US2574378A - Wire line tensioning device - Google Patents

Description

Nov. 6, 1 951 R. R. cRooKsToN WIRE LINE TENSIONING DEVICE Filed March 1, 1950 2 SHEETS-SHEET 1 COOLING PINS SHOCK ABSORBER TO BRAKE GYLS.

"ASTER GYL.

FIG. I. H

Van: ASSEMBLY A COOLING FINS BRAKE ASSEMBLY SPRING ASSEMBLY B 55 BRAKE onuu I snocx ABSORBER Robert R. Crooksfon,

ATTORNEY.

Nov. 6, 1951 R. R. CROOKSTON WIRE LINE TENSIONING msvxcs Filed March 1, 1950.

BRAKE ASSEMBLY FIG. 3.

SPRING 38 Asssrau INVENTOR. Robert R. Crooksfon FIG. 4.

ATTORNEY.

Patented Nov. 6, 1951 WIRE LINE TENSIONING DEVICE Robert R. Crookston, Houston, Tex., asslgnor, by

mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a corporation of Delaware Application March 1, 1950, Serial No. 146,977

Claims. I

The present invention relates 'to a device for maintaining a predetermined minimum tension on a wire line held in stretched relation. More particularly, the present invention relates to a device for maintaining a minimum tension on a wire line as the wire line is reeled onto a spool.

In the level winding" of wire line onto spools, a number of essential requisites are recognized, among the most important of which are first and second layer preparation, maintenance of proper fleet angle, and maintenance of minimum tension. For example, the Roebling Handbook, published by John A. Roeblings Sons Co., a wire line manufacturer, stipulates direction of first layer helix in accordance with direction of line winding and laying of the rope. Roebling, as well as other wire line manufacturers, recognizes that fleet angles should be maintained equally at between as a minimum per side and 1 as a maximum per side. This simply means that the arcs subtended by the wire line as it travels across the spool should exceed 1 and be less than 3 and that the line itself be the arcs perpendicular bisector when the line is in the mid-spool position.

Although wire line manufacturers, as well as field users, recognize the indispensability of minimum tension for successful level winding," no'adequate steps have been taken by field users for satisfying this requirement. The contingency necessitating the maintenance of a minimum tension is that a heavy load on the line in an outer layer of the reel tends to cause the line under tension to "cut into" any layers below that which are loosely spooled or wrapped. For example, if the first or second layer is loosely wrapped, successive layers may not cut into these layers until eight or ten or more layers are wrapped onto the spool. This condition can be corrected only by unreeling all the layers already on the spool and properly re-winding the wire line onto the spool.

Operators of rigs for drilling boreholes into subsurface formations are important users of wire lines. Such lines are used for various purposes such, for example, as swabbing, pulling cores, taking pictures, logging, etc. These lines must be wound and unwound from time to time onto spools, drums, or reels. It will be appreciated that the tension on the line as it is rewound will actually be less than, or at least different from, that tension required to comply with the conditions of "level winding unless a tensioning means is intentionally provided.

The object of the present invention is to provide a tensioning device which will maintain a minimum tension on a wire line which is being wound onto a drum reel or spool when the wire line is being normally used for such operations as swabbing, pulling cores, taking pictures. etc. A further object is to provide a tensioning device which will maintain a minimum of wire line tension forthe operation of stringing new lines. A further object is to provide a tensioning device which will permit a wire line to be unwound from a spool without maintaining any tension thereon but which will permit a minimum tension to be maintained on the wire line as it is being wound onto the spool or drum. A further object is to provide a tensioning device which will maintain a minimum tension on a wire line so that this requisite of "level winding" can be accomplished in the field.

