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newSchoolWorks.com › VoTechWorks › Building & Construction › Wind Turbine Technician › Wind Turbine Mechanics › Lesson 10
BLD-WIND70-02 · Lesson 10 · 14.5 clock hours · demo

Yaw System: Drive, Ring Gear, and Lubrication

Objective. Students describe how the yaw drive system orients the nacelle into the wind and demonstrate a yaw ring-gear visual inspection and lubrication task.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

What the Yaw System Does

A wind turbine generates maximum power only when the rotor faces directly into the wind. The yaw system continuously rotates the nacelle on top of the tower to keep that alignment as wind direction shifts. Without a properly working yaw system, energy capture falls and structural fatigue loads on the blades and tower increase.

How Active Yaw Control Works

Modern utility-scale turbines use active yaw control. A wind vane and anemometer mounted on the nacelle send wind-direction and wind-speed signals to the yaw controller (a dedicated programmable logic controller or part of the main turbine controller). When the controller detects a sustained misalignment beyond a set threshold, it commands one or more yaw motors to drive the nacelle.

Each yaw motor turns a pinion gear (a small-diameter drive gear). The pinion meshes with the large-diameter yaw ring gear that is bolted to the top of the tower. Because the ring gear is fixed to the tower and the nacelle rotates around it, the motor-pinion-ring arrangement acts as a high-ratio gear reduction: the motors turn fast but produce very high torque at low rotational speed, swinging the heavy nacelle smoothly.

Typical turbines have two to eight yaw motors arranged evenly around the ring gear. Multiple motors share the load and allow yaw movement to continue even if one motor fails.

Yaw Brake

Once the nacelle reaches the target heading, the yaw brake engages. The yaw brake is usually a hydraulic or spring-applied caliper that clamps a brake disc mounted on the yaw bearing or ring-gear assembly. The brake holds the nacelle steady against wind loading and prevents "hunting" (constant small corrections that would wear out the drive components). During yaw motion the brake is partially or fully released; during normal generation it is fully applied.

The braking sequence is: controller signals yaw movement → brake releases → motors drive pinion against ring gear → nacelle rotates → motors stop → brake re-applies.

Yaw Ring Gear and Lubrication

The yaw ring gear is a large open-face ring of alloy steel with precision-cut teeth. Unlike enclosed gearboxes that use circulating oil, the ring gear runs as an open gear and requires periodic application of open-gear compound. Open-gear compound is a thick, adhesive grease that clings to tooth flanks, protects against metal-to-metal contact, and resists fling-off during rotation.

OEM (original equipment manufacturer) procedures specify the product grade, application interval, coverage area (typically all tooth flanks around the full circumference), and quantity. Always follow the specific OEM procedure for the turbine you are servicing; do not substitute a product without engineering approval.

Visual Inspection of the Yaw Ring Gear

A ring-gear inspection is a scheduled preventive maintenance task. Inspectors use standardized condition codes to record what they find so that records are consistent across technicians and over time. Common conditions to look for:

Work systematically around the full ring gear circumference. Use adequate lighting (a headlamp and a flashlight to create cross-lighting highlights pitting). Record every condition on the inspection form before moving on.

Safety: Before You Begin

Lockout/Tagout (LOTO) is mandatory. The yaw motors and yaw brake are energized systems. Before any hands-on inspection or lubrication task, the turbine must be de-energized and locked out per your site-specific energy control procedure and OSHA requirements. Verify zero energy state before touching any drivetrain component.

PPE required for this task:

Dispose of used compound applicators and rags according to site waste-management procedures; open-gear compounds are petroleum-based and must not be poured down drains.

