Liebherr drive systems are particularly famous for their reliability, the compact structural design, and the ideal power-to-weight ratio. Electric motors to supplement the drives likewise come from Liebherr’s own development and manufacturing facilities and resources.

There are essentially no limits on the structural size of the drive elements. The product range includes components for supplying wind turbines from 750 kW through to large tur-bines with rated capacities of 7.5 MW, for operation on land as well as for offshore systems.

www.liebherr.com

Windpower Engineering & Development

For the wind-energy market, Vulkan manufactures rotor brakes that generate braking forces up to 96,000 N, and on yaw brakes, up to 434,000 N.

A yaw brake model, FHGE, comes in three models, -77, -90 and -120. Each features a small air gap and few moving parts for a short response times and fast braking. Additional features include a large brake-pad area and low brake-disc temperatures. For the wind-energy market, the company manufactures rotor brakes that generate braking forces up to 96,000 N and on yaw brakes, up to 434,000 N. Since 2002 the company has been working in the wind-energy market; supplying equipment such as brakes, couplings, composite shafts, hydraulic power units, along with online monitoring and diagnostic systems.

Manufacturer Vulkan Drive Tech recently received GL Certification for its hydraulic disc brake FHGE-120. According to technical manager Paulo Baraldi, the man responsible for the certification project, it has taken more than two years of development work on the FHGE-120. The brake has been working for more than a year in Germany on a pilot test. In addition to the recent GL Certification, Vulkan is also certified to ISO 9001 and ISO 14001 by TÜV Rheinland.

Vulkan USA
vulkan.com

Windpower Engineering & Development

MagnaShear motor brakes are totally enclosed from outside contaminants, with seal integrity for harsh and washdown environments. A modular design and assembly allows for ease of servicing and maintenance.

MagnaShear motor brakes use oil-shear technology for longer service life even in demanding applications such as pitch and yaw drives and generator-holding brakes in smaller wind turbines. The proven technology transmits torque between lubricated surfaces – thereby eliminating wear on friction surfaces. A patented fluid recirculation system dissipates heat – a common problem in dry-braking systems. Eliminating wear significantly increases service life and nearly eliminates adjustment.

Oil shear also provides a smooth “cushioned” stop which reduces shock to the drive system, further extending service life of downstream components. These totally enclosed brakes are impervious to moisture, dirt, and dust common in outdoor applications such as wind farms. Seal integrity makes them ideal for harsh environments and washdown applications.

Unlike dry brakes, oil-shear design uses a layer of automatic transmission fluid between the brake disc and drive plate. As the fluid is compressed, fluid molecules shear – thus imparting torque to the other side. This torque transmission causes the stationary surface to turn, bringing it up to the same relative speed as the moving surface. Because most work is done by the fluid particles in shear, wear is almost eliminated, which also eliminates need the adjustments common to dry braking systems.

MagnaShear brakes provide significantly longer service life, characterized by near maintenance-free operations. These motor brakes are available to accommodate a wide range of applications. Spring-set torque ratings are from 3 to 1,250 ft-lb. Motor brakes can be sized to the correct torque independent of the motor frame size or horsepower.

A “quick mount” features makes for quick and easy mounting to drive motors in NEMA frame sizes 56 to 449. They are shipped ready to install, without assembly or adjustments. They are also available pre-mounted on a motor for severe duty applications. MagnaShear motor brakes can be furnished to fit a NEMA or IEC frame motor, as a complete motor and brake assembly, or to mount on a machine frame or other special mounting configuration.

Force Control Industries
www.forcecontrol.com

Windpower Engineering & Development

Six Intorq brakes.

The requirements for pitch and yaw brakes include high reliability, long maintenance cycles, resistance to environmental influences, and a tight braking-torque tolerance. For pitch drives, one brake manufacturer has developed spring loaded devices as holding brakes that have to perform in a particular way in emergencies, even after a failure. The brake casing is encapsulated. Yaw drives use brakes from the company’s modular BFK458 series. In the event of a fault, the brakes act as a friction clutch to let the nacelle move with the wind. Offshore duty calls for high corrosion protection and durability. Encapsulated versions with special surface protection are used there.

