Patent Publication Number: US-2019195083-A1

Title: Backup device for link mechanism of steam turbine, steam turbine, and method for modifying steam turbine

Description:
BACKGROUND 
     Field 
     The present disclosure relates to a backup device for a link mechanism for a regulating valve or the like of a steam turbine, a steam turbine, and a method for modifying a steam turbine. 
     Description of the Related Art 
     An operation system of a regulating valve that regulates a flow rate of steam to be supplied to a steam turbine typically includes a link mechanism (for example, JP2013-72349A ( FIG. 11  and  FIG. 12 ) and JP8-200009A ( FIG. 2 )). 
     The operation system of the regulating valve used for drive control of the steam turbine includes an actuator with a motor, a pilot valve and a servomotor for obtaining oil pressure for driving the regulating valve, a regulating valve lever connected to a piston provided in the servomotor, and a feedback lever that coordinates with the regulating valve lever, and these components are coupled by links. The regulating valve lever and the feedback lever are coupled by a rod. 
     To ensure reliability of an operation of the steam turbine, it is important to prepare for damage due to vibration or aged deterioration of the link mechanism involved in an operation of the regulating valve or the like. 
     Then, the present disclosure has an object to provide redundancy to a link mechanism provided in a steam turbine. 
     SUMMARY 
     The present disclosure provides a backup device for a link mechanism, the link mechanism being configured to be involved in an operation of a valve provided in a steam turbine, and provided between the valve driven by oil pressure and an actuator to which an electric signal is supplied, wherein the backup device includes a locking portion configured to lock a contact portion of one of a first link and a second link included in the link mechanism by relative displacement of the first link and the second link caused by disjoining of the first link and the second link, on the other of the first link and the second link. 
     The backup device of the present disclosure preferably includes a stand-by member as the locking portion suspended from the first link and located immediately below at least a part of the second link. 
     “Located immediately below” herein refers to being placed below at least a part of the second link with a gap from at least the part. The stand-by member is located away from the second link in normal time when joining of the links is safe, but stands by near at least the part of the second link in preparation for damage to a joint or the like. 
     The backup device of the present disclosure further includes a support portion that is provided on the first link and suspends the stand-by member. 
     It is preferable that the backup device of the present disclosure includes a rod that suspends the stand-by member from the support portion, one end side of the rod is joined to the support portion by a pin, and the other end side of the rod is joined to the stand-by member by a pin. 
     In the backup device of the present disclosure, it is preferable that one end and the other end of the rod each have a rod end bearing into which the pin is inserted. 
     In the backup device of the present disclosure, it is preferable that a pin that joins the first link and the second link is used as the support portion, and the stand-by member is suspended from the pin. 
     In the backup device of the present disclosure, it is preferable that the stand-by member is located immediately below a contact stopper provided on an outer peripheral portion of the second link. 
     In the backup device of the present disclosure, it is preferable that at least one of the contact stopper and the stand-by member is formed into a spherical shape protruding toward the other. 
     In the backup device of the present disclosure, it is preferable that the stand-by member is located immediately below at least an end of the second link. 
     In the backup device of the present disclosure, it is preferable that the stand-by member is suspended from the support portion provided on an outer peripheral portion of the first link. 
     In the backup device of the present disclosure, it is preferable that the link mechanism includes a third link connected to the second link, and the backup device includes a first stand-by member as the stand-by member located immediately below the contact stopper provided on the outer peripheral portion of the second link, and a second stand-by member suspended from the support portion provided on the outer peripheral portion of the second link and located immediately below at least an end of the third link. 
     In the backup device of the present disclosure, it is preferable that an intermediate link included in the link mechanism couples the first link and the second link, and the stand-by member is suspended from the first link and located immediately below at least the end of the second link. 
     In the backup device of the present disclosure, it is preferable that the valve is a regulating valve configured to regulate a flow rate of steam to be supplied to the steam turbine, and the link mechanism includes a regulating valve lever rockable according to an opening of the regulating valve, and a feedback lever configured to coordinate with the regulating valve lever. 
     A steam turbine of the present disclosure includes the backup device described above. 
     The present disclosure also provides a method for adding a backup device including a stand-by member to a link mechanism. The link mechanism is configured to be involved in an operation of a valve provided in a steam turbine, and provided between the valve driven by oil pressure and an actuator to which an electric signal is supplied, wherein the method includes a locking structure adding step of adding a locking structure to a first link and a second link included in the link mechanism, the locking structure being configured such that a locking portion configured to lock a contact portion of one of the first link and the second link by relative displacement of the first link and the second link caused by disjoining of the first link and the second link is provided on the other of the first link and the second link. 
     In the modifying method of the present disclosure, the locking structure adding step includes an immediately-below placing step of suspending the stand-by member from the first link to place the stand-by member immediately below at least a part of the second link. 
     In the modifying method of the present disclosure, it is preferable that the locking structure adding step includes a step of providing a clamp on the first link, a step of providing a contact stopper on an outer peripheral portion of the second link, and a step of mounting one end side of a rod to the clamp, and in the immediately-below placing step, the stand-by member is mounted to the other end side of the rod, and the stand-by member is placed around the outer peripheral portion of the second link immediately below the contact stopper. 
     In the modifying method of the present disclosure, it is preferable that the locking structure adding step includes a step of providing a support portion on an outer peripheral portion of the first link, and a step of mounting one end side of a rod to the support portion, and in the immediately-below placing step, the stand-by member is mounted to the other end side of the rod below at least a part of the second link to be placed immediately below at least the part of the second link. 
     If a joint or the like that joins the links of the link mechanism is damaged to disjoin the links and thus disjoin the first link and the second link, the contact portion of one link is locked by the locking portion of the other link. 
     Then, the first link and the second link are joined based on engagement between the contact portion and the locking portion. As such, the backup device replaces the joining of the first link and the second link, thereby providing redundancy to the link mechanism provided in the steam turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a steam turbine and a link mechanism involved in an operation of a regulating valve of the steam turbine according to an embodiment of the present disclosure; 
         FIG. 2  is a perspective view of a regulating valve driving device including the link mechanism in  FIG. 1 ; 
         FIG. 3  is a perspective view of an upper backup device corresponding to a section III in  FIG. 1  and  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of a backup rod suspended from a clamp member provided on a regulating valve lever; 
         FIG. 5A  is a side view of a state of the regulating valve lever and the upper backup device corresponding to a maximum opening of the regulating valve, and  FIG. 5B  is a side view of a state of the regulating valve lever and the upper backup device corresponding to a minimum opening (closed); 
         FIG. 6  is a perspective view of a lower backup device corresponding to a section VI in  FIG. 1  and  FIG. 2 ; 
         FIG. 7  is a side view of the lower backup device viewed in the direction of arrow VII in  FIG. 6 ; 
         FIG. 8  is a schematic view mainly of a state of the upper backup device when a main rod is disengaged from the regulating valve lever; 
         FIG. 9  is a schematic view mainly of a state of the lower backup device when a feedback lever is disengaged from the main rod; 
         FIG. 10  is a schematic view of a backup device according to a first variant of the present disclosure; 
         FIG. 11  is a schematic view of a backup device according to a second variant of the present disclosure; 
         FIG. 12  is a perspective view of a backup device according to a third variant of the present disclosure; and 
         FIG. 13A  and  FIG. 13B  each show a backup device according to a fourth variant of the present disclosure,  FIG. 13B  being a perspective view of a section XIIIb of  FIG. 13A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, with reference to the accompanying drawings, an embodiment of the present disclosure will be described. 
