Abstract:
A hydraulically-actuated controller is disclosed for controlling delivery of hydraulic fluid from a variable-displacement, pressure-compensated pump to a hydraulically-powered reel on which is wound a cable or other flexible member extending from the reel to a tie point. Rotation of the reel is controlled appropriately to maintain the cable tautly between the reel and tie point as the distance therebetween changes. One use is on an electrically-powered mining vehicle, the power cable of which extends from a fixed power source to the vehicle-mounted reel. The controller is in a pay-in-mode whenever the distance between the reel and tie point is decreasing or remaining constant, and in a pay-out mode whenever said distance is increasing. During pay-in, pressurized hydraulic fluid is directed from the pump to the reel motor. During pay-out, pump output is occluded and fluid is directed through a restriction from the reel motor to the reservoir, the restriction serving to brake motor rotation. Two of four valves in the controller are hydraulically piloted in each position and biased in the pay-in position. A separate check valve ensures that pressure at the pilots is about equal during pay-in but unequal, favoring the pay-out position, when the reel is paying out cable.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to hydraulically-actuated controllers for delivering hydraulic fluid from a pressurized source to and from a hydraulic motor operable in both rotational directions. In particular, the hydraulic controller controls pay-in and pay-out actions of a cable or hose reel driven by a hydraulic motor in applications where the cable or hose extends from the reel to a remote terminus and the length of the unwound portion is variable and must be maintained in a taut condition between the reel and the terminus. A typical such application is in electrically-driven underground mining vehicles powered via an electrical cable from a remote power supply. 
     BACKGROUND OF THE INVENTION 
     In a number of industrial operations, it is necessary to have a cable, hose, or other flexible member (hereinafter collectively referred to as a &#34;cable&#34;) extending from a fixed terminus to a moving body, where the cable so extending must be taken up and maintained in a taut condition between the moving body and the fixed terminus. 
     One operation where this requirement is well-known is underground mining, where electrically-operated trackless vehicles and mining machines are becoming increasingly common. Such vehicles are typically connected to a main electrical supply via a cable extending from the vehicle to one of a number of regularly-spaced plug-in power sources, termed tie points. In order to prevent entangling the cable beneath the wheels of the vehicle or on obstacles while the vehicle is moving about, it is necessary to keep excess cable coiled on a reel or analogous device from which cable can be either payed out or payed in as the vehicle moves away from or toward the tie point, respectively. Usually, a number of tie points regularly spaced apart are available in the mine, thereby obviating the need for extremely long cables. Moving the vehicle long distances in the mine requires that the cable be unplugged from a more distant tie point and reconnected to a nearer tie point. Usually, mining machines are employed for periods of time in localized areas in the mine near a single tie point, where the tie point is located somewhere near the expected midpoint of vehicular travel at that location. Because there are many such power sources in a mine, economic considerations dictate that the take-up reel for the cable be mounted on the mining vehicle, not at each tie point. However, in other industrial applications it may be preferable to have the reel secured to a fixed location and the cable extending therefrom to a moving body or vehicle. 
     Whether the reel is located on the vehicle or is stationary, the mechanism that drives the reel must satisfy certain requirements. First, excess length of the cable must be kept wound upon the reel, preferably in an ordered lapped fashion to prevent kinking and damage to the cable. Second, the cable extending from the vehicle to the tie point must be maintained in a taut condition at all times. In other words, the reel must be driven to tautly pay in cable whenever the vehicle is moving toward the tie point, to keep the cable taut between the vehicle and tie point whenever the vehicle is remaining stationary, and to tautly pay out cable whenever the vehicle is moving away from the tie point. Third, all such motions of the reel should occur automatically. Fourth, the reel-like mechanism should be powerful to accommodate a heavy cable on a large, heavy reel. For example, an electrical cable for a modern mining vehicle can be three inches or more in diameter and weigh several tons when five hundred feet of it is fully coiled. A reel for such a cable can be up to five feet in diameter and five feet wide. Fifth, as experience has shown that hydraulically-driven reel mechanisms offer the best combination of power, quickness of response, and reliability, such hydraulic mechanisms must be capable of performing expected functions without excess heat generation. Such excess heat is a common problem with mobile hydraulic mining machinery constructed within difficult size and space constraints. Excess heat can cause premature failure of the hydraulic fluid (usually oil) and the hydraulic equipment. Improperly designed hydraulic equipment intended for high-power applications can experience catastrophic heat generation in a remarkably short period of time. 
