Patent GB-665387-A (GB)
Title: Improvements in winch control
IPC: B66C13/23
Filed: 1949-04-09
Abstract: 665,387. Winding and paying-out apparatus. DYNAMATIC CORPORATION. April 9, 1949. [June 1, 1948] No. 9689/49. Class 78 (v). [Also in Group XXXV] A tension-sensing device, located in a loadsupporting cable being payed out by a winch drum, is adapted to control an electric circuit supplying current to the coil of an eddy current slip coupling, one element of which is driven by the winch drum under the influence of the load supported by the cable, while the other element is constantly rotated at a uniform speed in the opposite direction, so that upon decrease of tension in the cable the current to the coil is increased, so increasing the braking effect of the coupling on the load, and vice versa. As applied to oil-well drilling the load is constituted by the drilling string, the drilling bit of which is pressed into the ground with a force which is maintained within safe limits by this means. Fig. 1. The drilling string is carried by a pulley block 1 suspended from a fixed block 3 by means of a cable 5 attached to a fixed point 7 at one end and wound on a drum 9 at the other. The cable is parted for insertion of a tensionsensing unit 35 communicating with a control box through a pipe 39. An electric circuit within the control box supplies current to the coil 25 wound on one member 19 of the eddy current slip coupling, this member being driven from drum 9 through a manual clutch 13, gear box 11, electrically controlled emergency brake 15 and generator 17. The other member 23 of the slip coupling is driven constantly in the opposite direction by a motor 29 through gear box 27. Fig. 2. The tension-sensing head consists of an annular bellows 51 one end of which is secured to a casing 43 to which a part of cable 5 is secured at 45, while the other end is provided with a plate on which bears a nipple 53 on the end of the other part of cable 5 which passes through a hole 49 in the casing 47. Fluid can be transferred between bellows 51 and bellows 41 in the control box through the pipe 39 while a spring, not shown, maintains bellows 51 full and bellows 41 empty in absence of tension in the cable 5. An arm 57 is operated by bellows 41 to control a potentiometer 61. Fig. 3. Control box circucit. A transformer has three output windings 65, 67, 69 in addition to valve heater supplies (not shown). Two gas filled rectifying valves 77, 79 are connected in opposition in circuit with winding 65 such that a rectified current is passed through the slip coupling coil 35 by valve 79 while valve 77 acts as a low impedance across coil 25 during the non-active periods of valve 79. The valve 79 has a grid 81 upon which is impressed a rectified and smoothed voltage which is obtained from winding 69, rectifying valve 90 and condenser 102 while a stabilising valve 187 is also included. This voltage is the resultant of a fixed voltage across potentiometer 64 and a voltage obtained across the potentiometer 61 actuated by the bellows 41 from the tensionsensing unit. Potentiometer 64 is manually adjusted to determine a tension of the cable 5 which is maintained substantially constant by operation of the apparatus. Valve 79 is found to respond over an increased range of grid voltages by superimposing on the smoothed rectified voltage an A.C. voltage out of phase with the anode voltage, this being effected by the winding 67, condenser 89 and potentiometer 85. Restricting maximum descent speed of load. The drill bit is prevented from sinking too quickly into soft strator since on subsequently encountering a hard obstruction damage to the bit would then occur. This is effected by the generator 17 which at high speeds generates a voltage which when transformed and rectified exceeds a fixed voltage obtained from the rectifying valve 90 through a potentiometer 95, and is arranged to make the grid 84 of a pentode valve 68 go negative. Valve 68 is disposed in circuit with the potentiometer 61 and normally has a low impedance since grid 84 is maintained positive by the fixed voltage across potentiometer 95. When a high speed is reached and grid 84 goes negative the current through potentiometer 61 is cut off making grid 81 positive and so causing a large current to pass to coil 25, braking the fall. The desired maximum speed is determined by adjustment of the potentiometer 95.