The tensioning device of the present invention consists essentially of a rotatable shaft flexibly supported at its opposite ends by biasing means, such as springs, above a frame member. A sheave is mounted on the shaft for free rotation on the shaft in one direction and is adapted for engaging with the shaft and for rotating the shaft when the sheave is rotated in the opposite direction. A braking assembly is mounted on the shaft on each side of the sheave. This braking assembly may be conventional in nature and may consist of a brake drum attached to the shaft and brake shoes adapted to engage with and disengage from the brake drum. The braking assemblies are adapted to be energized automatically unless the tension in the wire line is suilicient to hold down the shaft and sheave against the compression of the biasing means. So long as the tension in the wire line is not sufficient to hold the sheave and shaft down against the compression of the biasing means, the braking assemblies provide suflicient braking force to result in the desired tension. The sheave is adapted to receive on its outer periphery a sumcient number of turns of the wire line to assure the desired minimum tension on the tight line of the wire line even though the tension on the loose line is small. The tension on the tight line is determined by the well known wire line and belt equation which is as follows:

T1=tight line tension (pull on spool, drum, or

reel) Tz=1oose line tension (weight on free end of wire line) =Napierian logarithm base u=friction coefllcient (about 0.15 for wire line on steel or brass sheaves partially lubricated) 0=angular contact on sheave in radians.

If it is assumed that 3 /2 turns of the wire line are taken on the sheave and if the value of friction coefficient u is taken to be 0.15, the value of Ti in the foregoing equation is found to be:

This example demonstrates that a minimum of 2,000 pounds T1 tension is assured even for a dead weight of less than 100 pounds.

A tensioning device embodying the principles of the present invention is illustrated in the accompanying drawing in which:

Fig. 1 is a side view of one embodiment of the device of my invention;

Fig. 2 is a view of the embodiment shown in Fig. 1 taken along the line 11-11;

Fig. 3 is a fragmentary view, partly in cross section, of the embodiments shown in Fig. 1 taken along the line III-III of Fig. 1;

Fig. 4 is a cross sectional view of a portion of the embodiment shown in Fig. 3 taken along the line IV-IV of Fig. 3, the parts therein being shown in their respective relations when the sheave is rotated in a clockwise direction;

Fig. 5 is also taken along the line IV-IV of Fig. 3 but shows the parts therein in their respective relations when the sheave is moved in a counter-clockwise direction;

Fig. 6 is a view, partly in cross section and showing in detail the brake assembly A;

Fig. 7 is a detailed view of the brake cylinder illustrated in Fig. 6; and

Fig. 8 is a detailed view, partly in cross section, of the master cylinder shown in Fig. 1.

Referring to the drawing in which like numerals designate like parts and like letters designate like assemblies of parts throughout, II designates a shaft which is supported at each of its ends above a horizontal base by means of a compression spring assembly A and a compression spring assembly B. Mounted in the approximate center of the shaft is sheave I0. Mounted on shaft I I between sheave I and spring assembly A is brake assembly A, and mounted on shaft II between sheave I0 and spring assembly B is brake assembly B. Afiixed to shaft II on each side of sheave I0 are spacing collars I00 and I00. These spacing collars prevent the movement of sheave I0 longitudinally along shaft I I.

Sheave I0 consists of a circular rim portion IOI defining a U-shaped valley portion I02 on its outer periphery and a hub portion I03 joined to the rim portion by means of spokes I04. A pair of bearings I and I05 are disposed between the shaft I I and hub portion I03 of sheave I0, the said bearings being held in place by means of spacing rings I05 and I01, and closure plates I08 and I08. Closure plates I08 and I09 are secured to hub I03 and spacing rings I08 and I01 by means of a plurality of bolts I I0.

Sheave I0 is mounted on shaft I I so that sheave I0 can be freely rotated in one direction without also rotating shaft II, but which when rotated in the other direction will also rotate shaft II. In order to accomplish this purpose a, conventional free wheeling clutch mechanism I2 is mounted on shaft II within hub I03 of sheave I0.

Free wheeling clutch mechanism I2 consists of a member I3 mounted on shaft II for rotation therewith and is affixed to shaft II by suitable means such as by means of key I5. The outer surface I8 of member I3 carries a plurality of radially spaced indentations I1 disposed along its outer surface. In the drawing, four such indentations are shown although it will be understood that a lesser or greater number may be present. Each indentation I1 carries a roller bearing I8 which is normally free to move within indentation I1 between member I3 and the inner surface IQ of the hub I03 of sheave I0. It will be understood, of course, that a ball may be used instead of roller I8.

