FIELD OF INVENTION:
The present invention generally relates to the fastening arrangement of the wearing liner plates on the top-cover and pivot ring of guide apparatus assembly of hydro turbines. More particularly, the invention relates to a method for fastening the wearing liner plates on top-cover and pivot ring by a counter-sunk screw that can facilitate easy manufacturing and prevention of rework during the assembly. The invention further relates to a counter-sunk screw for affixing wearing liner plates on top covers and pivot rings of hydro turbines.
BACKGROUND OF INVENTION
Wearing liner plates are generally affixed to the bottom surface of top-covers (above guide vanes) and top surface of pivot ring (below guide vanes). These wearing liners are made of stainless steel plates for minimization of erosion due to silt content in water and move at a very high velocity past the guide vanes. With time, the liner plates erode leading to increase in clearances between the guide vane and the top-cover and between the guide vane and the pivot ring, thus causing decrease in turbine efficiency including increase in leakage past guide vanes. Following methods are generally adopted for affixing the liner plates:
1. By spot welding.
2. By hexagonal bolts and plugging with circular plates
3. By counter-sunk screws.

In all the above cases, one or other type of manufacturing problems, are encountered which may be summarized as under:
1. Spot Welding: In this case, a number of holes, located in one or more pitch circles, are drilled in the liner segments and after locating it properly on the top-cover/pivot surface, the holes are spot-welded so that liners get affixed firmly with the main component. Subsequently, the liners are face-machined for smooth finish. In such cases, it has been observed that after machining, a good number of blow holes appear on the surface, which require rectification by grinding and filling by weld metal resulting in wastage of time, effort and quality. Again, in case of replacement of the liners, an extensive grinding is required to detach the liner from the top-cover/pivot ring. But this design is quite popular among many turbine manufacturers all over the world.
2. Festeninq by Hexagonal Screws and plate plug: In this case, higher thickness of the liner plates are taken to accommodate the bolt head. Threaded holes are made in the main component and the liners are fastened with the help of hexagonal bolts followed by plugging the hole by welding of circular plates. Subsequently, the liners are face-machined for smooth finish. Here also, many times, cracks in the weld seams are detected after face machining which are difficult to rectify. Though, the process of replacement of the liners is much easier in this design, but it is discontinued due to manufacturing problems.

3. Fastening bv Counter-sunk Screws: In this design, the liners are fastened by counter-sunk screws. The screws are then locked by drilling a small size hole at the edge of the screw head followed by weld filling. After this process, the liners are face-machined for smooth finish. During face machining, as the cutting tool travels and comes over the screw heads, it hits the corner of the screw head causing the same to bend upward. Sometimes the screw opens up if the tool travels in the opening direction of the screw. Even the tool breaks up if it hits again on the bent corner of the screw heads. This results in lot of rework and wastage of valuable production time. Further, if the angle of screw heads does not match with that of holes in the liner plates, moon-shaped narrow gaps occur after machining, which require filling by weld metal. This process though very common, is not good from quality point of view.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a method for fastening wearing liner plates on top-cover and pivot ring in a hydro turbine, which eliminates the disadvantages of the prior art.
Another object of the invention is to propose a method for fastening wearing liner plates on top-cover and pivot ring in a hydro turbine by means of at least one counter-sunk screw, which eliminates the problem of the screw-head getting damaged during the face-machining by the cutting tool.

