Human Vibration, from theory to practice
From the proceedings of the MVSSA 2012 conference: ‘ENGINEERING: A Solution to mine ventilation challenges
The health and safety challenges that the mining industry faces are ever increasing. Humans interact on a daily basis with machinery in their everyday working environment.
Vibration exposure is associated with numerous health effects and injury. This paper discusses theoretical measurement- and exposure guidelines, associated with whole-body and hand-arm vibration experienced in the mining industry. A number of control measures are touched upon in an attempt to guide the mine occupational hygienist in the difficult task to reduce possible exposure to human vibration.
The mining industry is confronted with numerous challenges, of which one is to produce in a safe and healthy environment. Every day humans interact with machinery and contact with vibration is sometimes unavoidable. Unfortunately, human vibration is an emerging issue for the mining industry and it has been found that continuous exposure to mechanical vibration can lead to physical injury.
Vibration can be defined as regular, repeated movement of a physical object around a fixed point. Human vibration is then defined as the effect of mechanical vibration on the human body. Human vibration can be pleasant or unpleasant. Vibrations experienced when travelling in a vehicle on a bumpy road or when operating a power tool are more violent and classified as unpleasant and possibly harmful.
The harmfulness of vibrations depends on the intensity, the frequency and the time of exposure.
There are two types of human vibration:
- Whole-body vibration
- Hand-arm vibration*
Whole-body vibration (WBV)
Whole-body vibration is caused by vibration transmitted through the seat or the feet. Exposure to high levels of whole-body vibration is known to cause or aggravate back injuries depending on the magnitude of exposure. Activities that are known to cause whole-body vibration are commonly related to off-road work and operating of mobile equipment as well as standing operations such as standing on crushing equipment.
Along with vibration, other ergonomic factors may contribute to back pain caused by WBV, and should be considered. The risk will be increased where a person is exposed to additional ergonomic factors while being exposed to WBV.
Possible reasons for back pain in drivers/machine operators in addition to vibration exposure may include:
- poor design of controls, making it difficult for the driver to operate the machine or vehicle easily;
- incorrect adjustment by the driver of the seat position and hand and foot controls which may result in awkward position of operator;
- sitting for long periods without being able to change position;
- poor driver posture; and
- repeated manual handling and lifting of loads by the driver (2).
The risk increases where the driver or operator is exposed to two or more of these factors together.
Hand-arm vibration (HAV)
Hand-arm vibration is vibration transmitted by the use of vibrating hand-held power tools such as pneumatic drills, hammers and grinders. Vibration is transmitted to the hands and arms of the person operating the tool. Regular exposure to hand-arm vibration can cause health effects commonly known as hand-arm vibration syndrome. Hand-arm vibration syndrome is a general term used to broadly describe the physical damage to the hands, fingers, and related structures resulting from chronic exposure to excessive vibration (vascular and neuropathic effects). Regular exposure to hand-arm vibration can cause reduced grip strength, pain in the arms and shoulders, white finger syndrome and carpal tunnel syndrome. If left untreated, white finger becomes progressively worse and irreversible damage can occur. Symptoms include tingling and/or numbness in the fingers followed by finger blanching that often occurring at night or while not at work.
The effects of exposure to HAV are influenced by factors such as:
- acceleration and frequency of the vibration;
- duration of exposure during each work shift and number of years of exposure;
- state of tool maintenance;
- level of insulation on the tools;
- duration and frequency of work-rest periods;
- grip forces applied i.e. the tighter the grip, the more vibration is absorbed;
- surface area of the hand in contact with the source of vibration;
- hardness of material being contacted;
- posture of arm and hand during tool operation;
- type of handle on the tool;
- temperature of work environment, cold temperatures affects circulation;
- use of personnel protective equipment (gloves); and
- personal habits (smoking and use of drugs affects circulation) (5).
Units of measure
Vibration is defined by its magnitude and frequency. The magnitude of vibration could be expressed as the vibration displacement (in metres), the vibration velocity (in metres per second) or the vibration acceleration (in metres per second squared or m/s 2).
The amplitude and frequency of the vibration motion are measured in the three orthogonal directions (x, y and z) in terms of velocity. If you could watch a vibrating object in slow motion, you can see movements in different directions. The following terms are used to describe this movement:
A vibrating object moves back and forth from its normal stationary position. A complete cycle of vibration occurs when the object moves from one extreme position to the other and back again. The number of cycles that a vibrating object completes in one second is called frequency. The unit of frequency is hertz (Hz). One hertz equals one cycle per second.