Free wheeling clutch mechanism I2 is so arranged with respect to'sheave l0 and shaft II that sheave I0 may freely rotate on shaft II in one direction as, for example, in a clockwise direction as indicated by the arrow in Fig. 4 without turning member I3 and shaft II. This result is accomplished by reason of the shape of indentation I1 in the outer surface I8 of member I3. This shape is most clearly shown in Figs, 4 v

and 5 which show grooves I1 having surfaces 20 and 2I of different slopes with respect to a tangent drawn to the outer surface of member I3 at the point where surfaces 20 and 2| join outer surface I6. In this instance. surface 20 is shown as forming an angle of nearly with the tangent while surface 2I forms a relatively acute angle. In any event, the slope of surface 20 is of such magnitude that roller I8 will not wedge between the outer surface I5 of member I3 and the inner surface I3 of hub I83 of sheave I0 when sheave I0 is rotated in a clockwise direction. On the other hand, surface 2I is of such an angle that roller I8 will be caused to be wedged between surface 2I and the inner surface I9 of hub I03 when sheave I0 is rotated in a counter-clockwise direction as is shown in Fig. 5. Accordingly, when sheave I0 is rotated in a clockwise direction, member I3 and shaft II do not rotate; on the other hand, when sheave I8 is rotated in a counter-clockwise direction, member I3 and shaft II also rotate in a counter-clockwise direction in cooperation with said sheave.

Each of extremities 21 and 21' of shaft II is supported above a frame member 32 by spring assembly A and spring assembly B, respectively. The details of spring assembly A and spring assembly B are identical and, accordingly, only spring assembly A will be described in detail. The details of spring assembly A are shown most clearly in Fig. 3.

Referring to Fig. 3, extremity 21 of shaft II is shown as being circumscribed by bearing housing 28, ball bearing 23 and ball bearing 30 being positioned within said housing between shaft II and the housing. Bearing 28 and bearing 30 are spaced apart by spacer sleeve 3I. Nut 22 abuts against bearing 23 and prevents lateral movement of shaft II with respect to housing 28. It will be obvious that shaft II is freely rotatable within housing 28, Bearing housing 28 is supported above horizontal frame member 32 by means of spring 33, the upper end 34 of spring 33 abutting against a lower surface 35 of bearing housing 28; lateral movement of the upper end 34 of spring 33 is prevented by downwardly projecting spring positioning member 35. The lower end 31 of spring 33 abuts against horizontal frame member 32 and lateral movement of this end is prevented by upwardly projecting spring positioning member 38, member 38 forming a part of frame member 32. Frame member 32 is affixed to base member I22. Disposed on each side of spring 33 are shock absorbers 39 and 40 which are aillxed at their lower ends by means of nuts 39' and 40', respectively, to frame member 32 and at their upper ends to bearing housing 28 by means of connecting bolts 4| and 42, respectively, and nuts 43 and 44, respectively. Shock absorbers 39 and 40 are not essential ele ments of the present invention but have been found desirable in that they make for smoother operation.

In like manner, extremity 21' of shaft II is supported above horizontal frame member 32 by means of spring assembly B, shaft II being held against lateral movement within housing 28 of spring assembly B by means of nut 22.

The foregoing description makes it apparent that shaft II and the elements mounted thereon can be moved toward horizontal frame member 32 when suflicient pressure is exerted on the shaft to compress springs 33. However, the amount by which springs 33 can be compressed is limited by the abutment of the lower surface 49 of member 49 on the upper surface 32"01' frame member 32. Member 49 is a transversely extending member interconnecting vertically disposed yoke members 45 and 45'.

Yoke members 45 and 45' are identical in construction and, accordingly, only the details of member 45 will be given. Yoke member 45 defines a passage 46 and is mounted adjacent sheave I with shaft |I centrally disposed in passage 46. Ball bearing 24 fits within opening 46' of member 45 and spaces member 45 away from shaft II, permitting shaft II to rotate without also tending to rotate member 45. This is accomplished by mounting the inner race 23 of ball bearing 24 on the spacing collar I00, the outer race 26 supporting member 45. Circulating balls 25 are disposed between inner race 23 and outer race 26. Yoke members 45 and 45' are joined at their lower ends 50 and 50', respectively, by means of transversely extending member 49. The upper ends of yoke members 45 and 45 may be held in spaced relation by means of laterally extending bolts I20 and I2 I.

