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newSchoolWorks.com › VoTechWorks › Building & Construction › Wind Turbine Technician › Wind Turbine Mechanics › Lesson 5
BLD-WIND70-02 · Lesson 5 · 16.5 clock hours · lab

Main Shaft, Main Bearing, and Bedplate Inspection

Objective. Students perform a documented visual and tactile inspection of a main-shaft assembly and main-bearing housing, recording all observations on a standard maintenance checklist.

⚠ 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.

Main Shaft, Main Bearing, and Bedplate: Why They Matter

The main shaft (also called the low-speed shaft) transfers the rotational energy of the rotor hub into the drivetrain. It spins at rotor speed—typically between 5 and 20 RPM on a utility-scale turbine—and carries enormous bending and torsional loads. The main bearing supports the shaft radially and, depending on design, axially as well. The bedplate is the structural steel frame that mounts the entire drivetrain to the tower top. Failure in any of these three components can take a turbine out of service for weeks, so routine documented inspection is a core technician duty.

Lockout/Tagout Before You Touch Anything

Before any hands-on inspection of the drivetrain, you must follow the site-specific written Lockout/Tagout (LOTO) procedure. On a drive-train lab trainer the steps mirror real-world practice:

1. Notify affected personnel.

2. Identify every energy source (electrical, mechanical, pneumatic, hydraulic, gravitational).

3. Shut down the machine using normal stopping procedures.

4. Isolate each energy source at its control point (disconnect switch, valve, brake release).

5. Apply your personal lockout device and tag.

6. Release or restrain stored energy (bleed hydraulic pressure, block the rotor or shaft against rotation).

7. Verify zero-energy state—attempt to start, then confirm no movement.

Skipping or shortcutting LOTO is never acceptable. The main shaft can restart under wind load even when the generator is de-energized.

PPE for Drivetrain Inspection

For this lab you will wear at minimum: safety glasses or goggles, hard hat (inside a nacelle or near overhead components), cut-resistant gloves when handling bearing housings with sharp edges, and steel-toed boots. If the lab trainer is elevated, a full-body harness inspected before donning is required. Remove all rings, watches, and loose clothing before approaching rotating or stored-energy equipment.

Inspecting the Main-Bearing Housing

With the shaft locked out, work around the bearing housing systematically. Use the checklist as your guide—never rely on memory alone.

Cracks and corrosion. Use a flashlight and inspect the cast or fabricated housing for surface cracks, especially at welds, bolt-boss fillets, and any area where a previous repair is visible. Mark any crack with a paint pen and flag it on the checklist. Surface rust is normal on exposed steel; deep pitting that reduces wall thickness is a concern.

Grease-seal condition. The lip seals on each side of the bearing prevent grease from escaping and contaminants from entering. Look for cracking, extrusion, missing pieces, or excessive grease leakage beyond the seal face. A light film of grease near the seal lip is normal; puddles or dried grease on adjacent structure indicate seal failure.

Mounting-bolt torque markings. OEMs paint-mark or use witness marks across bolt heads and flanges at the factory-specified torque. If a mark is broken or offset, the bolt has moved. Do not attempt to re-torque during this inspection unless instructed; instead, record the finding. When re-torque is called for, use a calibrated torque wrench and the OEM specification.

Measuring Main-Shaft Radial Runout

Runout is the total indicator reading (TIR) of shaft surface variation as the shaft is rotated one full revolution. Excessive runout indicates a bent shaft, worn bearing, or improper seating.

Setup. Attach a magnetic-base dial indicator to a solid, non-rotating surface (bearing housing or bedplate). Position the contact point perpendicular to the shaft surface at the location specified in the lab procedure. Zero the indicator.

Measurement. Slowly rotate the shaft by hand through 360°. Record the highest positive and lowest negative reading. TIR = (maximum reading) − (minimum reading).

Serviceability limit. Compare your TIR to the limit given in the lab handout (which mirrors OEM format). If TIR ≤ limit, record "Acceptable." If TIR > limit, record "Out-of-tolerance" and flag the item—do not return the machine to service.

Units. Always record runout in inches or millimeters as specified. Mixing units is a common error; confirm your indicator scale before reading.

Condition Reporting

A professional condition report is brief, factual, and free of guesswork. Write in past tense, active voice, and identify components by OEM part name or location reference (e.g., "drive-end seal" not just "the seal"). State what you observed, where you observed it, and what action is recommended—not what you think caused it unless you are certain. Example opening: "Inspection of the drive-end grease seal on the main-bearing housing revealed circumferential cracking at the outer lip." Avoid phrases like "looks bad" or "probably fine."

Connecting to Your Credentials

This lab directly supports GWO Basic Safety Training (LOTO and PPE practices), NCCER Wind Turbine Tech (drivetrain inspection competencies and documentation standards), and OSHA 10 (recognizing and recording workplace hazards). Employers expect technicians to produce inspection records that can withstand audit—your checklist and condition report are professional documents from the moment you sign them.

