FIELD OF INVENTION
The present invention relates to retaining ring assembly in turbo-
generator. More particularly, it relates to an improved arrangement for
retaining ring assembly in turbo-generators in the event of replacing the
existing rings.
BACKGROUND AND PRIOR ART OF THE INVENTION
Retaining rings are fitted on general rotor end winding to keep the
winding restrained from centrifugal forces. These rings are highly
stressed components in a generator and are susceptible to failures
during unfavorable regimes of machine operation. Alternatively, the rings
of generators made in earlier days are made out of a material, susceptible
to stress corrosion cracking (SCC). In view of the SCC free material
available, most of the customers prefer ring placement during major
overhauls, even without noticing the failures.
The retaining ring assembly of a typical make consists of retaining rings,
ring nuts, axial arrestments and loose components. Problem with the
design as per prior art is that the procedure is cumbersome and involves

many components. Manufacture of the rings requires special purpose
turning lathes to form threads on the ring outer surface. This takes a
considerable cycle time and incurs higher costs to repair.
The present invention eliminates components viz, ring nuts and costly
axial arrestors. The improvised version eliminates the need to form
threads and hence the need to have a special purpose lathe. As per the
present invention, internal threading made on the ring nuts also will get
eliminated. The operating stress levels will reduce, thus making it
possible to reduce the ring sizes and the associated material cost. There
will be considerable reduction in manufacturing cycle time.
Retaining rings are used to protect the end winding from centrifugal
forces. These are fitted on either end of generator by shrink fitting. In
view of the rotor rotation, self weight and the end winding load acting at
its inner dia, the stresses induced are very high. In view of high stresses,
and presence of moisture in the cooling medium/environment in which
rings are in operation, they are susceptible for failure due to stress
corrosion cracking (SCC). New SCC free materials have been evolved and
the machines are fitted with the same. However, there are several rings
in operation which are SCC prone.

The retaining rings are subjected to another kind of failure when,
motoring takes place during generator operation. Healthiness of the rings
for reuse depends on the duration of failure and the extent of fault
currents. In either case, the rings need replacement.
As per prior art, the rings are shrink fitted on to the body of rotor (1) as
shown in Fig. 1 on its either end. The outer periphery of the rings (3) is
threaded to lock these rings from axial movement and this locking is
done with the help of ring nuts. Axial movement is caused due to force
exerted by end winding (2) due to thermal expansion. Set of axial arrestor
plates are given to restrain the ring nuts from axial movement. As per
invention shown in Fig.3, the number of components (ring nuts, axial
arrestor plate, grub screw) shown in Fig. 1 are eliminated. The retaining
ring is shrink fitted at two steps and the existing groove intended for
axial arrestor plate is utilized for a snap ring, the purpose of which is to
secure the retaining ring from axial movement. The snap ring being a
component made out of low carbon steel, simplifies the procurement and
manufacturing. Retaining rings are shrink fitted on either end of
generator.
Hence the proposed method as per invention simplifies the design and
eliminates some of the costly components. It also reduces the
procurement and manufacturing cycle times.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose an improved
arrangement for retaining ring assembly in turbo-generators in the event
of replacing the existing rings which eliminates few costly components
during assembly to make it economical.
Another object of the invention is to propose an improved arrangement
for retaining ring assembly in turbo-generators in the event of replacing
the existing rings which is capable of reducing manufacturing cycle time.
An yet another object of the invention is to propose an improved
arrangement for retaining ring assembly in turbo-generators in the event
of replacing the existing rings which is capable of avoiding thread
formation on retaining rings on special purpose lathes.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - shows a cross section of a turbo-generator
Fig. 2 - shows a retaining ring assembly as per prior art
Fig. 3 - shows the retaining ring assembly in accordance with the
invention

