ROPE TENSION DEVICE

Abstract: A novel rope tension damper device (60) is adapted to be installed, for example, on the deck of a ship (10) at a location between the winch (40) that controls the effective length of the rope and the fairlead (70) that guides the rope so that it does not snag or tangle as it is wound and unwound from the winch. The device comprises a co-planar of sheaves (102, 108) respectively mounted at opposite ends of a rotatable arm (110). The two sheaves are oriented with their respective rotational axes parallel to the rotational axis of the arm such that with the rope entering the device tangential to a point on the periphery of the first sheave (102) and exiting from the device tangential to a second point on the periphery of the second sheave (108), and with the arms longitudinal axis (3) more or less perpendicular to the orientation of the rope as it enters and exits from the device prior to the application of any tension, the rope within the device will be guided by the two sheaves into a generally S-shaped configuration which will be gradually distorted into a relatively straight configuration as the tension in the rope applies a torque couple to the arm and thereby causes it to rotate to an orientation more parallel to that of the rope outside the device. Preferably, a pair of such elastomeric torsional members (126, 128) are provided, one on either side of the rotatable arm (110) and concentric with its rotational axis. Shearing forces within the elastomer serve to generate a counter-torque or the rotating inner arm, thereby causing a variable damping force to be applied to the rope as the arm and sheave assembly is rotated from its zero load position. In accordance with a preferred embodiment of the invention, each torsion spring in a pair of hollow elastomeric cylinders (170, 172) coaxially nested about the arm's rotational axis, with a first end of the outer cylinder (172) being fixedly connected to a first end of inner cylinder (170) such that when one of the respective second ends is attached to the arm and the other of the respective second ends is attached to the outerframe (176), the torque generated by the inner elastomeric cylinder and the torque generated by the outer elastomeric cylinder are combined.

Description: 1 Rope Tension Device 2 3 Technical Field 4 5 The present invention relates generally to nautical 6 fittings and more particularly to a tension damper device 7 that may be used to absorb the sudden changes in tensile 8 forces to which a wire rope or other flexible line 9 securing a vessel to its mooring is subjected as the 10 vessel moves relative to the mooring. 12 Background Art 13 14 When a boat or other vessel is moored to a pier or 15 other mooring by means of a wire rope or other flexible 16 but relatively inelastic line, it is conventional to 17 provide the line with some type of device for maintaining 18 the line taut while allowing a predetermined amount of 19 resilient elongation in order to allow relative movement 20 between the vessel and its mooring. 21 22 U.S. Patent 4,022,450, entitled "Line Tensioning 23 Device," is representative of one such prior art device 24 which utilizes a pair of tension springs to maintain a -.25 resilient Z-shaped bend in the line that stretches into a • 26 more elongated shape when a predetermined tensile force is 27 applied to the line. 28 29 U.S. Patents 546,788 and 869,130, respectively 30 entitled "Cable Buffer" and "Yielding Boat Cleat", 31 disclose prior art mechanical devices in which one or more 32 coil springs are compressed as excessive tensions or 33 strains are applied to the hawser, cable or rope. 34 35 U.S. Patents 143,993 and 107,917, respectively 36 entitled "Windlasses" and "Improvements in Surge- Relievers," disclose other devices in which shocks in the cable caused by the movement of the vessel are absorbed by a plurality of rubber springs. it will be noted that the above-described prior art tension absorbing devices utilize spring elements which are compressed or elongated in a linear direction and accordingly there is an approximately linear relationship between the stress applied to the rope and the resultant increase in its effective length. U.S. Patent 4,273,066, entitled "Oil Storage Vessel, Mooring Apparatus And Oil Delivery For The Off-Shore Production of Oil," proposes various apparatus for mooring a vessel at sea to a deep water submerged mooring which includes both a mooring line (which depending upon the particular application may be either a floating- rope or a heavy chain) and a flexible length of oil transfer hose. The Patent makes mention of a manually or automatically controlled tension releasing (self-rendering) mooring winch, and a mechancial damping device (a damping roller) at columns 5 and 8. Column 15 of the Patent describes a mechanism called "The Damper" which is said to be designed and adjusted to absorb a portion of the shock cyclical loadings placed on the mooring line. The Damper is described as comprising a concave roller with butt ends over which the mooring line will pass, the roller being held at a specific height above and astern of the wire rope winch by two arms which pivot about an axis, the arms in turn being held in position by springs or hydraulically controlled rams. This same Patent also discusses the use of a mooring line to effect a resilient connection between a floating storage vessel and a service tanker with the required tension being provided by placing the propeller on the service tanker in its reverse power position to provide a predetermined amount of tension on the line and the oil hose is arranged in such a way that a relative motion between the vessels of up to a 150 feet (about 45 meters) may be accommodated (column 12). Mechanical tensioners have also been employed to maintain a more or less constant tension in endless drive belts such as are utilized to drive the various accessory subsystems associated with a modern automotive engine. Such belt tensioner devices are intended to compensate for the changes in the belt's length during its service lifetime while ensuring that a proper amount of tension is maintained in the belt to eliminate any slipping between the belt and the various pulleys around which it travels and at the same time without exerting an excessive tension force on the belt. U.S. Patent No, 4,285,676 and Reissue Patent No. 30,842, entitled respectively "Mechanical Belt Tensioner Construction" and "Tensioning Apparatus," disclose typical prior art endless drive belt tensioners wherein a torsional spring is provided at one