Industrial ERG

CHAPTER 10 VIBRATION

LEARNING OBJECTIVE

The students will be able to identify different sources of vibration, as well as the risks of permanent damage to the body, if exposed to high levels of vibration. The students will be able to identify the signs and symptoms involved in exposure to vibration and learn how to reduce vibration exposure.

INTRODUCTION

The topic of vibration and occupational exposure to vibration is complex, and this chapter provides a simplified overview of the topics.

Vibration can occur in any possible plane, and at any point a source of vibration can be generating vibration waves in more than one plane. Quite honestly, everyone living in a modern society, even those living in remote areas, experience some level of vibration almost constantly. For instance, the Earth vibrates at approximately 7.83 Hz (Schumann & Konig, 1954). Sound waves are vibration waves that are transmitted through air and oscillate our ear drums. We only perceive sound waves in range in frequency from relatively low frequencies (20 Hz) up to 20,000 Hz in young children. As people age, they generally lose the ability to hear higher frequency sounds. Vibrating equipment also generates sound waves, though, for this chapter, occupational exposure to the sound generated by the vibration is not the focus.

Occupational exposure to vibration can result from a wide range of equipment including grinders, drills, generators, motors, vehicles, compressors, fans, and vibration waves generated from flowing fluids and air.

DEFINITIONS

1. Vibration – The oscillating, reciprocating, or other periodic motion of a rigid or elastic body or medium forced from a position or state of equilibrium.

2. Frequency (F) – The frequency of a vibration, measured in hertz (Hz), is simply the number of to and fro movements made in each second. 100 Hz would be 100 complete cycles in 1 s.

3. Amplitude – The amplitude of vibration is the magnitude of vibration. A vibrating object moves to a certain maximum distance on either side of its stationary position. Amplitude is the distance from the stationary position to the extreme position on either side and is measured in meters (m). The intensity of vibration depends on amplitude.

4. Acceleration (A) – Acceleration is a measure of how quickly speed changes with time. The measure of acceleration is expressed in units of measure (meters or feet per second) per second or meters or feet per second squared (m/s2). The magnitude of acceleration changes from zero to a maximum during each cycle of vibration. It increases as a vibrating object moves further from its normal stationary position.

5. Speed/Velocity (V) – The speed of a vibrating object varies from zero to a maximum during each cycle of vibration. It moves fastest as it passes through its natural stationary position to an extreme position. The vibrating object slows down as it approaches the extreme, where it stops and then moves in the opposite direction through the stationary position toward the other extreme. Speed of vibration is expressed in units of measure per second (m/s). For instance, 1 ft/s or 1 m/s.

6. Wave length (λ) – The measure of peak to peak of one vibration cycle.

Resonance – Every object tends to vibrate at one particular frequency called the natural frequency. The measure of natural frequency depends on the composition of the object, its size, structure, weight, and shape. If we apply a vibrating force on the object with its frequency equal to the natural frequency, it is a resonance condition. A vibrating machine transfers the maximum amount of energy to the object when the machine vibrates at the object's resonant frequency.

The human body has natural resonance frequencies. Figure 10.1 shows some of the resonance frequencies of various parts of the human body.

Figure 10.1  Resonance frequencies of the human body

Figures 10.2 and 10.3 show diagrams of cyclical vibration waves and how these various vibration elements tie together.

Figure 10.2  Sigmoidal vibration wave

Figure 10.3  Changing velocity in a vibration wave

Two forms of occupational exposure may be distinguished: whole-body vibration (WBV), which is transmitted by mobile or fixed machines where the operator is standing or seated. Trucks, aircraft, heavy equipment, and boats are examples of things that cause WBV.

Hand-arm vibration (HAV) is transmitted from things in the hand to the hands and arms.

1. Whole-body vibration (WBV) is the mechanical vibration that, when transmitted to the whole body, produces risk to the health and safety of workers, in particular, lower back morbidity and trauma of the spine. Figure 10.4 shows a common military helicopter seat. These types of seats have no vibration dampening properties.

Figure 10.4  Helicopter seat

HAV is the mechanical vibration that, when transmitted to the human hand-arm system, entails risks to the health and safety of workers. In particular, vascular, bone or joint, neurological, or muscular disorders.

