Tuned vibration absorber for suppression of hand-arm vibration in electric grass trimmer

Prolonged use of electric grass trimmer exposes the user to the risk of hand-arm vibration syndrome. A simple approach for the suppression of hand-arm vibration in electric grass trimmer is presented. The proposed system is a tuned vibration absorber (TVA). Modal analysis and operating deflection shape analysis of the electric grass trimmer were carried out and a TVA was designed and fabricated for testing. The results indicated that minimum vibration level was related to the position of the TVA on the shaft of electric grass trimmer. The TVA was found to have best performance with 95% reduction on the acceleration level at position 0.025L. The results from modal analysis and operating deflection shape revealed that the presence of TVA has successfully reduced the large deformations of the handle where the node was shifted nearer to the handle location. The effect of TVA was also evaluated during field test involving grass trimming operation and subjective rating. The results indicated that average reduction of frequency-weighted rms acceleration in the Z_h- axis was 84% and 72% in X_h- axis for the cutting operation. For the no cutting operation, the reduction is 82% in Z_h- axis and 67% in X_h- axis. The presence of TVA in the electric grass trimmer has amplified the vibration level in Y_h- axis by 19% (no cutting) and 21% (cutting). From the field test, subjective rating of vibration perception consistently rate better for controlled electric grass trimmer.

Relevance to industry

The tuned vibration absorber when installed to the electric grass trimmer attenuated the vibration total value by 67%. This significantly reduces the risk of hand-arm vibration syndrome.     

An examination of the vibration transmissibility of the hand-arm system in three orthogonal directions

The objective of this study is to enhance the understanding of the vibration transmission in the hand-arm system in three orthogonal directions (X, Y, and Z). For the first time, the transmitted vibrations distributed on the entire hand-arm system exposed in the three orthogonal directions via a 3-D vibration test system were measured using a 3-D laser vibrometer. Seven adult male subjects participated in the experiment. This study confirms that the vibration transmissibility generally decreased with the increase in distance from the hand and it varied with the vibration direction. Specifically, to the upper arm and shoulder, only moderate vibration transmission was measured in the test frequency range (16 to 500 Hz), and virtually no transmission was measured in the frequency range higher than 50 Hz. The resonance vibration on the forearm was primarily in the range of 16–30 Hz with the peak amplitude of approximately 1.5 times of the input vibration amplitude. The major resonance on the dorsal surfaces of the hand and wrist occurred at around 30–40 Hz and, in the Y direction, with peak amplitude of more than 2.5 times of the input amplitude. At higher than 50 Hz, vibration transmission was effectively limited to the hand and fingers. A major finger resonance was observed at around 100 Hz in the X and Y directions and around 200 Hz in the Z direction. In the fingers, the resonance magnitude in the Z direction was generally the lowest, and the resonance magnitude in the Y direction was generally the highest with the resonance amplitude of 3 times the input vibration, which was similar to the transmissibility at the wrist and hand dorsum. The implications of the results are discussed.Relevance to industryProlonged, intensive exposure to hand-transmitted vibration could result in hand-arm vibration syndrome. While the syndrome's precise mechanisms remain unclear, the characterization of the vibration transmissibility of the system in the three orthogonal dimensions performed in this study can help understand the syndrome and help develop improved frequency weightings for assessing the risk of the exposure for developing various components of the syndrome.     

Experimental evaluation of uncertainty in hand-arm vibration measurements

Hand-arm vibration measurements are necessary for vibration exposure risk assessment and for the determination of vibration emission values in hand-guided machines. It has been reported that there is a high degree of uncertainty associated with this kind of measurement. The main goal of the present work is to investigate which factors cause uncertainty in hand-arm vibration evaluation and how much they contribute to the total uncertainty of the measurements. Some experiments were carried out in order to evaluate separately the factors relating to instrumentation and the methods of fixing accelerometers. The experiments were performed with handles belonging to real machines while being handheld by an operator. The results of this investigation show that the fixing method and the accelerometer behaviour are the two main sources of uncertainty. The total uncertainty of the measurements in this work, considering both instrumentation and fixing method, reaches up to 8% of the values measured. It is concluded that the adaptors for fixing the accelerometer with metallic stripe hose clips cause less uncertainty than handheld adaptors and should therefore be used as the first option.

