Abstract
Understanding noise and vibration
Unwanted sound from machine tools.
Unwanted sound is broadly termed as noise. Just as for sound, noise is experienced primarily when it is conducted or airborne, and the nature of noise can be broadband or tonal. Generated from machine tools, conducted noise can be physically felt through the floor or workbench, while airborne noise, often from the same source, is carried over through air and heard rather than felt. When generated in response to random excitation, noise mostly has a broadband nature, with the amplitude and frequency varying considerably over time. Noise can also have a tonal nature such as when generated by an engine, and consist of harmonics, which can be termed as the signature characteristic of a specific engine.
Conducted noise also leads to vibrations, a mechanical phenomenon that causes surfaces to oscillate about an equilibrium point. While in some cases vibrations may be desirable, others, especially those generated by machine tools, are undesirable. Vibrations, in turn, may create further noise and are generally energy wasters. Vibrations can be free or forced.
Free vibration happens when a mechanical system has been set in motion with an initial input and is allowed to vibrate freely. For instance, a hammer strike on a metal sheet may cause the sheet to vibrate freely. The metal sheet vibrates at one or more of its natural frequencies, eventually dampening and lapsing into motionlessness.
Instead of a single strike, if the hammer strike were repeated, the metal sheet would be forced to continue vibrating. Such a time-varying disturbance can cause forced vibration when applied to a mechanical system. The disturbance can be a random input, a transient input, or a periodic and steady-state input. Additionally, the periodic input can be a harmonic or a non-harmonic disturbance. Depending on the actual mechanical system, the response magnitude may vary. For instance, the application of a periodic, harmonic input may be disastrous for a linear system, if the frequency of its steady-state vibration response matches with the frequency of the applied force or motion. This is one reason soldiers marching on a bridge are ordered to break step, mainly to prevent a structural collapse brought about by the steady-state vibrations of their marching steps matching the natural frequency of the bridge.
Measurement of noise and vibration
To control or reduce noise and vibration, the parameters must be measurable and the measurements compared with established standards. According to Broch, 1 measuring vibration primarily requires monitoring vibratory displacement, velocity, and acceleration (peak or root mean square (RMS) values), using voltage, or charge type piezoelectric transducers. Electronic measuring instruments used for the purpose perform the operation by an integrating process. The relationship between the three parameters is a function of frequency.
While calibrated microphones are used for measuring airborne noise, conducted vibration is picked up with transducers producing an electrical output. Earlier, people used velocity-sensitive devices, but lately, the trend is toward the use of acceleration-sensitive transducers, also called accelerometers. The latter are physically much smaller, with a significantly wider frequency and dynamic range. This matches the requirements of modern machine tools that usually generate high-frequency vibration. For instance, high-frequency vibration from high-speed machinery carries valuable information about the condition of rolling elements such as ball, roller, needle bearing, gear teeth, and turbo machinery blades. Additionally, acceleration signals can be easily and validly integrated electronically to obtain velocity and displacement information.
Mechanical systems also produce appreciable displacements at low frequencies. This is often used when detecting and measuring unbalances in rotating machine parts, such as the displacements occurring at the frequency of shaft rotation during the balancing of a wheel.
Piezoelectric transducer.
Calibrated microphone.
Measuring amplifier and analyzer.
The electrical output of a microphone or accelerometer is fed to a preamplifier featuring very high input impedance and a low output impedance. Along with a variable amplification, the preamplifier has many facilities for conditioning the signal. The type of preamplifier used depends on whether the accelerometer is a voltage or charge source. The output of the preamplifier is suitable for recording on tape recorders or for feeding to a measuring amplifier or analyzer. Usually, the preamplifier is equipped with integrators and a range of high- and low-pass filters. This allows the measuring system to avoid interference from electrical noise or signals outside the linear portion of the frequency range of the accelerometer.
For measurements to be effective, microphones and accelerometers need to be periodically calibrated against international standards. Exciter systems with reference standard microphones and accelerometers are used for the purpose.
