Marine Reduction Drives With Continuous-Slip Hydraulic Clutch Control and Engine Torque-Up System Enhance Vessel Maneuverability
The application of medium- and high-speed diesel engines with narrow operating speed ranges, has resulted in vessel speeds that are too high at engine idle. To reduce the ship's speed, such as when docking or handling of fishing nets, the operator is forced to continually shift from ahead to neutral, or ahead to astern, resulting in an inefficient method of speed control. A continuous-slip drive and a torque-application control fixes that and enhances the operating speed range of todays modern propulsion systems.
By controlling the hydraulic pressure on the ahead or astern clutch, a continuous-slip control marine drive system provides closed loop speed control which increases the propeller shaft speed range from engine idle to zero rpm. During the slip mode the engine remains at idle while the clutch is slipped to deliver the reduced propeller speed. When a propeller speed above engine idle speed is needed, the pilot house control handle is moved forward and the slip-control system fully engages the clutch and the engine accelerates.
The slip-control system can be operated indefinitely, without damage to the clutch.
The torque transmitted during the slip mode is accomplished by shearing the oil film between the clutch plates. The heat generated during the slip mode is carried away by the cooling oil as it passes through the clutch and then dissipated through the reduction gear heat exchanger.
Control Maintained
The closed-loop slip is accomplished by a proportional-integral-derivative (PID) controller, which ensures that the selected propeller speed is maintained regardless of wave, wake, or current conditions. The controller compares the error between the operator selected speed and true propeller speed, and adjusts the clutch hydraulic pressure as required through a proportional slip-control valve. Figure 1 is a control logic schematic for the slip-control system. The programmability of the PID controller allows for the slip-control system to be adjusted for each specific application. This ensures smooth, stable propeller shaft speed control no matter how large or small the system masses and inertias may be. During sea trials the PID controller is programmed to match the propulsion system to the vessel. A small hand-held keypad is plugged into the PID controller program port, and the PID program values are entered. The keypad is removed once the system has been fine tuned.
During sea trials, the proportional, integral, and derivative gains are adjusted to yield smooth, stable propeller speed changes.
The proportional gain determines how large a signal is to be sent to the proportional slip-control valve. The proportional gain produces an output signal to the valve which is proportional to the error between the selected and true propeller shaft speed.
The integral gain controls the amount of propeller shaft speed drift that may occur during changes in wave, current, or vessel maneuvering. The integral gain multiplies the error between selected and true propeller speed over a time interval and sums this factor into the output signal to the slip-control valve.
The derivative error on the other hand controls the amount of overshoot or speed oscillation that the propeller exhibits as it approaches a new selected propeller shaft speed. By adjusting the derivative gain, smooth propeller speed changes are achieved without significant speed undershoot or overshoot.
Marine Drive Includes Propeller Shaft Brake
The continuous-slip hydraulic clutch control system is available in a Falk 3000 MRH marine reduction drive line. The Falk 3000 MRH marine drive incorporates surface hardened and ground gearing with internal hydraulic clutches. Also contained with the reduction gear, is the oil cooled, air-actuated propeller shaft brake. The location of the internal shaft brake greatly reduces the required shaft brake size since the brake is mounted on the end of the pinion shaft, taking advantage of the reduced shaft torque.
Figure 2 shows the orientation of the gearing in the Falk 3000 MRH marine reduction drive. The marine reduction drive contains two multi-disk oil-sheer clutches, one for ahead mode operation and a second for astern mode operation. Either clutch can be designated as ahead or astern in order to yield the required propeller shaft rotation for each particular application. Both hydraulic clutches are capable of unlimited continuous slip at engine idle speed, and maximum torque transmission at full engine speed. The proportional slip-control valve allows the clutch to slip while at engine idle speed, and fully pressurizes the clutch prior to acceleration of the engine.
Figure 3 is an unfolded or 'roll-out' view of the drive. Pinions A and B both mesh directly with the low-speed (propeller-shaft) gear at all times. Engine input is at the shaft at the upper right in Figure 2. At the right in Figure 2 are two clutches: one on the input shaft; the other on the secondary shaft of the pair of transfer (1:1 - ratio) gears. Only one clutch can be engaged at any one time. Thus, power flow can be from the input shaft through Pinion A to the low-speed gear, with Pinion B idling; or from the input shaft through the transfer gears, through Pinion B, to the low-speed gear, with Pinion A idling. In the second case, the low-speed gear turns in the direction opposite that of the first case.
