Applications on pneumatic:Noise from pneumatic equipment

Noise from pneumatic equipment

The general principles of Noise Reduction in compressors have been outlined in an earlier chapter; this chapter deals specifically with tools.

All pneumatic machines and tools tend to produce noise, caused in the main by the discharge of high pressure air from the exhaust port. In addition to the exhaust noise, there maay be mechanical noise, particularly from percussive tools.

There is a growing body of legislation limiting the noise from all types of equipment, and this places a duty on both the manufacturer and equipment user. The manufacturer is required to produce a tool which is within the prescribed legal limits, and the employer is obliged to pay attention to the noise exposure of the tool operator by limiting the working time or by providing suitable protection.

In considering the noise generated on a building site, it should be remembered that it is the total noise environment that is important when considering the exposure of the operator or the other site workers. To demonstrate the comparison between tools and compressors, Table 1 has been prepared.

Unsilenced compressors and road breakers are not available today (at least in the developed world), but the values in the table are useful in indicating the progress of silencing techniques. It can be seen that the breaker is by far the noisier piece of equipment and swamps the noise from the compressor. It makes very little sense to pay for an expensively silenced compressor and use it to operate a noisy breaker. As a general statement, the noise of a compressor can be reduced to any prescribed level, merely by adding progressively better treatment to its enclosure. A compressor can be as bulky as necessary without affecting its functioning; the only drawback is the expense. The same is not true of a breaker, which has to be both efficient and convenient to use. If a breaker cannot be handled, it will not find favour with the operator; so however well silenced it may be, ergonomic considerations will predominate in the choice of a tool.

When trying to reduce the total noise on a site it is clearly desirable to tackle first the most noisy equipment but there may be a practical limit to the noise reduction possible for certain kinds of equipment. The limit appears to have been nearly reached by a modem pneumatic breaker working on a conventional percussion system. Even the use of an hydraulic or electric breaker will not result in a quieter tool-an hydraulic breaker contrary to expectations is just as noisy as a pneumatic one, because most of the residual noise comes from the vibration of the drill bit.

Noise reduction in pneumatic tools

Most tool manufacturers apply some sort of noise reduction treatment to their tools. Figure I shows the proportion of noise from the various sources in a road breaker. It can be seen that the first and largest noise source to tackle is the exhaust noise, followed by the ringing noise from the steel, and then the internal clatter of the working parts. Fortunately exhaust noise is reasonably easy to suppress, at least in theory.

The principle to follow is to reduce the velocity of the jet noise from the exhaust port. The pressure ratio at exhaust is as high as 3 to I,which implies that the exhaust velocity is sonic and the noise is produced by turbulent mixing of the high velocity jet. The theory

on which the silencing of engine exhausts is based uses the assumption that the pressure variations are small. Such an assumption is not applicable to pneumatic tools and does not lead to useful designs. The technique that has proved most successful is diffusion of the exhaust stream by a gradually increasing cross section of the air passages. It has to be admitted that much of the design of exhaust mufflers is empirical, and most mufflers have been developed by trial and error. If the only problem were to reduce the exhaust noise, the solution would be easy- it would consist of a succession of expansion volumes joined by restricted passages. However the noise reduction must be achieved without changing the performance of the tool; so there must be no back pressure to impede the motion of the piston and no restrictions in the flow passages which could be clogged by ice formation. Balancing these factors is not easy.

Most manufacturers have found that a flexible plastic such as polyurethane is the most suitable material for manufacture of a muffler. Solid polyurethane is a very robust material which is capable of withstanding hard usage, and ideally it should form an integral part of the tool construction so that it cannot be removed without the tool ceasing to operate. Refer to the chapter on Contractors Tools for a description of the construction.

The next source of noise, particularly in a road breaker, is the steel "ring". As the impact stress wave passes down the tool stem, part of the energy will be absorbed by the road surface, but a proportion of it is reflected back and forth along the length of the tool. The stem is made of a high quality steel and so has a low internal damping, which ensures that it acts as an efficient radiator of noise. Although the energy from this source is small, it occurs at a single frequency and is subjectively very annoying. There have been attempts to suppress the noise from this source by the addition of damping rings to the stem of the tool. Such devices suffered from having a short life and have largely gone out of fashion.