     A steam turbine  1  ( FIG. 1 ) uses steam supplied from a boiler (not shown) or the like to rotate blades and thus output power. An electronic control unit  2  (governor) of the steam turbine  1  basically sets an appropriate opening of a regulating valve  3  to control a rotational speed of the steam turbine  1 . The steam turbine  1  includes the regulating valve  3  that regulates a flow rate of the steam to be supplied to the steam turbine  1 , and also a stop valve (not shown) that blocks supply of the steam to the steam turbine  1 . 
     Regulating Valve Driving Device 
     First, with reference to  FIG. 1  and  FIG. 2 , a brief description will be made to a configuration and an effect of a regulating valve driving device  10  used for drive control of the steam turbine  1 . 
     The regulating valve driving device  10  includes an actuator  11  with a motor, a pilot valve  12  and a servomotor  13  for obtaining oil pressure for driving the regulating valve  3  ( FIG. 1 ), a regulating valve lever  14  connected to a piston  131  provided in the servomotor  13 , and a feedback lever  15  that coordinates with the regulating valve lever  14 . 
     In the example in  FIG. 2 , the regulating valve driving device  10  is provided on a cover  19  of a bearing of the steam turbine  1 . 
     The components of the regulating valve driving device  10  are coupled by links (nodes). A plurality of links such as levers or rods of the regulating valve driving device  10  constitute a link mechanism  4  provided between the regulating valve  3  and the actuator  11 . 
     The actuator  11  converts a rotational displacement of the motor into a linear displacement and outputs the linear displacement to an output shaft  111 . 
     The pilot valve  12  includes a pilot cylinder  120  and a pilot piston valve  121  that vertically moves according to the output from the actuator  11 , and supplies control oil below or above a servomotor piston  131  in a cylinder  130  of the servomotor  13  according to an operation of the pilot piston valve  121 . The pilot valve  12  and the servomotor  13  are connected to a supply and discharge system (not shown) of the control oil. 
     The regulating valve lever  14  is rockable according to a lift amount with respect to a throttle hole  3 A of the regulating valve  3 , and pivotally supported on a member (not shown) at a position of a fulcrum  14 A in  FIG. 1 . An extent of rocking of the regulating valve lever  14  according to an electric signal input to the actuator  11  is shown by dashed-dotted lines in  FIG. 1 . The same applies to the feedback lever  15  and a lever  112 . 
     The feedback lever  15  is coupled to the regulating valve lever  14  via a rod  16 , and connected to the pilot piston valve  121  on a side of a roller  17  as a fulcrum. 
     It is assumed that the control unit  2  ( FIG. 1 ) inputs an electric signal to increase the opening of the regulating valve  3  (lift amount) to the actuator  11 . In that case, a position of the output shaft  111  of the actuator  11  is lowered, and a position of the roller  17  connected to the output shaft  111  by the lever  112  is also lowered. Thus, the pilot piston valve  121  is pushed by a holding portion  171  holding the roller  17  and lowered. 
     Then, the pilot piston valve  121  is opened, and the control oil supplied to the pilot cylinder  120  flows into the cylinder  130  of the servomotor  13  below the servomotor piston  131 . At this time, air is included above the servomotor piston  131  in the servomotor cylinder  130 , and the oil pressure in the cylinder  130  raises the servomotor piston  131 . 
     As the servomotor piston  131  is raised, the regulating valve lever  14  rocks upward around the fulcrum  14 A (clockwise in  FIG. 1 ). Simultaneously, as the rod  16  mounted to the regulating valve lever  14  is raised, the feedback lever  15  rocks upward around the output shaft  111  (counterclockwise in  FIG. 1 ). The feedback lever  15  rocks until the pilot valve  12  returns to a neutral position, and the pilot piston valve  121  connected to the feedback lever  15  returns upward. 
     The configuration and effect of the regulating valve driving device  10  as described above allows the regulating valve  3  to operate according to the lift amount indicated by the electric signal generated by the control unit  2  such as a speed governor and provided to the actuator  11 . 
     Outline of Backup Device 
     This embodiment is mainly characterized by including a backup device  20  ( FIG. 3 ) and a backup device  30  ( FIG. 6 ) that can replace joining of links to avoid an influence on the operation of the regulating valve  3  and further on the drive control of the steam turbine  1  even if a joint of the link mechanism  4  is damaged. 
     As described later, these backup devices  20 ,  30  may be added to the link mechanism  4  of the existing steam turbine  1 . 
     As shown in  FIG. 3 , the backup device  20  located on an upper end side of the rod  16  corresponds to a section III shown by dashed lines in  FIG. 1  and  FIG. 2 . The section III corresponds to a joining portion of the regulating valve lever  14  and the rod  16  and therearound. 
     As shown in  FIG. 6 , the backup device  30  located on a lower end side of the rod  16  corresponds to a section VI shown by dashed lines in  FIG. 1  and  FIG. 2 . The section VI corresponds to a joining portion of the rod  16  and the feedback lever  15  and therearound. 
     The backup device  20  and the backup device  30  are added to the link mechanism  4 , thereby ensuring a function of the link mechanism  4  even if the joint used for joining the links is damaged by wear or the like and the links are disjoined in the sections III and VI. 
     When the two links (the regulating valve lever  14  as a first link and the rod  16  as a second link) are disjoined, the upper backup device  20  ( FIG. 3 ) replaces the joining of the links  14 ,  16 . While the links are still safely joined, the backup device  20  does not replace the joining of the regulating valve lever  14  and the rod  16 . 
     The rod  16  in this embodiment includes a tubular turn buckle  160  ( FIG. 5 ) having a right thread on one end side and a left thread on the other end side and having an adjustable length, a rod end bearing  161  ( FIG. 5 ) provided at an upper end of the turn buckle  160  by a securing nut  16 A ( FIG. 5 ), a rod end bearing  162  ( FIG. 6 ) provided at a lower end of the turn buckle  160  by a securing nut  16 B ( FIG. 6 ), and a washer (not shown). 
     As shown in  FIG. 3 , the regulating valve lever  14  and the rod  16  are joined by a pin P 1  ( FIG. 3 ) inserted into the rod end bearing  161  ( FIG. 5 ). The rod end bearing  161  is placed in an internal space (not shown) similar to a hole  14 B in a regulating valve lever  14 ′ shown in  FIG. 13B , in a thick portion  141  of the regulating valve lever  14 . The pin P 1  is placed in a width direction perpendicular to a length direction of the regulating valve lever  14  so as to extend through the rod end bearing  161  ( FIG. 5 ) placed in the internal space and the regulating valve lever  14 . 