Applying Open-Gear Compound

Follow the OEM procedure step by step:

1. Confirm LOTO is in place and zero energy is verified.

2. Don appropriate PPE.

3. Using a stiff-bristle brush, apply compound to all tooth flanks around the full circumference of the ring gear.

4. Ensure the compound covers the contact band, tooth flanks, and tooth roots without leaving bare metal visible.

5. Do not over-apply to the point of creating buildup in the tooth valleys that could be flung onto brake surfaces.

6. Record the product name, lot number, date, your name, and coverage assessment on the lubrication schedule form.

7. Remove tools and materials, restore the area, and follow the re-energization steps in your energy control procedure.

Key terms10

Active yaw controlA powered system that uses sensor signals and motorized drives to rotate the nacelle and keep the rotor aligned with the wind, as opposed to passive (free-rotating) yaw used on small turbines.
Yaw ring gearThe large-diameter open gear ring fixed to the top of the tower; the yaw motor pinions mesh with it to rotate the nacelle.
Pinion gearThe small-diameter drive gear on each yaw motor shaft that engages the teeth of the yaw ring gear to produce high torque through gear reduction.
Yaw brakeA friction brake—typically hydraulic or spring-applied—that clamps the yaw bearing or ring-gear assembly to hold the nacelle at the commanded heading during power generation.
Open-gear compoundA heavy, adhesive lubricating grease formulated for open (unenclosed) gear surfaces; it clings to tooth flanks and resists fling-off during operation.
PittingA gear-tooth surface defect consisting of small fatigue craters; early-stage pitting is monitored, while advanced pitting requires engineering evaluation.
SpallingA more severe gear-tooth defect in which larger areas of surface material fracture and break away, often progressing from advanced pitting.
Condition codeA standardized alphanumeric or descriptive code used on inspection forms to record the observed state of a component consistently across technicians and inspection intervals.
Gear reductionThe mechanical arrangement in which a small drive gear (pinion) turns a larger gear (ring), multiplying torque while reducing rotational speed.
Lockout/Tagout (LOTO)An OSHA-required energy control procedure in which equipment is de-energized, locked, and tagged to prevent unexpected startup before maintenance or inspection work begins.
In class: Instructor presents a yaw system diagram and explains active yaw control: wind vane/anemometer signal, yaw controller, yaw motor(s), pinion gear, yaw ring gear, and yaw brake—students annotate their own diagram with torque direction, gear reduction path, and braking sequence. · Using a yaw ring-gear mockup, instructor demonstrates an inspection for tooth wear, pitting, and adequate grease coverage; students then perform their own inspection, recording surface condition on an inspection form using standardized condition codes. · Students apply open-gear compound to the yaw ring-gear mockup using a brush, following a mock OEM procedure specifying coverage area and product, and record the task on a lubrication schedule form.
Materials: Yaw system diagram/poster, yaw ring-gear mockup, open-gear compound (sample tube), application brush, inspection form with condition codes, lubrication schedule form, PPE kit
Check: Instructor-observed inspection: student correctly identifies and codes at least four of five deliberately placed condition examples on the mockup, and lubrication coverage is judged complete by instructor—4 of 5 conditions correctly coded to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Wind Turbine Tech – Turbine Systems
1. What is the primary purpose of the yaw system on a utility-scale wind turbine?
NCCER Wind Turbine Tech – Mechanical Systems
2. In an active yaw drive, the pinion gear is driven by the yaw motor and meshes with the yaw ring gear. What is the mechanical effect of this arrangement?
NCCER Wind Turbine Tech – Turbine Systems
3. What is the correct sequence of events when the yaw controller commands a heading change?
NCCER Wind Turbine Tech – Preventive Maintenance
4. During a yaw ring-gear inspection, a technician finds small fatigue craters scattered across the tooth flanks on roughly 20% of the teeth. Using standard condition terminology, this finding is best described as:
NCCER Wind Turbine Tech – Lubrication
5. Why is open-gear compound used on the yaw ring gear instead of standard machine oil?
OSHA 10 – Hazardous Energy Control
6. Before performing a hands-on lubrication task on the yaw ring gear, which safety step is required by OSHA and must be verified before touching any component?
NCCER Wind Turbine Tech – Lubrication
7. A technician applying open-gear compound to the yaw ring gear notices compound is building up heavily in the tooth valleys (roots) near a brake caliper. What is the primary concern with this condition?
GWO Basic Safety Training – Safety in the Wind Industry
8. During a ring-gear inspection a technician observes a crack at the root of one tooth. What is the correct immediate action?
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