Relevance to industry

A good accuracy in hand-arm measurements is the key for a correct assessment of the risk of developing HAVS (Hand-Arm Vibration Syndrome). Knowledge of the uncertainty factors will help the technicians who carry out the measurements to improve their quality.     

The design and development of suspended handles for reducing hand-arm vibration in petrol driven grass trimmer

The portable petrol driven grass trimmer is identified as a type of machine whose operator can be subjected to large magnitude of hand-arm vibration. These vibrations can cause complex vascular, neurological and musculoskeletal disorder, collectively named as hand-arm vibration syndrome. The vibration total level on the handle of grass trimmer of 11.30 m/s² was measured, and it has reached the exposure limit value of 5.0 m/s² for daily vibration exposure A(8). New suspended handles were designed to reduce the vibration level. Three different prototype handles with rubber mounts were designed. Handles were made of different materials, and the distance of rubber mounts were varied. From the study, it was observed that not all the handles with rubber mounts were effective in reducing hand-arm vibration. The reduction of vibration depended on the handle material and distance installed between rubber mount and vibration transmissibility of handle-isolation system. Subjective ratings of perception of vibration were measured, and the results indicated that operators were not fully aware of the level of vibration. A prototype handle that is made of heavier material results in the lowest hand-arm vibration of 2.69 m/s². The new handle has significantly reduced the vibration total value by 76% compare with the existing commercial handle.

Relevance to industry

Large numbers of workers are employed to perform grass trimming job in many developing countries. This paper presents the effect of handle types (commercial and prototype) on the commonly used grass trimmer.     

Synergistic effects of noise and hand-arm vibration on distortion product otoacoustic emissions in healthy subjects

An experimental study has been performed on the effect of exposure to noise (N), vibration (V), and combined noise and vibration (NV), on the distortion product otoacoustic emission (DPOAE) level. The aim of the study is investigating the possible synergistic effect of noise and vibration, increasing the risk of damage to the hearing function due to noise only.Twelve normal-hearing volunteers were randomly exposed to hand arm vibration (HAV) only, noise only, and HAV and noise simultaneously. The exposure consisted of five consecutive sessions interspersed with quiet periods. DPOAEs were recorded during the quiet intervals following each exposure session, and during recovery, and compared to their baseline level, and among different test treatments. An ad hoc acquisition system developed in Labview was used to record high frequency-resolution DPOAE spectra in a suitably short time. Time-frequency filtering, based on the wavelet transform, was used to separate the DPOAE components coming from different generation mechanisms.The three test treatments, N, V, and NV, yielded significantly different results. The condition affecting most significantly all DPOAE component levels was the exposure to vibration only, causing a statistically significant increase of the signal level during all the exposure periods, with a decreasing trend during recovery. Also the exposure to noise only, N, caused a DPOAE level increase, statistically significant for the long latency (LL) DPOAE component. Only when exposure to noise was in combination with vibration, the zero-latency (ZL) component was clearly suppressed during the exposure. The results suggest a synergistic adverse effect of noise and vibration on the cochlear function.Relevance to industryThe simultaneous exposure to noise and hand arm vibration is a very common industrial scenario. The risk that exposure to mild noise levels could interfere with the exposure to vibration, enhancing the adverse effect on the hearing function, should be considered to optimize prevention strategies at the workplace.     

Infrared thermography in the hands and feet of hand-arm vibration syndrome (HAVS) cases and controls