Example applications also include spring-applied brakes in pitch drives.

A bridge/half-wave rectifier has proven useful. After a period, the bridge/half-wave rectifiers switch over from bridge rectification to half-wave rectification. Depending on design, over-excitation or holding current reduction or both, it is possible to shorten the switching times or reduce self-heating (power reduction).

Company engineers have modified the insulation for this area of use and stored-spring brakes without partial discharge can be supplied as an option. The advantages of over-excitation and reduction of the holding current can be combined with each other in this case.

The image provides some explanation as to how the bridge/half-wave rectification functions and how reducing the holding voltage and over-excitation behaves when selecting a suitable coil voltage. When stored-spring brakes are operated from the DC link of a frequency inverter with pulse width modulation, partial discharge can occur and damage the coil system.

Intorq US Inc.

www.Intorq.info.com

Windpower Engineering & Development

The enclosed MagnaShear brakes are impervious to moisture, dirt, and dust. The design uses a layer of automatic transmission fluid between brake disc and drive plate. Along with heat removal and torque, the fluid serves to continually lubricate all components. MagnaShear brakes provide significantly longer service life than conventional designs and are characterized by virtually maintenance-free operations. These motor brakes accommodate a wide range of applications. Spring-set-torque ratings are available from 3 to 1,250 ft-lb. The brakes can be sized to a torque independent of the motor-frame size or horsepower.

Force Control Industries Inc.

forcecontrol.com

Windpower Engineering & Development

Brakes for wind turbines call for higher cycles rates, higher loads, greater reliability and often in more compact packages than those on conventional factory equipment.

Rotor brakes from Twiflex, Ltd., come assembled, provide high levels of reliability, easy electronic monitoring and maintenance, and are available with organic or metallic linings. Models are offered in a range of braking forces from 100 N to 1 MN. Rotor-brake models include the GMR (15 to 35 kN), the VCS (20 to 60 kN), and the VKSD (50 to 119 kN). VCS and VKSD brakes are available as both standard and floating models. Floating, single-sided brakes are mounted on sliding bushings to save space on the installation.

Slowing and halting an 80-m wind-turbine rotor involves converting its kinetic energy into heat. The same mechanical transfer occurs, for example, when stopping a large truck. A 40-ton mining truck, for instance, must be able to stop on a steep gradient. This involves a heavy load that opposes braking and provides a comparison to the aerodynamic torque delivered by a turbine rotor.
Let’s compare the emergency braking requirements of a 1.5-MW wind turbine under maximum wind conditions with those of a 40-ton mining truck. Imagine driving a fully loaded truck down a steep gradient of 25% (1:4) at a 85 mph when a road sign warns of a cliff a quarter mile ahead. The engineering required for effective braking in both cases is similar. Braking for the wind turbine is, in fact, more demanding. Consider that unlike vehicles, wind turbines:
• Have no drivers, so braking must be automatically controlled.
• Use brakes that must operate unmanned for extended periods.
• Must achieve high standards of reliability with extended service periods.
• Must operate under extreme conditions of desert heat or arctic cold.
• Can be sited offshore in salt atmospheres and high humidity, and temperature extremes. Brakes must withstand all these harsh conditions.

• Are located high above ground and sea level, making access difficult for maintenance.

Main rotor braking systems
Rotor brakes control overspeed, and provide parking and emergency braking. These brakes can be mounted on the rotor or low-speed shaft, on the generator (high-speed shaft), and in some cases on both shafts.
Low speed shaft braking is relatively straightforward in that a large disc brake, with a large friction lining area, is easy to accommodate. Unfortunately, installation here requires high braking torque.