     It is preferable that even if the joint (the pin P 1  and the rod end bearing  161 ) that joins the regulating valve lever  14  and the rod  16  is damaged and the rod  16  is disengaged from the regulating valve lever  14 , the upper end (the rod end bearing  161 ) of the rod  16  stays in the internal space of the regulating valve lever  14 . 
     When the two links (the rod  16  as a first link and the feedback lever  15  as a second link) are disjoined, the lower backup device  30  ( FIG. 6 ) also replaces joining of the links  16 ,  15 . While the links are still safely joined, the backup device  30  does not replace the joining of the rod  16  and the feedback lever  15 . 
     As shown in  FIG. 6 , the feedback lever  15  is joined to the lower rod end bearing  162  of the rod  16  by a pin P 2 . The pin P 2  is placed in a width direction perpendicular to a length direction of the feedback lever  15 . 
     As shown in  FIG. 2 , in this embodiment, an extending direction of the regulating valve lever  14  crosses an extending direction of the feedback lever  15 . An axis line of the pin P 2  provided in the feedback lever  15  crosses an axis line of the pin P 1  provided in the regulating valve lever  14 . 
     The rod end bearings  161 ,  162  are spherical bearings similar to rod end bearings  231 ,  232  shown in  FIG. 4 . 
     The joint using rod end bearings such as the rod end bearings  161 ,  162  can provide, between members to be joined, a degree of freedom in a direction around the axis of the pin and a degree of freedom in a rockable direction of a ball relative to a body of the rod end bearing. 
     It is preferable that the rod end bearings are used in the joining portions of the links so that the links of the link mechanism  4  are relatively smoothly displaced. 
     As described later, the backup devices  20 ,  30  each also includes a rod. To distinguish between the rods of the backup devices  20 ,  30  and the rod  16  that couples the regulating valve lever  14  and the feedback lever  15 , the rod  16  will be hereafter referred to as “main rod  16 ”. 
     As shown in  FIG. 1  and  FIG. 2 , an area from the regulating valve lever  14  through the main rod  16  to the feedback lever  15  corresponds to three links, and the two backup devices  20 ,  30  are provided in this area. 
     Now, configurations of the upper backup device  20  related to the regulating valve lever  14  as the first link and the main rod  16  as the second link, and the lower backup device  30  related to the main rod  16  as the second link and the feedback lever  15  as a third link will be described. 
     Upper Backup Device 
     The upper backup device  20  ( FIG. 3 ) is assembled to the regulating valve lever  14  and the main rod  16 . The upper backup device  20  includes a clamp  21  (first support portion) as a support portion provided on the regulating valve lever  14 , a first stand-by plate  22  (locking portion, first stand-by member) suspended from the clamp  21  and located immediately below a contact stopper  163  that forms a part of the main rod  16 , and a pair of rods  23 ,  23  that suspend the first stand-by plate  22  from the clamp  21 . 
     While the regulating valve lever  14  and the main rod  16  are joined, the upper backup device  20  is not involved in the operations of the link mechanism  4  and the regulating valve  3  because the first stand-by plate  22  is located away from a part (contact stopper  163 ) of the main rod  16 . 
     However, if the regulating valve lever  14  and the main rod  16  are disjoined due to damage to the joint or the like, the main rod  16  is disengaged from the regulating valve lever  14 , and the contact stopper  163  is supported by the first stand-by plate  22 . Thus, the regulating valve lever  14  and the main rod  16  are engaged via the upper backup device  20  including the first stand-by plate  22  and the clamp  21 . At this time, the upper backup device  20  replaces the joining of the regulating valve lever  14  and the main rod  16 . 
       FIG. 3  and  FIG. 5  show an example of the configuration of the upper backup device  20  having the effect described above. Now, specific configurations of the components of the upper backup device  20  will be described. 
     Clamp 
     The clamp  21  includes a pair of clamp members  211 ,  211  that hold the regulating valve lever  14  from opposite sides in the width direction, and restraining members  212  such as bolts and nuts that restrain the pair of clamp members  211 ,  211  from opposite sides. 
     The pair of clamp members  211 ,  211  both have a substantially L-shaped section except for a portion corresponding to the thick portion  141  of the regulating valve lever  14 , and are placed on lower flanges  142  of the regulating valve lever  14 . The clamp members  211 ,  211  are fastened to each other at two positions below the regulating valve lever  14  by the restraining members  212  having bolts extending through the clamp members  211 ,  211 . 
     The shape of the clamp members  211 ,  211  described above allows the pin P 1  to be withdrawn from the thick portion  141  with the clamp members  211 ,  211  being secured to the regulating valve lever  14 . If the pin P 1  is withdrawn, the upper backup device  20  acts like when the pin P 1  or the rod end bearing  161  is damaged. This allows the pin P 1  to be exchanged during the operation of the steam turbine  1 . 
     Also, each clamp member  211  is butted against a side wall of the thick portion  141  and fastened by a bolt  213 , and fastened to the lower flange  142  of the regulating valve lever  14  from below by a bolt  214 . Thus, the clamp  21  is secured to the regulating valve lever  14  so as not to move in the width direction of the regulating valve lever  14  and also in the length direction and a height direction (vertical direction) of the regulating valve lever  14 . 
     Rod 
     As shown in  FIG. 3 , the rods  23  are assembled to the pair of clamp members  211 ,  211 , respectively, and extend downward substantially along the main rod  16  on opposite sides of the main rod  16 . Each rod  23  is located radially away from the main rod  16 , and thus does not interfere with the main rod  16  even if the regulating valve lever  14  rocks. 
     As shown in  FIG. 4 , each rod  23  includes a turn buckle  230  having a right thread  23 R on one end side and a left thread  23 L on the other end side and having an adjustable length, a rod end bearing  231  provided at one end (upper end) of the turn buckle  230  by a securing nut  23 A, a rod end bearing  232  provided at the other end (lower end) of the turn buckle  230  by a securing nut  23 B, and a washer (not shown). 
     The rod  23  including the turn buckle is easily adjusted in length so that the link smoothly moves. 
     A rod that does not include the turn buckle  230  or the rod end bearings  231 ,  232  may be used. 
     The rod end bearing  231  is a joint that connects the regulating valve lever  14  and the turn buckle  230 , and a spherical bearing having a body  231 A and a ball  231 B as shown in  FIG. 4 . The ball  231 B has a cylindrical hole  231 D through which a pin  231 C ( FIG. 3  and  FIG. 4 ) is inserted. A lubricating liner may be provided between a spherical slide surface on an outer periphery of the ball  231 B and a slide surface on an inside of the body  231 A following the spherical slide surface. 
     The rod end bearing  232  and rod end bearings  331 ,  332  of the backup device  30  described later are configured similarly to the rod end bearing  231  described above. 
     As shown in  FIG. 3 , the rod end bearing  231  is placed in a middle between the two restraining members  212 ,  212  that fasten the pair of clamp members  211 ,  211 , on the outside of the clamp member  211 . The rod  23  is assembled to the clamp  21  by the rod end bearing  231  and the pin  231 C extending through the clamp member  211 . 