International standards related to noise and vibration
ISO, 2 the International Organization for Standardization, has published several standards related to measurement and analysis of noise and vibration. For instance, ISO 2017-1:2005 helps users when using a new or previously installed product, especially when the user has to solve a vibration problem. The standard establishes requirements for ensuring appropriate exchange of information between users, manufacturers, and suppliers of vibration sources and receivers with respect to the application of isolation systems.
Another standard, ISO 2017-3:2015, specifies the information to be exchanged between building owner, customer, and vibration isolation supplier. The standard is applicable during the design and construction of a new building in areas affected by vibrations generated by single or multiple sources. It gives appropriate responses to questions highlighted, such as why, what, when, and how mechanical systems may be isolated.
According to OSHA Pocket Guide, 3 NIOSH, The National Institute for Occupational Safety and Health, has recommended that “all worker exposures to noise should be controlled below a level equivalent to 85 dBA for eight hours to minimize occupational noise-induced hearing loss.”
Noise and vibration on the shop floor
Noise and vibration on the shop floor.
Shop floors today use sophisticated equipment and machine tools offering enhanced performance, repeatability, and precision. However, to realize these features, it is necessary to install them properly, meeting the specific requirement of the equipment. Since only about 1% of the value of the machinery goes into proper installation, not giving consideration to all facets of machine mounting amounts to ignorance or poor economy.
Lack of proper machine installation may provide inadequate support or isolation against noise, shock, and vibration, including inadequate means to attain and maintain an accurate degree of levelness. Apart from generating noise and vibration, this may shorten equipment life, lead to excessive maintenance, frequent realignment, and lost production.
Manufacturing operations and related activities in the plant often generate noise and vibration. One of the major sources of vibration is the resonance of the floor itself, but often overlooked. Foszcz 4 points out that when excited mechanically, building floors often resonate at several frequencies causing conducted vibration that can affect precision machine tools if the vibration frequencies are higher than 20–24 Hz.
Mounting methods to reduce conducted noise and vibration on the shop floor
On the shop floor, precision machinery can be termed as support-critical from an installation standpoint. For such support-critical machinery, the foundation is a critical factor in achieving optimum performance. For some machinery, it is enough to follow basic concrete design procedures for the foundation. Others may need to be bolted to the floor, to add to rigidity rather than support, and to prevent the machine from “walking” during operation. According to Foszcz, 4 the design of the foundation needs to be stiff enough for providing support, while at the same time, it must be able to react uniformly to dynamic loads.
In some cases, the above may prove inadequate and machinery may require total isolation to prevent the noise and vibration generated from disturbing neighboring equipment. This may require inertial block construction along with pneumatic isolation. Foszcz 4 has shown that by suspending the machine and its foundation system on deflecting elements that absorb energy, it is possible to reduce the transmission of shock and vibration by as much as 90%.
Isolation pads.
By inserting resilient material between the supporting structure and the vibrating source, it is possible to reduce structure-borne noise transmission. 5 Such vibration isolation techniques help in the control of noise, shock, and vibration from heavy machinery tools, large ventilation equipment, generators, process equipment, pumps, and delicate lab instruments. The industry uses a wide selection of such resilient material, including neoprene, fiberglass, springs, air mounts, and machine mounts. Using such materials, it is possible to have a variety of mounting methods (according to Foszcz 4 ) and vibration isolation systems (according to Kinetics Noise Control 6 ).
Use of pads
Since its early years, industry has been using felted fiber for absorbing shock and vibration from machines. Gradual refinements of this material have made it impervious to most industrial chemicals, oils, and moisture. In most cases, its life expectancy surpasses that of the equipment it supports. This material can be used along with isolation sleeves and anchor bolts. Two types of pads are commonly used.
Laminated pads.
With reference to Kinetics Noise Control, 5 pre-compressed, inorganic, inert fiberglass isolation pads with a coating of flexible elastomeric moisture barrier are effective in reducing shock transmission from impact producing machinery, such as punch presses. The fiberglass isolation pads can also reduce vibration produced by cooling towers, chillers, and pumps.
