Oil-shear slip clutches give ship a shove or a nudge, as needed

Feb. 1, 2000
Newer diesel engines with narrow operating speed ranges result in vessel speeds too high at engine idle. To reduce ship speed as in docking, an operator must shift continually from ahead to neutral, or ahead to astern — inefficient at best.

By controlling hydraulic pressure on the Ahead or Astern clutch, the continuous- slip marine drive provides closed-loop speed control, which increases the propeller shaft speed range from engine idle to zero rotary speed. During the slip mode, the engine remains at idle while the oil-shear 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; the slip control system fully engages the clutch and the engine accelerates.

You can operate the slip control system indefinitely without clutch damage. Torque is transmitted during slip by shearing the oil film between clutch plates. The oil passes through the clutch, carrying away heat generated during slip, and dissipates the heat through the reduction- gear heat exchanger.

Control no matter what

Closed-loop slip control is by means of a proportional-integral-derivative (PID) controller, to assure that selected propeller speed is maintained regardless of wave, wake, or current conditions. The controller compares the error between operator-selected speed and true propeller speed, and adjusts clutch hydraulic pressure as required through a proportional slip-control valve. Figure 1 is a logic schematic of the control system.

PID controller programmability lets the slip control system be adjusted for each application. This assures smooth, stable propeller shaft speed control no matter how large or small the system masses and inertias. During sea trials, the PID controller is programmed to match propulsion system and vessel. Program values are entered through a handheld keypad plugged into the controller program port. The keypad is removed once the system is fine-tuned.

During sea trials, proportional, integral, and derivative gains are adjusted for smooth, stable propeller speed changes.

Proportional gain determines how large a signal to send to the proportional slip-control valve. Proportional gain produces an output signal to the valve which is proportional to the error between selected and true propeller speeds.

Integral gain controls the amount of propeller speed drift that may occur during changes in wave, current, or vessel maneuvering. Integral gain multiplies the error between selected and true propeller speed over an interval and sums this factor into the output signal to the slip-control valve.

Derivative error, on the other hand, controls the amount of overshoot or speed oscillation of the propeller as it approaches a new selected speed. By adjusting derivative gain, smooth propeller speed changes are made without significant under or overshoot.

What’s in the box?

The continuous-slip hydraulic clutch control system works in a packaged marine drive. The drive contains surfacehardened and ground reduction gearing with the internal hydraulic clutches. Also within the reduction gear package is an oil-cooled, air-actuated propeller shaft brake. It is used during some shifting operations such as a forward-to-reverse crash-stop maneuver. The location of the internal shaft brake greatly reduces required brake size; the brake mounts on the end of the pinion shaft, taking advantage of the lower torque.

Figure 2 shows the reduction drive. It contains two oil-shear clutches: one for Ahead mode operation; the other, Astern. Either clutch can be designated as Ahead or Astern to produce the required propeller shaft rotation for each 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 lets the clutch slip at engine idle speed, and fully pressurizes the clutch prior to engine acceleration.

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Figure 2 is an unfolded or “rollout” view of the drive. In fact, 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 lowspeed 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.

More about controls

Slip system control is through a conventional marine-type control handle, which can be pneumatic or electronic. The handle is in the pilot house, with optional port and starboard wing control. The system can consist of a single control handle incorporating both slip control and locked-clutch control, or of two control handles, one for Ahead and Astern slip; the second, to control normal locked-clutch operation.

With single-handle control, Ahead and Astern slip is encountered as soon as the handle is shifted Forward or Aft out of Neutral. As the stroke increases, propeller speed is increased. At about 30-deg handle stroke, a second detent is encountered. The clutch is fully pressurized, ending clutch slip. With further stroke, the engine accelerates to produce selected propeller shaft speed. If low-speed maneuvering is not desired, the operator simply shifts out of neutral and immediately to the 30-deg detent.

In the two-handle system, the first is a neutral-center-position handle that controls ahead and astern slip while the engine remains at idle. The second handle is also a neutral center position control which fully pressurizes either the Ahead or Astern clutch when shifted from neutral to the first detent. As the operator then increases handle stroke, engine speed increases.

To facilitate troubleshooting, the slip control system has a monitoring and feedback system. The slip control unit is in a water-tight enclosure mounted near the reduction gear in the engine room. Within the enclosure is an LED readout which displays information such as selected propeller shaft speed, true propeller speed, engine speed, and the states of the reduction gear pressure and temperature switches.

During slip-clutch operation, the slip controller continuously monitors reduction gear lubricating oil pressure and temperature to assure that operating values are within preset limits. Should a problem arise, the controller terminates the slip mode, returns the propeller shaft speed to zero as the shaft brake is engaged, and displays a fault signal. The engine- room-mounted PID slip-controller display then indicates the exact reason for terminating the slip mode.

Torque up

To enhance maneuverability during vessel reversals, a marine drive can include a feature that loads the engine over a controlled rate. Called an “Engine Torque-Up System,” it greatly reduces shock to the propulsion system. During a vessel reversal, the system continues to apply the shaft brake during initial engagement of the Astern clutch. The locked shaft against the slipping clutch lets the engine produce torque over a longer, more controlled duration, while minimizing clutch heat build-up.

Figure 3 is an example of Astern clutch engagement using the system during a full-speed crash reversal of a tuna seiner. 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 engine fuel rate, causing the turbochargers to begin pumping, increasing engine torque output. As the shaft brake is released, engine speed falls from 500 rpm to engine idle at 350 rpm, and the propeller shaft accelerates in the Astern direction. With the turbo-chargers pumping, the engine is now developing enough torque to overcome the reversal.

Figure 4 is an example of a full-speed crash reversal without the engine torqueup system. It shows a reversal of a similar tuna seiner using a conventional marine control system. To overcome the high propeller back-torque in such a short period, engine speed must be boosted to 580 rpm. As the clutch engages, the propeller shaft brake is released prior to 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.

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The engine struggles to produce enough torque, falls to 250 rpm, and finally increases speed to complete the reversal. In such a reversal, the engine is loaded so abruptly that the turbochargers do not begin to increase engine torque output until well after the clutch has become fully engaged. This forces the engine to overcome propeller backtorque while in the aspirated mode, causing engine speed to fall well below idle speed, and possibly stalling and backdriving the engine.

Comparison of Figures 3 and 4 shows that the engine torque-up system allows reversal with much less shock to the engine, clutch, and other propulsion system components. Moreover, it allows reversal completion in a much shorter time, and with a shorter time delay.

The marine reduction drives with Continuous Slip Hydraulic Clutch Control and Engine Torque-Up System described in this article are by The Falk Corp., Milwaukee. For more about them, circle 406 on the reader service card.

If this article is helpful, please circle 407 on the reader service card.

Timothy Vail is a Project Engineer, Marine Engineering Group, The Falk Corp., Milwaukee.

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