     Several cable reel mechanisms have heretofore been patented. Slomer (U.S. Pat. No. 2,665,081) discloses a hydraulic controller intended for use with a positive-displacement, constant-output hydraulic pump, a hydraulic motor driven by the pump, and a reel driven by the motor. The controller includes an adjustable relief valve responsive to a change in the direction of hydraulic flow to vent excessive hydraulic pressure from the pump downstream to a fluid reservoir whenever the reel is stalled (vehicle stationary) or paying out cable (vehicle moving away from the power source). The only time the pump is not pumping fluid through the relief valve is when the reel is paying in cable at top speed. As a result, the Slomer device has serious disadvantages. In particular, because the reel motor is driven with a fixed-displacement pump, hydraulic fluid from the pump when not being used to drive the hydraulic motor must be externally shunted back to the fluid reservoir. Otherwise, the pump will stall or cause rupture of hydraulic conduits due to excessive pressure buildup. Unfortunately, such external shunting of hydraulic fluid from the pump causes rapid heating of the oil, especially at higher operating pressures. As a result, the Slomer device is only operable up to approximately 300 psig hydraulic pressure without overheating, which is many times less than operating pressures required on many types of modern equipment. 
     Maier (U.S. Pat. No. 4,114,827) discloses a hydraulic controller for a cable reel powered by a hydraulic motor supplied by a fixed-displacement, continuous-output pump. The controller includes a piloted and spring-biased bypass valve for unloading the pump whenever the vehicle is stationary or whenever cable is being payed out. The bypass valve pilot senses hydraulic pressure downstream of the reel motor, such pressure increasing during cable pay-out and when the reel is stalled. The controller also includes a pressure relief valve to ensure that pressure downstream of the pump never exceeds a preset limit (450 psig). Similar to the Slomer device, however, the Maier controller is designed for use with a fixed-displacement, constant-output hydraulic pump. As a result, excess pressure downstream of the pump during reel stall or cable pay-out must be externally shunted to the reservoir. Such shunting causes excessive heating of the hydraulic fluid unless the hydraulic pressure is maintained below a limit (450 psig) too low for many modern applications. 
     Carlson (U.S. Pat. No. 3,334,839) discloses another hydraulic system utilizing three fixed-displacement pumps: one to act as a reel-motion sensor, and two others to supply hydraulic pressure to the reel motor. The reel-motor pressure is controlled by a remote controlled relief valve that, in turn, is controlled by a system of shuttle valves. This system is less prone to heating than the Slomer and Maier systems, but is still limited only to about 450 psig or below. Further, the system is relatively complex with three fixed-displacement pumps instead of one, which decreases reliability. Finally, the 450 psig pressure limit is too low for many modern requirements. 
     The Lee patent (U.S. Pat. No. 2,912,184) discloses a hydraulic system for an entire mine haulage vehicle including its reel drive system. Two hydraulic pumps are employed to drive the reel, the first powered by an electric motor and the second by the vehicle&#39;s motion. The motor-driven pump is used for conditions requiring a relatively low hydraulic fluid flow rate, and the vehicle-driven pump is used for high-flow conditions. A disadvantage of this system is that the pumps are fixed-displacement types, which require that the pressures be kept low to avoid excessive heating of the system. 
     Oetringhaus (U.S. Pat. No. 4,511,100) and Pollman et al. (U.S. Pat. No. 4,537,364) disclose different types of closed-loop hydrostatic transmissions for cable reel drive systems that are controlled by electro-hydraulic servomechanisms. Since the hydraulic pumps are variable-displacement and bidirectional, there is no need to pump hydraulic fluid over relief at any time. As a result, both systems are capable of operating at pressures higher than 450 psig. Unfortunately, however, the systems incorporate a relatively large number of both electrical and mechanical components which render the systems complicated, expensive and less reliable than simpler all-hydraulic systems. 
     Modern, efficient underground mining operations, especially hard-rock mining but also including large-scale soft-rock (e.g., coal) mining, require much larger and more powerful vehicles and other equipment than in the past. Correspondingly, hydraulic systems on such vehicles and equipment must be much larger and more powerful than in the past. Where prior-art systems as described above operated satisfactorily at their pressure limits of 300-450 psig, modern systems require pressures of 2,000-3,000 psig or higher for satisfactory performance, which levels are simply beyond the capability of many prior-art systems. Further, since underground mining equipment must be reliable and maintenance services in most mines are irregular at best, relatively simple all-hydraulic systems not requiring electro-mechanical or other sophisticated sensors and feedback circuits have proven to be superior in terms of reliability and serviceability. 