The clatter from the internal working of the tool is reduced by the presence of the muffler itself, particularly if made of a flexible plastic.

The one source of noise which is very difficult to suppress is that produced by vibration of the material being worked. In the case of a road breaker, noise is produced by the shock waves in the road surface. In the case of a chipping hammer or a riveter, noise is generated by vibration of the casting or other component being worked.

Legislation on road breaker noise

Legislation in the European Community limits the sale of pneumatic breakers to those which meet the prescribed noise levels. The EEC Commission Directives, to which reference should be made, are 84/537/EEC and 85/409/EEC. In the U.K. these Directives are implemented by SI 1985:1968 and by the appropriate legislation in other European countries; they quote the maximum levels of noise and the proper test methods to be used. It should be noted that the marketing or use of breakers and compressors (as well as certain other construction equipment) which emit noise in excess of the permitted levels is a criminal offence. Breakers which satisfy the regulations have to bear an approved mark specifying the sound power level (refer to the chapter on Contractor's Tools for an illustration of this). In order to establish conformity with the Directive, the measurements have to be determined at a Test Station approved by the appropriate authority inside the Member Country of the Community. A breaker approved in one country can be freely

imported into another without further inspection. In the U.K., the authority is the Department of Trade and Industry. The laboratories which are able to perform the tests are commercial bodies (rather than Government Laboratories, as in some countries of the Community) and are inspected by NAMAS (National Measurement Accreditation Serv­ ice). At the present time only the bodies listed in Table 2 are approved to perform tests. They are in commercial competition, so the charge for performing the tests will vary.

It should be understood that the noise level measured according to the Directive is not necessarily typical of the actual noise that is likely to be experienced by an operator of a tool or by a passer-by. The test is performed for type approval of the breaker alone, and so it is devised in such a way that the steel ring and the radiated noise from the concrete block are suppressed. The quoted value is the sound power level emitted by the breaker, in the rather artificial test arrangement shown in Figure 2. In order to assess the actual noise exposure in practice, sound pressure measurements should be taken. The type approval test is useful in comparing one breaker with another, but should not be used to determine the noise environment on a particular site.

Measurement of noise from tools other than road breakers

So far, road breakers and compressors are the only items of pneumatic equipment subject to legislation. To measure the noise emitted by other items, the only available test procedure is the CAGI-Pneurop Code, which should be studied for further details. This code defines the measurement procedure for all kinds of pneumatic equipment. The readings have to be reported in terms of the sound pressure level. The measurement distance from the noise source is I m for tools and 7 m for compressors and other large equipment, and the measurement points are situated on the sides of an (assumed) enclosing parallelepiped. This code is useful for assessment of the actual noise experienced by the operator or by the public, since it measures the noise from all sources. It has been shown that when this Code is applied to compressors it gives results (when the sound pressure readings are converted to sound power) that are as accurate as the EEC method. For the reasons given above, this is not the case for road breakers.

Vibration of pneumatic tools

Another health hazard for the user of a percussive tool is vibration. Almost any power tool will generate vibration which, if the level is high enough and the exposure is sustained for long enough, will affect the health of the operator. The main disease caused by vibration is known by a variety of names - Vibration White Fingers, Raynaud's Disease of Occupational Origin or Vibration Syndrome. The Health and Safety Executive prefer the term Hand Arm Vibration Syndrome (HAYS), which includes Vibration White Fingers (VWF). VWF is characterised by intermittent blanching of one or more of the fingers, due to impaired circulation, which gets progressively worse with continued exposure to vibration. There is still much that is unknown about the disease, but the generally accepted view is that it is caused by vibration damage to the peripheral arteries in the fingers; the nerves are also affected. For a discussion on the various methods that are available for the diagnosis of VWF refer to "Hand-Arm Vibration- HS(G)88" published by HSE Books. These methods are not suitable for routine workplace surveillance. Attacks of White Finger are often precipitated by cold. They last about an hour and may be associated with considerable pain as the attack is terminated.

Even if the use of vibrating tools docs not result in the disease of White Fingers, the operator may still be adversely affected by the presence of vibration- he is likely to tire earlier and be less effective in his work- so it is sensible to take all reasonable steps to reduce the level of vibration.