Table 10.1 shows a listing of the common sources of vibration and the areas they affect.

Table 10.1  Sources of Vibration

Industry	Type of Vibration	Common Source of Vibration
Agriculture	Whole body	Tractors
Military, commercial, and general aviation	Whole body	Aircraft
Military and others	Whole body and hand-arm	Boats
Boiler making	Hand-arm	Pneumatic tools
Construction	Whole bodyHand-arm	Heavy equipment vehicles Pneumatic tools, jackhammers
Diamond cutting	Hand-arm	Vibrating hand tools
Forestry	Whole body Hand-arm	Tractors Chain saws
Foundries	Hand-arm	Vibrating cleavers
Furniture manufacture	Hand-arm	Pneumatic chisels
Iron and steel	Hand-arm	Vibrating hand tools
Lumber	Hand-arm	Chain saws
Machine tools	Hand-arm	Vibrating hand tools
Mining	Whole body Hand-arm	Vehicle operation Rock drills
Riveting	Hand-arm	Hand tools
Rubber	Hand-arm	Pneumatic stripping tools
Sheet metal	Hand-arm	Stamping equipment
Shipyards	Hand-arm	Pneumatic hand tools
Shoe-making	Hand-arm	Pounding machine
Stone dressing	Hand-arm	Pneumatic hand tools
Textile	Hand-arm	Sewing machines, looms
Transportation	Whole body	Vehicles

Potential Injuries Resulting from Vibration Exposure

Raynaud's syndrome or phenomenon was first described as “a condition, a local syncope (loss of blood circulation) to the fingers and hand” (Raynaud, 1888). A more modern term for Raynaud's syndrome is Hand Arm Vibration (HAVS). These terms, along with vibration-induced white finger describe the same condition. Early stages of HAVS are characterized by tingling or numbness in the fingers. Temporary tingling or numbness during or soon after use of a vibrating hand tool is not considered HAVS. To be diagnosed as HAVS, these neurologic symptoms must be more persistent and occur without provocation by immediate exposure to vibration. Other symptoms of HAVS include blanching, pain, and flushing. The symptoms usually appear suddenly and are precipitated by exposure to cold. With continuing exposure to vibration, the signs and symptoms become more severe, and the pathology may become irreversible.

The severity of HAVS can be measured using a grading system developed by Taylor (1974). After a clinical observation and an interview, a worker can be placed into one of the categories shown in Table 10.2.

Table 10.2  Stages of Hand Arm Vibration Syndrome

Stage	Condition of Fingers	Work and Social Interference
00	No tingling, numbness, or blanching of fingers	No complaints
OT	Intermittent tingling	No interference with activities
ON	Intermittent numbness	No interference with activities
TN	Intermittent tingling and numbness	No interference with activities
01	Blanching of a fingertip with or without tingling and/or numbness	No interference with activities
02	Blanching of one or more fingers beyond tips, usually during winter	Possible interference with nonwork activities; no interference at work
03	Extensive blanching of fingers; during summer and winter	Definite interference at work, at home, and with social activities; restriction of hobbies
04	Extensive blanching of most fingers; during summer and winter	Occupation usually changed because of severity of signs and symptoms

Source: Adapted from Taylor (1974).

Another classification scheme is from the Stockholm Workshop and is shown in Table 10.3 (Gemne et al., 1987).

Table 10.3  Stockholm Workshop Classification of Stages of Raynaud's Syndrome

colspan="2">The Stockholm Workshop Classification Scale for Sensorineural Changes in Fingers Due to Hand-Arm HAVS
Stage	Symptoms
OSN	Exposed to vibration but no symptoms
1SN	Intermittent numbness, with or without tingling
2SN	Intermittent or persistent numbness, reduced sensory perception

Source: Adapted from Gemne et al. (1987).

There are several other factors that can affect the development of HAVS and are listed in Table 10.4.

