Linear Actuator Encourages Smooth Sailing

Design option for engine throttle arm control system

—by Bob Jeter, applications engineer, Amacoil, Inc.

A diesel engine powering a large sea-going vessel is subject to severe vibration, and a harsh operating environment. Vibration can cause mechanical throttle linkage to loosen. This situation creates play so that movement of the throttle adjustment arm does not necessarily translate into the desired increase or decrease in engine speed. The accuracy of the throttle is compromised, and the system requires ongoing preventative maintenance.

Design engineers at a large, European shipbuilder developed an electronically controlled motion control system that uses a rolling ring linear drive assembly to control advances and retractions of the throttle adjustment arm. When the master throttle lever on the ship’s bridge is moved, a transmitter potentiometer attached to the lever sends an electronic signal to the variable speed gearmotor located inside the metal casing. The output shaft of the gearmotor holds a spur gear that meshes with a gear on the rolling ring linear drive shaft, causing the shaft to rotate and the drive “nut” to move. A metal bracket connects the drive nut to the throttle adjustment arm that, in turn, slides in and out through a gasket in the watertight metal apparatus casing as required to open or close the throttle.

Moving the master throttle lever on the bridge transmits an electronic signal to a receiver potentiometer connect by a rack-and-pinion set-up to the throttle adjustment arm. The system continually compares and equalizes the resistance value of the transmitter potentiometer to the resistance value of the receiver potentiometer. The equalized signal received on the bridge tells personnel that the desired throttle setting has been achieved, while an electronic sensor triggers a switch that shuts off the throttle gearmotor. Moving the speed control on the bridge reactivates the system, turning the motor back on.

The rolling ring drive was attractive to the ship’s engineers because sudden changes to the speed control are translated smoothly and instantaneously through the system, without mechanical backlash. The slightest rotation of the drive’s shaft is immediately transmitted — with no play — to the rolling ring bearings and linear movement occurs. The system’s accuracy is thereby enhanced, in turn improving handling of the vessel. The smooth drive shaft also means there are no threads to wear out and cause play in the throttle. Moreover, the only maintenance required is periodic light lubrication of the shaft.