end of a lever arm for pivotally moving an idle pulley at the other end of the lever arm in a direction generally perpendicular to the belt's longitudinal orientation as it extends between adjacent fixed pulleys to thus compensate for changes in the belt's length by increasing the effective distance between the fixed pulleys. The torsional spring may comprise a plurality of coil springs (such as is shown in Patent 4,285,676) or, alternatively, may be a hollow cylinder of elastomer material which exerts the required rotational force as the result of the elastomer being subjected to a torsional shear (such as is shown in Reissue 30,842) . u.S. Patent 3,817,113, entitled "Chain Drive with Idler Wheel Tensioning Means Biased by Elongated Pads," is y ^ E ( OMPI m 1 particularly related to a chain drive for textile machines 2 which may have to be abruptly stopped and restarted such 3 that the chain thereby experiences considerable strain. A 4 torsion effect results as a plurality of rubber pads are 5 distorted in cross section as the tension device is 6 rotated. 7 8 i It will be appreciated that dampers that have been 9 adapted for use with continuous belt chains and other 10 similar drive systems will never be required to handle the 11 particularly large forces and constant pounding that are 12 associated with many nautical applications. Accordingly, 13 the reactive force resulting from the application of an 14 unbalanced torque (rather than a balanced torque couple) 15 will probably not materially affect the reliability of 16 such devices when used for their intended application. 17 18 U.S. Patent 2,087,253, entitled "Tilting Mechanism 19 Especially For Chairs," illustrates the use of a 20 cylindrical body of rubber that is highly compressed and 21 confined between concentric tubular members so as to in 22 effect be bonded to their surfaces such that relative 23 rotary movement between the tubular members puts the 24 rubber body under torsional strain. An adjusting ,25 mechanism is provided to vary the torsional strain imposed •26 upon the rubber and thus the force required to tilt the 27 seat of a chair supported by such a resilient rubber 28 arrangement. Such a form of construction is believed to 29 be exemplary of a number of different applications in 30 which a cylindrical rubber bushing is utilized to 31 accommodate a limited rotational movement between adjacent 32 structural elements, while at the same time biasing the 33 mechanism to its normal position. It should be noted that 34 such a bushing, although cylindrical in shape, does not 35 employ what is hereinafter referred to as "longitudinal 36 torsional shear" and that furthermore the "radiating" 1 torsional shear within the rubber is concentrated in its 2 hub portion, rather than being distributed uniformly. 3 4 Disclosure of the Invention 5 6 novel rope tension damper device is adapted to be 7 installed at an intermediate position along the length of 8 a wire rope or other similar flexible line. For example, 9 the tension device may be attached to the deck of a ship 10 at a location between the winch that controls the 11 effective length of the rope and the fairlead that guides 12 the rope so that it does not snag or tangle as it is wound 13 and unwound from the winch. Alternatively, the device may 14 be installed in the vicinity of a fixed end of the rope 15 preferably between a fairlead and the cleat to which the 16 rope is attached. 17 18 The actual device comprises a co-planar pair of 19 sheaves respectively mounted at opposite ends of a 20 rotatable arm. The two sheaves are oriented with their 21 respective rotational axes parallel to the rotational axis 22 of the arm such that with the rope entering the device 23 tangential to a point on the periphery of the first sheave 24 and exiting from the device tangential to a second point .25 on the periphery of the second sheave, and with the arm's • 26 longitudinal axis more or less perpendicular to the 27 orientation of the rope as it enters and exits from the 28 device prior to the application of any tension, the rope 29 within the device will be guided by the two sheaves into a 30 generally S-shaped configuration which will be gradually 31 distorted into a relatively straight configuration as the 32 tension in the rope applies a torque couple to the arm and 33 thereby causes it to rotate to an orientation more 34 parallel to that of the rope outside the device. One or 35 more elastomeric torsion springs effect the connection of 36 the rotatable inner arm to an outer fixed frame, with 1 shearing forces within the elastomer serving to generate a 2 counter-torque on the rotating inner arm, thereby causing 3 a variable damping force to be applied to the rope as the 4 arm and sheave assembly is rotated from its zero load 5 position. 6 7 in accordance with a preferred embodiment of the 8 invention, the elastomeric torsion spring has the 9 configuration of a pair of hollow cylinders, one 10 concentrically nested inside of the other, and has its 11 cylindrical axis coaxial with the arm's rotational axis, 12 with a first end of the outer cylinder being fixedly 13 connected to a first end of the inner cylinder such that 14 when one of the respective second ends is attached to the 15 arm and the other of the respective second ends is 16 attached to the outer frame, the torque generated by the 17 inner elastomeric cylinder and the torque generated by the 18 outer elastomeric cylinder are combined. Since each of 19 the two nested cylinders need accommodate only a portion 20 of the rotation of the arm relative to the outer frame, 21 the corresponding length of each of the cylinders will be 22 shorter for a given maximum shear angle within the 23 elastomer, thereby permitting a more compact design to be 24 constructed for a given set of operational parameters. -25 "26 Preferably, a pair of such elastomeric torsional 27 members are provided, one on either side of the rotatable 28 arm and concentric with its rotational axis. 29 30 In accordance with other embodiments of the invention, 31 the elastomeric member may be configured either as an 32 elongated cylinder with one end of the cylinder attached 33 to the rotatable member and the other end of the cylinder 34 attached to the fixed member or as a disk having a hub 35 portion attached to one of the members and its rim 36 attached to the other of the members. OMPI 1 As a result of the design geometry, as an increase in 2 tension in the rope rotates the arm and elongates the S- 3 shaped bend, a correspondingly lesser leverage is 4 associated with the torque couple applied to the arm by 5 the tension in the rope. Since the elastomer provides a 6 counter-torque approximately proportional to the angular 7 displacement of the arm, the device will always maintain 8 at least a slight bend in the cable and thus be able to 9 absorb further momentary increases in the tension applied 10 to the cable. Furthermore, by appropriate initial 11 adjustment of the orientation of the rotatable arm under 12 zero torsion load conditions relative to the orientation 13 of the rope as it enters and exits the device, it is 14 possible to vary the operational characteristics of the 15 device and in particular the rate at which the effective 16 length of the rope changes with respect to a predetermined 17 increment of tension. 