Patent Number: 054715130
Section: claims

1. A control rod drive comprising: means for coupling to a control rod;  a drive assembly which is rotatable in response to electrical power, said drive assembly comprising first threaded means;  second threaded means connected to said coupling means for transforming rotation of said drive assembly into vertical translation of said coupling means;  first means for moving said first threaded means into threaded engagement with said second threaded means;  second means for moving said first threaded means out of engagement with said second threaded means; and  means for applying a force which causes said first means to move said first threaded means into engagement with said second threaded means, wherein said second means move said first threaded means out of engagement with said second threaded means when application of said force ceases.  means for coupling to a control rod;  drive means which are rotatable in response to electrical power;  means for transforming rotation of said drive means into vertical translation of said coupling means;  first means for moving said drive means into engagement with said rotation transforming means;  second means for moving said drive means out of engagement with said rotation transforming means; and  means for applying a force which causes said first means to move said drive means into engagement with said rotation transforming means, wherein said second means move said drive means out of engagement with said rotation transforming means when application of said force ceases, said rotation transforming means comprise a drive screw and said drive means comprise a segmented drive nut which engages said drive screw when said force is applied.  means for coupling to a control rod  drive means which are rotatable in response to electrical power;  means for transforming rotation of said drive means into vertical translation of said coupling means;  first means for moving said drive means into engagement with said rotation transforming means;  second means for moving said drive means out of engagement with said rotation transforming means; and  means for applying a force which causes said first means to move said drive means into engagement with said rotation transforming means, wherein said second means move said drive means out of engagement with said rotation transforming means when application of said force ceases, said first means comprise a piston and said force applying means comprise a source of pressurized fluid. 2. The control rod drive as defined in claim 7, wherein said second threaded means comprise a drive screw and said first threaded means comprise a segmented drive nut which engages said drive screw when said force is applied. 3. The control rod drive as defined in claim 1, wherein said first means comprise a piston and said force applying means comprise a source of pressurized fluid. 4. The control rod drive as defined in claim 1, wherein said second means comprise a compression spring arranged to urge said first threaded means out of engagement with said second threaded means. 5. A control rod drive comprising: 6. The control rod drive as defined in claim 5, wherein said second means comprise compression spring means arranged to urge said segmented drive nut out of engagement with said drive screw. 7. A control rod drive comprising: 8. The control rod drive as defined in claim 7, further comprising compression spring means arranged to urge said piston in a direction opposite to a direction in which said force is exerted on said piston. 9. The control rod drive as defined in claim 7, wherein said rotation transforming means comprise a drive screw and said drive means comprise a segmented drive nut which engages said drive screw when said force is applied. 10. The control rod drive as defined in claim 9, wherein said drive means further comprise a first circular tube having a first radius, a second circular tube having a second radius less than said first radius, upper and lower means for supporting said first and second circular tubes in concentric relationship to form an annular channel therebetween, a drive shaft connected to said lower means and a motor coupled to said drive shaft, said annular channel being in flow communication with source of pressurized fluid; wherein said drive screw is displaceable in a circular channel of said second circular tube; and wherein said piston is annular, said annular piston being displaceable in said annular channel is response to said force applied by said pressurized fluid. 11. The control rod drive as defined in claim 10, wherein said segmented drive nut penetrates said second circular tube and has a first inclined cam surface, and said piston has a second inclined cam surface which abuts said first inclined cam surface when said force is applied. 12. The control rod drive as defined in claim 10, wherein said annular channel and a circular channel of said second circular tube are in fluid communication via a vent port.