     The pin  231 C is placed in parallel with the pin P 1  near (immediately below) the joint (the rod end bearing  161  and the pin P 1 ) that joins the regulating valve lever  14  and the main rod  16 . 
     Lengths of the rod  23  and the rod  33  of the backup device  30  described later are shorter than a length of the main rod  16 . Thus, a natural frequency of each of the rod  23  and the rod  33  is different from a natural frequency of the main rod  16 . 
     Thus, even if the rod end bearing or the pin located at the end of the main rod  16  is damaged due to resonance in the main rod  16  during the operation of the steam turbine  1 , resonance occurring in the rods  23 ,  33  can be avoided to prevent damage to the rod end bearing or the pin related to the rods  23 ,  33 . 
     Contact Stopper (Contact Portion) 
     The contact stopper  163  ( FIG. 3 ) is provided on an outer peripheral portion of the turn buckle  160  of the main rod  16  and thus integrated with the main rod  16 . 
     The contact stopper  163  is split into two pieces so as to hold the turn buckle  160  from opposite sides in a diametrical direction of the turn buckle  160 . Notches (not shown) in the outer peripheral portion of the turn buckle  160  are held between a semicircular first piece  163 A and a substantially semicircular second piece  163 B of the contact stopper  163 , and the first piece  163 A and the second piece  163 B are fastened by a bolt  163 C. Thus, the first piece  163 A and the second piece  163 B are secured to the outer peripheral portion of the main rod  16 . 
     The first piece  163 A and the second piece  163 B are placed in the pair of notches (not shown) in the outer peripheral portion of the turn buckle  160 , thereby preventing the contact stopper  163  from shifting downward. The same applies to split pieces of a support stopper  311  ( FIG. 6 ) described later. 
     If the main rod  16  includes a portion that faces an upper surface of the first stand-by plate  22  and can come into contact with the first stand-by plate  22  when the main rod  16  is disengaged from the regulating valve lever  14 , for example, a large diameter portion that faces the upper surface of the first stand-by plate  22 , the portion can replace the contact stopper  163 , thereby eliminating the need for the contact stopper  163 . The portion that can come into contact with the first stand-by plate  22  does not always need to be placed entirely around the main rod  16 . 
     The support stopper  311  described later can be also replaced by an appropriate portion provided in the main rod  16 , if any. 
     As shown in  FIG. 5 , the contact stopper  163  preferably has a spherical contact surface  163 D protruding toward the first stand-by plate  22 . 
     The contact surface  163 D comes into contact with the first stand-by plate  22  if the main rod  16  is disengaged from the regulating valve lever  14  by its own weight when the regulating valve lever  14  and the main rod  16  are disjoined, for example, due to damage to the rod end bearing  161  or the pin P 1 . 
     When the contact stopper  163  comes into contact with the first stand-by plate  22 , the spherical contact surface  163 D contributes to an increase in a degree of freedom between the main rod  16  to which the contact stopper  163  is secured and the regulating valve lever  14  provided with the clamp  21  from which the first stand-by plate  22  is suspended. 
     Stand-by Plate 
     The first stand-by plate  22  ( FIG. 3  and  FIG. 5 ) is placed immediately below the contact stopper  163 . The first stand-by plate  22  is placed below a part (contact stopper  163 ) of the main rod  16  with a gap from the part. 
     The first stand-by plate  22  receives the contact stopper  163  that forms the part of the main rod  16  disengaged from the regulating valve lever  14  due to damage to the joint or the like. At this time, the contact stopper  163  is locked in contact with the first stand-by plate  22  to prevent the main rod  16  from moving further downward. 
     As shown in  FIG. 3 , the first stand-by plate  22  is suspended from the regulating valve lever  14  via the clamp  21  by the pair of rods  23  with stability in position. The first stand-by plate  22  is assembled to the lower rod end bearings  232  of the rods  23  by pins  232 E ( FIG. 3 ). The first stand-by plate  22  and the pair of rods  23  are entirely suspended from the regulating valve lever  14  via the clamp  21 . 
     The first stand-by plate  22  is placed around the outer peripheral portion of the turn buckle  160  of the main rod  16 . Unlike the contact stopper  163  described above, there is a gap between the outer peripheral portion of the turn buckle  160  and the first stand-by plate  22 , and thus the first stand-by plate  22  does not restrain the main rod  16 . 
     Like the contact stopper  163 , the first stand-by plate  22  may include two split pieces on the opposite sides of the turn buckle  160  in the diametrical direction. 
     The first stand-by plate  22  of this embodiment includes substantially rectangular flat half-split bodies  22 A,  22 B. 
     The half-split bodies  22 A,  22 B are fastened by a bolt  22 C with the turn buckle  160  being held therebetween. A hole  220  ( FIG. 8 ) through which the turn buckle  160  is placed is formed between the half-split bodies  22 A,  22 B. A diameter of the hole  220  is larger than an outer diameter of the turn buckle  160 . 
     As shown in  FIG. 5A  corresponding to a maximum opening of the regulating valve  3  and  FIG. 5B  corresponding to a minimum opening thereof, the positions of the regulating valve lever  14  and the main rod  16  are changed according to the opening of the regulating valve  3 , and a dimension of the gap between the contact stopper  163  and the first stand-by plate  22  is also changed. 
     Thus, while the regulating valve lever  14  and the main rod  16  are safely joined (normal time), an appropriate clearance C 1  that can avoid a contact between the contact stopper  163  and the first stand-by plate  22  is provided between the contact surface  163 D of the contact stopper  163  and a flat upper surface of the first stand-by plate  22  located immediately below the contact surface  163 D. The clearance C 1  may be determined in view of vibration displacement transmitted from the bearing or the like of the steam turbine  1  to the upper backup device  20  via the regulating valve driving device  10  provided near the bearing. 
     A large clearance C 1  increases an amount of shift when the main rod  16  is disengaged from the regulating valve lever  14  as compared to before the links of the regulating valve lever  14 , the main rod  16 , and the feedback lever  15 , or the like are disengaged. Thus, the clearance C 1  is preferably determined to the extent that the operations of the links are not hindered. 
     The clearance C 1  can prevent an influence on the operations of the links of the link mechanism  4  in normal time. 
     Relative displacement and contact between the contact stopper  163  and the first stand-by plate  22  due to disturbance are not excluded. 
     Like a positional relationship between the contact stopper  163  and the first stand-by plate  22 , an appropriate clearance that can avoid contact between the first stand-by plate  22  and the turn buckle  160  may be also set between a peripheral edge of the hole  220  ( FIG. 8 ) in the center of the first stand-by plate  22  and the outer peripheral portion of the turn buckle  160 . Then, the turn buckle  160  can smoothly move through the hole  220  in the first stand-by plate  22  as the regulating valve lever  14  rocks. 
     Lower Backup Device 
     Next, as shown in  FIG. 6 , the lower backup device  30  assembled to the main rod  16  and the feedback lever  15  will be described. 
