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This invention generally relates to runner hub of a Kaplan type hydro turbine which
accommodates the runner blades alongwith trunnions. More particularly, this invention relates to
a composite two piece runner hub which has a shorter manufacturing cycle.
BACKGROUND AND PRIOR ART:
In a typical hydroelectric plant having Kaplan turbine, runner hub is a vital
component of the runner which converts hydraulic energy of water into rotational mechanical
energy. A typical runner hub performs the following functions:-
1. The outer top portion transfers the torque generated by the blades to turbine shaft which
drives the generator.
2.The outer middle portion houses the sealing arrangement and the outer bushes for the blade
trunnion.
3. The outer bottom portion holds the runner cone.
4. The inner top portion has a counter bore which aligns the hub with the shaft.
5.The inner middle portion accommodates inner bushes for the runner blade trunnions.
6.To the inner bottom portion is attached the piston of the hydraulic cylinder which drives the
runner blade turning mechanism.
In the prior art, manufacture of runner hub calls for a single piece casting. A single piece
casting of runner hub has the following disadvantages/drawbacks:-
1.) Casting complications of core support arise due to molten metal flow to the heavy central
portion during casting. The central heavy portion is thus defect prone. Casting defects in the
heavy central portion are quite often inaccessible for repair and sometimes lead to rejection of
the casting altogether.
2.) Machining the long central bore which is required for oil passage for the hydraulic
servomotor is very difficult and again results in faults and rejection.

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3.) On account of the large size and weight of casting and casting complications, only a limited
number of casting suppliers are capable of supplying this item which leads to procurement at
uneconomical prices, and also results in a long lead time for procurement of the casting.
The two-piece runner hub in accordance with the invention seeks to overcome the drawbacks of
the prior art.
OBJECTS OF THE INVENTION:
It is therefore an object of the invention, to provide a runner hub which eliminates the earlier
casting complications of the heavy central portion which was cast integral with the outer portion.
Another object of the invention is to provide a runner hub which allows easier machining of the
central bore.
Yet another object of the invention is to reduce the weight of single piece casting and the casting
complications so that it is feasible to procure much faster and at more economical prices.
A further objective of the invention to provide runner hubs with different materials for the outer
and the inner portions.
Yet another objective of the invention is to reduce the cycle-time for its manufacture.
DESCRIPTION OF THE INVENTION:
The runner hub of a Kaplan turbine in accordance with the invention is a two piece composite
item with the outer profile portion being a casting and the central inner portion a forging. The
inner and outer portions have a spigot fit and are bolted to the shaft flange. The central portion
is a forging, in which the bore is conveniently machined by trepanning operation.
In projects where a stainless steel runner hub is required to combat silt erosion, only the outer
profiled portion, which in is contact with silty water is provided as a

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stainless steel casting, whereas for the inner central portion a highly economical steel forging is
provided instead of stainless steel casting.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS:
Fig-1 shows the runner arrangement with single piece runner hub casting as in prior art.
Fig-2 shows a sectional view of the single piece runner hub casting as in prior art.
Fig-3 shows the runner arrangement with two piece composite runner hub.
Fig-4 shows the two piece composite runner hub in accordance with the invention.
Fig-5 Shows the runner hub outer portion casting in accordance with the invention.
Fig-6 Shows the runner hub central inner portion forging in accordance with the invention.
The invention will view be described in details with the help of the accompanying drawings.
Fig-1 Shows runner hub of prior art of Kaplan type hydro turbine runner which converts hydraulic
energy into rotational mechanical energy.
The outer top portion of runner hub has reamed holes and threaded holes for coupling with
flange of turbine shaft. Shear bushes fitted in the reamed holes transfer torque to turbine shaft.
The studs fitted in the threaded holes transfer the weight and hydraulic thrust of runner blades
to the shaft.
The outer middle portion of runner hub accommodates the runner blade sealing against flowing
pressurized water outside the hub and the oil inside the hub.
The outer bottom portion has threaded holes to which is bolted the runner cone. The inner top
portion has a counter bore which aligns the hub with shaft. The inner middle portion
accommodates inner bushes for blade trunnions. The inner bottom portion has threaded holes to
which is bolted the piston of hydraulic cylinder of the turning mechanism.
In the prior art the runner hub comprising of the outer and inner portions was a large single
piece integral casting as shown in Fig-2. Such a large single piece casting with a very heavy inner
central portion had the following casting complications/problems
a) Core support
b) Poor molten metal flow to thick central portion