Protection of operators from vibration damage

Pneumatic tools which are known to have caused White Fingers include road breakers, riveters, chipping hammers, rock drills and grinders (both hand-held and pedestal). The damage caused by White Fingers is generally considered to be irreversible, so any worker who complains of attacks should be removed from use of vibrating tools and placed in an environment where he is not likely to be subject to cold. Regular checks should be made on those operators who regularly use these tools. Apart from the use of specially designed tools with reduced vibration, the following measures are recommended:

• The tool should be held as lightly as possible consistent with proper control.

• Wearing of gloves to keep warm. Note: there is little evidence that gloves, by themselves, do much to reduce the magnitude of vibration.

• Keep the workshop warm and ensure that operators do not use tools before their hands are properly warm.

• Chisels should be kept sharp and grinding wheels properly dressed.

• Regular periods of rest allow the hands and arms time to recover and circulation to be restored.

Note that there is a recent standard ISO I 0819- Hand Arm Vibration- Methods for the measurement and evaluation of vibration transmission of gloves at the palm of the hand; this can be used to compare gloves, but this is still at a development stage and should be applied with caution.

Acceptable levels of vibration

Several attempts have been made in recent years to assess safe levels of vibration. Any investigation into this subject can only be epidemiological, ie vibration injuries and the exposure which causes them can only be assessed after they have occurred; there seems to be no reliable predictive method of determining the chance of an individual sustaining vibration injury.

There are some standards which have been prepared by the International Standards Organisation to which reference should be made. The ISO Standard covering the assessment of human exposure to hand transmitted vibration is ISO 5349 (also BS 6842 and DDENV 25349) which embodies the best current knowledge on vibration exposure. The method of expressing vibration level is by use of a weighted root mean square acceleration which takes into account the whole vibration spectrum. Legislation is not yet in place which specifies maximum vibration levels of a tool, although attention is drawn to the Machinery Directive 89/392/EEC (amended by 91/368/EEC) implemented in the U.K. by The Supply of Machinery (Safety) Regulations 1992 (SI 1992:3073). This Directive requires that the instructions supplied with the tool must state that tests have been done and either the rms acceleration does not exceed 2.5 m/s2 or if it does, the value must be stated; the test regime under which these measurements are made must be the appropriate one for the tool being tested, see below. Most manufacturers now take 2.5 rn/s2 as a target for vibration levels of their tools.

Figure 3 gives the recommendations ofiSO 5349. It should be noted that the sustainable

vibration depends on the amount of exposure during a working day and on the frequency of rest periods. Any recommendations must be provisional in the light of present knowledge on the subject, but most authorities accept the general validity of the data. The standard gives much useful information on the precautions which should be taken to prevent vibration injuries.

The operator is standing on a scale

Measurement of tool vibration

The usefulness of vibration standards necessarily depends on an exact method of determining the vibration of the tools. Standards for the measurement of handle vibrations of percussive tools have been issued for chipping hammers, rivetting hammers, rock drills, rotary hammers, grinding machines, paving breakers, hammers for construction work, impact drills, impact wrenches and orbital sanders, most of which are operated by compressed air. These are to be found in ISO 8662 (BSEN 28662).

There are two fundamental problems in the measurement of vibrations in percussive tools. The first is the establishment of a consistent means of absorbing the energy of the tool. It is not practical to allow the tool to operate in the same way as it would in practice because it would be impossible to ensure consistency; in a road breaker, for example, the variability of the concrete would make comparisons between different testing stations impossible. It might be thought that the energy absorbing method illustrated in Figure 2 would be satisfactory, but this has not been adopted by ISO. Instead the percussive energy is absorbed in a steel tube full of hardened steel balls as illustrated in Figure 4. Tests on this method have shown that the reflected energy from the absorber is of the order of 15 to 20 %, which is typical of a working situation. In ISO 8662-5, detailed dimensions are given for various sizes of tool. The down force, expressed in newtons, to be applied by the operator is to be 15 times the value of the mass of the tool in kilograms; this is in addition to the weight of the tool.