Yoke member 45 defines horizontally extending arm 80 and a vertically extending arm 8| which joins the end 82 of horizontally extending arm 80. Yoke member 45' defines horizontally extending arm 80 and vertically extending arm 8| vertically extending arm 8| joining horizontally extending arm 80 and 82. A port 83 passes transversely through yoke member 45 at the juncture of horizontally extending arm 80 and vertically extending arm 8|. A similarly located port 83' traverses member 45'. A vertically extending frame member 84 defines a pair of spaced hinge members 85 and 86, each defining a port 81 and 88, respectively, in register with ports 83 and 83. A pin 89 passes through ports 83, 83, 81, and 86. Vertically extending frame member 84 forms a part of base member I22 and, accordingly, spaced hinge members 85 and 86 are in fixed relation to frame member 32. Accordingly, pin 89 acts as a pivot about which members 80 and 80 and 8| and 8| may move arcuately. The lower ends 90 and 90' of vertically extending members 8| and 8|, respectively, are pivotly connected with piston rod 9| by means of pivot member 92.

Master cylinder I30 comprises cylinder housing I 3| which is afiixed to frame member 32 as by means of bolts I 32. Master cylinder housing |3| defines a reservoir I 33 and a cylindrical piston chamber I34, cylindrical piston chamber I34 cylindrical piston chamber I34 communicating with-reservoir I33 by means of ports I35 and I36.

A refill port I31 communicates with reservoir I33 for recharging said reservoir with hydraulic fluid. Piston rod 9| is joined at one end to members 8| and 8| by means of pivot pin 92 and its other bailed end I38 fits slidably into socket I39 formed by piston I40. Piston I40 is slidably mounted within cylindrical piston chamber I34 in fluid-tight relation with the inner walls thereof. Piston I40 is biased toward the piston rod end of piston chamber I34 by means of spring MI. The head end of cylindrical piston chamber I34 defines an outlet port I43 which fluidly communicates with conduits 11 and 11'.

When the head end I46 of piston I40 is to the right of port I36, hydraulic fluid in conduits I 43, 11, and 11 and in the head end of piston cylinder I34 is free to fiow into reservoir I33 through port I36. When, on the other hand, piston I40 is moved toward the head end of piston chamber I34 a suflicient distance to blank 01f port I36, then the fluid trapped in the head end of cylinder I34 is forced out through port I43, through conduits 11 and 11. The function of port I35 is to permit the equalization of pressure in reservoir I33 and the piston rod end of piston cylinder I34. Spring I4I tends to bias piston I40 toward the piston rod end of piston chamber I34.

A brake assembly A is mounted on shaft ll between yoke member 45 and bearing housing 28 of spring assembly A. A brake assembly B is mounted on shaft I between yoke member 45' and bearing housing 28' of spring assembly B. Brake assembly B is identical in construction to brake assembly A and, accordingly, the details of brake assembly A only will be described. Brake assembly A includes a brake drum 52, the outer surface 'of which may carry a plurality of radially extending, angularly spaced fins 53. Brake drum 52 is secured to annular disc 54 by means of a plurality of bolts 55, annular disc 54 being mounted on shaft I I and affixed thereto as by means of key 56. One edge 51 of annular disc 54 abuts against spacer ring 58 which circumscribes shaft II which, in turn, abuts against inner race 23 of ball bearing 24, the other edge 59 of annular disc 54 abutting against ball bearing 30 of spring assembly A. A pair of arcuate brake shoes 60 and 6| carrying friction brake lining 62 and 62', respectively, on their outer arcuate surfaces is disposed within brake drum 52, brake shoes 60 and 6| being so positioned that brake linings 62 and 62' are in proximity to the inner cylindrical surface 63 of brake drum 52. Ends 64 and 65 of brake shoes 60 and 6|, respectively, are secured to securing ring 41 by means of pivot pins 66 and 66', respectively, a spacer 9 being interposed between said ends and securing ring 41. The other ends 61 and 68 of brake shoes 60 and 6|, respectively, are adapted to be moved to and from each other by a suitable means, such as by a conventional double acting hydraulic element 69, to engage and disengage friction linings 62 and 62' with inner surface 63 of brake drum 52. A spring 5 interconnecting brake shoes 60 and 6| is adapted to bias ends 61 and 68 of said shoes toward each other.