Table 10.4  Factors That Can Influence the Development of Hand Arm Vibration Syndrome

colspan="3">Factors That Influence the Effect of Vibration on the Hand
Physical Factors	Biodynamic Factors	Individual Factors
Acceleration of vibration	Grip forces – how hard the worker grasps the vibrating equipment	Operator's control of tool
Frequency of vibration	Surface area, location, and mass of parts of the hand in contact with the source of vibration	Machine work rate
Duration of exposure each workday	Hardness of the material being contacted by the handheld tools, for example, metal in grinding and chipping	Skill and productivity
Years of employment involving vibration exposure	Position of the hand and arm relative to the body	Individual susceptibility to vibration
State of tool maintenance	Texture of handle-soft and compliant versus rigid material	Smoking and use of drugs Exposure to other physical and chemical agents
Protective practices and equipment including gloves, boots, work-rest periods	Medical history of injury to fingers and hands, particularly frostbite	Disease or prior injury to the fingers or hands

Whole-body vibration (WBV) can contribute fatigue, insomnia, stomach problems, headache, and “shakiness” shortly after or during exposure. These symptoms are similar to those that many people experience after a long car or boat trip. After daily exposure over a number of years, WBV can affect the entire body and result in a number of health disorders such as back injuries. Sea, air, or land vehicles cause motion sickness when the vibration exposure occurs in the 0.1–0.6 Hz frequency range. Bus and truck drivers found that occupational exposure to WBV could have contributed to a number of circulatory, intestinal, respiratory, muscular, and back disorders. The combined effects of body posture, postural fatigue, dietary habits, and WBV are the possible contributors for these disorders. Studies show that WBV can increase heart rate, oxygen uptake, and respiratory rate and can produce changes in blood and urine. Many studies have reported decreased performance from workers exposed to WBV.

VIBRATION MEASUREMENT

Vibration measurement is complex due to its many components – displacement, velocity, acceleration, and frequencies. In addition, vibration can occur in any possible plane. Also, each of these components can be measured in different ways – peak-to-peak, peak, average, RMS (Root Mean Square), each of which can be measured in the time domain (real-time, instantaneous measurements with an oscilloscope or data acquisition system) or frequency domain (vibration magnitude at different frequencies across a frequency spectrum), or just a single number for “total vibration”.

VIBRATION MONITORS

The following provides brief discussion of various types of measurements that are of concern when measuring vibration. This discussion is adapted from information obtained from the Centers for Disease Control (CDC) website (CDC, 2012). Human response to vibration is dependent on several factors including frequency, amplitude, direction, point of application, time of exposure, clothing and equipment, body size, body posture, body tension, and composition. A complete assessment of exposure to vibration requires the measurement of acceleration in well-defined directions, frequencies, and duration of exposure. The vibration will generally be measured along three (x, y, and z) axis.

A typical vibration measurement system includes a device (accelerometer) to sense vibration, a recorder, a frequency analyzer, a frequency-weighting network, and a display such as a meter, printer, or recorder. The accelerometer produces an electrical signal in response to vibration. The size of this signal is proportional to acceleration applied to it. The frequency analyzer determines the distribution of acceleration in different frequency bands. The frequency-weighting network mimics the human sensitivity to vibration at different frequencies. The use of weighting networks gives a single number as a measure of vibration exposure (i.e., units of vibration) and is expressed in meters per second squared (m/s2).

Measurement for Hand-Arm Vibration

Exposure measurement for HAV will generally be conducted for workers using handheld power tools such as drills, grinders, needle guns, and jackhammers. The first step is to determine the type of vibration that will be encountered because a different accelerometer will be used depending on whether an impact (e.g., jackhammer or chipper) or nonimpact (e.g., chain saws or grinders) tool is being used. The accelerometer will be attached to the tool (or held in contact with the tool by the user) so the axis are measured while the worker grasps the tool handle. The z-axis is generally from the wrist to the middle knuckle, the -axis is from the top of the hand down through the bottom of the hand and wrapped fingers, and the y-axis runs from right to left across the knuckles of the hand. The measurement should be made as close as possible to the point where the vibration enters the hand.