A full array of caliper designs is available from Twiflex, Ltd. to meet yaw-braking requirements of any size wind turbine. All brake models are reliable, hydraulically activated, and direct applied. Models T20 and T40 with up to 40 kN braking force, feature two-bolt side mounting and are intended for light to medium-duty applications. Model VCH with 60 kN, featuring four-bolt center mounting, works well in medium sized turbines. Model VKH with 118 kN and base mounted caliper is designed for larger, heavy-duty turbine applications.

Generally speaking, the most cost-effective position is on the high-speed shaft between the gearbox and the generator. The high increase ratios of wind-turbine gearboxes produce a large reduction in output torque. In many cases, a serious criteria regarding brake selection is choosing a friction liner area of sufficient size to ensure adequate heat dissipation during emergency stops.
The energy which must be dissipated is the same wherever the brake is placed meaning the total lining area must be the same. It also means the brake-pad area must be sufficient to control the temperature rise.
These requirements are more difficult to meet on the high-speed shaft because speed and space will be limiting factors with regard to the maximum disc diameter and brake selections. Nevertheless, braking on the high-speed shaft braking has been used on many turbines rated up to 750 kW, although as the industry develops higher capacity turbines, the trend is leaning towards rotor-shaft braking.
A further consideration regarding brake position is the possibility of gear tooth damage. If brakes are installed on the gearbox-output shaft and the turbine is stationary, gusts are likely to cause the rotor to transmit a rocking motion within the backlash of the input and output gears. Without forced lubrication between the mating teeth this effect could ultimately result in fretting and expensive gear damage

A series of high-torque, electrically released, spring engaged, static holding brakes can withstand the severe conditions in the pitch drive of large turbines. This brake series, from Warner Electric, is typically smaller in diameter than the motor assembly and adds minimal length to the overall package. For example, Model ERS-68 brake, rated at 135 Nm, is 6.5-in. diameter and only slightly over 2-in. long. “This brake is rated for 15,000 to 30,000 dynamic stops, depending on coil and voltage required, far exceeding the typical design life criteria of 500 to 1,000 stops,” says Warner Electric Engineer Rich Silvestrini. An additional benefit from an electric brake is its short reaction time, 0.20 sec or less, making it a superior choice for wind pitch drive systems. The reliable design of this brake style easily dissipates the heat generated far and above that required by the normal duty cycle.

Torque for rotor brakes
The braking torque level for rotor brakes is a crucial consideration that must be calculated during initial stages of brake design. The maximum permissible braking torque on the rotor shaft is usually imposed by the blades, or their anchorage to the gearbox input shaft. On the other hand, braking on the high-speed shaft braking is usually related to the maximum permissible gear-tooth loading.
There is also a minimum level of braking torque below which the variable nature of the frictional forces under different operating conditions could place the turbine rotors at risk.
It is therefore important to allow an adequate window of safety, or service factor, to ensure that the brakes will always operate effectively and under all climatic conditions. To achieve an adequate service factor it is helpful to consider how braking performance can vary with the same predetermined level of braking torque. For example, suppose a 500-kW turbine has:

Rotor Inertia: 163,000 kgm²

Aerodynamic torque: 100,000 Nm Rated rotor speed: 50 rpm

If the brake is applied during an emergency at 20% over-speed, the rise in disc temperature and stopping time will vary depending on the chosen service factors.
Maximum brake-path temperatures shows how these change using different service factors relative to a comfortably accepted factor of 2.00, that is:

2.0 = T_b/T_L

where

T_b = brake torque and T_L = load torque.
In this case, the calculated braking torque is 200,000 Nm.
It can be seen on the graph Maximum brake path temperatures that 3T_L gives the minimum temperature rise and, although this is true for all values of inertia, speed, and load torque, it is to a certain extent dependent on the thermal properties of the disc.
Notice also that temperature rise and stopping time vary by only small amounts when the service factors are changed from 1.5T_L to 3T_L. In fact in the case of temperature the rise is only 6%.
This is certainly not the case when applying service factors of 1.5T_L to 1.05T_L. In fact, both stopping time and temperature rise increase rapidly.
Although precise figures for this steep increase will vary with the actual inertias, speeds, and aerodynamic torques, it is clearly and potentially hazardous to design a brake within this region.
Other factors, apart from the composition of the liner material, affect the achievable friction level. A summary includes:

• Bedding and conditioning of the liners
• Dirt on the braking surfaces
• Condensation
• Oil on the braking surfaces
• Rubbing speed and pressure
• Disc temperature
• Disc surface finish and hardness
• Wear debris on liner surfaces
Because wind turbines operate unmanned, it is not possible to monitor all of these conditions. Consequently an allowance must be made when calculating a safe torque level.
Experience shows that molded brake-pad materials can lose 50% of their friction, even under apparently good conditions. This fact suggests that a ratio of T_b/T_L +2 should be regarded as a minimum.

The ratio of braking torque to aerodynamic torque provides one guide for selecting brakes.

Criteria for required braking torque can be summarized as:
• Minimum torque rating T_b/T_L = 2.0
• Adequate pad area
• Acceptable rubbing speed
• Liner material compatible with maximum disc temperature

“Almost all wind turbine rotor brakes are of the fail-to-safe design, being spring-applied and hydraulically released,” says Jon Cooksley, Sales and Marketing Director at Twiflex, Ltd., UK, “They incorporate powerful springs which directly, or through an independently mounted thruster, apply force to press each brake liner against a disc. The brakes are released by compressing the springs with high pressure hydraulic oil supplied from a power pack.”

Brakes for yaw control
Yaw brakes provide an effective means of smoothly controlling a wind turbine nacelle as it rotates “up wind” or yaws. They are usually installed as drag brakes and operate by controlling back pressure, which in turn controls the degree of spring force and therefore braking torques.
Under normal operational conditions, a horizontal axis wind turbine can be stopped by moving the blades out of the wind. However, the mechanism that controls this action usually relies on electricity and would be inoperative in the event of a power failure. While it is possible to design a control system to operate without electrical power, it would be cost-prohibitive. Also, adequate response time could present a serious problem when an emergency stop is required in high winds. Without an electrical load to restrain free acceleration yaw controls may not be fast enough to prevent dangerous over-speeding under gusty conditions. A braking system must also be 100% reliable because should power fail during high winds, brakes become the last line of defense in preventing a catastrophe.
An anemometer signals a change in wind direction which energizes the motor driving the gear ring on the yawing system. The motor is de-energized by a further signal when the yaw mechanism reaches a best up-wind position and stops.
Typically, there are four to eight yaw motors per turbine. The brakes usually mount to the back end of the drive motors and are commonly positioned on the underside of the yaw gear ring. “Varying wind strengths cause varying motor loading and therefore determine the accuracy of the nacelle stop relative to the change in wind direction” said Edouard Haffner, Engineer at Warner Electric, France. “Motor load can be effectively controlled regardless of wind strength by installing a permanently applied, electromechanically released brake on the gear-ring face and varying its drag from the signal actuated by the rise or fall in motor current.”
This ensures accurate nacelle positioning and best operating efficiency. The design eliminates potential damage from erratic movement with the gear backlash and the brake is an effective clamp to lock the mechanism in position.
Wind-turbine engineers agree that a
mechanical disc brake is the best solution in terms of reliability, simplicity of manu-facture, ease of servicing, and initial cost. Disc brakes are renowned for their excellent performance in hostile environments which is why they are used in cranes, heavy vehicles, and other safety-critical applications. Another reason is that a disc brake requires little physical space relative to the braking force it provides.
Depending on turret size and therefore the required clamping torque, caliper brakes may be used in multiples of 2 to 24. Turret brakes typically provide a combined clamping (holding) force ranging from 50 kN to 500 kN.