     Hence, there is a need for a hydraulically powered reel mechanism capable of operating at 2,000 psig hydraulic pressure or higher without experiencing destructive heat buildup. 
     There is also a need for such a system operable only via, and responsive entirely to, hydraulic pressure for minimal complexity and maximal reliability. 
     There is also a need for such a system sufficiently powerful to operate the large heavy reels and cables required with modern industrial machinery. 
     SUMMARY OF THE INVENTION 
     The present invention is a novel hydraulic system and hydraulically-actuated controller for a reel used to windably store, pay in and pay out a flexible cable extending from the reel to a tie point so as to keep the cable taut as the distance between the reel and the tie point changes or remains constant. The system and controller are especially suited for heavy, high-capacity power cable reels on underground mining vehicles where high hydraulic pressures are required. 
     The reel is powered by a hydraulic motor supplied by a source of pressurized hydraulic fluid. The controller is hydraulically coupled at a first port to the source of pressurized hydraulic fluid, at a second port to a hydraulic motor powering the reel, and at a third port to a hydraulic fluid reservoir. The controller assumes a &#34;pay-in&#34; mode whenever the distance between the reel and the tie point is either decreasing or remaining constant, and a &#34;pay-out&#34; mode whenever said distance is increasing. 
     The controller includes at least first, second, third, and fourth valves intercoupling the first, second, and third ports. The first and second valves are each two-position valves that are hydraulically piloted in both the &#34;pay-in&#34; and &#34;pay-out&#34; positions, and spring-biased in the &#34;pay-in&#34; position. The first and second valves permit flow of pressurized hydraulic fluid from its source to the motor during the pay-in mode so as to apply a hydraulic force to turn the reel in a pay-in direction. 
     Responding to a transient pressure spike that occurs when the cable begins to be payed out from the reel, the first and second valves, with the help of the fourth valve (which is a one-way check valve), shift to their respective &#34;pay-out&#34; positions. This shift to pay-out causes the flow of hydraulic fluid from its source to be fully occluded by the first valve and the hydraulic fluid from the motor to be routed through the second valve and a flow-restriction imparted by the third valve to the fluid reservoir. A significant pressure drop across the &#34;pay-out&#34; position of the second valve ensures that the first and second valves remain in their respective &#34;pay-out&#34; positions so long as the reel is paying out. The flow restriction imparted by the third valve applies sufficient back pressure to the motor to impart a dynamic hydraulic braking to it during pay-out. 
     The present invention is particularly suitable for use with a variable-displacement, pressure-compensated pump serving as the source of pressurized hydraulic fluid because such pumps are not adversely affected when the flow of hydraulic fluid downstream of them is fully occluded, as occurs by the first valve during pay-out. Because the present invention does not pressure relieve the pump during pay-out, it is capable of operating without overheating at the high hydraulic pressures required of such reel systems used in modern industrial applications such as hard-rock mining. 
     One object of the present invention is to provide a controller for use with hydraulically-powered reel mechanisms capable of operating at 2,000 psig hydraulic pressure or higher without experiencing destructive heat buildup. 
     Another object of the present invention is to provide such a controller for use with a pressure-compensated variable displacement pump. 
     Another object is to provide such a controller operable only via, and responsive entirely to, hydraulic pressure. 
     Another object is to provide such a controller that substantially instantaneously shifts from a pay-in mode to a pay-out mode and from a pay-out to a pay-in mode as required to ensure that the cable extending from the reel to a tie point is maintained in a taut condition at all times during operation. 
     Another object is to provide such a controller having a simplicity and reliability suitable for use on underground mining equipment and vehicles. 
     Another object is to provide a hydraulic system including such a controller, a variable-displacement pressure-compensated hydraulic pump, and a hydraulic motor drivably coupled to a cable reel, the system mounted to an electrically powered mining vehicle and operable at about 2,000 to 3,000 psi hydraulic pressure. 