Laminated isolation pads can absorb noise, shock, and vibration generated by high load equipment, says the Kinetics Noise Control. 5 There are two types—one, a blend of ozone-resistant rubber elastomers, and the other, a blend of laminated neoprene. Neoprene pads have pocket patterns and do not require adhesives, bolts, holes, or tools for installation. Simply laying them on the floor and placing the machinery on them is adequate to prevent walking or creeping machines.
Use of leveling mounts
According to Foszcz, 4 this device compensates for uneven floors. It combines leveling capability with ease of installation through pads. The device has a base with an attached elastomer that can be slipped under the foot of the machine. A screw inserted from the top can be used for fast, simplified, precise leveling. Such mounts can be of various types, with and without the leveling facility. For instance, the mounts can have fiberglass isolators, spring isolators, and air isolation mounts.
Fiberglass isolation mounts.
Spring isolator.
Housed spring isolator.
According to the Kinetics Noise Control,5,6 fiberglass isolator mounts come with load plates and mounting brackets. These are specifically designed for bolt-down applications such as for high-speed motors, axial fans, vent fans, and similar equipment.
Kinetics Noise Control5,6 also depict spring isolators that are restrained, freestanding, or housed. The spring element, complete with high frequency vibration control noise pads have adjustable top load plates with leveling bolts. Restrained spring isolators are usually assembled within a welded steel housing that limits vertical movement of the isolated equipment. The freestanding spring isolators, with stable steel springs in between an upper load plate and a lower load plate, can also have noise plate assemblies for control of high-frequency vibration.
According to Foszcz, 4 and Kinetics Noise Control,5,6 housed spring isolators are ideal for isolating mechanical equipment that has to be frequently started up and shut down, such as compressors and engine generators. These isolators consist of telescoping aluminum or cast iron housings with high-deflection stable springs assembled along with noise pads bonded to the lower load surface. The top housing contains the adjusting and leveling bolts.
Foszcz 4 and Kinetics Noise Control 5 go on to show that pneumatic, elastomeric vibration mounts offer low, natural frequency isolation for protecting sensitive equipment from floor-borne vibration. The mounts come with automatic leveling controls and custom mounting.
Reducing airborne noise
Composites, barriers, and damping are very effective in controlling airborne noise. 6 This usually involves reducing noise generated by sources such as machine access, ventilation, raw material input and parts overflow, lighting, fire protection, and installation that do not support process disruption. Noise control products can be broadly classified as belonging to two groups—flexible and rigid.
Barriers and damping.
Flexible products used for control of airborne noise include curtain enclosures, sound absorbers, barrier materials, composites, and sound damping. If flexible products are not enough for curbing the noise to the desired extent, rigid products such as acoustic enclosure systems, barrier wall systems, and silencers supplement them.
Smart-structure technology for actively controlling noise and vibration
A smart-structure technology for actively controlling noise and vibration involves five key elements—the material used for structure, distributed sensors, and actuators, strategies for control, and electronics for power conditioning. These components allow the smart structure to respond to changing environmental and operational conditions.
The smart-structure technology can actively control noise, vibration, and deformations. Apart from machine tools, applications involve space systems, fixed-wing and rotary-wing aircraft, optical systems, automotive, medical systems, and other infrastructure.
The technology involves use of microprocessors for analyzing the response from sensors, while using integrated control algorithms for commanding actuators to apply localized damping/displacements/strains to alter the elasto-mechanical response of the system. The structure has highly integrated—bonded or embedded—actuators and sensors that do not cause any significant changes in the structural stiffness or mass of the system. Most of the actuators and sensors are solid state and based on smart materials.
New smart materials for actively reducing noise and vibration
Smart materials.
Piezoelectric ceramics have always been the first choice in active noise and vibration reduction. These materials are popular because of their easy commercial availability, fast response times, and the ability to generate large forces. Moreover, they come in the form of fibers, patches, and stacks, enabling them to be easily integrated into structural components.
Electrostrictive ceramics and magnetostrictive material are other less popular commercial smart materials used for active noise and vibration reduction. Compared with piezoelectric ceramics, these materials are limited by their low actuator stroke.