Double-acting hydraulic brake cylinder 69, which is shown most clearly in Fig. '1, consists of a cylinder 1| containing piston 13 connected by piston rod 14 to brake shoe 60 and of piston 15 connected by piston rod 16 to brake shoe 6|. Conduit 11 fluidly connects with the interior of cylinder 1| into the space between pistons 13 and 15. Pistons 13 and 15 are adapted to move slidably within cylinder 1| in fluid-tight relation it is to maintain upon the wire line.

with the inner walls thereof. Cylinder II is secured against movement within brake assembly A by attachment to securing ring 41 by means of bracket 1 and bolts 8. The interior portion of brake assembly A is protected against the entry of foreign matter, such as dust and grit, by means of brake rim which is appropriately secured to securing ring 41 as by means of bolts I and pins 66 and 66.

In Fig. 2 of the drawing, wire line I I0 is shown as wrapped around the outer U-shaped peripheral surface I02 of sheave I0. While three turns of wire line 0 are shown on sheave I0, it will be appreciated that a lesser or greater number of turns may be wrapped upon sheave I0. The number of turns which are wrapped upon sheave I 0 will depend upon the magnitude of the tension In any event, U-shaped peripheral surface I02 of sheave I0 must be sufficiently broad to accommodate at least one turn of wire line ||0.

Having fully described the various parts of the device of my invention and their relation to each other, the operation of the device will now be described. When a wire line 0 is run over sheave I0 so as to rotate sheave I0 in a clockwise direction, sheave I0 free wheels, that is, rotates freely on shaft |I without rotating said shaft. Accordingly, the device of the present invention maintains no drag whatsoever upon wire line I I0 when it is passed over sheave I0 in such a manner as to rotate sheave I0 in a clockwise direction. On the other hand, when wire line H0 is passed over sheave I0 in such a manner as to rotate sheave |0 in a counter-clockwise direction, rollers I8 of clutch mechanism I2 are wedged in between the inner surface I9 of sheave I0 and surface 2| of member I3 so that member I3 rotates in cooperation with sheave I0. Inasmuch as member I3 is affixed to shaft II by means of key I5, shaft II also rotates. Not only does the motion produced by drawing wire line I I0 over sheave I0 rotate shaft I I, but the tension on wire line 0 tends to move shaft II and the parts mounted thereon, including brake assemblies A and B, downwardly against the upward thrust exerted by springs 33 of spring assemblies A and B provided the weight suspended by the loose line 0-1) of wire line 0 is sufficiently great. Rapid flex of springs 33 is prevented by shock absorbers 39 and 40. 'If the weight suspended from the loose line IIO-b of wire line 0 is sufliciently great, springs 33 of spring assemblies A and B will be compressed until the lower surface 49 of member 50 abuts against the upper surface 32'- of frame member 32. The abutment of member 50 against frame member 32 prevents, of course, further compression of springs 33. The letter a indicates the distance which member 50 moves while springs 33 move from their non-compressed state when the loose line 0-1) of wire line 0 is not under tension to the fully compressed state of springs 33, and the magnitude of a will depend, for a set of springs of given compressive strength, upon the minimum tension it is desired to maintain on the tight line I I0-a of wire line I I0.

When member 49 abuts against frame member 32. brake linings 62 and 62' are disengaged from the inner surface 63 of brake drum 52 in a manner which will be hereinafter described. However, while sheave I0 is being rotated in a clockwise direction by movement of wire line I I0 thereover and before member 50 abuts against frame member 32, brake linings 62 and 62' of brake shoes 60 and BI, respectively, are in engaging contact with the inner surface 63 of brake drum 52. The engagement of brake shoes 60 and 6| with brake drum 52 provides a drag on the rotation of shaft II, and the magnitude of the braking force is so adjusted that the drag on shaft II results in the desired minimum tension in the tight line ||0-a of wire line 0, irrespective of the tension in the loose line IIO-b of wire line ||0. Heat generated during periods of braking is dissipated by means of fins 53 disposed on the outer periphery of brake drum 52.