     These and other objects and features of the invention will be apparent from the following description and accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a hydraulic system and controller according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Construction 
     Referring to FIG. 1, the hydraulic controller 10 of the present invention is shown hydraulically coupled to a variable-displacement, unidirectional, pressure-compensated pump 12 and a bidirectional hydraulic motor 14. Preferably, the hydraulic controller 10 is incorporated into a manifold block 16 having a first port 18 hydraulically coupled via conduit 20 to the outlet port 22 of the pump 12, a second port 24 hydraulically coupled via conduit 26 to the first port 28 of the motor 14, and a third port 30 hydraulically coupled via conduit 32 to the hydraulic fluid reservoir 34. Note that the reservoir 34 is also coupled to a pump inlet 36 via a conduit 38 with in-line filter 37 and to a second port 40 of the motor 14 via a conduit 42. 
     In a preferred embodiment, the hydraulic motor 14 is coupled directly to a smaller sprocket 44 linked by a roller chain drive 46 or analogous mechanical linkage to a larger sprocket 48 coupled directly to the reel 50. The reel 50 is used to wind a length of cable 52 extending to a fixed tie point (not shown). (As used herein, &#34;cable&#34; refers generally, unless otherwise specified, to either electrical cable or other suitable flexible member wound on the reel 50 and extending therefrom to a fixed terminus, or tie point.) The means of coupling the reel 50 to the motor 14 as herein described is preferable for applications such as mining vehicles because a smaller but faster hydraulic motor may be used to turn a reel of a given size and weight than would be required if the coupling between the motor 14 and reel 50 were direct-drive. Even though modern mining vehicles carry large reels (up to five feet in diameter) holding a heavy load of wound cable (up to 3,500 pounds or more), problems with large centrifugal forces generated by a rapidly rotating reel are avoided because such vehicles travel a maximum of about five miles per hour. At such low vehicular velocities, a large reel rotates slowly. Use of a small, fast motor with appropriate gear-reduction to drive the reel allows the use of a smaller pump. 
     In the case of mining vehicles, the entire mechanism diagrammed in FIG. 1 is typically mounted on the rear of the vehicle (not shown). In other applications, the mechanism of FIG. 1 may be mounted adjacent to the fixed terminus of the cable (not shown), with the cable 52 terminating distally on a moving vehicle or other moving body (not shown). 
     To form even laps 54 of cable across the width of the reel 50, it is desirable to employ a level-wind mechanism 56, especially on large reels accommodating long or thick cables 52. The level-wind mechanism 56 is similar in design and function to that utilized, for example, on certain types of fishing reels, only much larger. The level-wind mechanism is typically actuated via mechanical linkage to the reel 50 or reel motor 14 (linkage not shown). 
     The present invention is usable with any variable-displacement, pressure-compensated pump 12 having a suitable pumping rate and pressure rating for the particular combination of motor 14, reel 50, cable 52 and anticipated maximum rate of pay-in and pay-out. This type of pump does not require an external bypass loop for pressure relief upon restricting or occluding the flow of hydraulic fluid downstream thereof. As a result, they may be used at high pressure (2,000 psig or more) without any external pressure relief without danger of overheating or pressure runaway. The present invention cannot be used with a fixed-displacement pump. 
     If the reel-drive system is situated on an electrically powered mining vehicle, the pump 12 is typically driven by an electric motor (not shown). However, in other applications of the present invention, other types of pump drive motors may be used, such as an internal combustion engine. 
     The hydraulic motor 14 can be any suitable rotary actuator operable in a first direction in response to upstream hydraulic pressure. When caused to rotate in the second, or opposite, direction, the hydraulic motor 14 pushes hydraulic fluid in the upstream direction (toward the pump 12). In the embodiment of a cable reel for a mining vehicle, the hydraulic motor 14 is preferably a gear-within-gear (&#34;Gerotor&#34;) type. 
     Continuing, the controller 10 comprises minimally a first (or control) valve 58, a second (or pressure differential) valve 60, a third (or pressure-relieving) valve 62, and a first check valve 64, all hydraulically intercoupled with one another, as described below. Although it is preferable to contain the above-named valves in a manifold block 16, such containment is not necessary. Other components that are desirable under certain uses of the present invention, but not required for other uses, include a second check valve 66, an adJustable pressure regulator 68, a third check valve 70, a pressure sampling port 72, and a manually-actuated bypass valve 74, all described in further detail below. 
     In the preferred embodiment shown, the outlet port 22 of the pump 12 is coupled via conduit 20 to the second check valve 66. The second check valve 66 is oriented so as to only allow passage of hydraulic fluid in a direction away from the pump 12, thereby protecting the pump 12 from reverse hydraulic flow, regardless of operating conditions. 
