DC Electrics - DC Motors — Page 117, Lesson 116

I want to walk you through linear actuators, actuator brakes, clutches, and the visual indicators used with them. This is all part of how DC motors are applied to do real work on an aircraft. Let's start with linear actuators. A linear actuator is a device that produces a straight-line push or pull motion. Inside, it has a small reversible DC motor. That motor is coupled through a reduction gear to a screw jack. When the motor turns, the reduction gear slows it down and increases torque, and the screw jack converts that rotary motion into linear motion, extending or retracting a ram or plunger. These are used for any operation that requires a push/pull action. Common examples on an aircraft include operating flaps, the undercarriage, trim tabs, and even as inching controls for oil cooler shutters. 'Inching' means making very small, precise movements. How is the actuator controlled? For full up/down operations, like raising or lowering the landing gear, selector switches are used. But for small movements, such as trimming, a spring-loaded self-centring ‘OFF’ switch is used. This switch has a centre 'off' position. If you move it one way from centre, it supplies power to the actuator motor to operate in one sense, say extend. If you move it the other way, it supplies power for the opposite sense, say retract. When you let go, the spring returns the switch to centre, and the motor stops. Now, two limit switches control the extent of travel and the direction. They also operate visual indicators for the pilot. Each limit switch is positioned at one end of the actuator's travel. When the actuator reaches full travel in one direction, the respective limit switch opens, which stops the motor. That prevents the actuator from trying to go further and damaging itself. With an inching actuator, both limit switches will be closed at any time the actuator is not at a full travel position. This is important because it allows the motor to be reversed by the inching control switch. If one limit switch were open, the circuit would be broken and the motor couldn't run in that direction. Let's move on to actuator brakes. Many actuators are fitted with electromagnetic brakes. Their purpose is to prevent over-travel when the motor is switched off. Without a brake, the momentum of the motor and the load could cause the actuator to coast past its intended stop position. The design of brake systems varies, but in all cases the brakes are spring-loaded to the ‘on’ condition when the motor is de-energized. That means when there is no power, the spring applies the brake, holding the actuator in place. The operating solenoids are connected in series with the armature. So, immediately power is applied to the motor, the solenoid is energised, which withdraws the brake against the spring force, allowing the motor to turn. This is a fail-safe design: if power is lost, the brake automatically engages. Next, actuator clutches. Friction clutches are incorporated in the transmission systems of actuators. Their purpose is to protect the actuator against the effects of mechanical over-loading. If the actuator jams or hits an obstruction, the clutch slips instead of transmitting the full force, which prevents damage to the gears or motor. Now, let's look at visual indicators used with linear actuators. Where no intermediate stopping positions between the actuator limits are required, press-to-test lights or magnetic indicators are used. These simply show that the actuator has reached one end or the other. In situations where movement either side of a datum, or between open and closed, is to be shown, position indicators with a graduated scale are fitted. This gives the pilot a continuous readout of the actuator's position. For visual indicators used with rotary actuators, these indicate to the pilot the position of the actuated equipment. Typically, this would be for fuel or oil valves. These valves are only ever in the ‘OPEN’ or ‘SHUT’ position — there are no intermediate positions. In both cases — for linear and rotary actuators — an indication of either a loss of power supply, or that the actuator is travelling between selected positions, will be required. The pilot needs to know if the actuator is moving, or if it has stopped because of a power failure. Finally, indicator lights. These are usually of the ‘press-to-test’ type. The pilot can apply finger pressure on the front glass of the lamp unit. This enables the filament to be tested without operating the control switches of the actuator. So you can check that the bulb itself is working, without actually moving the actuator.