18 19 Brief Description of the Drawings 20 21 Figure 1 is a diagramatic representation of a rope • 22 tension damper device in accordance with the present 23 invention in use aboard a ship to absorp sudden changes in 24 tension in the rope used to attach the ship to a mooring .25 buoy; Figure 2 is a isometric view of the damper device of '26 Figure 1 together with a portion of the rope associated 27 therewith; Figure 3 is a side elevational view partially 28 in cross section of the damper device of Figure 2; Figures 29 4a and 4b are a pair of isometric diagrams of a cylinder 30 of elastomeric material showing the geometric relationship 31 between the shear and the torsion before and after the 32 cylinder has been subjected to a torsional shear; Figure 5 33 is a isometric view of a second embodiment of a rope 34 tension damper device constructed in accordance with the 35 present invention wherein the elastomeric torsion elements 36 are nested thereby resulting in a more compact H 1 unit; Figure 6 is a side elevational view partially in 2 cross section of the embodiment of Figure 5; Figure 7 is a 3 isometric view of a third embodiment of a rope tension 4 damper device; Figure 8 is a side elevational view 5 partially in cross section of the embodiment of Figure 7; 6 Figures 9 through 14 are geometric diagrams schematically 7 showing a rope tension damper device in accordance with 8 the present invention with the angle between the arm and 9 the external orientation of the rope being respectively 10 minus 30 degrees, 0 degrees, 15 degrees, 45 degrees, 90 11 degrees, and 120 degrees; and Figure 15 is a set of graphs 12 showing the relationship of the tension applied to the 13 rope as a function of the displacement when the initial 14 zero load orientation of the arm with respect to the rope 15 is respectively 45 degrees, 15 degrees, 0 degrees, and 16 minus 30 degrees. 17 18 Presently Preferred Embodiments for Carrying 19 Out the Invention 20 21 Reference should now be made to the drawings, and in 22 particular to Fig. 1 thereof, which it will be recalled is 23 a diagrammatic representation of a device in accordance 24 with the present invention used to absorb changes in -25 tension in a ship's mooring rope. It will be seen that " 26 the ship (10) is secured to a mooring buoy (20) by means 27- of a mooring rope (30). Typically, the mooring rope (30) 28 will be a twisted cable of steel strands having an overall 29 diameter of approximately 5 centimeters and will be 30 capable of withstanding a considerable tensile force but 31 may also be any other suitable type of rope or a chain. 32 The cable (30) thus will normally be more than adequate to 33 secure a relatively large vessel (for example, a 100 ton 34 oil tanker) relative to its mooring (20). However, in 35 high seas, the action of the waves will cause the vessel 36 (10) to pitch and roll, which in turn will result in 1 relative movement between the vessel (10) and its mooring 2 (20). Accordingly, as the vessel (10) moves towards its 3 mooring, there will be a tendency for slack to develop in 4 the mooring rope (30). Conversely, as the vessel (10) 5 moves away from the mooring (20), any slack in the cable 6 (30) will be taken up and unless appropriate means is 7 provided to prevent the buildup of excess tension, the 8 cable (30) will suddenly have to withstand a sudden 9 potentially large force impulse as the momentum of the 10 vessel (10) is transferred to its mooring (20) through the 11 now taut rope (30). To some extent, the effects of such 12 an increase in the tension " to which the rope (30) is 13 subjected may be avoided by means of prior art spring- 14 loaded shock absorber devices such as have been described 15 above; furthermore, the winch (40) may be designed and 16 operated so as to offer only a limited resistance to 17 increases in the tension within the cable (30) before it 18 automatically allows the cable to unwind. However, it 19 should be appreciated that such expedients are operable 20 only within a limited range of operational parameters and 21 there still remains a need for an appropriate device that 22 will reliably dampen and absorb the effects of increases 23 in the rope's tension. before the tension forces have built 24 up to the point where they may equal or exceed the tensile 25 strength of the rope or of its termination. , 26 27 As shown in the Figure, the vessel's mooring is a buoy 28 (20) which also is subject to the action of the waves. 29 However, the buoy (20) will normally be anchored to the 30 seabed by means of a system of anchors and/or pipes (not 31 shown in the Figure) maintained in tension by the buoyant 32 action of the buoy (20). Although it is possible to 33 minimize the effects of wave action on the buoy (20) by it 34 such that the greater part of its volume is sufficiently 35 deep to be reasonably free from severe wave action, it 36 nonetheless will be appreciated that the buoy (20) also ϋ> λ P I will experience some motion that will be out of phase with the pitching and rolling of the vessel (10) and thus will further add to the relative motion between the vessel (10) and its mooring (20). A first end of the cable (30) is wound about a winch (40) on the deck of the vessel (10), while the other end of the cable (30) is attached to the buoy (20) by means of a suitable cable attachment (50); the winch (40) thus functions to secure — albeit in an adjustable fashion — the cable (30) relative to the vessel (10). A novel tension device (60) is installed on the vessel (10) such that it is in an intermediate position between the winch (40) and a fairlead (70) which may be provided at the gunnel of the vessel to control the orientation of the exiting portion (80) of the cable (30) relative to the tension device (60). The portion (90) of the cable (30) entering the tension device (60) will normally be at a proper orientation relative both to the winch (40) and to the tension device (60) by virtue of the fact that the cable (30) extends directly from the winch (40) to the tension device (60) and both are commonly mounted on the deck of the ship (10). Furthermore, the entering portion (90) of the cable (30) and the exiting portion (80) are oriented more or less parallel to one another but spaced apart such that as tension is applied to the cable (30) a torque couple will be applied to the tension device (60) , the magnitude of which being determined not only by the tension in the rope (30) but also by the vertical displacement of the entering portion (90) relative to the exiting portion (80) . At this point, it should be noted that although particularly adaptable for use for securing an oil tanker to a buoy provided at the upper end of a riser leading from an undersea oil well, thereby locating the vessel 1 relative to the mooring buoy while oil is pumped from the 2 well to the tanker by means of a flexible oil line (not 3 shown in the Figure), the tension device (60) may also 4 advantageously be used when the vessel is to be attached 5 to fixed moorings such as provided by a pier or a fixed 6 platform mounted on pilings penetrating the ocean floor 7 (for example, in shallower regions such as may be found 8 along the continental shelf) , as well as when a first 9 vessel is to be tied to a second vessel (for example, a 10 tugboat to a barge, or a floating storage vessel moored in 11 the vicinity of an undersea well to a service tanker while 12 the service tanker is off-loading oil from the storage 13 vessel) . 14 15 Reference should now be made to Figs. 2 and 3 which 16 are respectively isometric and side-elevational views of 17 the damper (60) shown in Fig. 1. The rope (30) is guided 18 by an upper sheave (102) from its generally horizontal 19 orientation extending between the winch (40) and the 20 tension device (60) over the front portion (104) of the 21 upper sheave (102), to the rear portion (106) of a lower 22 sheave (108), then finally to the lower horizontal portion 23 of the rope (30) as it extends from the tension device 24 (60) towards the fairlead (70) (Fig. 1). .25 • 26 The upper sheave (102) and the lower sheave (108) are 27 rotatably mounted at the respective upper and lower ends 28 of a rotatable arm assembly (110). As shown in the 29 Figures, the longitudinal orientation of the arm assembly 30 (110) when at rest (zero tension in the cable) is more or 31 less vertical and thus more or less perpendicular to the 32 horizontal orientation of the respective incoming and 33 exiting portions (90, 80) of the cable (30). 34 35 in the embodiment illustrated in Figs. 2 and 3, the 36 arm assembly (110) comprises a pair of spaced plates, 1 namely, a left plate (112) and a right plate (114), 2 connected to each other at their respective upper ends by 3 means of an upper sheave pin (116) and at their respective 4 lower ends by means of a lower sheave pin (118). The 5 upper sheave (102) includes a hub bushing (120) to permit 6 the upper sheave (102) to rotate coaxially about the upper 7 pin (116), while the lower sheave (108) includes a similar 8 hub bushing (122) so as to permit it to be similarly 9 rotatably mounted about the lower pin (118). Each of the 10 two sheave pins (116, 118) is fixedly secured to only one 11 of the two side plates (114) comprising the arm assembly 12 (110); its free end slides in and out of a bushing 13 provided as part of the opposite plate (112). By this 14 means, a certain amount of variation in the spacing 15 between the left plate (112) and the right plate (114) may 16 be accommodated. 17 18 The left arm plate (112) is connected to a left outer 19 frame member (124) by means of a left elastomeric 20 torsional spring (126). A right elastomeric torsional 21 spring (128) similarly connects the right plate (114) to a 22 right outer frame member (130). Referring specifically to 23 Fig. 3, it may be seen that the left torsion spring (126) 24 and the right torsion spring (128) are substantially .25 identical to one another, each being in the shape of a •26 cylinder and each being concentric about a common axis 27 (132) which intersects the arm assembly (110) at 28 approximately ninety degrees (%τr radians) . 29 30 Referring now to th'e left elastomeric torsion spring 31 (126) shown in cross section in Fig. 3, it will be seen 32 that the torsion spring assembly comprises a relatively 33 thick outer end plate (134), an intermediate portion (136) 34 and an inner end plate (138) somewhat thinner than the 35 outer end plate (134). The intermediate spring portion 36 (136) is molded of an appropriate elastomeric material * such as neoprene or other rubber-like material resistant to the effects of sunlight, salt water and other related adverse environmental conditions such as may be encountered in a maritime environment. The intermediate portion (136) also comprises a plurality of rigid reinforcing plates (140) , each of which is thin, relatively flat and oriented generally perpendicular to the axis (132) about which the arm assembly (110) rotates. The elastomer portion (142) of the elastomeric springs (126, 128) is bonded to the reinforcing disks (140) and to the two end plates (134, 138) such that when the inner end plate (138) is rotated about the axis (132) and the outer end plate (134) is held stationary, a torsional shearing action is imparted to the elastomer material (142) which in turn generates a counter torque about the axis (132) in an amount that is substantially proportional to the magnitude of the rotation of the inner end (138) with respect to the outer end (134) . The function of the reinforcing plates (140) is to impart anisotropic characteristics to the elastomeric spring and in particular, to stiffen the elastomer against unwanted flexing without reducing the torsion spring's ability to accommodate the winding and unwinding of one end relative to the other about the central axis (132) of the cylinder (136) (see also the discussion of Fig. 4 hereinafter) . This is accomplished by orienting the reinforcing disk (140) parallel to the direction of the intended shear. It will also be appreciated that the reinforcing disk (140) has a further effect of minimizing the tendency of the elastomeric assembly to be subjected to changes in length or diameter in response to simple external compressive and tensile forces. Although it will be appreciated that when a torque couple is applied to the reinforced elastomeric assembly (136) to produce a shear in a direction perpendicular to the central axis (132), OM 1 the individual reinforcing plate will have no appreciable 2 effect thereon. 