Control System Options
The control of the slip system is through a conventional marine-type control handle, which can be either pneumatic or electronic. The handle is mounted in the pilot house, and has as optional port and starboard bridge wing control. The system can consist of a single control handle which incorporates both the slip control and the locked-clutch control, or it can be comprised of two control handles, one handle for ahead and astern slip, and the second handle to control normal locked-clutch operation.
When using the single handle control system, ahead or astern slip is encountered as soon as the handle is shifted forward or aft, out of the neutral position. As the stroke of the handle is increased, the propeller speed is increased. At approximately 30-degree handle
stroke, a second detent is encountered at which point the clutch is fully pressurized ending
the clutch slip. As the handle is stroked further the engine now accelerates to produce the selected propeller shaft speed. If the low-speed maneuvering is not desired, the handle is simply shifted out of neutral and immediately to the 30-degree detent position, at which point the clutch is fully pressurized with the engine at idle speed.
In the two-handle system, the first handle is a neutral-center position handle that controls ahead and astern slip while the engine remains at idle speed. The second control handle is also a neutral center position control handle which fully pressurizes either the ahead or astern clutch when shifted from the neutral position to the first detent. As the operator increases the handle stroke, engine speed increases.
To facilitate any troubleshooting, the slip-control system incorporates a monitoring and feedback system. The slip control unit is housed in a water-tight enclosure and is mounted in the engine room, near the reduction gear. Within the enclosure is an LED read-out which displays information such as selected propeller shaft speed, true propeller shaft speed, engine speed, and the status of the reduction gear pressure and temperature switches.
During slip-clutch operations, the slip-controller continually monitors reduction gear lube-oil pressure and temperature to assure that operation values are within preset limits. If a problem should arise, the slip-controller will terminate the slip mode, return the propeller shaft to zero as the shaft brake is engaged, and will display a "fault" signal to the operator.
The engine-room mounted PID slip-controller display will then indicate the exact reason for the termination of the slip mode.
Reversals -- Enhanced Maneuverability
To greatly enhance maneuverability during vessel reversals, Falk marine drives can incorporate the patented engine torque-up system. This feature loads the engine over a controlled rate which greatly reduces the shock to the propulsion system. During a vessel reversal, the control system continues to apply the shaft brake during the initial engagement of the astern clutch. The locked shaft against the slipping clutch allows the engine to produce torque over a longer, more controlled duration, while minimizing
clutch-heat build up.
Figure 4 shows an example of the astern clutch engagement during a full-speed crash reversal of a tuna seiner using the engine torque-up system. In the example, engine speed is boosted to 500 rpm, and the astern clutch begins pressurization. The accelerating engine comes under load as the shaft brake remains engaged and the astern clutch begins to transmit torque. This initial load on the engine causes the fuel rack to increase the fuel rate to the engine, causing the turbochargers to begin pumping, increasing the torque output of the engine. As the shaft brake is released the engine speed falls from 500 rpm to engine idle at 350 rpm, and the propeller shaft accelerates in the astern direction. With the turbochargers pumping, the engine is now developing sufficient torque to overcome the reversal.
An example of a full-speed crash reversal without the engine torque-up system is shown in Figure 5. This graph shows a reversal of a similar tuna seiner using a standard marine control system. To overcome the high propeller back-torques in such a short period of time, the engine must be boosted to 580 rpm. As the clutch engages, the propeller shaft
brake is released, prior to the torque transmission within the clutch. The propeller begins to
accelerate in the ahead direction, driven by the entrained water within the wheel. As the engaging astern clutch begins to transmit torque, the propeller must be stopped and reversed. The engine struggles to produce sufficient torque, falls to 250 rpm and finally increases speed to complete the reversal.
In a reversal of this type, the engine is loaded so abruptly, that the turbochargers do not begin to increase the engine torque output until well after the clutch has become fully engaged. This forces the engine to overcome the propeller back-torque while in the aspirated mode, causing the engine speed to fall well below idle speed, and possibly stalling and back-driving the engine.
In comparing Figures 4 and 5, it can be seen that the with the engine torque-up system, the reversal is completed with much less shock to the engine, clutch, and other propulsion system components. And, the torque-up system allowed the reversal to be completed in a much shorter time, and with a shorter time delay.
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