The efforts to generate materials with larger actuator strokes have resulted in the development of new and promising materials, especially in the form of electroactive polymers and ferromagnetic shape memory alloys. However, these new materials presently have low mechanical stiffness, producing comparably low forces.
Piezoelectric ceramics
Jordan and Ounaies 7 say the most widely used piezoelectric material, lead zirconate titanate, or PZT, exhibits very high dielectric and piezoelectric properties. This material can be used interchangeably as a sensor and actuator, as it responds to both direct and converse piezoelectric effects. When subject to physical strain, the direct piezoelectric effect causes the material to produce a voltage. Likewise, when placed in an electric field, the converse piezoelectric effect causes the material to become physically strained.
For active noise and vibration reduction tasks, piezoelectric ceramics are used in the form of thin sheets or stripes/fibers. These are embedded or attached within composite structures or stacked to form discrete piezostack actuators.
Piezoelectric polymers
Kawai 8 says that piezoelectric polymers such as polyvinylidene fluoride (PVDF), along with its copolymers tetrafluoroethylene (TFE), and trifluoroethylene (TrFE) show strong piezoelectric activity. These are semi-crystalline fluoropolymers presently representing the commercially available and state of the art in piezoelectric polymers. Similar to PZT, the PVDF can also differentiate between direct and conversion effects, which makes it suitable for use both as sensor and actuator.
However, PVDF contracts in the direction of applied electric field instead of elongating as the PZT does. PVDF has 10–20 times smaller strain constants as compared with those of PZT. Additionally, the low stiffness of PVDF significantly reduces its authority over the structure. This limits the applications of PVDF to low-force requirements.
PVDF has certain advantages over PZT. With lower density and higher processing flexibility, PVDF is tough, can be readily manufactured into large areas, and can be cut and formed into complex shapes.
Electrostrictive ceramics
According to Monner, 9 unlike PZT, ceramic substances such as lead magnesium niobate (PMN), although not polarized, exhibit a change in length when subjected to an electric field. Again, unlike piezoelectric ceramics, electrostrictive ceramics increase in length in the presence of both negative and positive electric fields. However, the temperature dependence of electrostrictive ceramics—operation temperature less than 40°C—makes them of less practical importance compared with piezoelectric ceramics.
Magnetostrictive materials
Monner 9 is of the opinion that materials such as nickel, cobalt, iron, and their alloys exhibit magnetostrictive effects. Rods of these materials exhibit a change in diameter in the presence of a magnetic field and vice versa. Of late, the discovery of Terfenol-D by a research group at the US Naval Ordnance Lab (NOL) has led to the manufacture of actuators with an improved performance compared with that offered by piezoelectric stack actuators.
Shape memory alloy
According to Monner, 9 shape memory alloys (SMAs) such as the gold-cadmium alloy and the nickel-titanium alloy are easily deformed at lower temperatures, but can recover their original shape and rigidity when heated to above the transformation temperature. Materials exhibiting two-way shape memory effect can return to their preset shape when heated to above the transformation temperature, but change to a certain alternate shape when cooled. The material requires several cycles of special training for it to memorize the alternate shape.
Available in the form of wires, rods, tubes, ribbons, and thin sheets, the superelastic hysteresis effect makes SMA suitable for structural damping, when used for active reduction of noise and vibration. By generating internal compressive stress in a structure, SMA can change the eigenfrequencies of the structure in certain specialized applications.
Other emerging smart materials
According to Monner, 9 research in the electroactive polymers (EAP) field is adding new smart materials to augment those already available for reducing noise and vibration actively. Among them are ferromagnetic shape memory alloys (FSMAs), EAPs, electrostrictive polymers, electrostrictive graft elastomers, electrostatically stricted polymer (ESSP), ionomeric polymer–metal composites, conductive polymers, carbon nanotube actuators, and others.
Electric EAPs have advantages in noise and vibration reduction because of their fast response times and large strain generation. However, the voltages required presents a challenge to electronic drive circuits. One way of circumventing the high-voltage requirements is to use only thin films of the materials.
Actions and recommendations for controlling vibration and noise
According to the Noise Control Tool, 10 over the years, several typically tested and verified methods have emerged for controlling noise and vibration at the source. However, these are selectively applicable, meaning some methods may give better results in specific application. A summary follows.