The magnitude of the braking force applied to inner surface 63 of brake drum 52 by means of brake shoes 60 and 6| is automatically controlled by regulating the amount of fluid supplied to double-acting brake cylinders 69 of braking assemblies A and B through conduits TI and 11', respectively. The amount of fluid supplied to double-acting cylinder 68 is, in turn, controlled by master cylinder I30 which supplies hydraulic fluid to double-acting brake cylinders 69 in response to changes in compression of springs 33. When springs 33 are in their non-compressed state, piston I40 of master cylinder I30 is at its forward limit of travel toward the head end of piston cylinder I34 as indicated by the dotted lines in Fig. 8. When piston I40 is in this position, the hydraulic fluid in the head end of piston cylinder I34 has been forced through conduits I43 and each of conduits "I1 and I1 into double-acting brake cylinder 69 of brake assemblies A and B. At such time brake shoes 63 and 6| of brake assemblies A and B are in their full engaging position and provide maximum drag on the rotation of shaft II and this drag is of suflicient magnitude to produce the desired tension in the tight line IIO-a of wire line IIO. If the tension on the loose line |I0-b of wire line I I0 is only sufficient to partially compress springs 33, then the braking force exerted by braking assemblies A and B is of such magnitude as to supply the desired tension in the tight line I Il-a of wire line 0.

The magnitude of the braking force exerted by brake shoes 60 and 6| of brake assemblies A and B is correlated with the force exerted by springs 33 of spring assemblies A and B by means of yoke members 45 and 45'. This follows from the fact that as springs 33 are compressed, yoke members 45 and 45' move downwardly. Since horizontally extending arms 00 and and vertically extending arms 8| and 0| of yoke members 45 and 45', respectively, are pivoted to upright frame member 04 by pin 03, the lower ends and 80 of vertically extending arms 0| and 8|, respectively. move arcuately in a direction away from master cylinder I30. Since piston rod SI of master cylinder I3! is pivotly attached to the lower ends 90 and 90 of vertically extending arms 3| and 0|, respectively, by means of pin 92, piston rod 9| also moves in the same direction. Follower spring I in master cylinder I30 forces piston I40 to move in cooperation with piston rod 9|, thereby permitting hydraulic fluid to flow from doubleacting brake cylinders 69 of brake assemblies A and B through conduits 'Il, I1, and I43 into the head end of piston cylinder I34 of master cylinder 30. The movement of lower ends 96 and 90' of vertically extending members 8| and II, respectively, continues in the aforementioned direction until member 50 abuts against frame member 32, at which time piston I40 of master cylinder I30 reaches the full open position indicated in Fig. 8, uncovering port I36. When port I38 is uncovered by piston I44,-hydraulic fluid in double-acting brake cylinders 69. conduits I1, 11', and I43 is no longer under pressure and is free to flow into reservoir I33 of master cylinder I30. Since no pressure is then being exerted against brake shoes 6| and GI by double-acting brake cylinders 69 of brake assemblies A and B, spring retracts brake shoes 60 and BI into a fully disengaged position with respect to brake drums 52. I

Master cylinder I30 may be refilled with hydraulic fluid from time to time through conduit I31 which fluidly communicates with reservoir I33. Port I35 permits the hydraulic fluid in reservoir I33 to have access to the piston rod side of piston I40 so as to provide lubrication for piston I40 in its travel within piston cylinder I34.

From the foregoing description it will be ob vious that by selecting springs 33 of proper compressive strength and by correlating the braking forces exerted by brake assemblies A and B with the compressive strength of said sprin s, the device of my invention may be constructed so that it will exert any desired minimum tight line tension on a wire line.

In the embodiment illustrated and described, hydraulic rather than pneumatic brakes have been employed. It will be understood, of course, that pneumatic brakes will also function in the described combination although hydraulic brakes are preferred because of the greater degree of compactness which can be secured through their use.

Having fully illustrated and described the present invention, what I wish to claim as new and useful and desire to secure by Letters Patent is:

1. A device for maintaining a minimum tension on a wire line comprising, in combination, a rotatably mounted shaft supported by a plurality of springs in compression, a sheave mounted on said shaft for free rotation on said shaft in one direction and adapted for engaging with and for rotating said shaft when said sheave is rotated in the opposite direction, said sheave being adapted to receive on its outer periphery at least one turn of said wire line, a brake drum aifixed to said shaft for rotation therewith, and a braking element adapted to be engaged with and disengaged from said brake drum in response to changes in the degree of compre sion of said springs for resisting the rotary motion of said sha t w en in enga ement with aid brake drum.