3 4 in the embodiment illustrated in Figs. 2 and 3, each 5 of the outer ends (134) is secured in position with 6 respect to its corresponding outer frame member (124, 130) 7 by means of a plurality of fasteners (144) . The fasteners 8 (144) engage a corresponding plurality of bores (146) in 9 the outer end plate (134). Since the fasteners (144) and 0 the respective bores (146) are spaced at regular intervals 1 (for example, 15 ) about a circumference of a circle, it 2 will be appreciated that it will be possible to adjust the 3 initial angular orientation of the arm assembly (110) and 4 the two sheaves (102, 108) with respect to the orientation 5 of the rope as it exits (80) and enters (90) the tension 6 device (60) by selecting which particular pin (144) is to 7 be inserted into which particular bore (146) . The minimum 8 increment of such an adjustment will be determined by the 9 angular spacing between adjacent fasteners (144); however, 0 if a finer adjustment is required, it will be evident to a 1 skilled artisan that such a need may be accommodated by 2 providing an additional adjustment means are provided to -3 reduce the amount of a single increment. 4 5 A plurality of bolts (148) are provided to clamp the 6 respective left and right plates (112, 114) forming the 7 arm assembly (110) to the respective inner end plates 8 (138) of the respective left and right elastomeric torsion 9 spring assemblies (126, 128). In the embodiment 0 illustrated, the respective surfaces of inner end plates 1 (138) facing the arm assembly (110) are provided with a 2 suitable groove into which the respective arm plate (112, 3 114) is partially countersunk, thereby preventing any 4 rotational slippage between the end plate (138) and its 5 respective arm plate (112, 114). Accordingly, the 6 individual bolts (148) which secure the two end plates 1 (138) to their respective arm plates (112, 114) are not 2 exposed to excessive side shear as a result of the torque 3 transmitted from the elastomeric springs (126, 128) to the 4 arm assembly (110) and vice versa. In the embodiment 5 illustrated there is further provided the capability of 6 adjusting the height of the arm assembly (110) relative to 7 the elastomeric springs (126, 128) and the outer frames 8 (124, 130) merely by loosening the arm securing bolts 9 (148) and sliding the arm (110) along the aforementioned 10 grooves in the inner end plate (138) to the desired 11 position, whereupon the bolts (148) may be again 12 tightened. In that regard, it will normally be preferable 13 to center the arm assembly (110) with respect to the 14 central axis (132) of the cylindrical torsion element 15 (136) and to orient the winch (40) and the fairlead (70) 16 (see Fig. 1) relative to the entering and exiting portions 17 (90, 80) of the cable (30) in symmetric fashion such that 18 only a pure torque couple is applied to the elastomeric 19 springs (126, 128) and they are not subjected to excessive 20 translational forces. 21 22 The torque generated by an elastomeric torsion element 23 may be expressed by the equation 24 M = a G I / £r -25 where a equals the angular displacement of one end plate "26 relative to the other in radians; G equals the shear 27 modulus of the elastomeric material in kilograms per 28 square meter (Kg/m); I is the polar moment of inertia in IT 29 quadratic meters (m.) and L is the length of the elastomer 30 in its axial direction expressed in meters (m) . (For a 31 solid cylinder, the polar moment of inertia I will be 32 equal to πR , where R is the radius of the cylinder in 33 meters) . 34 35 Since one degree equals 0.01745 radian and a typical 36 shear modulus is 225 psi (about 1.6 x 10 Kg/m ) , i t 1 therefore may be calculated that a pair of elastomeric 2 cylinders such as have been shown in Figs. 2 and 3 having 3 a radius of 24 inches (approximately 0.61 meters) and a 4 length of 46 inches (approximately 1.17 meters) will 5 produce a combined counter-torque of approximately 520 6 meter-kilograms per degree of rotation. 7 8 Referring now to Fig. 4, which it will be recalled is 9 intended to show the geometric relationship between the 10 shear and the torsion, it will be seen that a square 11 element (150) may be defined on the outer circumference of 12 an elastomeric cylinder prior to the application of any 13 torsional shear (152A) such that the square will lie 14 against a straight line A-B connecting points A and B 15 respectively on the outer and inner ends (154, 156) of the 16 cylinder (152) and parallel to the axis (158) of the 17 cylinder. However, when one end (154) is rotated with 18 respect to the other end (156) by an angle (cylinder 19 152B) such that the line connecting point A with point B 20 is no longer a straight line parallel to the axis (158) 21 but rather is a helical spiral (160), then the square 22 element (150) will be distorted (sheared by the angle a ) 23 into a rhombus (162) whose respective vertices will be ( n - ) and (Jjπ + a ). For a typical elastomer material, "25 the maximum shear angle through which it may be ' 26 subjected without any loss of its elastic properties may 27 be conservatively limited to be 45 degrees ( τr radians) . 28 Using the above-stated typical values (L equals 1.17 29 meters, R equals 0.61 meters) , it may be calculated that 30 the corresponding maximum rotation β of the one end plate 31 (134) with respect to the other (138) will be 32 approximately 1.9 radians (109 degrees). 33 34 Reference should now be made to Figs. 5 and 6 of the 35 drawings, which it will be recalled illustrate another 36 embodiment of the invention wherein a more compact unit than that shown in Fig. 1 is provided as a result of the elastomeric torsion elements being nested. From the above discussion of Fig. 4, it will be recalled that conservative design practice makes it desirable that no portion of the elastomer be subject to a shear strain greater than 45 degrees. Accordingly, in the embodiment of Figs. 2 and 3, if the arm assembly (110) is to be provided with the capability of deviating 109 degrees in either direction from its rest position, it is necessary for the diameter of the cylinder to be approximately equal to its length. Furthermore, from the above-stated formula for the torque generated by an elastomeric element as it is twisted about an axis, it will be noted that the torque is a function of the polar moment of inertia about that axis. Accordingly, the interior of the cylinder makes a relatively