     The second check valve 66 is coupled to the adjustable pressure regulator 68, which allows the pressure downstream from it to be preset at a desired level equal to or less than the pressure at the pump outlet 22. Such pressure reduction may be required if, for example, the pump 12 is employed to provide pressurized hydraulic fluid for not only the reel-drive mechanism but also for other vehicle-mounted mechanisms such as brakes or digging equipment that require an even higher operating pressure than the reel drive mechanism. If included, the pressure regulator is typically adjusted to regulate downstream pressure within the range of about 1,500 to 2,500 psig when the upstream pressure is about 2,500 psig. Pressure regulator 68 would not be required if the pump 12 powered only the reel-drive mechanism and were pressure compensated at a level specifically for that purpose. 
     The first (or control) valve 58 has an inlet port 76, an outlet port 78, and an exhaust port 80. The inlet port 76 is coupled to the pressure regulator 68 (if a pressure regulator is used). The first valve 58 has a first, or &#34;pay-in&#34; position 82 and a second, or &#34;pay-out&#34; position 84. In other words, the first valve 58 is a two-position, three-way valve. The first valve 58 also includes a spring bias 86 which, in the absence of any opposing force, urges the valve to remain in the first or pay-in position 82. The first valve 58 also includes a first hydraulic pilot 88 serving the first position 82 and a second hydraulic pilot 90 serving the second position 84. 
     The second (or pressure-differential) valve 60 has an outlet port 92 coupled via conduit 94 to the outlet port 78 of the first valve 58, and an inlet port 96 coupled via conduit 98 to the second port 24 of the controller 10. The second valve 60 has a first, or normally-closed, position 100 and a second, or open, position 102. In other words, the second valve 60 is a two-position, two-way valve. The second valve 60 also includes a spring bias 104 which, in the absence of any opposing force, urges the valve to remain in the first, or normally-closed, position 100. The second valve 60 also includes a first hydraulic pilot 106 serving the first position 100 and a second hydraulic pilot 108 serving the second position 102. 
     Conduit 94 not only connects the outlet port 78 of the first valve 58 with the outlet port 92 of the second valve 60, but also with the first pilot 88 of the first valve 58 through conduit 110 and with the first pilot 106 of the second valve 60 through conduits 112 and 114. Conduit 98, downstream of the second valve 60, is connected to both the second pilot 90 of the first valve 58 through conduit 116 and the second pilot 108 of the second valve 60 through conduit 118. Such common connections of first pilots 88, 106 together and both second pilots 90, 108 together ensure that both valves 58 and 60 shift substantially simultaneously, as described further below. 
     First check valve 64 is coupled to the intersection of conduits 112 and 114, allowing hydraulic flow therefrom through first check valve 64 only in the direction toward the intersections of conduits 116, 118, 98 and 120. 
     The third valve 62 of the controller 10 is an adjustable pressure-relieving valve having an inlet port 122 coupled via a conduit 124 to the exhaust port 80 of the first valve 58, and an outlet port 126 coupled via a conduit 128 to the third port 30 of the controller 10. The third valve 62 regulates the pressure upstream from it to a value preset by the operator. Pressure vented from both the pressure regulator 68 and the third valve 62 passes through the conduit 128 to the reservoir 34. 
     The optional manually-actuated bypass valve 74, if included, is coupled between conduits 98 and 128 through conduits 130 and 132, respectively. Bypass valve 74 is a two-position, two-way valve that is normally closed during automatic operation of the reel 50, but is manually openable via handle 134 to pass pressurized hydraulic fluid therethrough from the second port 24 of the controller 10 directly to the reservoir 34, thereby relieving pressure at the motor inlet port 28 to about atmospheric pressure. Although not required, this valve is particularly advantageous during maintenance of the reel mechanism or if the operator for any reason wishes to manually turn the reel 50 without disconnecting the drive chain 46. 