The frequency-weighting network for HAV is given in the International Organization for Standardization (ISO) standard ISO 5349-1 (Mechanical Vibration – Measurement and Evaluation of Human Exposure to Hand-Transmitted Vibration – Part 1: General Requirements). The human hand does not appear to be equally sensitive to vibration energy at all frequencies. The sensitivity appears to be highest around 8–16 Hz (hertz or cycles per second), so the weighting networks will generally emphasize this range. Vibration amplitudes, whether measured as frequency-weighted or frequency-independent acceleration levels (m/s2), are generally used to describe vibration stress (American National Standards Institute, American Conference of Governmental Industrial Hygienists (ACGIH), ISO, and the British Standards Institution). These numbers can generally be read directly from the human vibration meter used. The recommendations of most advisory bodies are based on an exposure level likely to cause the first signs of Stage II Hand-Arm HAVS (white finger) in workers.

OSHA does not have standards concerning vibration exposure. The ACGIH has developed threshold limit values (TLVs) for vibration exposure to handheld tools. The exposure limits are given as frequency-weighted acceleration. The frequency weighting is based on a scheme recommended in ISO 5349-1. Vibration-measuring instruments have a frequency-weighting network as an option. The networks list acceleration levels and exposure durations to which, ACGIH has determined, most workers can be exposed repeatedly without severe damage to the fingers. The ACGIH advises that these values be applied in conjunction with other protective measures, including vibration control.

The ACGIH recommendations are based on exposure levels that should be safe for repeated exposure, with minimal risk of adverse effects (including pain) to the back and the ability to operate a land-based vehicle. Table 10.5 shows the ACGIH vibration exposure TLVs.

Table 10.5  ACGIH Vibration Exposure Limits

colspan="3">Threshold Limit Values for Exposure to Hand Arm Vibration in Any of the X, Y, or Z Planes
Daily Exposure (h)	colspan="2">Dominant Frequency of Weighted-Component of Acceleration Which Shall Not Be Exceeded (RMS)
	m/s2	G's
4–8h	4	0.40
2–4	6	0.61
1–2	8	0.81
Less than 1	12	1.22

Source: Used with permission from ACGIH.

Whole-Body Vibration

The measurement of WBV is important when measuring vibration from large pieces of machinery that are operated in a seated, standing, or reclined posture. WBV is measured across three (x, y, and z) axis. The orientation of each axis is as follows: z is from head to toe,  is from front to back, and y is from shoulder to shoulder. The accelerometer must be placed at the point where the body comes in contact with the vibrating surface, generally on the seat or against the back of the operator.

The measurement device is generally an accelerometer mounted in a hard rubber disc. This disc is placed in the seat between the operator and the machinery. Care should be taken to ensure that the weight of the disc does not exceed more than about 10% of the weight of the person being measured.

One of the important issues with vibration measurement is ensuring the measuring equipment is properly calibrated. Vibration equipment will not generally be calibrated by the user. These devices will generally be sent back to the manufacturer for calibration on an annual basis.

Special Considerations

The ISO standard suggests three different types of exposure limits for whole body vibration, of which only the third type is generally used occupationally and is the basis for the ACGIH TLVs:

1. The reduced-comfort boundary is for the comfort of passengers in airplanes, boats, and trains. Exceeding these exposure limits makes it difficult for passengers to eat, read, or write when traveling.

2. The fatigue-decreased proficiency boundary is a limit for time-dependent effects that impair performance. For example, fatigue impairs performance in flying, driving, and operating heavy vehicles.

3. The exposure limit is used to assess the maximum exposure allowed for WBV. There are two separate tables for exposures. One table is for longitudinal (foot to head; z-axis) exposures, with the lowest exposure limit at 4–8 Hz based on human body sensitivity. The second table is for transverse (back to chest and side to side;  and  axes) exposures, with the lowest exposure limit at 1–2 Hz based on human body sensitivity. A separate set of “severe discomfort boundaries” is given for 8-h, 2-h, and 30-min exposures to WBV in the 0.1–0.63 Hz range.

REDUCING VIBRATION EXPOSURE

Engineering Controls

The engineering controls related to controlling vibration include the following:

· Installing equipment that vibrates less

· Proper maintaining the equipment

· Remotely operating the vibrating equipment

· Changing the process. For instance, protecting steel to prevent rusting; therefore, reducing or eliminating the need to sand or grind it before finishing.

