Generator Rotor Stub Shaft Repair

image credit: MD&A
Jay Eldridge's picture
Vice President Sales & Marketing MD&A

Bachelor of Science (BS) in Mechanical Engineering with a Mathematics Minor from Union College.  28 years’ experience working in the power generation industry.  Almost 17 of those years were...

  • Member since 2013
  • 43 items added with 61,829 views
  • Nov 16, 2020

Mechanical Dynamics and Analysis (MD&A) was retained by a utility to perform a stub-shaft repair of a generator rotor collector-end shaft. The customer reported that the generator rotor had suffered a collector ring failure and also a ground to the main shaft. Major arcing and heat damage occurred to the generator rotor forging shaft.

The utility weighed several repair options. These included rotor welding, which would have been time-consuming and expensive, and field replacement, which presented an unacceptably long lead time and high cost. A third option—stub shaft repair—proved to be the best choice in terms of cost and downtime for the revenue-generating asset. MD&A designed and performed the repair, which involved severing the rotor outboard of the collector-end journal and then installing a bolted-on stub shaft.

As part of the repair, the generator rotor shaft end was removed and the bolted-on stub shaft was fitted in the journal outboard seal area. To ensure that no hydrogen leak would occur around this flange face, an O-ring groove and O-ring were installed.

Also, MD&A experts prepared calculations to ensure that the new stub-shaft bolted connection could support the assembly torque and the exciter that bolts to the shaft. These computations used the original shaft end bore, the key holding the shrunk-on coupling, and the shrunk-on coupling bolting to the exciter.

Stub Shaft Design Concept

Machining of the rotor shaft end included severing the rotor just outboard of the collector-end journal in the middle of the outboard oil seal diameter. Once severed, the shaft-end face was milled, drilled, and tapped to allow for a bolted flange, an alignment female spigot, and an O-ring seal groove.

Machining the new stub shaft included all reverse-engineered features of the original collector shaft end. The forging was bored to the original shaft bore diameter to include the bore plug fit. The flange face was machined to include 14 thru holes and a male spigot fit-in support of bolting up to the original shaft. The shaft was machined to include all the fit diameters for the collector rings, the fan hub, and the shrunk-on coupling. Four radial holes were drilled for the collector ring radial stud assembly.

After it was final-machined, the stub shaft was dimensionally inspected and subjected to a non-destructive examination by MD&A. Before the rotor was returned to the customer, final quality checks were completed, including visual, dimensional and electrical tests.

MD&A provides comprehensive refurbishment and repair of utility generator stators and fields, including collector ring machining and high-voltage bushing refurbishment. Call MD&A today at +1 (314) 880-3000.

Also be sure to sign up for our MD&A Insight e-newsletter delivered quarterly.  Fill in sign up form and SUBSCRIBE!



Tour our state-of-the-art 250,000 square foot MD&A Turbine-Generator Repair Facility! Through over 30 years, we’ve earned a reputation as the trusted, one-stop-shop for power generators.


Connect with MD&A

Fill out this form to receive more information from MD&A.

Jay Eldridge's picture
Thank Jay for the Post!
Energy Central contributors share their experience and insights for the benefit of other Members (like you). Please show them your appreciation by leaving a comment, 'liking' this post, or following this Member.
More posts from this member


Spell checking: Press the CTRL or COMMAND key then click on the underlined misspelled word.

No discussions yet. Start a discussion below.

Get Published - Build a Following

The Energy Central Power Industry Network is based on one core idea - power industry professionals helping each other and advancing the industry by sharing and learning from each other.

If you have an experience or insight to share or have learned something from a conference or seminar, your peers and colleagues on Energy Central want to hear about it. It's also easy to share a link to an article you've liked or an industry resource that you think would be helpful.

                 Learn more about posting on Energy Central »