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 speed of a vibrating object varies from zero to a maximum during each cycle of vibration. Speed is expressed in units of meters per second (m/s). Acceleration is a measure of how quickly speed changes with time and therefore, acceleration is expressed in meters per second squared (m/s2). The magnitude of acceleration changes from zero to a maximum during each cycle of vibration. It increases as the vibrating object moves further from its normal stationary position.
Numerous research studies have evaluated the effect of over-exposure to human vibration in the working environment and results have been applied in establishing international standards. The International Standards Organisation (ISO) consequently compiled the following standards relevant for the evaluation of human vibration:
- ISO 2631-1 (1997) Mechanical vibration and shock – Evaluation of human exposure to whole body vibration – Part 1: General requirements;
- ISO 5349-1 (2001) Mechanical vibration – Measurement and evaluation of human exposure to hand-transmitted vibration – Part 1: General requirements; and
- ISO 5349-2 (2001) Mechanical vibration – Measurement and evaluation of human exposure to hand-transmitted vibration – Part 2: Practical guidance for measurement at the workplace.
The primary purpose of these international standards is to define methods for the measurement of human vibration. These standards together with additional literature form the basis of this paper in order to formulate a measurement approach for human vibration (1).
The mentioned ISO standards define daily vibration exposures in terms of an exposure action value (EAV) and an exposure limit value (ELV). The EAV is the amount of daily exposure to whole-body vibration above which action is required to reduce risk. The ELV is the maximum amount of vibration an employee may be exposed to on any single day (3).
The following exposure guidance is given by the ISO standards for vibration:
|HAV||2.5 m/s2||5 m/s2|
|WBV||3 m/s2||6 m/s2|
|VDV||8.5 m/s1.75||17 m/s1.75|
Daily exposures to vibration may be assessed in terms of “Daily vibration exposure, A(8)” or “Vibration dose value, VDV”.
The A(8) requires an exposure time and is the continuous equivalent acceleration, normalised to 8 hours, based on the root-mean-square averaging of the acceleration signal and is measured in m/s 2.
VDV is a cumulative dose on the 4th root-mean-quad of the acceleration signal and is measured in m/s 1.75.
The vibration dose value (VDV) provides an alternative measure of vibration exposure. The VDV was developed as a measure that gives a better indication of the risks from vibrations that include shocks (VDV is more sensitive to high peak vibrations). The units for VDV are metres per second to the power 1.75 (m/s 1.75), and unlike rms vibration magnitude, the measured VDV is a cumulative value (it increases with measurement time) (6).
In order to assess the daily vibration of workers a proper risk assessment should be conducted. The purpose of the vibration risk assessment is to enable a valid decision to be made about the measures necessary to prevent or adequately control the exposure of workers.
The risk assessment should:
- identify where there may be health or safety risks for which vibration is the cause or a contributory factor;
- estimate workers’ exposures and compare with exposure action and exposure limit values; and
- identify the available risk controls.
Along with vibration, other ergonomic factors may contribute to the risk and will increase where a person is exposed to additional ergonomic factors while being exposed to vibration.
The daily vibration exposure of workers needs to be estimated and in order to do that the total daily duration of exposure to the vibration source must be known. In most instances vibration exposures will be interrupted by periods without vibration exposure, e.g. truck loading and waiting times. Usually, the vibration that occurs when the vehicle is travelling will dominate vibration exposures. Work patterns, therefore need to be carefully considered.
Vibration magnitude is the frequency weighted acceleration value in the highest of three orthogonal axes (x, y or z). This can be obtained by using manufacturer’s data, if available. Vibration exposure, however, is very dependent on factors such as the quality of road surfaces, the speed the vehicle is driving, the manner in which the vehicle / equipment is operated etc. Initial exposure assessment data should, therefore, rather be confirmed by measuring the actual vibration magnitudes.
As mentioned before, the parameter used to assess vibration is the peak particle velocity expressed in millimetres per second (mm/s).The root-meansquare (rms) vibration magnitude is expressed in terms of the frequency-weighted acceleration at the seat of a seated person or the feet of a standing person, it is expressed in units of metres per second squared (m/s 2). The rms vibration magnitude represents the average acceleration over a measurement period.
Vibration should be measured according to the coordinate system originating at a point from which vibration is considered to enter the human body, i.e. the seat or the feet (Figure 1).