small contribution to the total torque. Taking both the above factors into account, it may be readily appreciated that by nesting two hollow elastomeric cylinders, one within the other, with a first end of the outer cylinder being fixedly connected to a first end of the inner cylinder, the torque generated by the inner elastomeric cylinder and the torque generated by the outer elastomeric cylinder will be combined such a way that only a portion of the rotation of the arm assembly (110) relative to the outer frame need be accommodated by each of the two nested cylinders. The embodiment shown in Figs. 5 and 6 take advantage of these considerations. In these Figures, the various reference numbers previously utilized with respect to the embodiment of Figs. 1 and 2 have been retained but the prime symbol ( ' ) has been appended to distinguish otherwise similar elements of the two embodiments. (A similar convention will be applied to the embodiment of Figs. 7 and 8 using the double prime symbol ( " ) ) . ~ ' Referring now specifically to Figs. 5 and 6, with Fig. 5 being an isometric view (partially broken away to better show the arm assembly (110)) and Fig. 6 being a side elevational view partially in cross section as indicated by the line 6-6 in Fig. 5, it will be seen that this embodiment is much more compact in the direction of the longitudinal axis (132* ), although for a given set of operational parameters, its -overall height is determined by the vertical displacement between entering cable portion (90) and exiting cable portion (80) and thus will be substantially identical to the corresponding vertical dimensions of the previously described embodiment. The function of the solid cylindrical elastomeric torsional element (126) (Figs. 2 and 3) is now performed by a nested pair of hollow torsional elements, namely, an inner hollow cylindrical element (170) and an outer hollow cylindrical element (172). An outer tubular shell (174) rigidly mounted to a hollow base (176) functionally corresponds to- the left outer frame (124) of the previous embodiment. From the cross sectional view of Fig. 6, it can be seen that an inwardly protruding annular rim (178) is fastened to an inboard annular end plate (180) of the outer elastomeric element (172) by means of an appropriate mechanical fastener (182). Although symbolized in the drawings by a bolt (182), it should be appreciated that other fastening means such as welding and/or riveting may also be employed to provide the required relatively rigid connection between the outer shell (174) and the inboard end plate (180). The outboard end of outer elastomeric element (172) is terminated by an outer annular end plate (184) which is rigidly fastened by a plurality of bolts (or other appropriate fastening means) (186) to the outboard annular end plate (188) of the inner elastomeric element (170). The inboard end of the inner elastomeric element (170) is terminated by an inboard end plate (138 » ) to which is anchored the left arm (112 1 ) of the rotatable arm assembly (110 1 ). From the Figure it may also be seen that the radial thickness of the inner hollow cylinder (170) is somewhat greater than that of the outer hollow cylinder (172). As noted previously, the torque • produced by such an elastomeric element for a given torsional twisting about its longitudinal axis is a linear function of the polar moment of inertia and in fact increases as the fourth power of the radius. Since the outer torsional element (172) should share the torque and rotational movement of the arm assembly (110) in an equal fashion with the inner torsional element (170), it accordingly should have about the same stiffness as, and thus should be somewhat thinner than, the inner. element. Both the inner and outer elastomeric elements (170, 172) employ the same reinforced integrally molded structure as that of the elastomeric element (136) shown in Fig. 3 and in particular are preferably molded of a rubber-like material capable of withstanding the adverse effects of the maritime environment (such as neoprene) , the rubber being reinforced by a plurality of plates of a suitable reinforcing material such as stainless steel, aluminum, or Kevlar embedded in the elastomer during the molding process and oriented perpendicular to the device's main axis of rotation (132'). However, the individual reinforcing elements are not disk-shaped but rather are in the respective shapes of outer annuluses (190) and inner annuluses (192). (In that regard, it may be observed that the embodiment of Fig. 2 and Fig. 3 may optionally be modified by the substitution of a annulus-shaped reinforcing element for the disk-shaped element (140) shown in the Figure, since the reinforcing function of the disk (140) is required more towards the periphery of the cylinder than in its middle) . 1 It will be appreciated that the right-hand elastomeric 2 spring of the second embodiment (60*) inside the right- 3 hand tubular shell (190) and supported by the right-hand 4 base (192) is the mirror image of the corresponding 5 structure discussed previously with respect to the left- 6 hand portion of the device. In order to provide 7 additional rigidity, an upper connection link (194) may be 8 advantageously provided to connect the corresponding upper 9 portions of the left-hand tubular shell (174) and the 10 right-hand tubular shell (190). Such a linking member 11 (194) will also provide a means whereby the entire device 12 (60') may be conveniently hoisted and will serve as a 13 guide to maintain the cable entering portion (90) in its 14 proper relationship with respect to upper sheave (102') 15 when the arm assembly (110*) is in its at-rest position 16 and there will accordingly be the possibility of slack in 17 the cable (30). 18 19 Reference should now be made to Figures 7 and 8, which 20 it will be recalled illustrate yet another possible 21 embodiment of the present invention that is substantially 22 different from the two previously described embodiments 23 that utilize the elastomeric damping effect of a 24 cylindrical body in which the torsional shearing is 25 distributed along the axial (longitudinal) direction of •26- the torsional element, with one end of the element being 27 secured to a rotatable arm and the other end of the 28 torsional element being attached to a fixed frame. By way 29 of contrast, the embodiment shown in Figs. 7 and 8 30 utilizes a different configuration of an elastomeric 31 torsional element in the shape of a tapered disk in which 32 the shearing strain is distributed radially from a hub 33 portion adjacent the axis of the disk to a circumferential 34 portion at its outer periphery, with a substantial 35 tapering in the disk's thickness from its hub to its 36' periphery whereby the shearing action will be essentially 1 uniform throughout the elastomer. The former type of 2 shearing may be thought of as "longitudinal torsional 3 shear" while the latter may be termed "radiating torsional 4 shear." 