     The lower backup device  30  includes a support stopper  311  provided on the outer peripheral portion of the main rod  16 , a joining plate  312  supported on the support stopper  311 , a second stand-by plate  32  suspended from the joining plate  312  and located immediately below a front end  151  of the feedback lever  15 , and a pair of rods  33 ,  33  that suspend the second stand-by plate  32  from the joining plate  312 . 
     The lower backup device  30  includes a support portion (second support portion) including the support stopper  311  and the joining plate  312 , and the second stand-by plate  32  as a second stand-by member. 
     While the main rod  16  and the feedback lever  15  are joined, the lower backup device  30  is not involved in the operations of the link mechanism  4  and the regulating valve  3  because the second stand-by plate  32  is located away from the front end  151  of the feedback lever  15 . 
     However, if the main rod  16  and the feedback lever  15  are disjoined due to damage to the joint or the like, the feedback lever  15  is disengaged from the main rod  16  by its own weight, and the front end  151  is supported by the second stand-by plate  32 . Thus, the main rod  16  and the feedback lever  15  are engaged via the lower backup device  30  including the second stand-by plate  32  and the support portion  31 . At this time, the lower backup device  30  replaces the joining of the main rod  16  and the feedback lever  15 . 
       FIG. 6  and  FIG. 7  show an example of a configuration of the lower backup device  30  having the effect described above. Now, specific configurations of the components related to the lower backup device  30  will be described. 
     Support Stopper 
     The support stopper  311  is secured to a predetermined position in the length direction of the main rod  16 . 
     Like the contact stopper  163  described above, the support stopper  311  includes a first piece  311 A and a second piece  311 B. The first piece  311 A and the second piece  311 B are fastened by a bolt (not shown) with notches (not shown) in the outer peripheral portion of the turn buckle  160  of the main rod  16  being held therebetween. 
     In this embodiment, an upper end of the rod  33  is assembled to the joining plate  312  rather than the support stopper  311  for the following reason. 
     Assembling a rod end bearing  331  to the support stopper  311  by a pin  331 C without the joining plate  312  is related to the fact that a phase around an axis of the turn buckle  160  of the main rod  16  is not constant due to an assembly tolerance or the like of the main rod  16 . If the phase around the axis of the turn buckle  160  were within an extent that assembling of the rod  33  is not hindered, the rod end bearing  331  could be assembled to the support stopper  311 . 
     Actually, the phase around the axis of the turn buckle  160  cannot be predicted due to the assembly tolerance or the like, and thus phases of the notches formed in the outer peripheral portion of the turn buckle  160  (positions of the pair of notches in a direction around the axis of the rod) cannot be predicted either. 
     On the other hand, phases of the pair of rods  33 ,  33  are determined by a configuration in which the rods  33 ,  33  are placed on the opposite sides of the feedback lever  15  in the width direction to suspend the stand-by plate  32 . The pin  331 C inserted into the upper end of each rod  33  is placed in parallel with the pin P 2 . 
     If the position of the pin  331 C matches a boundary between the first piece  311 A and the second piece  311 B placed to hold the turn buckle  160  therebetween according to the position of the notches due to the phase of the turn buckle  160 , the rod end bearing  331  cannot be assembled to the support stopper  311 . 
     To eliminate the restriction on assembling of the rod  33  as described above, the support portion  31  includes the two members of the support stopper  311  and the joining plate  312  as in this embodiment. 
     As shown in  FIG. 7 , the support stopper  311  preferably has a spherical support surface  311 D protruding toward the joining plate  312 . 
     The support surface  311 D contributes to an increase in a degree of freedom between the main rod  16  provided with the support stopper  311  and the joining plate  312  and the feedback lever  15  supported by the second stand-by plate  32 . 
     Joining Plate 
     The joining plate  312  is placed around the outer peripheral portion of the turn buckle  160  above the support stopper  311  and supported from below by the support stopper  311 . 
     Like the first stand-by plate  22  described above, the joining plate  312  may include two split pieces. 
     Substantially rectangular flat half-split bodies  312 A,  312 B of the joining plate  312  are fastened by a bolt  312 C with the turn buckle  160  being held therebetween. A hole  312 D through which the turn buckle  160  is placed is formed between the half-split bodies  312 A,  312 B. A diameter of the hole  312 D is larger than an outer diameter of the turn buckle  160 . 
     Rod 
     As shown in  FIG. 6 , the rods  33  are assembled to the half-split bodies  312 A,  312 B of the joining plate  312  and extend downward substantially along the main rod  16  on the opposite sides of the main rod  16 . 
     Each rod  33  is configured similarly to the rod  23  in  FIG. 4 . Specifically, the rod  33  also includes a turn buckle  330 , a rod end bearing  331  provided at one end (upper end) of the turn buckle  330  by a securing nut  33 A, a rod end bearing  332  provided at the other end (lower end) of the turn buckle  330  by a securing nut  33 B, and a washer (not shown). 
     The rod  33  is assembled to the joining plate  312  by the pin  331 C inserted into the rod end bearing  331 . 
     Second Stand-By Plate 
     The rectangular second stand-by plate  32  of the lower backup device  30  is placed immediately below near a joining portion of the main rod  16  and the feedback lever  15  (the front end  151  here) with a gap from the front end  151 . 
     The second stand-by plate  32  receives the front end  151  of the feedback lever  15  disengaged from the main rod  16  due to damage to the joint or the like. 
     The second stand-by plate  32  is suspended from the joining plate  312  by the pair of rods  33 . The second stand-by plate  32  is assembled to the lower rod end bearings  332  of the rods  33  by pins  332 E. The second stand-by plate  32  and the pair of rods  33  are entirely suspended from the main rod  16  via the joining plate  312  and the support stopper  311 . 
     The pin  332 E is placed in parallel with the pin P 2  near the joint (the rod end bearing  162  and the pin P 2 ) that joins the main rod  16  and the feedback lever  15 . 
     The pin  332 E located in the lower end of the rod  33  is located below the pin P 2 . On the other hand, in the upper backup device  20  described above, the pin  231 C located in the upper end of the rod  23  is located below the pin P 1  so as to extend through the clamp members  211 ,  211  placed to allow the pin P 1  to be withdrawn. Comparing the rod  23  of the upper backup device  20  with the rod  33  of the lower backup device  30  in view of such a configuration, the rod  23  is relatively short and the rod  33  is relatively long. 
     A pair of walls  321 ,  322  rise from an upper surface of the second stand-by plate  32 . Between the pair of lever members  15 A,  15 B of the feedback lever  15 , the rod end bearing  162  at the lower end of the main rod  16  is placed between the walls  321 ,  322  rising from the second stand-by plate  32 . 
     It is preferable that even if the joint that joins the rod  16  and the feedback lever  15  is damaged and the feedback lever  15  is disengaged from the main rod  16 , the lower end (rod end bearing  162 ) of the main rod  16  stays in a space surrounded by the walls  321 ,  322  and the lever members  15 A,  15 B. 
     As shown in  FIG. 7 , while the main rod  16  and the feedback lever  15  are safely joined, an appropriate clearance C 2  that can avoid contact between the front end  151  and the second stand-by plate  32  is provided between the front end  151  of the feedback lever  15  and the upper surface of the second stand-by plate  32 . The clearance C 2  may be set similarly to the clearance C 1  between the contact stopper  163  and the first stand-by plate  22  described above. 