This study evaluated IR thermography during 15 min of re-warming after cold water immersion in accordance with ISO guidelines (12 °C for 5 min) in cases with the vascular component of HAVS and controls. The purposes of the study were: (1) To evaluate the performance of IR thermography as a diagnostic test for the vascular effects of HAVS in the fingers, (2) To determine if there were any temperature differences in the toes between cases and controls and, if so, to evaluate the IR thermography as a diagnostic test for toe vascular effects in HAVS. A total of 39 HAVS cases and 46 controls were included in the study. In the fingers, an ROC analysis indicated that the highest areas under the curve (AUC) were obtained at 7 min of re-warming for the fingertips, middle phalanges and proximal phalanges. However, at these locations and time period of re-warming the sensitivities and specificities were not sufficiently high. For example a sensitivity of 66.7% was associated with specificities of 60.9%–67.4%. In the toe analysis 33 (84.6%) cases but only 3 controls (6.5%) reported increased cold intolerance in the feet and this difference was statistically significant (p < 0.001). These 33 cases were then compared to the 46 controls. In the ROC analysis for the toes, the AUC's were lower than those for the fingers and the estimated sensitivities and specificities were worse for the toes than for the fingers. However some statistically significant colder temperatures were found in cases in comparison to controls towards the end of the re-warming period. IR thermography after the ISO recommended cold water stimulation conditions did not perform well as a diagnostic test in the hands or feet. The statistically significant colder temperatures in the toes towards the end of re-warming are relevant to our understanding of foot vascular effects associated with vibration. In industry there is recent interest in determining the effects of foot transmitted vibration (FTV). Many workers have mixed HAV and FTV and it is important to account for feet effects secondary to HAVS in the hands when evaluating the effects of FTV.     

Distributed vibration power absorption of the human hand-arm system in different postures coupled with vibrating handle and power tools

This study presents vibration power absorption (VPA) of different hand-arm substructures in the bent-arm and extended arm postures excited by broadband random and power tool vibrations. VPAs are estimated using biomechanical models of the hand-arm system derived from both the driving-point mechanical impedance and distributed vibration transmissibility. VPAs due to the vibrations of selected hand-held power tools are also estimated. The results show that distributed VPAs of the arms are greater below 25 Hz than those of the hand (fingers and palm) for both postures, while the hand VPAs are greater above 100 Hz, although the values are smaller than those below 25 Hz. The power absorbed during the extended arm posture is about 2.5 times greater than the power absorbed with the bent-arm posture for similar hand forces and vibration magnitude. Similar trends are observed in distributed VPAs due to broadband random as well as typical tool excitations, while the VPA due to tool vibration revealed peaks near the operating frequencies of the power tools and their harmonics. Furthermore, the percentage of power absorbed in different hand-arm substructures was dependent on the operating speed of the power tools, the higher the operating speed the higher the power absorbed in the hand and vice versa. The results showed relatively lower VPA values in the fingers and palm in the 60–160 Hz range than those obtained for the arms in the 5–16 Hz range. The study revealed the need for different frequency weightings for assessment of potential injury risk of different hand-arm substructures.Relevance to industryOperators of hand-held power tools are exposed to hand-transmitted vibration and the associated potential injuries. This study showed that the extended arm posture should be avoided when operating hand-held power tools because large vibration power is absorbed in the human hand-arm system, which may cause hand-arm injury.     

The relationships between hand coupling force and vibration biodynamic responses of the hand-arm system

This study conducted two series of experiments to investigate the relationships between hand coupling force and biodynamic responses of the hand–arm system. In the first experiment, the vibration transmissibility on the system was measured as a continuous function of grip force while the hand was subjected to discrete sinusoidal excitations. In the second experiment, the biodynamic responses of the system subjected to a broadband random vibration were measured under five levels of grip forces and a combination of grip and push forces. This study found that the transmissibility at each given frequency increased with the increase in the grip force before reaching a maximum level. The transmissibility then tended to plateau or decrease when the grip force was further increased. This threshold force increased with an increase in the vibration frequency. These relationships remained the same for both types of vibrations. The implications of the experimental results are discussed.

Practitioner Summary: Shocks and vibrations transmitted to the hand–arm system may cause injuries and disorders of the system. How to take hand coupling force into account in the risk assessment of vibration exposure remains an important issue for further studies. This study is designed and conducted to help resolve this issue.     