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Fig. 1 shows a turbo-generator. Generator rotor (1) carries the filed
winding and this is the main component on which various other
components are mounted to accomplish its function. Rotor end winding
(2) is the winding formed on either end of the rotor active length. The end
winding (2) is encompassed by the retaining rings (3) in the present
invention. Snap ring (4) is used to arrest the axial movement of the
retaining ring when the generator is operating at its rated speed.
As shown in Fig.2 of prior art, the retaining ring (3) will restrain the end
winding (2) from flying away due to centrifugal forces. The ring nut (5) is
intended to axially lock the rings from movement. The fit between the
retaining ring (3) and ring nut (5) is by thread fit. The retaining ring
threads (6) are externally formed and the ring nut threads (7) are
internally formed. Axial arrestor (8) is used to protect the ring nut from
moving away along with retaining ring, away from rotor body and grub
screw (9) in intended to restrain the ring nut (5) form moving towards
rotor body.

Fig.3 shows the retaining ring (3) as per invention. Retaining ring (3)
encompasses the end winding (2). The ring is shrink fitted on the rotor
body (1). Axial expansion of the retaining ring is restrained by the snap
ring (4).
As per prior art, the fit between retaining rings (3) and ring nuts (5) is a
threaded fit, with external threading on the retaining rings and internal
threading on the ring nuts. The ring nuts (5) in turn are axially locked by
assembling axial locking plates (8). While threading on the ring nuts (5)
are made on the nuts outside the rotor, external threading on the
retaining rings are made after shrink fitting. As the generator rotor
length is quite long running in meters and the retaining thread dia is the
largest dia in a generator, thread forming cannot be done on all longer
center lathes. Special purpose lathes are required to form the threads.
Hence the design as per prior art constrains the utilization of the
machine tools available in a shop floor.
The invention uses the same rotor body (1) stepped geometry and same
groove for the axial locking plates (8). The retaining rings (3) are shrink
fitted at two steps as shown in Fig.3. The existing groove is utilized for
mounting a snap ring (4).

In view of the elimination of thread cutting and introduction of double
stepped shrink fits with snap rings (4) assembled in the existing groove,
the ring design gets simplified. The invention, the arrangement of which
is shown in Fig.3, also eliminates ring nuts (5) and axial arrestment
plates (8). The machining process that involves threading of the outer dia
of retaining rings (3) and internal threads of ring nuts (5) get eliminated.
The ring nuts (5) and axial arrestment plates (8) are made out of high
alloy steel to withstand the high induced stresses due to rotation and
their higher diameters. As per the invention, the ring nuts and axial
arrestors are eliminated. Snap ring (4) made out of mild steel is employed
to restrain the ring (3) from axial movement. As the invention is for the
rotors that come up for repair, the existing rotor geometry is
implemented. Modified arrangement, shown in Fig.3 with stepped shrink
fits and snap ring arrangement is an improvised version of the retaining
ring assembly compared to the ring design as per prior art.

WE CLAIM
1. An improved arrangement for retaining ring assembly in turbo-
generators in the event of replacing the existing rings comprising:
a plurality of retaining rings (3) encompassing the end winding (2)
for restraining the said end winding from flying away;
at least one snap ring (4) for arresting the axial movement of the
said retaining ring;
characterised in that the existing groove for axial locking plates is
utilized for mounting the said snap ring.
2. An improved arrangement for retaining ring assembly in turbo-
generators in the event of replacing the existing rings as claimed in
claim 1, wherein the retaining ring (3) is shrink fitted at two steps.
3. An improved arrangement for retaining ring assembly in turbo-
generators in the event of replacing the existing rings as claimed in
claim 1, wherein the snap ring (4) is made of mild steel.
4. An improved arrangement for retaining ring assembly in turbo-
generators in the event of replacing the existing rings as claimed in

claim 1, wherein the retaining rings (3) are fitted on either end of
generator.

An improved arrangement of retaining ring (3) assembly consists of a plurality of retaining rings shrink fitted at two steps on either side of a generator rotor end winding (2) wherein a snap ring (4) is adapted in the existing groove to arrest the axial movement of the said retaining ring (3).