The following article will guide you about how to control noise from pneumatic tools in metal industry.
There are three broad classifications of pneumatic tools:
2. Piston and
The noise generated by the use of these tools is due to:
1. Pneumatic exhaust.
2. Tool noise: impacts, rotation.
3. Tool/workpiece interaction.
Most of the direct noise from the tool is due to pneumatic exhaust. The exhaust noise of almost all tools, except for newer models, exceeds 90 dBA under both no-load and load conditions. The pneumatic tool industry was engaged in an increased power per pound race in the early sixties, which lead to an increase in tool noise.
This complicates the situation greatly.
There are, however, several methods for quieting exhaust noise:
1. Piped away exhaust.
2. Expansion volume (muffler).
3. Diffusion at exit.
4. Tortuous path.
Piped away exhaust systems are by far the most effective where noise is the only consideration. However, due to cost and physical constraints, piped away exhaust systems are generally not the most practical solution to the problem. Piped away systems remove the exhaust from a tool by means of a light-weight hose coupled to the handle.
The exhaust air then passes through a muffler or manifold. The major advantage to this system is that it removes exhaust noise to a place remote from the operator and then silences the noise effectively. More than one tool can be connected to a muffler in a piped away exhaust system.
The second solution to tool exhaust noise is the use of an expansion volume or muffler. This involves the use of an expansion chamber around the exhaust port of the tool to reduce the noise. The system works well, but does have several drawbacks. The first is that mufflers increase tool size and weight. Mufflers also require maintenance to keep their exhaust ports open and free.
The third and fourth solutions to the exhaust noise problem are usually used in conjunction. Diffusion at the exit reduces the exhaust velocity, which in turn, reduces the noise. This is accomplished by using meshes, sintered metals, felts and open-cell plastic foam. The major problem with this solution is that clogging occurs at the exhaust port and a pressure drop of 4-8 psi occurs at the exit. The tortuous path method involves sending the exhaust air in a tortuous path through the tool to the exhaust port. This also lowers exhaust velocity and noise.
Tool mechanism noise, other than exhaust noise, is another major noise problem. The driving mechanism in a pneumatic tool creates a large amount of acoustical and vibrational energy. Most of this sound is shielded by the tool itself. Noise from rotary tools using vane-type motors is usually present in the exhaust.
The easiest method to lower the noise level of pneumatic tools is to buy new tools. However, many plants are presented with the problem of tools that work well except that the original exhaust ports have deteriorated. A 10 dBA reduction may be achieved by placing perforated sheet metal over the open exhaust port of vertical grinders where the port is badly damaged. An additional 2 dBA reduction may be obtained by making the thickness of the perforated metal equal to the diameter of the holes.
Pin grinders are very similar to vertical grinders in their noise generation. A basic diffuser can be made for pin grinders by placing 10 ppi foam in the exhaust ports.
Percussive tools, however, present another problem. There are two types of percussive pneumatic tools. One type used a free reciprocating piston to strike repeated blows on a tool held in the end of the barrel. The other type reciprocates a captive piston to pound or ram.
Essentially, both mechanisms have a device which strikes a chisel. The chisel, after being struck, then radiates cylindrical sound waves. Noise level of various chisels is shown in Table 16.1. From this it is seen that the sound from a pneumatic tool can be decreased slightly by proper chisel selection.
In one of the study, sound level measurements were performed during chipping operations with a normal and newly sharpened chisel. A sound level reduction of 5 dBA was measured with the sharpened chisel. The employee noise exposure time can also be reduced by use of sharp chisels, since cleaning will be more efficient.
Two other potential methods for quieting percussive tool noise are to coat the chisel with lead or with a visco-elastic material. Neither of these treatments has been found to hold up under load conditions. Coating the chisel with lead also decreases the tool efficiency, as the mass in increased. Special alloy steel chisels have been tried, but the cost is prohibitive for large-scale use and significant noise reductions have not been documented.
Vibration damping material placed between the piston and the chisel has had some limited success. Once again, with this application, tool efficiency is lost.
In a recent test a slag hammer in free operating mode, three different heads were evaluated with regard to associated sound levels. A weld flux scaler head resulted in a sound level of 95 dBA, a damped chisel resulted in a sound level of 86 dBA and a cold chisel resulted in a sound level of 98 dBA.