     The controller 10 serves to regulate and control the delivery of pressurized hydraulic fluid from the pump 12 to the motor 14 to cause the reel 50 to &#34;pay in&#34; or wind the cable 52 thereon. When the reel 50 is &#34;paying out&#34; or unwinding cable, such as, for example, when a mining vehicle on which the reel is located is moving away from the cable tie point, the controller 10 serves to regulate and control the delivery of pressurized hydraulic fluid passing therethrough from the motor 14 to the reservoir 34. In response to hydraulic pressure changes resulting from the motor 14 either rotating in a pay-in direction, remaining stalled, or rotating in a pay-out direction, the controller 10 automatically shifts back and forth between a &#34;pay-in&#34; mode and a &#34;pay-out&#34; mode as required to keep the cable taut between the reel 50 and the fixed terminus of the cable. The controller 10 is in a &#34;pay-in&#34; mode whenever the vehicle is moving either toward the cable tie point or remaining stationary. The controller 10 is in a &#34;pay-out&#34; mode whenever the vehicle is moving away from the cable tie point. 
     Operation 
     During both the pay-in and pay-out modes, the pump 12 draws hydraulic fluid from reservoir 34 through the filter 37 and conduit 38 and delivers the pressurized fluid through conduit 20 to the first, or pump, port 18 of the hydraulic controller 10. The fluid then passes through the second check valve 66 and the pressure regulator 68 to the inlet port 76 of the first valve 58. 
     Pay-In Mode 
     When the controller 10 is in the pay-in mode, the first valve 58 is biased by spring 86 to remain shifted to the first position 82. Hydraulic fluid then passes through the first valve 58 from the inlet port 76 to the outlet port 78 and through conduit 94 to the outlet port 92 of the second valve 60. From conduit 94, hydraulic fluid also passes through conduit 110 to pressurize the first pilot 88 of first valve 58 and through conduit 112, first check valve 64 and conduit 116 to pressurize the second pilot 90 of first valve 58. Because the pressure drop across first check valve 64 is only about 5 psi, the pressures at the first pilot 88 and the second pilot 90 are about equal, thereby allowing the first valve 58 to remain in the first position 82 under the influence of the bias spring 86. So long as first valve 58 remains shifted to the first position 82, the valve is in a &#34;pay-in&#34; position. 
     In the pay-in mode of the controller 10, hydraulic fluid also passes through conduit 114 to the first pilot 106 of the second valve 60 and, after passing through first check valve 64, conduits 120 and 118 to the second pilot 108 of the second valve 60. Because the pressure drop across first check valve 64 is only about 5 psi, the pressures at the first pilot 106 and the second pilot 108 are about equal, thereby allowing the second valve 60 to remain in the normally-closed position 100 under the influence of the bias spring 104. So long as second valve 60 remains shifted to the normally closed position 100, the valve is in a &#34;pay-in&#34; position. 
     Continuing in the pay-in mode, hydraulic fluid also passes through conduit 98 to the second port 24 of controller 10, through conduit 26, then to the inlet port 28 of the motor 14. Pressurized hydraulic fluid passing through the motor 14 to the outlet port 40 thereof causes the motor 14 to rotate, thereby rotating in combination the sprocket 44, the sprocket 48 coupled thereto via the roller chain 46, and the reel 50 in a pay-in direction. As the reel 50 rotates in a pay-in direction, the cable 52 is wound upon the reel 50. After passing through the motor 14, hydraulic fluid then passes through the conduit 42 to the reservoir 34. 
     Hydraulic fluid cannot pass out of the controller 10 other than as described above during the pay-in mode because the third check valve 70 prevents escape of fluid through the sample port 72 (if included). Also, the bypass valve 74, unless manually opened, prevents any bypass of pressurized hydraulic fluid away from the motor 14. 
     As can be seen, therefore, valves 58 and 60 remain in the pay-in mode unless a high-pressure event occurs somewhere between the inlet port 96 of the second valve 60 and the inlet port 28 of the motor 14. When used as a cable reel for an electric-powered mining vehicle, the hydraulic system of the present invention during pay-in has a hydraulic pressure at the motor inlet port 28 of about 2,000 psig. This pressure is sufficient to turn a large reel (e.g., five feet in diameter) holding up to two tons of cable as required to keep the cable 52 taut during any conditions during which the reel 50 is paying in the cable. 
     Cable Reel Stationary 
     During conditions in which the reel is stationary, the controller 10 remains in the pay-in mode. The hydraulic pressure at the motor inlet port 28 remains at the same level (about 2,000 psig) as when the reel 50 was paying in cable. This is because first valve 58 and second valve 60 remain in their first, or &#34;pay-in,&#34; positions whenever the reel is stationary. More specifically, the pressures at all four pilots 88, 90, 106 and 108 are about equal, thereby allowing the bias springs 86 and 104 to keep the first and second valves 58 and 60, respectively, in their pay-in positions. As a result, hydraulic pressure is conducted to the motor inlet 28 just as if the reel were expected to pay-in cable. Such pressure is sufficient to ensure that the cable 52 extending from the reel 50 to the tie point is maintained in a taut condition. 