5 6 As in the previously described embodiment, an inner 7 arm assembly (110") is provided with an upper sheave 8 (102") and a lower sheave (108"), with each of the two 9 sheaves respectively pivoted at opposite ends of the arm 10 assembly by means of an upper pivot pin (116") and a lower 11 pivot pin (118") such that the two sheaves (102", 108") 12 will be free to rotate about their respective axes and 13 parallel to a central axis (132") intersecting the arm 14 assembly (110") in the region between the two sheaves. 15 16 A left tapered disk elastomeric element (200) is 17 ' attached to the left plate (112") of the rotatable arm 18 (110") such that the hub portion (202) of the elastomeric 19 element (200) will rotate about the device's principal 20 axis (132") in accordance with the rotation of the arm 21 assembly (110"). The periphery portion (204) of the 22 elastomeric unit (200) is clamped by means of an outer 23 circumferential strap (208) secured by suitable bolts or 24 other fastening means (210) to the corresponding left side 25 frame (124"). In a similar fashion, a substantially •26 identical tapered disk elastomeric element (206) is 27 provided at the right hand of the device with its hub 28 portion fixed relative to the arm assembly's right-hand 29 plate (114") and with its peripheral portion secured to 30 the right hand frame member (130") by means of a second 31 mounting strap (208). Referring in particular to the 32 cross section view of the left side elastomeric element 33 (200) visible at the left hand of Fig. 8, it will be noted 34 that each of the elastomeric elements (200, 206) is 35 provided with a plurality of tubular reinforcing elements 36 (212), each concentric about the device's axis (132"). 1 The operational characteristics of a rope tension -2 device constructed in accordance with the present 3 invention will now be discussed with particular reference 4 to the geometric diagrams of Figures 9 through 14 and the 5 graph of Figure 15. 6 7 Figures 9 through 14 it will be recalled are intended 8 to schematically show a typical rope tensioner device in 9 accordance with the present invention (60, 60', or 60") 10 with the angle between the arm (110, 110* 110") and the 11 external orientation of the rope (80, 90) being rotated to 12 respectively minus 30 degrees (Fig. 9), 0 degrees (Fig. 13 10), 15 degrees (Fig. 11), 45 degrees (Fig. 12), 90 14 degrees (Fig. 13) and 120 degrees (Fig. 14). 15 16 Referring now specifically to Fig. 9, it will be seen 17 that the Figure represents symbolically the upper sheave 18 (102), the lower sheave (108), the arm (110) and a portion 19 of the rope (30) (including an upper portion (90) where it 20 enters the device from the winch and a lower exiting 21 portion (80) where it exits the device on its way to the 22 fairlead (70)). Both the upper sheave (102) and the lower 23 sheave (108) have the same respective radius "R" and are 24 mounted apart from one another along the rotatable arm .25 (110) at a fixed distance "A". Since entering portion ■ 26 (90) and exiting portion (80) of the cable (30) will be 27 oriented more or less horizontally during the intended use 28 of the device (see Fig. 1), it may be seen that the 29 tension will apply a torque couple to the tension device 30 whose magnitude is equal to the tension "T" times the 31 vertical displacement "V" between the incoming and exiting 32 portions of the cable. Figures 9 through 14 correspond to 33 the rope tension device of the present invention in use 34 with the arm (110) at various angles of inclination with 35 respect to the vertical; however, in the general case, the 36 tension force T is not necessarily applied to the device_ 1 in the horizontal direction and the angle of interest will 2 then be measured with respect to a line perpendicular to 3 the orientation of the incoming rope portion (90) and the 4 exiting rope portion (80) (or in the event that these 5 portions are not parallel, then a line perpendicular to 6 the bisector of the angle therebetween) . 7 8 Referring now specifically to Fig. 9, which is 9 intended to show the geometrical characteristics of the 10 device when the arm is inclined backwards in an amount 30 11 degrees from the vertical, it will be seen that the 12 vertical distance V is somewhat less than that shown in 13 Fig. 10 when the arm (110) is oriented vertically. A 14 simple geometric calculation will show that the vertical 15 distance V in Fig. 10 is equal to 2R -4- A while in Fig. 9 16 it is equal to 2 R + A cos (30°) . Furthermore, the 17 effective length of the cable (30) will increase by an 18 amount S = S, + S 2 = 2 R Q + A sin (β) between the minus θ 19 radian position shown in Fig. 9 and the 0 position shown 20 in Fig. 10 (or between 0 and plus θ radian). 21 22 Referring now to Fig. 11, which shows the arm (110) 23 now oriented at an angle of " plus 15 degrees with respect 24 to the vertical, it will be seen that the vertical .25 displacement V is now equal to 2R - A cos (15°) while the • 26 effective length of the cable (30) will have been further 27 increased by an amount (1/6)TJR. 28 29 The corresponding calculations for Figures 12 and 13, 30 which show the arm (110) further rotated as a result of 31 successive increases in the tension T will show that the 32 corresponding vertical distance V will be respectively 33 A cos (45 ) + 2R and 2R while the corresponding elongation 34 S will be respectively A sin (45°) + τR and A +ττ R. 35 36 1 Referring now to Figure 14, it will be seen that the 2 tension of the cable (30) has been increased to the point 3 where the angle is increased to 120 degrees, a value which 4 will normally be in excess of the normal operating range 5 of the elastomeric torsion element which it will 6 be recalled (at least with respect to the numerical 7 examples given with respect to the embodiment of Figs. 2 8 and 3) is designed with a normal maximum rotational 9 deflection of 109 degrees. In any event, the vertical 10 displacement V will be reduced to an amount equal to 11 2R - A sin (30°) and the elongation S will be (4/3) tr R. 12 13 Reference should now be made to Fig. 15, which is a 14 graph showing the operational characteristics of a rope 