A still another object of the invention is to propose a method for fastening wearing liner plates on top-cover and pivot ring in a hydro turbine by means of at least one counter-sunk screw, which prevents the possibility of getting the cutting tool damaged during the face machining .
A further object of the invention is to propose a method for fastening wearing liner plates on top-cover and pivot ring in a hydro turbine by means of at least one counter-sunk screw, which reduces the wastages and production cycle by eliminating the re-work due to damage of the cutting tool and/or the screw head.
A still another object of the invention is to propose an improved counter-sunk screw for affixing wearing liner plates on top-cover and pivot ring in hydro turbines which eliminates the disadvantages of the prior art.
SUMMARY OF INVENTION
In the present invention, the basic concept constitutes fastening of the liners by counter-sunk screws. Here, the configuration of the counter-sunk screws have been modified with the edge of the screw head made flat with 15° taper and the holes in the liner plates are maintained to accommodate a normal counter sunk , screw having an included angle of 90° to match the screw heads. After the screw is tightened up to a designed torque, the V-slot of 60° existing around the screw heads, is weld deposited. Later, the liner face is machined as usual.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- A typical Section through a Turbine.
Fig. 2- Configuration of affixing holes in a liner segment according to prior art
Fig. 3- Spot-welding of liner plates on top-cover/pivot ring according to prior art.
Fig. 4- Fastening of liners by hexagonal bolts and plate plug according to prior art
Fig. 5- Fastening of liners by counter-sunk Screws according to prior art.
Fig.6- a counter-sunk screw according to the invention
Fig.7-Fastening of liners by the counter-sunk Screw according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure-1 shows a typical sectional arrangement of a Francis type hydro turbine showing the location of wearing liners (3) on top-cover (2) and pivot ring (5).
Figure 2 shows the configuration of affixing a plurality of holes (6) in a liner segment (3) on the top-cover/pivot ring (2,5) for guide vanes (1).
Fiogure-3, shows how a liner (3) is attached to the top-cover/pivot ring (2,5) by spot-welding, and further shows the finally machined surface (7).

Figure-4 shows how a thicker liner (3) is fastened to the top-cover/pivot ring (2,5) by a hexagonal head bolt (8) followed by welding of a plate plug (9).
Figure -5 shows how a liner (3) is fastened to the top cover/pivot ring (2,5) by a counter-sunk screw (10). A locking (11) is done by welding.
Figure-6 shows a counter-sunk screw (11) with tapered head according to the invention as opposed to a counter-sunk screw (10) of prior art.
Figure-7 shows how a liner (3) is fastened to the top-cover/pivot ring (2,5) by a counter sunk screw followed by welding (12) around the screw head according to the invention.
ADVANTAGES OF THE INVENTION:
No defect in an around the vicinity of screw
No damages to cutting tool
Smooth finish after machining
Chances of localized erosion due to unevenness/irreparable defects of surface
minimized
Easy replacement of liners possible at later stage.

WE CLAIM:
1. A method for fastening wearing liner plates (3) on top-cover and pivot ring in
hydro turbine by adapting improved counter-sunk screws, the method is
comprising:
- providing counter-sunk screw (11) having edge of the screw-head flattened with 15° taper;
- providing liner plates (3) each comprising a plurality of holes, each of the holes is configured to accommodate a typical counter-sunk screw having (11) an included angle of 90° to match said screw head;
- fastening the liner plates (3) at a predetermined torque on the top cover (2) and the pivot ring (5) by the counter-sunk screws (11);
- weld-depositing the v-slot of 60° still available around the screw-head; and -face-machining the liners for smooth finish.
characterized in that the counter sunk screws having a hexagonal head with a notch in between which breaks after tightening wherein the weld is deposited in the v-slot around the counter sunk screws.
2. An improved counter-sunk screw (11) for affixing wearing liner plates (3) on
top-cover (2) and pivot ring (5) in hydro turbines, the improvement is
characterized by comprising a screw head having edges flattened with 15° taper.

ABSTRACT

A METHOD FOR FASTENING WEAR LINER PLATES ON TOP COVER AND PIVOT RING OF HYDROTURBINES BY ADAPTING IMPROVED COUNTER SUNK SCREWS
The invention relates to a method for fastening wearing liner plates on top-cover
and pivot ring in a hydro turbine by adapting improved counter-sunk screws,The
method is providing counter-sunk screws having edge of the screw-head
flattened with 15° taper, providing liner plates each comprising a plurality of
holes, each of the holes is configured to accommodate a typical counter-sunk
screw having an included angle of 90° to match said screw head, fastening the
liner plates at a predetermined torque on the cover and the pivot ring by the
counter-sunk screws, weld-depositing the v-slot of 60° still available around the
screw-head, and face-machining the liners for smooth finish.