A dashpot is a mechanical device, a damper 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). The process and instrumentation diagram (P&ID) symbol for a dashpot is .
The two most common types of dashpots are linear and rotary ones.
Linear dashpots — or linear dampers — are used to exert a force opposite to a translation movement. They are generally specified by stroke (amount of linear displacement) and damping coefficient (force per velocity).
Similarly, rotary dampers will tend to oppose any torque applied to them, in an amount proportional to their rotational speed. Their damping coefficients will usually be specified by torque per angular velocity. One can distinguish two kinds of viscous rotary dashpots:
- Vane dashpots which have a limited angular range but provide a significant damping torque. The damping force is the result of one or multiple vanes moving through a viscous fluid and letting it flow via calibrated openings.
- Continuous rotation dashpots which aren't limited in their rotation angle but provide a smaller damping coefficient. These use the friction generated by the shearing forces induced in the viscous fluid itself by the difference in motion between the dashpot's rotor and stator.
Eddy current damper
A less common type of dashpot is an eddy current damper, which uses a large magnet inside a tube constructed of a non-magnetic but conducting material (such as aluminium or copper). Like a common viscous damper, the eddy current damper produces a resistive force proportional to velocity.
Dashpots frequently use a one-way mechanical bypass to permit fast unrestricted motion in one direction and slow motion using the dashpot in the opposite direction. This permits, for example, a door to be opened quickly without added resistance, but then to close slowly using the dashpot. For hydraulic dashpots this unrestricted motion is accomplished using a one-way check-valve that allows fluid to bypass the dashpot fluid constriction. Non-hydraulic rotatry dashpots may use a ratcheting gear to permit free motion in one direction.
A dashpot is a common component in a door closer to prevent it from slamming shut. A spring applies force to close the door, which the dashpot offsets by forcing fluid to flow through an orifice, often adjustable, between reservoirs, which slows 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 provides a steady, gentle motion until the access door has fully opened.
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 carburetors, where the return of the throttle lever is cushioned just before the throttle fully closes, then is allowed to fully close slowly to reduce emissions. The British SU carburettor's main piston carries a stepped needle. This needle is held in the fuel flow orifice. The manifold vacuum causes this piston to rise allowing more fuel into the airflow. The SU's dashpot has a fixed hydraulic piston, damping the main piston as it moves upward. A valve in the piston disables the damping as the main piston returns.
Relays can be made to have a long delay by utilizing a piston filled with fluid that is allowed to escape slowly. Electrical switchgear may use dashpots in their overcurrent sensing mechanism to reduce reaction speed to brief events, thus making them less sensitive to false-triggering during transients whilst still remaining sensitive to sustained overloads. Another use is for delaying the closing or opening of an electrical circuit. Such a dashpot timer might be used for example for timed staircase lighting.
Anti-stall mechanisms in internal combustion engines are aimed to prevent stalling of the engine at low rpm. Anti-stall mechanisms use dashpots to arrest the final closing movement of the throttle.
Dashpots are used as models of materials that exhibit viscoelastic behavior, such as muscle tissue. Maxwell and Kelvin–Voigt models of viscoelasticity use springs and dashpots in series and parallel circuits respectively. Models containing dashpots add a viscous, time-dependent element to the behavior of solids, allowing complex behaviors like creep and stress relaxation to be modeled.
- Oxford English Dictionary. Oxford University Press.
dash-pot, n. a contrivance for producing gradual descent in a piece of mechanism or for preventing vibration or sudden motion, consisting of a cylinder or chamber containing liquid in which a piston moves; a hydraulic buffer.
- Mark H. Holmes (2009). Introduction to the Foundations of Applied Mathematics. Springer. p. 329.
the resistance force is proportional to the velocity
- "Dashpot Types". www.kinetrol.com. Retrieved 5 August 2020.
- Mike Plissi. "Update on eddy-current damping experiments" (PDF). Laser Interferometer Gravitational-Wave Observatory (LIGO). Archived from the original (pdf) on 2010-07-25. Retrieved 2010-05-29.
A magnet moving inside a non-magnetic conductive tube has its motion retarded. Retardation force is proportional to velocity of magnet- viscous damping.
- Sodano; Bae; Inman; Belvin (June 2006). "Improved Concept and Model of Eddy Current Damper" (PDF). Transactions of the American Society of Mechanical Engineers. 128: 294–302. Archived from the original (pdf) on 2010-07-28.
This process of the generation and dissipation of eddy current causes the system to function as a viscous damper
- Starin; Neumeister (19–21 September 2001). "Eddy Current Damping Simulation and Modeling". Proceedings of the 9th European Space Mechanisms and Tribology Symposium. 480: 321–326. Bibcode:2001ESASP.480..321S. ISBN 92-9092-761-5.
One major advantage of ECD's is their linearity
- Henry A. Sodano (May 5, 2005). "Development of Novel Eddy Current Dampers for the Suppression of Structural Vibrations" (pdf). Virginia Polytechnic Institute and State University. Retrieved 2020-09-26.
This damping force can be described as a viscous force due to the dependence on the velocity of the conductor.
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