15 tension damper device utilizing an elastomeric torsional 16 element subjected to longitudinal torsional shear with the 17 four curves corresponding respectively to the operational 18 characteristics of the device (60) when the arm (110) has 19 been adjusted to an at-rest condition (that is to say, 20 zero tension T in the cable (30)) with an initial 21 inclination of respectively minus 30 degrees (lower curve 22 (300)), 0 degrees (curve 302)), plus 15 degrees (curve 23 (304)) and plus 45 degrees (curve (306)). These 24 correspond respectively to the geometric configuration •25 indicated diagrammatically in Figs. 9, 10, 11 and 12. It • 26 will be seen accordingly the first curve (300) is almost 27 linear and provides a displacement S of approximately 4 28 meters when a tension of approximately 50 metric tonnes is 29 applied to the cable (30) (or conversely a tension T of 30 approximately 50 tonnes will result in a displacement S of 31 approximately 4 meters) . The 0 degree curve (302) , by way 32 of contrast, starts to climb steeply when the displacement 33 S is in excess of 3 meters and/or the tension T is in 34 excess of 40 tonnes. The 15 degree curve (304) starts to 35 climb steeply at an even lesser displacement S and/or 36 tension T. The 45 degree curve (306) is even more extreme. The horizontal dotted line (308) corresponding to approximately 65 metric tonnes and the vertical dotted line (310) corresponding to slightly more than 3.0 meters indicate a typical set of design parameters. From the various graphs it may be seen that an initial orientation of arm (110) at an angle intermediate 0 degrees and plus 15 degrees will result in a smooth transition offering a noticeable increase in resistance as the design parameters are reached but will permit some additional displacement if the operational parameters are limited without resulting in an excessively high rate of increase in the tension T. Although three different embodiments of a novel rope tension damper device have been described in detail, it should be apparent to the skilled artisan in light of the foregoing description that other variations and embodiments may be constructed that do not depart from the various inventive concepts which underlie the present invention. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims. OMPI

Claims: 1 Claims 2 3 1. A tension damper device for a wire rope or other 4 relatively inelastic flexible line comprising: 5 a frame; 6 a rotatable arm having a first rotational axis; 7 an elastomeric torsion spring oriented about said 8 first rotational axis of said arm for connecting 9 arm to said frame; 10 a first sheave rotatably mounted with respect to said 11 arm having a second rotational axis parallel to 12 said first rotational axis of said arm and 13 intersecting said arm at a first location; and 14 a second sheave rotatably mounted to said arm at a 15 second location spaced apart from said first 16 location and having a third rotational axis 17 parallel to said first rotational axis of said 18 arm; 19 whereby said rope as it passes through said device may 20 be guided by said first sheave and by second sheave 21 into a generally S-shaped configuration which will be 22 gradually distorted into a relatively straight 23 configuration as the tension applied to said rope is 24 increased and said arm is thereby caused to rotate 25 about said first rotational axis. *•*-*» 26 27 2. The tension damper device of Claim 1 wherein said 28 elastomeric torsion spring comprises a cylindrical 29 body in which the torsional shearing is distributed in 30 the longitudinal direction of the torsional element to 31 effect a longitudinal torsional shearing action, said 32 cylinder having a first end adapted to rotate with 33 said arm and a second end secured against rotation 34 with respect to said frame. 35 36 OMPI 3. The tension damper device of Claim 2 further comprising a second frame and a second elastomeric torsion spring, said second elastomeric torsion spring being substantially identical to the first said elastomeric torsion spring in operational characteristics, one end of said second elastomeric torsion spring being adapted to rotate with said arm and the other end of said second elastomeric spring being secured against rotation with respect to said second frame, said second frame being spaced apart from said first frame and separated therefrom by said arm. 4. The tension damper device of Claim 1 wherein said elastomeric torsion spring comprises at least a pair of cylindrical elastomeric torsional elements, at least one of said elements being hollow and adapted to accommodate a second of said elements in concentric nesting fashion, a first end of said hollow cylindrical element being rigidly secured to a selected one of said frame and said arm, a second end of said hollow cylindrical element being rigidly secured to a first end of said second cylindrical element and a second end of said second cylindrical element being connected to said arm. 5. The tension damper device of Claim 4 wherein said connection between said second end of said cylindrical element and said arm is a rigid connection. 6. The tension damper device of Claim 5 wherein said connection between said second end of said second cylindrical element and said arm is a flexible connection comprising at least one additional elastomeric element. OMPI 7. The tension damper device of Claim 6 wherein a second such nested arrangement is provided for securing said arm to a second frame spaced apart from said first frame and separated therefrom by said arm. 8. The tension damper device of Claim 1 wherein said elastomeric torsion spring has a hub portion and a periphery portion such that as said hub portion is rotated relative to said periphery portion the elastomer within said element is subjected to a radiating torsional shearing action essentially uniform throughout the elastomer, one of said hub and said periphery portions being connected to said arm and the other of said hub and said periphery portions being connected to said frame. 9. The tension damper device of Claim 1 wherein said elastomeric torsion spring is reinforced by means of a plurality of rigid annular disks. 10. The tension damper device of Claim 1 wherein said elastomeric torsion spring is reinforced by means of a plurality of rigid tubes.

Inventors: PEPPEL GEORGE WALTER, SULLIVAN PAUL EUGENE, PEPPEL, GEORGE, WALTER, SULLIVAN, PAUL, EUGENE
Applicants: MURDOCK MACHINE & ENG, MURDOCK MACHINE AND ENGINEERING COMPANY
Application year: 1983
Main classification: B65H77
IPC codes: B63B21, B66D1, F16F1