The second problem is the mounting of the accelerometer on the tool handle. Most modern tool handles are covered in resilient handgrips to reduce high frequency vibrations; the attachment of an accelerometer to these is unreliable, so ISO recommend the use of a rigid adaptor clamped to the handles. Mainly through the work done by Pneurop, the correct techniques for vibration measurement have been established and incorporated in ISO 8662. One feature of the vibration spectrum which has to be borne in mind is the high level of shock present which, unless precautions are taken, can seriously affect the accuracy of the readings. Vibration readings of the order of a few metres per second have to be measured in the presence of short-period shocks several thousand times higher.

Pneurop found that the only satisfactory measurement technique is as shown in Figures 4 and 5. A piezo-electric accelerometer is mounted on a mechanical filter which isolates the high shocks (a mechanical filter is a special accelerometer mount, with a rubber insert, which has a flat response well beyond the measurement frequency). The use of so-called shock accelerometers has been found to be ineffective in this application; all the shock accelerometers that have been tried suffer from a phenomenon known as d.c. shift, resulting in false readings. The analysis equipment must be ofhighquality; an FM recorder should be used, and the amplifier must give indication of signal overload. For further advice on the mounting of accelerometers, refer to ISO 5348 (BS 7129).

The difficulties inherent in the measurement of vibration makes it imperative that any manufacturer or importer of tools must chose a test laboratory familiar with the technique described above. Such a laboratory may be an in-house facility or a commercial company prepared to do the work. There are as yet no laboratories accredited by NAMAS. It appears that the only one currently able to do this work is situated at ISVR Southampton University, although some manufacturers have their own equipment.

Vibration reduced equipment

Some pneumatic tools have been designed with a degree of vibration isolation and their use should be encouraged where they are available. Most manufacturers now supply this kind of equipment.

There are several methods which are used to suppress the vibration of percussive tools;

most of the work has been done on the road breaker. Modern tools have comfortably shaped handles, usually made of rubber or plastic or they have resilient grips which help to take out the high frequency "sting". If the grips are removable, they must be regularly inspected and replaced when worn. A rather more elaborate form of the same idea uses spring bushes and hinges to support the handles; an example of these can be found in the chapter on Contractor's Tools.

The predominant low frequency vibration occurs at the operating frequency of the tool and is the hardest to suppress. One method uses spring/mass isolation of the handles; the spring can be a metallic helical spring or it can use the compressed air as a spring. The sprung mass attached to the handles has to be fairly large to give a cut-offfrequency at the operating frequency of the tool (about 16 Hz for a road breaker); another method incorporates an internal mass which moves in opposition to the piston, so as to neutralise the external vibration.

These methods can be effective, and a reduction of the order of 90% is realisable, but a word of warning should be given to anyone considering a purchase. Any artificial flexibility introduced into the handle is bound to affect the response of the operator, and while on a test rig the tool may behave very well, it may fail to meet the test of user acceptance. Because of the extra complexity in construction, the tool may be heavier and more expensive. Special tool bits may also be required. The assessment of vibration exposure is a complex matter, depending not only on the magnitudeofvibration of the tool handles, but also on other factors such as the grip force. Itwould make little sense to reduce vibration yet at the same time require the operator to apply a greater force to keep the tool on the work surface.

Other pneumatic tools

Chipping hammers have also been made available with a degree of vibration isolation, see Figure 6. This tool has a piston on which the air pressure constantly acts on its rear surface, so the reaction force on the handle is constant; air is alternately admitted to the front of the piston and exhausted from it; the piston reciprocates under the unbalanced forces. The force in the front chamber acts only on the tool bit, which is not felt by the operator. Shock reflections from the chisel are cushioned by the front collet. This tool is claimed to have a considerable reduction in vibration when compared with a conventional tool.

Hand held grinders also have high vibration levels. As mentioned in the chapter on Industrial Tools, vibration is produced mainly by imbalance of the wheel so it is important to keep the grinding wheel regularly dressed and balanced. In the chapter on Industrial

Tools, a grinder is illustrated which incorporates a set of bearing balls which can compensate for imbalance. Fortunately the measurement of acceleration does not the involve the same high shock levels as in percussive tools, so the accelerometer mounting is not as much of a problem.

For further information refer to "Handbook of Noise and Vibration Control" published by Elsevier.