Hand-arm vibration syndrome (HAVS) and musculoskeletal symptoms in the neck and the upper limbs in professional drivers of terrain vehicles--a cross sectional study

This study compares the prevalence of symptoms of Hand-arm vibration syndrome (HAVS) and musculoskeletal symptoms in the neck and the upper limbs, between professional drivers of terrain vehicles and a referent group. 769 male professional drivers of forest machines, snowmobiles, snowgroomers and reindeer herders and 296 randomly selected male referents completed a questionnaire about symptoms of HAVS and musculoskeletal symptoms in the neck and the upper limbs. They also gave information about their lifetime exposure duration driving terrain vehicles and their nicotine use. Prevalence odds ratios (POR) were determined and adjusted for age and nicotine use. Results show that there is a relation between exposure to driving terrain vehicles and some of the symptoms of HAVS (POR: 1.2–6.1). Increased odds of musculoskeletal symptoms in neck, shoulders and wrists were also found (POR 1.2–6.4), and it seemed to be related to the cumulative exposure time.     

Power tiller: vibration magnitudes and intervention development for vibration reduction

The operators of power tiller are exposed to a high level of vibration originating from the dynamic interaction between the soil and the machine. The vibration from the power tiller is transmitted from the handle to hands, arms and shoulders. In the present study, experiments were conducted in three operational conditions i.e. transportation on farm roads, tilling with cultivator and rota-tilling with rota-vator. The highest vibration values were observed in x-direction in all the experiments. The maximum vibration rms values for x-direction were 5.96, 6.81 and 8.00 ms⁻² in tilling with cultivator, transportation and rota-tilling respectively. Three materials were used for intervention development to reduce vibration magnitude. The maximum reduction of 25.30, 31.21 and 30.45% in transportation; 23.50, 30.64 and 20.86% in tilling with cultivator and 24.03, 29.18 and 25.52% in rota-tilling were achieved with polyurethane (PU), rubber and combination of PU and rubber intervention. It was found that the maximum vibration reductions were achieved with the rubber in all three operational conditions. The average exposure time for occurrence of white finger syndrome increased by 28–50% with incorporation of intervention in different operations. Physiological and postural parameters also improved with incorporation of interventions.     

The effects of vibration-reducing gloves on finger vibration

Vibration-reducing (VR) gloves have been used to reduce the hand-transmitted vibration exposures from machines and powered hand tools but their effectiveness remains unclear, especially for finger protection. The objectives of this study are to determine whether VR gloves can attenuate the vibration transmitted to the fingers and to enhance the understanding of the mechanisms of how these gloves work. Seven adult male subjects participated in the experiment. The fixed factors evaluated include hand force (four levels), glove condition (gel-filled, air bladder, no gloves), and location of the finger vibration measurement. A 3-D laser vibrometer was used to measure the vibrations on the fingers with and without wearing a glove on a 3-D hand-arm vibration test system. This study finds that the effect of VR gloves on the finger vibration depends on not only the gloves but also their influence on the distribution of the finger contact stiffness and the grip effort. As a result, the gloves increase the vibration in the fingertip area but marginally reduce the vibration in the proximal area at some frequencies below 100 Hz. On average, the gloves reduce the vibration of the entire fingers by less than 3% at frequencies below 80 Hz but increase at frequencies from 80 to 400 Hz. At higher frequencies, the gel-filled glove is more effective at reducing the finger vibration than the air bladder-filled glove. The implications of these findings are discussed.     

Identification of effective engineering methods for controlling handheld workpiece vibration in grinding processes

The objective of this study is to identify effective engineering methods for controlling handheld workpiece vibration during grinding processes. Prolonged and intensive exposures to such vibration can cause hand-arm vibration syndrome among workers performing workpiece grinding, but how to effectively control these exposures remains an important issue. This study developed a methodology for performing their analyses and evaluations based on a model of the entire grinding machine-workpiece-hand-arm system. The model can simulate the vibration responses of a workpiece held in the worker's hands and pressed against a grinding wheel in order to shape the workpiece in the major frequency range of concern (6.3–1600 Hz). The methodology was evaluated using available experimental data. The results suggest that the methodology is acceptable for these analyses and evaluations. The results also suggest that the workpiece vibration resulting from the machine vibration generally depends on two mechanisms or pathways: (1) the direct vibration transmission from the grinding machine; and (2) the indirect transmission that depends on both the machine vibration transmission to the workpiece and the interface excitation transformation to the workpiece vibration. The methodology was applied to explore and/or analyze various engineering methods for controlling workpiece vibrations. The modeling results suggest that while these intervention methods have different advantages and limitations, some of their combinations can effectively reduce the vibration exposures of grinding workers. These findings can be used as guidance for selecting and developing more effective technologies to control handheld workpiece vibration exposures.     

