Reducing Frictional Resistance

Reducing friction between rotating, sliding, or moving parts in mechanical systems frequently results in smoother operation and lower noise output. Similarly, reducing flow resistance in fluid distribution systems results in less noise radiation.

Four of the more important factors that should be checked to reduce frictional resistance in moving parts are the following (see Figure 6.6.3):

Alignment: Proper alignment of all rotating, moving, or contacting parts results in less noise output. Good axial and directional alignment in pulley systems, gear trains, shaft coupling, power transmission systems, and bearing and axle alignment are fundamental requirements for low noise output. Polish: Highly polished and smooth surfaces between sliding, meshing, or contacting parts are required for quiet operation, particularly where bearings, gears, cams, rails, and guides are concerned. Balance: Static and dynamic balancing of rotating parts reduces frictional resistance and vibration, resulting in lower noise output (see Figure 6.6.4). Eccentricity (out-of-roundness): Off-centering of rotating parts such as pulleys, gears, rotors, and shaft and bearing alignment causes vibration and noise. Likewise, out-of-roundness of wheels, rollers, and gears causes uneven wear resulting in flat spots that generate vibration and noise.

The key to effective noise control in fluid systems is streamline How. This fact holds true for all their systems including air flow in ducts or vacuum cleaners and water flow in plumbing systems. Streamline flow is simply smooth, nonturbulent, low-friction flow.

Two factors that determine whether flow is streamline or turbulent are the speed of the fluid and the cross-sectional area of the flow path, that is, the pipe or duct diameter. The rule of thumb for quiet operation is to use a low-speed, large-diameter system to meet a specified flow capacity requirement. However, even such a system can inadvertently generate noise if certain aerodynamic design features are overlooked or ignored. A system designed for quiet operation employs the following features (see Figure 6.6.5):

Low fluid speed: Low fluid speeds avoid turbulence, one of the main causes of noise.

Smooth boundary surfaces: Duct or pipe systems with smooth interior walls, edges, and joints generate less turbulence and noise than systems with rough or jagged walls or joints. Simple layout: A well-designed duct or pipe system with a minimum of branches, turns, fittings, and connectors is less noisy than a complicated layout. Long-radius turns: Changes in flow direction should be gradual and smooth. A recommendation is for turns to have a curve radius equal to about five times the pipe diameter or major cross-sectional dimension of the duct. Flared sections: Flaring of intake and exhaust openings, particularly in a duct system, tends to reduce flow speeds at these locations, often with substantial reductions in noise output. Streamlined transition in flow path: Changes in flow path dimensions or cross-sectional areas should be gradual and smooth with tapered or flared transition sections to avoid turbulence. A good rule of thumb is to keep the cross-sectional area of the flow path as large and uniform as possible throughout the system. Minimal obstacles: The greater the number of obstacles in the flow path, the more tortuous, turbulent, and noisy the flow. All other required and functional devices in the path, such as structural supports, deflectors, and control dampers, should be as small and streamlined as possible to smooth out the flow patterns.

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