Key terms8

Main shaft (low-speed shaft)The shaft connecting the rotor hub to the gearbox or generator; rotates at rotor speed and transmits drivetrain torque.
Main bearingThe large rolling-element or plain bearing that supports the main shaft radially and (in some designs) axially against rotor loads.
BedplateThe structural steel frame that sits atop the tower and supports the entire nacelle drivetrain, including the main bearing housing and gearbox.
Radial runout (TIR — Total Indicator Reading)The difference between the maximum and minimum dial-indicator readings taken at a fixed axial location as the shaft is rotated one full revolution; indicates shaft straightness and bearing condition.
Lockout/Tagout (LOTO)A mandatory energy-control procedure that physically isolates all hazardous energy sources and prevents re-energization while work is performed, in accordance with OSHA requirements.
Grease lip sealA flexible seal pressed into the bearing housing bore that retains lubricating grease inside the bearing and excludes contaminants; inspected for cracking, extrusion, or excessive leakage.
Witness mark (torque mark)A paint stripe or scribe line applied across a bolt head and mating surface at the correct torque; if the mark is broken or offset, the fastener has loosened.
Serviceability limitThe maximum allowable value for a measured parameter (such as runout) as defined by the OEM; a reading beyond this limit requires the component to be removed from service or repaired.

Worked examples4

1. A dial indicator is zeroed on the main shaft. After one full rotation the maximum reading is +0.006 in and the minimum reading is −0.002 in. The serviceability limit is 0.010 in TIR. Is the shaft acceptable?
  1. Identify the maximum reading: +0.006 in
  2. Identify the minimum reading: −0.002 in
  3. Calculate TIR: TIR = maximum − minimum = 0.006 − (−0.002) = 0.006 + 0.002 = 0.008 in
  4. Compare to the serviceability limit: 0.008 in ≤ 0.010 in
Answer: TIR = 0.008 in, which is within the 0.010 in limit. Record as Acceptable.
2. On the same shaft the indicator is re-zeroed and a second run gives a maximum of +0.007 in and a minimum of +0.002 in. Calculate TIR and classify.
  1. Maximum reading: +0.007 in
  2. Minimum reading: +0.002 in
  3. TIR = 0.007 − 0.002 = 0.005 in
  4. Compare to the serviceability limit of 0.010 in: 0.005 in ≤ 0.010 in
Answer: TIR = 0.005 in. Acceptable. Note: both readings were positive because the zero was set at a high point; TIR arithmetic is the same regardless of sign.
3. A different shaft gives maximum +0.009 in and minimum −0.004 in against a serviceability limit of 0.010 in. Classify the reading and state the required action.
  1. TIR = 0.009 − (−0.004) = 0.009 + 0.004 = 0.013 in
  2. Compare: 0.013 in > 0.010 in
  3. Classification: Out-of-tolerance
  4. Required action: Flag the item on the checklist and condition report; do not return to service without engineering review or component replacement per OEM guidance.
Answer: TIR = 0.013 in, exceeds the 0.010 in limit. Record as Out-of-tolerance and flag for corrective action.
4. A technician converts a runout reading from inches to millimeters. The TIR is 0.008 in. What is the equivalent in millimeters? (Use 1 in = 25.4 mm.)
  1. Multiply: 0.008 in × 25.4 mm/in
  2. 0.008 × 25.4 = 0.2032 mm
  3. Round to three decimal places as is conventional for precision measurements: 0.203 mm
Answer: 0.008 in = 0.203 mm. Always confirm which unit the OEM limit is expressed in before comparing.

The math in this lesson2

Learners compute total indicator runout 0.006 − (−0.002) = 0.008 in from dial readings.
7.NS.1N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Following a written LOTO procedure, students isolate the main shaft on a drive-train lab trainer, then inspect the main bearing housing for cracks, corrosion, grease-seal condition, and mounting-bolt torque markings, recording each finding on the checklist. · Using a dial indicator on a magnetic base, students measure main-shaft radial runout at a specified location, compare the reading to a given serviceability limit, and classify the reading as acceptable or out-of-tolerance. · Students photograph two findings (one normal, one flagged as a potential concern) and write a one-paragraph condition report using prescribed technical language consistent with OEM documentation style.
Materials: Drivetrain lab trainer (or sectioned main-shaft/bearing assembly), dial indicator with magnetic base, feeler gauges, inspection checklist forms, calibrated torque wrench, camera or phone, PPE kit
Check: Completed inspection checklist reviewed by instructor: all 15 checklist items addressed, dial-indicator reading recorded with correct units, condition report uses accurate terminology—pass requires all items attempted and no safety step skipped.

Practice check8

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

OSHA 10 / GWO Basic Safety Training — energy control and hazard recognition
1. What is the FIRST action a technician must take before touching the main shaft on the drive-train lab trainer?
NCCER Wind Turbine Tech — drivetrain inspection and measurement
2. A dial indicator reads a maximum of +0.011 in and a minimum of −0.003 in during a shaft runout check. What is the TIR?
NCCER Wind Turbine Tech — fastener inspection and documentation
3. During main-bearing housing inspection a technician finds that a paint witness mark across a mounting bolt head is broken and offset by about 5 degrees. What does this indicate?
NCCER Wind Turbine Tech — component condition assessment
4. Which condition at a main-bearing grease lip seal is considered NORMAL and does NOT require a defect flag on the checklist?
NCCER Wind Turbine Tech — measurement and documentation
5. A TIR measurement of 0.007 in is compared to a serviceability limit of 0.010 in. What is the correct classification and checklist entry?
NCCER Wind Turbine Tech — technical documentation
6. When writing a condition report in OEM documentation style, which sentence best describes a cracked grease seal?
OSHA 10 / GWO Basic Safety Training — hazardous energy identification
7. Which stored-energy source is most likely to remain on the main shaft drivetrain even after the generator electrical disconnect has been opened?
NCCER Wind Turbine Tech — precision measurement
8. A student sets up a dial indicator on the bearing housing and rotates the shaft. The indicator needle swings between +0.004 in and +0.009 in. What is the TIR?
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