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c) Central portion inaccessible for repair.
The central portion was defect prone and often led to rejections.
On account of such a large heavy defect prone casting there were very few foundries to cast it
and, they took lot of time to deliver a good casting, thereby delaying the manufacturing
schedules on account of its long cycle-time for manufacture.
Also it was very difficult to machine the central bore required for oil passage to
servomotor/hydraulic cylinder.
In the two-part hub (1) according to the invention as shown in Fig-3&4, an innovative approach
has been adopted viz. that of splitting the runner hub into two portions namely, the outer
spherical profiled portion (1.1) in contact with water and the inner portion (1.2). The outer
spherical profiled portion is conveniently made as a casting. The inner heavy central portion with
relatively simpler shapes is made as a forging.
Making the outer profile portion (1.1) as a separate barrel type casting as shown in Fig-5 instead
of a single casting of outer & inner portions has the following advantages:-
a) The core support becomes much simpler and easier
b) The molten metal flow pattern is much better

c) The casting is easily accessible from inside from the point of view of non-destructive checks
and up gradation if required
d) On account of simpler & lighter casting, suppliers are able to deliver the casting in much less
time.
Making the inner central heavy portion (1.2) as a forging as shown in Fig-6 has the following
advantages:-
1) Since forgings do not require any special tooling or patterns, they are delivered much faster;
so that a lot of machining operation on it which can be carried out independent of the casting of
the outer profiled portion are carried out.

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2) The inner portion is more like a shaft forging and the central bore (1.3) required for oil
passage to servomotor, is very conveniently machined by the forge master as a trepanning
operation of shafts. In the earlier single piece casting, machining of the long central bore was a
difficult operation.
3) Due to the convenience of carrying out parallel machining operations separately, availability
of the central forging much in advance of the casting and machining of central bore at forge
master's works, the total machining operations are finished much earlier and made available for
assembly well in time. Thus the delivery cycle of the runner assembly is reduced.
4) As compared to earlier cast design where the inner portion was defect prone the forging of the
innovated design has a very sound material.
The two-part runner hub (1) in accordance with the present invention provides another
noteworthy advantage over the single piece cast runner hub of the prior art. It is possible to offer
different materials for the two parts.
The outer portion (1.1) is subjected to erosion due to the silt and particulate matter present in
the water being used to drive a Kaplan type turbine. This erosion is a matter of great concern, as
the water approaching the outer part is under pressure and is flowing at high velocity. It is
therefore preferable that the outer portion be made of stainless steel, which is much more
erosion resistant.
In the prior art, to achieve this one had to go for the entire hub casting in stainless steel, calling
for huge costs.
In accordance with the present invention however, as the outer portion (1.1) is cast as a
separate part, only this portion can be cast with stainless steel, the inner portion (1.2) remaining
of forged steel. This offers considerable cost advantage.

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WE CLAIM
1. A composite two piece runner hub (1) for Kaplan type hydro turbine comprising an outer
potion (1.1) and on inner portion (1.2) having a central bore (1.3), characterized in that the inner
portion (1.2) is a forging.
2. A composite two piece runner hub (1) as claimed in claim 1, wherein the outer portion (1.1) is
selectively cast with stainless steel.
3. A composite two piece runner hub (1) as claimed in claim 1, wherein the central bore (1.3) is
created in the inner Dortion (1.2) bv treDannina.

A composite two piece runner hub (1) for Kaplan type hydro turbine comprising an outer
potion (1.1) and on inner portion (1.2) having a central bore (1.3), characterized in that the inner portion (1.2) is a forging.