     Description on Backup Function of Backup Device 
     Damage to the joint or the like due to vibration, impact, or aged deterioration may disjoin the links. 
     As shown in  FIG. 8 , if damage to the pin P 1  or the rod end bearing  161  ( FIG. 4 ) occurs and the main rod  16  is disengaged from the regulating valve lever  14 , the main rod  16  is disengaged by its own weight. Then, the contact stopper  163  secured to the main rod  16  is displaced by a distance of the clearance C 1  to the first stand-by plate  22  suspended form the regulating valve lever  14  by the clamp  21 , and supported by the first stand-by plate  22  immediately after the main rod  16  is disengaged. 
     Then, based on engagement between the contact stopper  163  and the first stand-by plate  22  supporting the contact stopper  163 , the main rod  16  provide with the contact stopper  163  and the regulating valve lever  14  provided with the clamp  21  that suspends the first stand-by plate  22  are joined via the upper backup device  20 . Specifically, joining of the regulating valve lever  14  and the main rod  16  by the pin P 1  and the rod end bearing  161  is switched to joining of the regulating valve lever  14  and the main rod  16  by the backup device  20 . 
     Between the clamp  21  of the upper backup device  20  and the first stand-by plate  22  that has come into contact with the contact stopper  163 , a degree of freedom is provided based on joining by the pin  231 C and the rod end bearing  231  ( FIG. 3 ) located near the pin P 1  and joining by the rod end bearing  232  and the pin  232 E on a lower end side of the rod  23 . A degree of freedom corresponding thereto is also provided between the regulating valve lever  14  provided with the clamp  21  and the main rod  16  provided with the contact stopper  163  supported by the first stand-by plate  22 . 
     Further, since only a part of the spherical contact surface  163 D comes into contact with the first stand-by plate  22 , friction between the contact stopper  163  and the first stand-by plate  22  can be reduced to allow changes in position of the contact stopper  163  relative to the upper surface of the first stand-by plate  22  with the contact stopper  163  and the first stand-by plate  22  being in contact. 
     Thus, the spherical contact surface  163 D increases the degree of freedom between the regulating valve lever  14  and the main rod  16 . 
     The same advantage as described above can be obtained if at least one of the contact surface  163 D and the upper surface of the first stand-by plate  22  has a spherical shape. 
     The front end  151  of the feedback lever  15  joined to the main rod  16  by the pin P 2  and the lower backup device  30  suspended from the support portion  31  (the support stopper  311  and the joining plate  312 ) provided on the main rod  16  are also slightly displaced downward by the distance of the clearance C 1  that has existed between the contact stopper  163  and the first stand-by plate  22  before the main rod  16  is disengaged. 
     Similarly to the upper backup device  20 , also between the support portion  31  of the lower backup device  30 , the rod  33 , and the second stand-by plate  32 , a degree of freedom is provided based on joining by the rod end bearings  331 ,  332  and the pins corresponding thereto. The link joining in the lower backup device  30  also provides a degree of freedom between the main rod  16  provided with the support portion  31  and the feedback lever  15  supported by the second stand-by plate  32 . 
     Further, similarly to the spherical contact surface  163 D described above, the spherical support surface  311 D of the support stopper  311  increases the degree of freedom between the main rod  16  and the feedback lever  15 . 
     The same advantage as described above can be obtained if at least one of the support surface  311 D and the lower surface of the joining plate  312  has a spherical shape. 
     From the above, even after the backup device  20  replaces the joining of the regulating valve lever  14  and the main rod  16 , substantially the same degree of freedom as before the replacement is ensured between the links including the regulating valve lever  14 , the main rod  16 , and the feedback lever  15 . 
     Specifically, even if the links including the regulating valve lever  14 , the main rod  16 , and the feedback lever  15  are relatively displaced as the main rod  16  is disengaged by the distance of the clearance C 1 , a sufficient degree of freedom that absorbs the relative displacement between the links and covers a movable range of the link mechanism  4  from the minimum opening to the maximum opening of the regulating valve  3  is ensured by the link joining in the backup devices  20 ,  30 . Thus, even after the upper backup device  20  replaces the pin P 1  and the rod end bearing  161 , the function of the link mechanism  4  can be ensured while the displacement is transmitted from the regulating valve lever  14  via the main rod  16  to the feedback lever  15 . Specifically, the operation of the steam turbine  1  can be continued while the opening of the regulating valve  3  is controlled via the link mechanism  4 . 
     Next, with reference to  FIG. 9 , a case where damage to the pin P 2  or the rod end bearing  162  ( FIG. 6 ) located at the lower end of the main rod  16  occurs will be described. 
     In this case, if the feedback lever  15  is disengaged from the main rod  16  by its own weight, the lower backup device  30  joins the main rod  16  and the feedback lever  15 . 
     At this time, the front end  151  of the feedback lever  15  is displaced by the distance of the clearance C 2  to the second stand-by plate  32  suspended from the joining plate  312  and supported by the second stand-by plate  32 . 
     Then, based on engagement between the front end  151  of the feedback lever  15  and the second stand-by plate  32  supporting the front end  151 , the feedback lever  15  and the main rod  16  that suspends the second stand-by plate  32  from the support portion  31  are joined via the lower backup device  30 . 
     On the upper end side of the main rod  16 , the upper backup device  20  is still suspended from the regulating valve lever  14  joined to the main rod  16  by a safe joint. At this time, it is preferable that the contact stopper  163  is not in contact with the first stand-by plate  22 . 
     Also when the lower backup device  30  operates, substantially the same degree of freedom as before the lower backup device  30  operates is ensured between the links including the regulating valve lever  14 , the main rod  16 , the feedback lever  15  based on the joining by the pins and the rod end bearings at the opposite ends of the rod  23  provided in the upper backup device  20  or the rod  33  provided in the lower backup device  30  and the spherical support surface  311 D being formed on the support stopper  311 . 
     Specifically, even if the links including the regulating valve lever  14 , the main rod  16 , and the feedback lever  15  are relatively displaced as the feedback lever  15  is disengaged by the distance of the clearance C 2 , a sufficient degree of freedom that absorbs the relative displacement between the links and covers a movable range of the link mechanism  4  from the minimum opening to the maximum opening of the regulating valve  3  is ensured by the link joining in the backup devices  20 ,  30 . Thus, the lower backup device  30  can join the main rod  16  and the feedback lever  15 , and the operation of the steam turbine  1  can be continued while the function of the link mechanism  4  is ensured and the opening of the regulating valve  3  is controlled. 
     The steam turbine  1  including both the upper backup device  20  and the lower backup device  30  as in this embodiment can achieve backup of the link mechanism  4  both even when the upper end of the main rod  16  and the regulating valve lever  14  are disjoined and even when the lower end of the main rod  16  and the feedback lever  15  are disjoined. 
     However, only the upper backup device  20  may be provided in the steam turbine  1  in preparation for disjoining at the upper end of the main rod  16 , or only the lower backup device  30  may be provided in preparation for disjoining at the lower end of the main rod  16 . 
