Dashpot

A dashpot, which is also known as a damper, is a mechanical device designed to resist motion. It achieves this through a process called viscous damping. When an object moves, the dashpot creates a resistive force that acts in the opposite direction of that motion. This force is proportional to the velocity, or the speed of the movement. As the device works, it absorbs kinetic energy, which is the energy of motion. This energy is then converted into heat.

Dashpots are essential for controlling movement and are often used alongside springs in dynamic systems.

The mechanism of a dashpot relies on the relationship between speed and resistance. In a typical setup, the force produced by the dashpot is directly related to how fast the object is moving. This relationship is often described as being linear. In a linear system, the output force, denoted as F, is proportional to the velocity, denoted as v. Engineers use a constant of proportionality, often labeled as c, to define this relationship. However, real-world dashpots are frequently non-linear. In these cases, the force is proportional to the velocity raised to an exponent, known as alpha. This exponent can vary between 0.2 and 2.0. Because of this complexity, engineers must use numeric methods rather than simple calculus to analyze non-linear dampers.

There are several distinct types of dashpots categorized by how they move. Linear dashpots, or linear dampers, exert force against translation movement, which is movement in a straight line. These are usually specified by their stroke, the amount of linear displacement possible, and their damping coefficient.

Rotary dashpots are different because they oppose torque, which is a twisting force. These are measured by torque per angular velocity. Within the rotary category, there are two main kinds. Vane dashpots use one or multiple vanes moving through a viscous fluid. These vanes move through calibrated openings to provide a significant damping torque over a limited angular range. Continuous rotation dashpots allow for unlimited rotation angles. They provide a smaller damping coefficient by using friction from shearing forces in the fluid between a rotor and a stator.

Another specialized version is the eddy current damper. This device is less common but very effective for specific tasks. It uses a large magnet placed inside a tube made of a conducting but non-magnetic material, such as copper or aluminum. Like a viscous damper, it produces a resistive force proportional to velocity. Because it is a frictionless method when near rest, it is a perfect choice for balance scales. This allows the scales to come to a quick and steady rest without the wear of physical contact.

Many dashpots are designed for one-way operation to allow for versatility. This means the device can permit fast, unrestricted motion in one direction while providing slow, controlled motion in the other. For hydraulic dashpots, this is done using a one-way check-valve. This valve allows fluid to bypass the constriction during fast movement.

Non-hydraulic rotary dashpots might use a ratcheting gear to achieve this same effect. A common example is a door closer. A spring provides the force to close the door, but the dashpot forces the fluid through an orifice to ensure the door closes slowly instead of slamming.

Dashpots serve critical roles in many different industries and technologies. In the automotive industry, they are key components in shock absorbers. These devices cushion a vehicle against vibrations from uneven road surfaces. They are also used in carburetors, such as the Zenith-Stromberg, to cushion the return of the throttle lever.

This helps reduce emissions by allowing the throttle to close slowly. In much larger scales, dashpots are used to arrest steam catapults on aircraft carrier decks. They are even used in electrical switchgear. In these systems, they can reduce reaction speeds to brief events, preventing false-triggering during transients.

Beyond mechanical movement, dashpots are vital in structural engineering and science. Large dashpots are added to buildings and bridges to protect them from wind vibrations and earthquakes. A famous example occurred with the Millennium Bridge in London. After the bridge was found to wobble when people walked on it, repairs were made using such technology. Scientists also use dashpots to model viscoelasticity. This is the behavior of materials like muscle tissue. By using Maxwell and Kelvin–Voigt models, which combine springs and dashpots in series or parallel, researchers can model complex behaviors like stress relaxation and creep.