Vibration-Dampened Tools

Tools can be designed or mounted in ways that help reduce the vibration level. For example, using vibration-dampened chain saws reduces acceleration levels by a factor of about 10. These types of chain saws must be well maintained to ensure the dampened qualities. If a tool is dropped or otherwise damaged, it needs to be inspected, repaired, or discarded. Maintenance must include periodic replacement of shock absorbers. Some pneumatic tool companies manufacture antivibration tools such as antivibration pneumatic chipping hammers, pavement breakers, and vibration-dampened pneumatic riveting guns.

Most reparable tool manufacturers list the level a tool vibrates as it comes out of the box. Tools that vibrate less should be purchased, if they have enough power to do the job.

Vibration Absorbing Gloves

Conventional protective gloves (e.g., cotton, leather), commonly used by workers, do not reduce vibration that is transferred to workers' hands when they are using vibrating tools or equipment. Vibration absorbing gloves are made of a layer of viscoelastic material. Actual measurements have shown that such gloves have limited effectiveness in absorbing low-frequency vibration, the major contributor to vibration-related disorders. Therefore, they offer little protection against developing vibration-induced white finger syndrome. However, gloves do provide protection from typical industrial hazards (e.g., cuts, abrasions) and from cold temperatures that, in turn, may reduce the initial sensation of white finger attacks. Also, some of these types of gloves require the user to grip tools with a stronger grip that might lead to other occupational injuries.

Safe Work Practices

Along with using vibration-dampened tools and vibration absorbing gloves, workers can reduce the risk of HAVS by the following safe work practices:

· Employ a minimum hand grip consistent with safe operation of the tool or process.

· Wear sufficient clothing, including gloves, to keep warm.

· Avoid continuous exposure by taking rest periods.

· Rest the tool on the work piece whenever practical.

· Refrain from using faulty tools.

· Maintain properly sharpened cutting tools.

· Consult a doctor at the first sign of vibration disease and ask about the possibility of changing to a job with less exposure.

Employee Education

Training programs are an effective means of heightening the awareness of HAVS in the work place. Training should include proper use and maintenance of vibrating tools to avoid unnecessary exposure to vibration. Vibrating machines and equipment often produce loud noise as well. Therefore, training and education in controlling vibration should also address concerns about noise control.

Whole-Body Vibration

The following precautions help to reduce WBV exposure:

· Limit the time spent by workers on a vibrating surface.

· Mechanically isolate the vibrating source or surface to reduce exposure.

· Ensure that equipment is well maintained to avoid excessive vibration.

· Install vibration-dampening seats.

The vibration control design is an intricate engineering problem and must be set up by qualified professionals. Many factors specific to the individual work station govern the choice of the vibration isolation material and the machine mounting methods.

KEY POINTS

· HAV can cause various health issues, including HAVS.

· Vibration is difficult to measure, and the measuring equipment can be very costly.

· The specification sheet for good quality tools contains information about the vibration level of tools.

· Vibration absorbing gloves can reduce the transmission from the tool to the hands.

· Internally dampened tools can help reduce the potential for exposure to vibration.

REVIEW QUESTIONS

1. A worker is grinding on a pipe. The tool's specifications state that the tool vibrates at 10 m/s2. How many hours during the day can the worker operate the tool under ACGIH guidelines?

2. What types of injuries could a helicopter pilot experience from exposure to whole body vibration?

3. How might a company prevent all vibration-related hand-arm injuries?

4. Vibration exposure can occur off the job as well as on the job. What types of off-the-job activities could contribute to vibration exposure?

5. A company is planning to purchase a new piece of industrial equipment. What things could you suggest that would help reduce vibration exposure?

REFERENCES

1. Gemne, G. et al. (1987). Environment and Health. Scandinavian Journal of Work, 13, 4, 275–278.

2. Raynaud, M. (1888). Local Asphyxia and Symmetrical Gangrene of the Extremities. London: New Sydenham Society.

3. Schumann, W. O. and Konig, H. (1954). The Observation of Atmospherics at the Lowest Frequencies. Retrieved from The Healers Journal: http://www.thehealersjournal.com/2012/05/21/the-schumann-resonance-earths-powerful-natural-vibration/#sthash.qrXSOhWM.dpuf

4. Taylor, W. (1974). The Vibration White Finger. London: Academic Press.