If it is not feasible to obtain precise alignment of the vibration transducers with the preferred basicentric axes, the sensitive axes of transducers may deviate from the preferred axes by up to 15° where necessary. For a person seated on an inclined seat, the relevant orientation should be determined by the axes of the body, and the z-axis will not necessarily be vertical.
Transducers shall be located so as to indicate the vibration at the interface between the human body and the source of its vibration. Vibration which is transmitted to the body shall be measured on the surface between the body and that surface. Three principal areas for seated persons are described:
- the supporting seat surface
- the seat-back; and
- the feet
As an extension, the steering wheel vibrations can also be measured in the same run.
Vibration measurements should be made to represent the vibration throughout the operator’s working period. If achievable, measurements should be made over periods of at least 20 minutes, where shorter measurements are unavoidable they should normally be at least three minutes long and, if possible, they should be repeated to give a total measurement time of more than 20 minutes (I).
On completion of the assessment of the vibration risk to employees, one needs to decide if they are likely to be exposed above the daily EAV or if they are likely to be exposed above the daily ELV.
Measuring exposure levels of HAV is complicated. An accelerometer is used to measure the vibration from a power tool that is converted to an electrical output. This output is modified to account for the range of frequencies that can cause harm to the hand and arm, which is a frequency weighted value that is measured in m/s 2.
Vibration measurements should be taken at a point close to where VIbration enters the hand. Measurements should be taken in three axes (x, y and z) and the alignment of these axes of the accel-erometer should be precise. Figure 2 shows the basicentric axes in which HAV exposure should be conducted.
Hand-transmitted vibration is measured at the contact point between the hand and the tool and sine cannot assume that the tool has a dominant axes, the measurement will take place in all three of the mentioned axes (4).
Whole body vibration
The decision on the actions required should be in proportion to the risk identified. It is necessary to consider whether it is the WBV exposures that are contributing most or the additional factors such as manual handling or postural strain that may be sig-nificant. A higher priority needs to be given to con-trol the specific stressor.
Actions for controlling risks could include the following:
- Train and instruct operators and drivers on:
- Adjustment of suspension seats, to minimise vibration;
- Adjust the seat position and controls correct-ly, where adjustable, to provide good lines of sight, adequate support and ease of reach for foot and hand controls;
- Adjust the vehicle speed to suit the ground conditions to avoid excessive bumping and jolting;
- Steer, brake, accelerate smoothly; and
- Follow worksite routes to avoid travelling over rough, uneven or poor surfaces.
- Choose machinery suitable for the job:
- Select vehicles and machines with the appropriate size, power and capacity for the work and the ground conditions; and
- Consult suppliers for advice.
- Maintain machinery and roadways:
- Make sure that paved surfaces or site roadways are well maintained, i.e. potholes filled, ridges levelled, rubble removed etc.;
- Maintain vehicle suspension systems correctly i.e. cab, tyre pressures, seat suspension; and
- Replace solid tyres on machines before they reach their wear limits.
In addition, work schedules to avoid long periods of exposure in a single day and allow for breaks where possible can be introduced. Health monitoring in terms of back problems can be conducted (2).
Risk controls for HAV can include:
- Alternative work methods which eliminate or reduce exposure;
- Select the lowest vibration tool that is suitable and can do the work efficiently;
- Mechanise or automate the work;
- Equipment that is unsuitable, too small or not powerful enough is likely to take much longer to complete the task and expose employees to vibration for longer than is necessary;
- Work equipment is likely to be replaced over time as it becomes worn out, and it is important that you choose replacements, so far as is reasonably practicable, which are suitable for the work, efficient and of lower vibration;
- Get your employees to try the different models and brands of equipment;
- Find out about the equipment’s vibration reduction features (consider maintenance);
- Train purchasing staff on the issues relating to vibration;
- Improve the design of workstations to minimise unnatural position of hands, wrists and arms;
- Use devices such as jigs and suspension systems to reduce the need to grip heavy tools tightly
- Do not use blunt or damaged concrete breakers etc.;
- Limit the time that your employees are exposed to vibration, several shorter periods are preferable;
- Provide your employees with protective clothing when necessary to keep them warm and dry; and
- Gloves are available to protect hands form excessive vibration exposure (7).
Despite the complexity of the assessment and evaluation of human vibration exposures, it is true that exposures can be managed without compromising production, health or safety. Together with continual improvements to manual handling, posture, prolonged operation, maintenance, training etc. the risks can be reduced.
D. Senekal & T. van Dyk Kobus Dekker Occupational Hygiene Consultancy, South Africa
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