The use of chipping tools appears to be inherently noisy, while it is reported that noise level reductions of up to 20 dBA can be achieved by the substitution of needle scalers for chipping hammers for certain operations. It should be pointed out that all needle scaler are not quiet and that selection of the proper drive mechanism and needle type must be made in order to achieve minimum noise levels.
Assessment of needle scalers must be made with regard to production compatibility as well as noise reduction.
Potential advantages of the use of needle scalers include:
1. Needle scalers are reported to provide better weld finishes.
2. Needle scalers provide some stress relieving of the weld seam.
3. Needles do not require sharpening.
4. Needles last longer than chisels when used properly (up to 500 hours).
One reported disadvantage of the needle scaler in the experiments conducted was the failure of the tool air exhaust to blow away powder which accumulates in the weld seams. The front of slag hammers provides this function. To solve this problem, the side ports of needle scalers may be blocked to provide front exhaust from these tools also.
Needle scalers may have the following disadvantages:
1. Employees may show a psychological reluctance to change.
2. Weld cleaning may require more time with needle scalers than with slag hammers (however, there have also been reports that needle scalers are faster).
3. Needle scalers are not applicable for all types of welds.
As with chipping hammer chisel selection, the proper selection of needles for needle scalers may reduce the noise level. In a recent test of three needle scalers during a slagging operation, it was found that up to an 8 dBA reduction in the noise level may result by using 2 mm needles instead of 3 mm needles.
Impact wrenches are not always a major noise problem, although some of the larger models can become a noise hazard.
There are five steps to be taken for effective noise control of impact wrenches:
1. Contact the manufacturer of the wrench to find out whether retrofit mufflers are available.
2. Pipe away the exhaust from the auxiliary exhaust port if facilities for piped away exhaust are available.
3. Insert 10 ppi foam in the exhaust port.
4. One element of impact wrench tool noise is caused by operator misuse. It is common practice for tool operators to keep a wrench working on a nut or bolt after optimal tightening has been achieved. This practice causes the tool to emit excessive noise. The only solution to this problem is to educate the impact wrench operator not to continue this incorrect mode of operation.
5. If possible, replace old tools with automatic shut-off angle nutrunners.
Air cylinders are simple to silence. Noise is generated by two mechanisms: the exhaust and the stroke. The exhaust can be muffled or piped away and pad can be placed on the cylinder to cushion the impact of the return stroke. Several cushioned cylinders are commercially available.
In every plant with a pneumatic air system, the problem of air leaks arises. Air leaks are a problem that requires continuing attention because they can generate noise levels of over 90 dBA as well as wasting expensive compressed air.
In addition to the noise generated by the pneumatic tool exhaust, noise is also generated by vibrations induced into the work piece due to grinding or chipping. This noise may be reduced by application of a vibration damping material to the vibrating surface.
There are two basic configurations for the application of damping treatment to structures to increase the loss factor – free layer damping and constrained layer damping. Free layer damping, also referred to as extensional or surface damping, is the most commonly and easily applied.
Damping treatment of this type may be a viscous substance which is applied similarly to an automobile undercoating or may be in the form of a sheet or tile which is bonded to the structure being damped. Constrained layer damping involves sandwiching a layer of viscoelastic material between the structure being damped and an outer constraining layer. This type of damping finds application where structural members are quite thick or where a large vibration reduction is required.
The selection of vibration damping treatment is dependent upon:
1. Material type.
2. Material thickness.
3. Panel size.
4. Required vibration reduction.
It should be pointed out that it is not necessary to treat an entire surface area to achieve effective damping.
Vibration damping may be installed on structures being ground or chipped, utilising the following concepts:
1. Placement on a viscoelastic fixture.
2. Application of a damping panel by means of clamps, magnets or fixturing.
Our analysis has indicated that vibration damping may not provide significant vibration or noise reduction for chipping operations of large parts for the following reasons:
1. ‘For a single degree-of-freedom system, it can be shown that when the ratio of the pulse duration to the system natural period is much less than one, the maximum response can be reduced by increasing the mass of the structure. On the other hand, when the ratio of the pulse duration to the system natural period is much greater than one, i.e., the force is applied slowly, the maximum response occurs while the force is acting.