2. A device for maintaining a minimum tension on a wire line comprising, in combination, a rotatably mounted shaft supported by a plurality of springs in compression, a sheave mounted on said shaft for free rotation on said shaft inone direction and adapted for engaging with and for rotating said shaft when said sheave is rotated in the opposite direction, said sheave being adapted to receive on its outer periphery at lease one turn of said wire line, a brake drum affixed to said shaft for rotation therewith, and a braking element adapted to be engaged with said brake drum for applying an alterable braking force whenever said springs exert a bias on said shaft less than that required to maintain a minimum tension on said wire line, said alterable braking force being of suflicient magnitude to maintain said minimum tenson, said braking element being adapted to be disengaged from said brake drum when said springs exert a bias on 10 said shaft suflicient to maintain a minimum tension on said wire line.

3. A device for maintaining a predetermined minimum tension on a wire line comprising, in combination, a frame member, a first spring mounted on said frame member with one end of said spring abutting against said frame member, a second spring mounted on said frame member with one end of said spring abutting against said frame member, said second spring being spaced from said first spring, a shaft rotatable about its longitudinal axis mounted on said first and second springs, said springs biasing said shaft away from said frame member, a sheave mounted on said shaft for free rotation on said shaft in one direction and adapted for engaging with and for rotating said shaft when said sheave is rotated in the opposite direction, said sheave being adapted to receive on its outer periphery at least one turn of said wire line, a brake drum mounted on said shaft for rotation therewith, a braking element adapted to be engaged with said brake drum for applying an alterable braking force thereto when said springs exert a bias against said shaft less than that ,required to maintain said predetermined minimum tension on said wire line, said alterable braking force being of suflicient magnitude to maintain said predetermined minimum tension, said braking element being adapted to be disengaged from said brake drum when said springs exert a bias suflicient to maintain said predetermined minimum tension on said wire line.

4. A device for maintaining a predetermined minimum tension on a wire line comprising, in combination, a frame member, a first helical spring mounted on said frame member with one end of said spring abutting against said frame member, a second helical spring mounted on said frame member with one end of said spring abutting against said frame member, said second helical spring being spaced from said first helical spring, a shaft rotatable about its longitudinal axis mounted on said first and second helical springs, said springs biasing said shaft away from said frame member, a sheave mounted on said shaft for free rotation on said shaft in one direction and adapted for rotating said shaft when said sheave is rotated in the opposite direction, said sheave being adapted to receive on its outer periphery a plurality of turns of said wire line, a brake drum mounted on said shaft for rotation therewith, said brake drum having a cylindrical surface coaxially arranged with and radially spaced from said shaft, a yoke member supported by said shaft and extending radially therefrom, said yoke member being pivotally attached to said frame member and being arranged to limit the travel of said shaft in its movement toward said frame member against the bias exerted by said springs, a first brake shoe having an arcuate friction surface engageable with the cylindrical surface of said brake drum, a second brake shoe having an arcrate friction surface engageable with the cylindrical surface of said brake drum, one end of said first and second brake shoes being pivotally hinged to said yoke member, the other end of said first brake shoe being attached to a first movable e ement arranged within said double-acting hydraulic cylinder and the other end of said second brake shoe being attached to a second movable element arranged within said double-acting hydraulic cyl inder, a master cylinder containing hydraulic fluid aflixed to said frame member, a piston 11 slidably and aealably arranged in said master cylinder, said piston being pivotaily'connected by means of a piston rod to said yoke member and longitudinally movable within said master cylinder by said yoke member, a first conduit fluidly connectlnz said master cylinder with said first hydraulic cylinder, and a second conduit fluidly connecting said master cylinder with said second hydraulic cylinder.

5. A device for maintaining a minimum tension on a wire line comprising, in combination, a rotatably mounted shaft supported by flexible biasing means, a sheave mounted on said shaft for free rotation on said shaft in one direction and adapted for en aging with and for rotating said shaft when said sheave is rotated in the opposite direction, said sheave being adapted to 12 receive on its outer periphery at least one turn of said wire line, and braking means associated with said shaft and adapted to be operatively connected with and disconnected from said shaft in response to changes in the degree of bias exerted by said biasing means for resisting the rotary motion of said shaft when operatively connected with said shaft.

ROBERT R. CROOKSTON.

REFERENCES CITED UNITED STATES PATENTS Name Date Chapman May 25, 1943 Number

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