Dampers

Damper is a mechanical device, which resists motion via viscous friction.The resulting force is proportional to the velocity, but acts in the opposite direction,slowing the motion and absorbing energy. It is commonly used in conjunction with a spring (which acts to resist displacement).

Types Two common types of dampers exist - linear and rotary. Linear dampers are generally specified by stroke (amount of linear displacement) and damping coefficient (force per velocity). Rotary dampers will have damping coefficients in torque per angular velocity. A less common type of damper is an eddy current damper, which uses a large magnet inside a tube constructed of a non-magnetic but conducting material . Like a common viscous damper, the eddy current damper produces a resistive force proportional to velocity.

Damper frequently use a one-way mechanical bypass to permit fast unrestricted motion in one direction and slow motion using the damper in the opposite direction. This permits, for example, a door to be opened quickly without added resistance, but then to close slowly using the damper. For hydraulic damper this unrestricted motion is accomplished using a one-way check-valve that allows fluid to bypass the damper fluid constriction. Non-hydraulic damper may use a ratcheting gear to permit free motion in one direction.

Applications

A damper is a common component in a door closer to prevent it from slamming shut. A spring applies force to close the door and the damper dashpot, implemented by requiring fluid to flow through a narrow channel between reservoirs (often with a size adjustable by a screw), slows down the motion of the door. Consumer electronics often use dashpots where it is undesirable for a media access door or control panel to suddenly pop open when the door latch is released. The dashpot slows the sudden movement down into a steady and gentle movement until the access door has opened all the way under spring tension. Dashpots are commonly used in dampers and shock absorbers. The hydraulic cylinder in an automobile shock absorber is a dashpot. They are also used on carburettors, where the return of the throttle lever is cushioned right before the throttle fully closes and then is allowed to fully close slowly to reduce emissions of sudden deceleration (compared to deceleration without a dashpot) Large forces and high speeds can be controlled by dashpots. For example, they are used to arrest the steam catapults on aircraft carrier decks. Relays can be made to have a long delay by utilizing a piston filled with fluid that is allowed to escape slowly. Some high energy motor starter contactors have used the dashpot. The Allen West type, for example, uses a hydraulic piston. The rod of the piston is part of the built in 'over current' function. The current sensing coil and rod act as a solenoid. The rod is the plunger in the over current coil. During over current the plunger moves up and releases the dropout lever cutting power to the motor. During motor start, current in-rush time is short in comparison with dashpot action and so start current cannot pull the plunger as quick. The damping of the current coil plunger prevents the motor start current from unlatching the dropout lever. On three phase units there is one dashpot for each of the three contactors so excess current in either phase will drop all three contactors as the dropout lever encompasses all contactors.