Designing a Linear Motion System for a Ship's Throttle
by Bob Eisele, Amacoil, Inc.
Linear movement of the throttle arm in a ship's diesel engine needs to be smooth and backlash-free. Engine vibration can cause throttle linkage to loosen creating "play," which decreases throttle arm movement accuracy, compromising the integrity of the ship's speed control. Additionally, the ongoing maintenance is time consuming.
To address these issues, engineers at a large, European shipbuilder, developed a unique engine throttle arm motion control system. The electronically controlled system employs a rolling ring linear motion assembly to control advances and retractions of the throttle adjustment arm.
The basic set-up combines a manually operated main throttle control on the bridge, with an eletcromechanical control system in the engine room. A transmitter potentiometer on the ship's bridge is linked to the main throttle control. When the pilot moves the main control, the transmitter potentiometer sends an electronic signal to a variable speed gearmotor housed in a water-tight metal casing in the engine room. The signal from the transmitter potentiometer generates the appropriate gearmotor rotation.
As illustrated, the output shaft of the gearmotor holds a spur gear that meshes with another gear mounted on the rolling ring linear drive shaft. So as the gearmotor rotates, it causes rotation of the linear drive shaft. The rolling ring linear drive converts this rotary motion input, from the shaft, into linear output causing the drive "nut" to move. The throttle adjustment arm, attached via a metal bracket to the nut, advances or retracts depending on the travel direction of the nut.
A rack-and-pinion links the throttle arm to the receiver potentiometer. As the throttle arm moves, it changes the resistance of the receiver potentiometer. The resistance values of the transmitter and receiver potentiometers are continuously compared. When the values are identical, an electronic sensor triggers a switch that shuts off the gearmotor. Moving the speed control on the bridge reactivates the system and switches the motor back on.
The metal casing which houses the motion control system is sealed against the environment. Furthermore, a rolling ring drive system does not use a screw - the rolling ring drive shaft is threadless. Should particulate contaminants somehow enter the system, the absence of threads means the particles can not be trapped on the shaft and cause clogging or jamming which could damage the system and cause downtime for repair work.
The smooth drive shaft also means there are no threads to wear out. Worn screw threads would cause too much play in the throttle reducing accuracy and eventually requiring system shut down to replace the screw. Using a rolling ring linear drive on a threadless shaft extends the life of the system. Periodic light lubrication of the shaft is the only maintenance required.
For accuracy in throttle adjustments, there can be no backlash when moving the throttle adjustment arm. Rolling ring bearings maintain constant, point contact with the drive shaft. As a result, the most minute rotation of the shaft is immediately transmitted - with no play - to the bearings and linear movement occurs.
This feature of rolling ring linear drives was attractive to the design engineers working on the throttle control, because sudden changes to the speed control are smoothly and instantaneously transmitted through the system. The system's accuracy is enhanced and ultimately this improves handling of the vessel.