Reducing vibration and mechanical noise
Simply tightening parts or panels, and balancing rotating parts, can often reduce vibration and mechanical noise. In some cases, installing isolation dampers and or using flexible connectors for piping helps. If changing the speed of rotating parts does not help in reducing vibration and noise, it may be necessary to replace toothed gears with helicoidal ones or replace metal parts with plastic alternatives.
Reducing transmission of sound through ground and structures
Installing isolation dampers is one way of isolating a source generating low-intensity sound or vibration. According to the Kinetics Noise Control, 5 for sound of higher intensities, it may be necessary to isolate the source structurally to prevent/reduce sound or vibration form being transmitted through the floor or other structures.
Reducing effect of shocks and impacts
A lot can be gained by changing work habits. For instance, allowing two objects to ease into contact prevents the generation of an impact noise. Metal parts falling perpendicularly on another metal part generate more shock noise than when allowed to fall at an angle—it helps to tilt the surface on which the parts are falling. Reducing the falling distance also generates shock and impact noise of lower intensity.
Reducing the effect of metal surfaces and containers
Metal surfaces and containers may vibrate in sympathy with the conducted/airborne noise and vibration generated elsewhere. This mostly happens if the natural frequency of the surface happens to match the tones of the offending noise. If feasible, the vibrating surfaces and or containers may be constructed out of “noise-less” steel, usually with a rubber sheet sandwiched between two steel sheets.
Less expensive methods may also be used such as using wood, plastic, or other non-metals for constructing the surfaces and containers. If it is not feasible to replace the metal surfaces and containers, covering them with noise dampening material may prevent them from vibrating.
Reducing room reverberations
A noisy machine in a highly reflecting room may be very annoying to the operator. Adding absorbing materials to the walls and ceiling helps to reduce the noise and vibration from being reflected. This method also helps in reducing noise transmitted to adjoining rooms. Kinetics Noise Control 5 is of the opinion that using a sufficiently dense material to enclose the vibrating part of the machine may also help in reducing the noise transmitted.
Reducing airborne noise
Apart from conducted noise and vibration, machine tools may also generate airborne noise. Kinetics Noise Control 6 is of the view that barrier walls, sound barriers, and absorbent materials used individually or in combination are highly effective in reducing such airborne noise.
Regular maintenance
Regular maintenance of machine tools goes a long way in reducing generation of unwanted noise and vibration, by timely replacing machine parts that otherwise may have become loose during operation or failing from being simply worn out.
Rooms and enclosures also need regular maintenance for providing continued isolation. Vibrations may cause cracks and gaps to appear around partitions and barriers through which sound leaks can seriously reduce the effectiveness of the barrier. Most susceptible areas are the flexible connections and gaps where ceilings and walls meet. Regular maintenance is necessary to fill these cracks and gaps in walls and sound barriers.
Ensuring smoother airflows in ducts
Smooth airflow in ducts generates far less noise than when the air is moving irregularly. It is important to select the fan judiciously for producing smooth airflow. It is equally important to ensure there are no obstructions or sharp turns in the duct, as this may cause turbulence and hence generate noise.
Increasing the distance between noise source and worker
Even without obstructions, sound levels gradually decrease as sound waves travel away from the source. Every doubling of distance from the source reduces the sound level by a quarter of the original level.
According to OSHA Pocket Guide, 3 OSHA guidelines warn that exposure to high levels of noise can permanently damage a person’s hearing. Provided it is feasible, simply increasing the distance between the noise source and the worker will reduce the level appreciably.
Conclusion
Loud noise and vibration reduce work productivity and contribute to workplace accidents as it makes it difficult to hear warning signals. Although damage to hearing can be prevented, there is no reversal from hearing loss caused by noise—the damage is permanent. Additionally, hearing loss usually occurs gradually, so the realization it is happening, comes very late.
Therefore, it is important to reduce exposure to hazardous noise from machine tools by planning for potential exposure before stating activities.
Footnotes
Note
This article was compiled by NVW staff from public domain sources.