     Method for Modifying Existing Steam Turbine 
     Next, a method for modifying the steam turbine  1  that has a backup function that can replace the link joining in the link mechanism  4  by adding the backup devices  20 ,  30  to the existing steam turbine will be described. 
     Examples of procedures for adding the upper backup device  20  ( FIG. 3 ) and the lower backup device  30  ( FIG. 6 ) will be described below. Both for the upper backup device  20  and the lower backup device  30 , a locking structure adding step including the following procedures is performed. 
     Adding Upper Backup Device (FIG.  3 ) 
     (1) The clamp members  211 ,  211  of the clamp  21  are secured to the regulating valve lever  14  (a clamp providing step). 
     (2) The first piece  163 A and the second piece  163 B of the contact stopper  163  are placed on the opposite sides of the turn buckle  160  of the main rod  16  in the diametrical direction, and the first piece  163 A and the second piece  163 B are fastened. Thus, the contact stopper  163  is secured to the outer peripheral portion of the main rod  16  (a contact stopper providing step). 
     (3) The upper ends of the rods  23  are mounted to the clamp members  211 ,  211  by the pins  231 C (a rod mounting step). 
     (4) The half-split body  22 A of the first stand-by plate  22  is mounted to the lower end of one rod  23  by the pin  232 E, and the half-split body  22 B is mounted to the lower end of the other rod  23  by the pin  232 E. The half-split bodies  22 A,  22 B are suspended from the clamp  21  by the rods  23 . 
     (5) The turn buckle  160  is held between the half-split bodies  22 A,  22 B and the half-split bodies  22 A,  22 B are fastened to assemble the first stand-by plate  22 . Then, the first stand-by plate  22  suspended from the clamp  21  is placed immediately below the contact stopper  163  and around the turn buckle  160  (the items (4) and (5) above correspond to an immediately-below placing step). 
     By the above, adding of the upper backup device  20  to the existing steam turbine is completed. 
     Adding of Lower Backup Device (FIG.  6 ) 
     (1) The turn buckle  160  of the main rod  16  is held between the first piece  311 A and the second piece  311 B of the support stopper  311 , and the first piece  311 A and the second piece  311 B are fastened. Thus, the support stopper  311  is secured to the outer peripheral portion of the main rod  16 . 
     (2) On the support stopper  311 , the turn buckle  160  is held between the half-split bodies  312 A,  312 B of the joining plate  312 , and the half-split bodies  312 A,  312 B are fastened. Thus, the joining plate  312  is supported on the support stopper  311  (the items (1) and (2) above correspond to a support portion providing step). 
     (3) The upper ends of the rods  33  are mounted to the half-split bodies  312 A,  312 B of the joining plate  312  by the pins  331 C (a rod mounting step). 
     (4) The second stand-by plate  32  is mounted to the lower ends of the pair of rods  33 , 33  below the feedback lever  15  by the pins  332 E. Then, the second stand-by plate  32  suspended from the joining plate  312  is placed immediately below the feedback lever  15  (an immediately-below placing step). 
     By the above, adding of the lower backup device  30  to the existing steam turbine is completed. 
     By the procedures for adding the backup devices  20 ,  30  described above, the contact stopper  163 , the first stand-by plate  22 , the support stopper  311 , and the joining plate  312  are members that can be assembled around the main rod  16 , and thus there is no need for an operation for inserting the main rod  16  through an annular member. Thus, the backup devices  20 ,  30  can be installed in the existing links without a need to disassemble the links such as to remove the pin P 1  or the pin P 2  to disengage the main rod  16  from the regulating valve lever  14  or the feedback lever  15 . Thus, a modifying operation for adding the backup devices  20 ,  30  can be performed while driving of the regulating valve  3  is controlled via the link mechanism  4  without stopping the operation of the existing steam turbine. 
     For Newly Produced Steam Turbine 
     The steam turbine  1  including the backup devices  20 ,  30  may be newly produced, not limited to the addition of the backup devices  20 ,  30  by modifying the existing steam turbine as described above. 
     In that case, the upper ends of the rods  23  are mounted to the regulating valve lever  14  by the pins  231 C, and the rods  23  can suspend the first stand-by plate  22  from the regulating valve lever  14 . Thus, there is no need for the clamp  21 . 
     Depending on the order of assembly of the components of the main rod  16 , the regulating valve lever  14 , the feedback lever  15 , and the backup device  20 , there is no need for the first stand-by plate  22  or the contact stopper  163  to be split. 
     This also applies to the support stopper  311  or the joining plate  312  of the lower backup device  30 . 
     First Variant 
       FIG. 10  shows a lower backup device  80  according to a first variant of the present disclosure. 
     The same components as those in the embodiment described above are denoted by the same reference signs. 
     In the first variant, a joining plate  81  is supported in an upper position via rods  33  by a pin P 2  having an increased length, and the joining plate  81  is locked by a stopper  82  when a feedback lever  15  is disengaged from a main rod  16 . 
     Specifically, in the first variant, the joining plate  81  corresponds to a contact portion and the stopper  82  corresponds to a locking portion. A clearance C 2  is set between the joining plate  81  and the stopper  82 . The joining plate  81  may be configured similarly to the joining plate  312  in the embodiment described above ( FIG. 6 ). The stopper  82  may be also configured similarly to the support stopper  311  in the embodiment described above ( FIG. 6 ). 
     Second Variant 
       FIG. 11  shows an upper backup device  40  and a lower backup device  50  according to a second variant of the present disclosure. 
     The upper backup device  40  is different from the upper backup device  20  in the embodiment described above mainly in that a pair of chains  43  instead of the pair of rods  23  are used to suspend a stand-by member  42 . 
     The upper backup device  40  includes a clamp  21 , the stand-by member  42 , and the pair of chains  43  (only one is shown). 
     The chains  43  are mounted to clamp members  211 ,  211  secured to a regulating valve lever  14  and support the stand-by member  42 . 
     The stand-by member  42  includes a stand-by portion  421  located immediately below a contact stopper  163  secured to a main rod  16 , and a wall  422  that surrounds an outer peripheral portion of the main rod  16  and prevents positional displacement of the stand-by portion  421 . 
     The upper backup device  40  operates similarly to the upper backup device  20  in the embodiment described above. 
     The lower backup device  50  includes a support stopper  311  secured to the main rod  16 , and a box-like stand-by member  52  that is suspended from the support stopper  311  and has a stand-by portion  52 A located immediately below a front end  151  of a feedback lever  15 . A lower surface of an upper wall  52 B of the stand-by member  52  is supported by the support stopper  311 , and the stand-by member  52  is entirely suspended from the support stopper  311 . 
     A support surface  311 D of the support stopper  311  has a spherical shape, and thus the stand-by member  52  is preferably rockable while being supported by the support stopper  311 . 
     The stand-by member  52  and the stand-by member  42  described above are preferably split so that they can be also mounted to the existing main rod  16 . 
     The lower backup device  50  operates similarly to the lower backup device  30  in the embodiment described above. 