     Pay-Out Mode 
     Considering further the example of a mining vehicle equipped with the present invention, at the moment the vehicle begins to move away from the cable tie point, the cable extending tautly thereto exerts a torque to the reel 50 in a pay-out direction. The reel 50 thereby urges the motor 14 to rotate in a pay-out direction, which causes a pressure spike (about 150 psig) above the pay-in pressure between the motor inlet 28 and the inlet port 96 of valve 60. This pressure spike will be conducted to both the second pilot 108 of the second valve 60 and the second pilot 90 of the first valve 58. However, because the first check valve 64 will not pass fluid therethrough in an upstream direction and because the second valve 60 is instantaneously still in the normally closed position 100, first pilots 88 and 106 of the first and second valves 58 and 60, respectively, do not experience the pressure spike as do the second pilots 90 and 108. This momentary pressure imbalance is sufficient to cause the second valve 60 to shift to the open, or &#34;pay-out,&#34; position 102 against the force exerted in combination by the bias spring 104 and first pilot thereof 106, and valve 58 to simultaneously shift to the second, or &#34;pay-out,&#34; position 84 against the force exerted in combination by the bias spring 86 and first pilot thereof 88. 
     After so shifting to their respective &#34;pay-out&#34; positions, valves 58 and 60 remain so shifted so long as the vehicle is moving away from the cable tie point because second valve 60 has a considerable pressure drop therethrough from the inlet port 96 to the outlet port 92 (about 150 psig), which maintains the pressure imbalance at second pilots 90 and 108 relative to first pilots 88 and 106. Hence, so long as the reel 50 is paying out cable, pressurized hydraulic fluid passage from the pump 12 is fully occluded or blocked at the inlet port 76 of the first valve 58. Hydraulic fluid from the motor 14 passes through the second valve 60 from the inlet port 96 to the outlet port 92 thereof, through the conduit 94, through the first valve 58 from the outlet port 78 to the exhaust port 80 thereof, through conduit 124, through the third (pressure-relieving) valve 62, through conduit 128 and into the reservoir 34. 
     Because the output of hydraulic fluid from the pump 12 is effectively turned off during pay-out, the hydraulic pressure at the motor inlet 28 drops from the momentary &#34;spike&#34; maximum of about 2,150 psig (in the mining-vehicle embodiment of the present invention) to about 1,500 psig after valves 58 and 60 shift to their respective &#34;pay-out&#34; positions. The latter pressure is maintained because of the pressure drop across second valve 60 (about 150 psi) from the inlet port 96 to the outlet port 92 thereof in combination with the pressure drop across the third valve 62 (which is adjustable). 
     The 1,500 psig back pressure applied to the motor 14 during pay-out serves to hydraulically brake motor rotation. Such dynamic braking prevents the reel 50 from paying out more cable than required to keep the cable 52 taut between the reel 50 and the cable tie point. 
     The moment the reel 50 ceases paying out cable, the pressures at pilots 88, 90, 106, and 108 become about equal, allowing the bias springs 86 and 104 to shift the first and second valves, respectively, to the first and normally closed positions (pay-in positions), respectively. 
     If at any time one wishes to instantaneously reduce the hydraulic pressure at the motor inlet port 28 to about atmospheric pressure without disconnecting the drive chain 46, handle 134 of the bypass valve 74 is turned to shift the valve 74 to the open position. Such pressure relief may be necessary to perform maintenance or to manually turn the reel 50. 
     If one wishes to measure the pressure between the inlet port 96 of the second valve 60 and the motor inlet port 28, a pressure gauge or similar device is coupled to the pressure sample port 72. The gauge must be capable of opening check valve 70 after coupling. Such pressure measurements during pay-in and pay-out may be particularly helpful during adjustment of pressure regulator 68 and third valve 62. 
     Having illustrated and described the principles of my invention in a preferred embodiment and variations thereof, it should be apparent to those skilled in the art that the present invention may be modified in arrangement and detail without departing from the principles thereof. I claim as my invention all modifications coming within the scope and spirit of the following claims.