In the latter case, the response is inversely proportional to stiffness, i.e., increasing the stiffness should reduce response and hence, reduce noise. When the duration of the force is equal to one-half the natural period of the system, a pseudo resonance exists. Control of resonant response can be achieved by detuning the system or adding damping. As pointed out by Harris and Crede, however, a tenfold increase in the fraction of critical damping produces a decrease in maximum response on only about nine per cent.’
2. The thickness of some parts (above ½”) would render conventional damping ineffective, even if applicable. A constrained layer damping would be extremely difficult to conform to the irregular shape of many parts.
An experiment was conducted at a plant to determine the influence of vibration damping treatment on noise levels generated by the use of a chipping hammer. Three ⅛ ” plates of approximately 2 square foot area were treated with an area of 0, 20 and 100 per cent with a trowel-on damping compound.
The sound levels measured during chipping were:
It should be pointed out that the damping compound was out completely dry on the 100 per cent damped plate and was of a thickness slightly less than ⅛”. From these results, however, it is not expected that a thicker damping treatment would provide significantly greater noise reduction.
In many operations, castings or other metal products are placed on workbenches where they are chipped or ground. Vibrations are transmitted directly from the castings which are being chipped or ground to the metal workbench top. The tables, being of large surface area and relatively light weight, are excellent radiators of sound.
Sound radiation from the table may be reduced by isolating the work piece vibrations by means of a rubber or soft plastic lining placed on the surface of the table. For effective isolation, any rubber material of thickness not less than ⅜” may be used. The primary requirement of the rubber material is wear ability. In this regard, either conveyor belting or a wear resistant rubber should be satisfactory.
If the workbench is the sight of a welding/grinding operation and the table is used as a ground, placement of constrained layer damping material on the underside of the work surface should be considered.
A less effective but satisfactory alternative to the use of rubber table tops would be table tops constructed of wood. Where wood top benches are presently in use, their use should be continued.
The highest sound levels incurred during chipping operations are when parts are not supported or in contact with a work surface area. Employees should be instructed that parts should be placed on the work surfaces in a manner to provide maximum surface area support.
The practice of chipping parts on wood horses or on surfaces where support of the part is not allowed should be eliminated. It may be necessary to provide additional workbenches to accommodate all chipping requirements which may occur at any given time.
A series of experiments were conducted to determine the influence on noise generation of the manner in which parts are clamped or supported during the grinding and chipping operations. Parts being ground at grinding booths in a cleaning room were clamped by means of pneumatic vises.
It was initially theorised that insertion of a vibration absorbing viscoelastic material or wood between the metal vise and part would result in absorption of a significant amount of vibrational energy, thereby achieving noise reduction. A series of experiments indicated that this was not the case. Measured sound levels were 1 dBA higher with rubber or wood inserts than with the metal vise alone. Thus the increased sound levels were due to the decreased rigidity of the work piece in the semi-rigid mounting.
Experiments related to clamping for the chipping operations also indicated that noise levels were not significantly affected by the method or type of clamping.
Damping of vibrations of large castings while being ground is a surprisingly simple process. By placing the casting in a sandbox, vibrations are damped to a degree and they are not transmitted to the normal work surface.
The following sound levels were reported for chipping on a metal casting in a recent experiment:
A pneumatic tool can and will make more noise when it is not in proper operating condition. One of the main causes of tool degeneration is water in the air lines. This can be corrected by air dryers and water traps.
A regular maintenance programme for pneumatic tools should be maintained. All rotary tools should also be checked for proper operating speeds. A tool which is being operated in a higher rpm range than that for which it was designed will create higher noise levels.
To insure minimum operating noise levels of all grinders, it is advisable that each grinder be inspected for noise levels whenever it is brought to the maintenance department for repair or inspection.
The acoustical inspection should include:
1. Checking the sound level with a sound meter.
2. Checking the rpm.
3. A visual inspection of the muffler or exhaust ports.
If excessive sound levels are observed, the grinder should be repaired or the exhaust muffler should be replaced. The filter-lubricator is probably the most important component of a pneumatic system. It is the last defense against water and particulate matter. The lubrication added will lower the sound level of a tool by 5 dBA or more and as a side benefit, greatly increase the life of the tool.