     Similarly to the embodiment described above, the second variant can provide redundancy to a link mechanism  4 . 
     Third Variant 
     The backup devices  20 ,  30 ,  40 ,  50  described above all replace the joining of the two links directly joined. For example, the upper backup device  20  replaces the joining of the regulating valve lever  14  as the first link and the main rod  16  as the second link. 
     On the other hand, a backup device  60  according to a third variant of the present disclosure in  FIG. 12  is assembled to a first link (regulating valve lever  14 ) and a second link (feedback lever  15 ′) via an intermediate link (main rod  16 ′) to replace both joining of the first link  14  and the intermediate link  16 ′ and joining of the intermediate link  16 ′ and the second link  15 ′. 
     Specifically, the backup device  60  serves as both the upper backup device  20  and the lower backup device  30 . 
     A length of the main rod  16 ′ in  FIG. 12  is shorter than the length of the main rod  16  in the embodiment described above. However, the main rod  16 ′ may have any length as long as a degree of freedom can be ensured between the links to the extent that movement of a link mechanism  4  is not hindered after the joint is damaged. 
     The backup device  60  includes a clamp  21  secured to the regulating valve lever  14 , a stand-by plate  32  located immediately below a front end  151  of a feedback lever  15 , and a pair of rods  23  that suspend the stand-by plate  32  from the clamp  21 . 
     The backup device  60  does not include members corresponding to the contact stopper  163  and the stand-by plate  22 , or the support stopper  311  and the joining plate  312 . 
     If the main rod  16 ′ is disengaged from the regulating valve lever  14  due to damage to a pin P 1 , a rod end bearing  161 , or the like, the main rod  16 ′ is displaced by its own weight by a distance of a clearance C set between the stand-by plate  32  and the front end  151  of the feedback lever  15 . Thus, the front end  151  of the feedback lever  15  joined to a lower end of the main rod  16 ′ is locked by the stand-by plate  32 . Also thereafter, an operation of a steam turbine  1  can be continued while the link mechanism  4  is normally operated because of a degree of freedom provided by the link joining in the backup device  60 . 
     Also, if the feedback lever  15  is disengaged from the main rod  16 ′ due to damage to a pin P 2 , a rod end bearing  162 , or the like, or if a shaft of a turn buckle  160  of the main rod  16 ′ or the like is broken, similarly to the above, the front end  151  of the feedback lever  15  is locked by the stand-by plate  32 , and thus the operation of the steam turbine  1  can be continued while the link mechanism  4  is normally operated. 
     Fourth Variant 
       FIG. 13A  and  FIG. 13B  show a regulating valve lever  14 ′ as a first link and a rod  16  as a second link. 
     As shown in  FIG. 13B , a rod end bearing  161  located at an upper end of the rod  16  is placed in a hole  14 B of the lever  14 ′. The rod end bearing  161  and a pin P 1 ′ join the lever  14 ′ and the rod  16 . 
     The fourth variant is characterized in that a plate  72  as a contact portion is suspended from the pin P 1 ′ as a support portion provided in the lever  14 ′. The pin P 1 ′ is used to achieve a compact structure as compared to the structure of the embodiment described above in  FIG. 3 . 
     The pin P 1 ′ extends through the lever  14 ′ and protrudes from opposite side surfaces of the lever  14 ′ in a width direction. 
     A backup device  70  according to the fourth variant includes the pin P 1 ′ and the plate  72  suspended from the pin P 1 ′. 
     A nut  161 N is secured to a shaft  161 A of a body of the rod end bearing  161  of the main rod  16 . The nut  161 N and the plate  72  are in contact with each other. The plate  72  is integrally assembled to the main rod  16 . The nut  161 N and the plate  72  may work in the integrated manner. 
     Rod end bearings  73  that support the plate  72  above locking nuts  731  as locking portions are placed over portions of the pin P 1 ′ protruding to opposite sides of the lever  14 ′. The locking nut  731  is secured to a shaft  73 A of a body of the rod end bearing  73 . 
     A clearance C is set between the plate  72  and the locking nut  731 . 
     The rod end bearing  73  is also provided with another nut  732  above the plate  72 . A gap exists between the nut  732  and the plate  72 . The nut  732  restricts inadvertent upward movement of the plate  72 . 
     The plate  72  extends in the width direction of the lever  14 ′. Shafts of the rod end bearings  161 ,  73 ,  73  extend through the plate  72 . 
     If a portion  161 B surrounding a ball of the body of the rod end bearing  161  is broken by wear or the like, the rod  16  is disengaged with the pin P 1 ′ remaining in the lever  14 ′. At this time, the plate  72  provided on the rod  16  is displaced downward by the distance of the clearance C and locked by the locking nut  731 , and thus the backup device  70  replaces joining of the lever  14 ′ and the main rod  16 . 
     The nut  732  is placed with a predetermined gap from the plate  72  so that movement of the link mechanism  4  including the lever  14 ′ and the main rod  16  is not hindered after the backup device  70  replaces the joining of the lever  14 ′ and the main rod  16 . The nut  732  is not always needed. 
     The fourth variant may be applied also to a joining portion of the lower end of the rod  16  and the front end of the feedback lever  15 . 
     Other than those described above, the configurations provided in the embodiment described above may be chosen or changed to other configurations without departing from the gist of the present disclosure. 
     The structure of the link mechanism to which the backup device of the present disclosure is applied is not limited to that of the embodiment described above, but may include any combination of levers or rods. Also, the valve related to the link mechanism in the present disclosure is not limited to the regulating valve of the steam turbine, but any valve may be used. 
     In the embodiment described above, as shown in  FIG. 3 , the stand-by plate  22  is suspended from the clamp  21  provided on the regulating valve lever  14  using the rods  23 , but if the stand-by plate  22  can be directly suspended from the regulating valve lever  14  without the clamp  21 , there is no need for the clamp  21 . For example, the stand-by plate  22  can be suspended from the regulating valve lever  14  by mounting the rods  23  to the thick portion  141  of the regulating valve lever  14 . 
     Besides, the stand-by plate  22  may be suspended from the regulating valve lever  14  using a chain or a rope passed through a hole in the regulating valve lever  14  without the clamp  21  or the rod  23 . Further, a member including a portion corresponding to the stand-by plate  22  may be directly mounted to the regulating valve lever  14 . Specifically, the backup device of the present disclosure may only include a stand-by member suspended from the first link and located immediately below at least a part of the second link. 
     Also for the configuration in  FIG. 6 , if the main rod  16  includes a component replacing the support portion  31  including the support stopper  311  and the joining plate  312 , there is no need for the support portion  31 . 
     In the backup device  30  ( FIG. 6 ) according to the embodiment described above, a spring having appropriate spring stiffness may replace the rod  33 . 
     It is assumed that the stand-by plate  32  and the joining plate  312  are coupled via the spring. In that case, as the main rod  16  and the feedback lever  15  are disjoined and the feedback lever  15  is displaced downward by its own weight, the spring mounted to the feedback lever  15  is elastically deformed, and the stand-by plate  32  locks the feedback lever  15  by an elastic force of the spring.