Blade pitch control braking consideration.
Large horizontal axis wind turbines “pitch” or angle their rotor blades for best efficiency. The rotor blades are also pitched or feathered to minimize rotation in high winds and for turbine maintenance. “Pitch drives can be driven electrically or hydraulically,” says Warner Electric Sales Manager-OEM Tim Heikkinen. “Electric is most common, which lends itself to a cleaner, more compact design. In addition, the electric drive is more accurate and can be easily programmed to meet a variety of application variables. In either case, a power-off holding brake built into the drive serves as an added safety feature, as well as for dynamic braking in emergency pitch conditions.”
The general layout of an electric pitch drive includes: an electric motor, (ac, dc, or servo), a position sensor (encoder or resolver), and a power-off holding brake. Control logic releases the brake, drives the motor, senses the position, stops the pitch operation, and engages the brake to hold the blades in a predetermined position. The motor drives a large ring gear integral to each blade, typically with a gear ratio in the 1,000:1 range. The pitch drive must be a compact package because there is limited space to mount the assembly in the turbine’s nose cone.
When selecting a brake for the pitch drive, allowance must be made for sufficient torque in a compact package. Typically, the brake must not be larger in diameter than the motor and position sensor, and must not add excessive length to the drive system.
Design life must also be factored in to components selection. A large-scale turbine can have an effective design life of twenty years, so individual components and packaged systems must meet or exceed this standard. The brake has to withstand a minimum of full speed dynamic stops, (up to 3,000 rpm, at 135+Nm) to be considered for incorporation into the package.
The number of estimated emergency pitch stops in a 20-year life is generally defined between 500 and 1,000. Due to the large inertia these stops create, the engineer must account for thermal dissipation and peak energy input. A properly designed disc and caliper brake can meet torque and thermal specifications. However this style of brake tends to be quite large in diameter and can be difficult to mount in a limited space. A flange mounted electrically-released/spring-engaged standard motor brake can meet the space requirement, but normally falls short in the torque and thermal specifications. More robust brakes have been designed to meet the higher standards needed in this type of application. WPE

Windpower Engineering & Development

All four models of Twiflex yaw brakes function as static-holding brakes for keeping the nacelle positioned into the wind.

A full array of caliper designs is available from Twiflex, Ltd. to meet the yaw braking-force requirements of any size wind turbine. All brake models are durable, hydraulically activated, and direct applied. Models T20 and T40 deliver up to 40 kN braking force, feature two-bolt side mounting, and are intended for light to medium-duty applications. Model VCH provides 60 kN, features four-bolt center mounting, and works well in medium-sized turbines. Model VKH generates 118 kN, and is a base mounted caliper for larger, heavy-duty turbines.

There typically are four to five yaw motors per wind turbine. The brakes mount to the back end of the drive motors and are commonly positioned on the underside of the yaw gear ring. Twiflex is a member of the Altra Industrial Motion family of power transmission companies.

Windpower Engineering & Development

The brakes come in a range of braking forces from 100 N to 1 MN to meet the torque requirements of the most common turbines. Rotor-brake models include the GMR (15 to 35 kN), the VCS (20 to 60 kN), and the VKSD (50 to 119 kN). VCS and VKSD brakes are available as standard and floating models. Floating, single-sided brakes are mounted on sliding bushings to save space.

Twiflex spring-applied, hydraulically-released, caliper brakes are typically mounted to a turbine’s ma

Twiflex Model GMR-SH disc calipers generate a braking force of 35 kN. The line includes three models to generate braking forces of 15 to 119 kN.

in rotor shaft, between gearbox and generator, and used primarily as safety brakes during emergency stops under high wind conditions. All units are engineered to handle the large output torque generated by the high ratios found in wind-turbine gearboxes. The brake models are in operation today on hundreds of wind turbines around the world. Twiflex is a member of the Altra Industrial Motion family of power transmission companies.

Windpower Engineering & Development