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Noise and Noise Control

Noise and Noise Control. Sound

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Noise and Noise Control

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    1. Noise and Noise Control BAE 515 – Lecture 14

    2. Noise and Noise Control Sound – Air pressure waves which causes the human ear drum to vibrate – sound is created by anything which causes pressure changes in undisturbed air. Prior to passage of the Walsh-Healy Act, hydraulic system “noise” was buried in overall machinery noise levels. Promulgation of OSHA regulations has caused noise levels to decline, exposing hydraulic systems to criticism. Sound waves have two measureable characteristics: Amplitude – a measure of loudness Frequency – pitch of noise

    3. Noise and Noise Control Amplitude of sound is measured in decibels (db); frequency is measured in cycles per second or Hertz (Hz) OSHA regulations on noise level and worker exposure are shown in Fig 19.2

    4. Noise and Noise Control Sound level meters filter high frequency noise out of the measurement, OSHA regulations stipulate the use of “A” scale measurements as shown in Fig. 19.3 Sound pressure levels contribute to ear damage while high frequency noise is irritating.

    5. Sources of Noise in Hydraulic Systems Two types of noise: Fluid born - noise transmitted through the fluid Structure born – noise transmitted through structural elements of machine Sources of noise: Fluid-induced – noise generated by high pressure gradients (cavitation and pump pressure ripple) Mechanical-induced – noise generated by impacting mechanical parts (gear teeth, dynamic unbalance caused by vanes sticking in slots, vibrating lines)

    6. Sources of Noise in Hydraulic Systems Most common source of noise in a hydraulic pump is caused by rapid changes in fluid pressure In all pumps, fluid pressure increased in the chamber which moves fluid from inlet to outlet port. Improperly designed pumps allow this change in pressure, to take place too abruptly, thereby increasing the potential for noise

    7. Sources of Noise in Hydraulic Systems Compression and decompression pressure changes at pump outlet causes pressure ripple. Pump ripple amplitude is further magnified when mechanical components are excited at their natural frequencies.

    8. Sources of Noise in Hydraulic Systems Aeration and cavitation case noise. Mineral-based oils generally contain 9% dissolved air, which remains in solution at typical system pressures. Cavitation is caused when fluid passes through a restriction resulting in an increase in velocity, and drop in static pressure. Air held in solution forms bubbles which are suspended in solution. As pressure increases in the pump compression zone, these bubbles collapse and the air goes back into solution. This collapse causes pressure fluctuations and loud noises.

    9. Sources of Noise in Hydraulic Systems

    10. Sources of Noise in Hydraulic Systems Typical noise levels for hydraulic pumps are as follows:

    11. Sources of Noise in Hydraulic Systems The probability of cavitation increases as air is added to the oil, this usually occurs in the inlet line. When cavitation occurs, sound levels may jump 10 to 15 db with a characteristic shrill. High fluid velocity, velocity changes, and turbulence can also cause cavitation in hydraulic motors and valves. Air entrainment in oil is in proportion to air pressure above the oil.

    12. Sources of Noise in Hydraulic Systems

    13. Sources of Noise in Hydraulic Systems Pump cavitation can be controlled by: Suction line velocities should be kept below 5 ft/s or 1.5 m/s Pump inlet lines should be short Pump inlets should have a minimum number of bends and joints Place reservoir above pump inlet when possible to flood the inlet Use a low pressure-drop inlet filter or strainer Use a low viscosity oil

    14. Sources of Noise in Hydraulic Systems Pressurized reservoirs do not reduce the potential for out-gassing in the supply line to the pump

    15. Sources of Noise in Hydraulic Systems Air leaks on the inlet side of the pump cause noise. Some mobile application utilize pressurized reservoirs to eliminate this problem Another source of air ingestion is around cylinder rod seals when loads are dropped quickly. High vacuums created on the supply side of the cylinder pull air past the seals.

    16. Reservoir Design Best place to prevent air bubbles from becoming entrained in fluid is the reservoir. Design Considerations Reservoir capacity should be twice the total pump capacity for two minutes Reservoir should be baffled to allow enough retention (dwell) time to allow bubbles to dissipate Tank return lines should be as far from pump inlet as possible When adequate reservoir size or baffling is not possible, sloping screens can be used to help separate air from oil

    17. Reservoir Design

    18. Reservoir Design A 60 mesh screen installed at a 300 angle from the horizontal will effectively remove up to 90% of air bubbles Return lines must enter the reservoir below the oil level, or jet action will entrain air A diffuser can be used to break up the returning flow stream Exit velocity must be less than 2 ft/s Diffuser opening should be a maximum of 40% of surface area

    19. Reservoir Design

    20. Reservoir Design Reservoir baffles should contain holes so that oil does not slosh or pour over the top of baffle on mobile machinery, thereby entraining more air

    21. Pump Noise Pump noise is composed of many single frequency components (i.e., pump unbalance, drive motor, couplings, etc.) Largest noise energy component is at pumping frequency Shaft speed times number of pumping elements: pistons, vanes, or gear teeth Noise amplification occurs at harmonics Small pumps tent to peak (radiation of energy) near the sixth harmonic

    22. Pump Noise

    23. Noise Radiation Pumps, being relatively small, are poor noise radiators. Motors, reservoirs, and fluid conductors are good radiators of pump noise Valves are another source of noise generation, the most common type of noise is “hissing,” which is associated with cavitation Squeals can be caused by imperfection in valve seats Pump pulsations can also cause poppet valves to rattle

    24. Noise Reduction Techniques Pumps may be operated at lower speeds (i.e, electric motors rated at 1800 rpm can be replaced with 1200 rpm motors) Pumps can be enclosed in a sound-reducing enclosure Metal drive couplings should be replaced and properly aligned. Isolate pump using resilient mounts (isolation mounts) Pump mounting plate mass can be increased (plate thickness >= 1.0 in) Connections to reservoir should be flexible hose Add isolation mounts to reservoir

    25. Noise Reduction Techniques

    26. Noise Reduction Techniques Align drive couplings between pump and motor Use acoustic filters on high pressure lines (accumulator, pulse filter, single frequency devices, and reactive muffler) Isolate lines from the rest of the machine (line supports, bulk head fittings, or flexible hose)

    27. Noise Reduction Techniques

    28. Noise Reduction Techniques

    29. Noise Reduction Techniques

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