Viscous damping uses resistance to sudden movement and shear force to reduce vibrations in machines and structures. Clear liquids like silicones, water, or hydrocarbon oils are used. Bingham plastics, like cornstarch and water, can also have damping effects.
When thick liquids are suddenly poured or moved, they move slowly due to inertia, which is the resistance to the sudden movement. If the same liquid is placed between two plates that are rapidly being rotated in opposite directions, the liquid will resist the rotational motion due to its shear force or tear resistance. Both of these effects can be used in viscous damping equipment, used for shock absorbers and vibration control.
When a viscous liquid is exposed to shear, it attempts to stay in place and will only move when the shear force overcomes the resistance. The movement of the liquid creates heat that may need to be removed to avoid equipment damage. Large shock absorbers often contain heat sinks or radiator fins to allow air to cool them.
The effects of viscosity are speed dependent, meaning they will exhibit greater resistance when the forces of movement are greater. Very small vibrations may not be removed well with viscous damping systems alone. Many systems use a combination of springs and viscous fluids to provide a better range of vibration reduction. Damping is commonly used in parts of the world where earthquakes are common and buildings may be constructed with large dampers that isolate them from the ground. When an earthquake occurs, resistance to motion limits the amount of vibration transmitted to the building structure.
A linear viscous damping system is often used in the construction of seismic systems. The shock absorber typically contains a piston within a cylinder filled with a viscous fluid. Small ports or holes allow fluid to move back and forth in the cylinder, but most of the fluid resists any piston movement. The cylinder may be connected to construction steel and the piston shaft connected to a foundation block installed in the ground.
Shear effects can be used in devices called rotary dampers, which use a fluid between two parallel plates. Rotary dampers can be used to reduce vibration in machines and will work as a speed brake to slow down moving equipment. They can be placed under the equipment in a horizontal position to dampen vibrations in multiple directions.
Viscous damping uses clear liquids, such as silicones, water or hydrocarbon oils. Mixtures of liquids and solids can have damping effects, but the effects are different. The addition of solids can create a plastic Bingham effect, i.e. a low resistance to flow after an initial shear stress has been overcome. One example is a mixture of cornstarch and water, which pours easily but acts as a solid when shaken or tapped. Tomato ketchup is another Bingham plastic, which is why it stays in the bottle when turned upside down, but will flow easily with a shake or tap onto the container.
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