The vibration transmissibility and driving-point biodynamic response of the hand exposed to vibration normal to the palm

Prolonged, intensive exposure to vibrations from palm and orbital sanders could cause finger disorders. They are likely to be associated with the biodynamic responses of the fingers. Although the biodynamic responses of the hand-arm system have been studied by many researchers, the detailed biodynamic responses distributed in the hand substructures have not been sufficiently understood. To advance the knowledge in this aspect and to aid in the development of improved finite element models of the substructures, this study simultaneously measured the overall driving-point biodynamic response and the distribution of vibration transmissibility at the fingers and back of the hand exposed to a flat plate vibration (as an approximate simulation of the operations of the palm and orbital sanders) and examined the relationship between these two measures of biodynamic responses. Ten subjects (five males and five females) participated in the experiment. A scanning laser vibrometer was used to measure the distributed vibration. This study confirmed that the distributed hand responses generally varied with locations on each finger, vibration frequencies, and applied hand force. Two major resonances were observed in the vibration transmissibility. At the first resonance, the transmitted vibrations at different locations were more or less in phase; hence, this resonance was also observed in the driving-point biodynamic response that measures the overall biodynamic response of the system. The second resonance was observed at the fingers. Because this resonant frequency varied greatly among the fingers and the specific segments of each finger, it is difficult to identify this resonance in the driving-point biodynamic response. The implications of the findings for further model developments and applications are discussed.

Relevance to industry

This study enhanced the understanding of the biodynamic responses of the fingers and hand exposed to vibrations on a contact surface with a large effective radius such as that found on palm and orbital sanders. The results can also be used to develop and/or validate models of the substructures of the hand-arm system, which can be further used to help design and analyze these tools and associated anti-vibration devices. The results may also be applicable to help develop location-specific frequency weightings to assess the risks of the finger vibration exposure.     

An analytical investigation of an energy flow divider to attenuate hand-transmitted vibration

A five-degrees-of-freedom (DOF) bio-mechanical model of the hand-arm system is developed to study the vibration transmissibility characteristics of the human hand-arm. The model parameters are identified from the characteristics of vibration transmitted to the hand, forearm and upper arm, measured in the 10–200 Hz frequency range under a constant 25.0 N grip force. A concept of an energy flow divider is proposed to reduce the flow of vibration energy into the hand. The coupled hand-arm-divider is modeled as a six-DOF dynamical system and the response characteristics are evaluated for handle excitations caused by a palm-grip orbital sander. The response characteristics of the coupled hand-arm-divider model are compared to those of the hand-arm model to demonstrate the potential performance benefits of the proposed energy flow divider. The hand-transmitted vibration is further assessed using the overall weighted acceleration response, and it is concluded that the proposed energy flow divider can reduce the magnitude of hand-transmitted vibration considerably.     

Tool-specific performance of vibration-reducing gloves for attenuating palm-transmitted vibrations in three orthogonal directions

Vibration-reducing (VR) gloves have been increasingly used to help reduce vibration exposure, but it remains unclear how effective these gloves are. The purpose of this study was to estimate tool-specific performances of VR gloves for reducing the vibrations transmitted to the palm of the hand in three orthogonal directions (3-D) in an attempt to assess glove effectiveness and aid in the appropriate selection of these gloves. Four typical VR gloves were considered in this study, two of which can be classified as anti-vibration (AV) gloves according to the current AV glove test standard. The average transmissibility spectrum of each glove in each direction was synthesized based on spectra measured in this study and other spectra collected from reported studies. More than seventy vibration spectra of various tools or machines were considered in the estimations, which were also measured in this study or collected from reported studies. The glove performance assessments were based on the percent reduction of frequency-weighted acceleration as is required in the current standard for assessing the risk of vibration exposures. The estimated tool-specific vibration reductions of the gloves indicate that the VR gloves could slightly reduce (<5%) or marginally amplify (<10%) the vibrations generated from low-frequency (<25 Hz) tools or those vibrating primarily along the axis of the tool handle. With other tools, the VR gloves could reduce palm-transmitted vibrations in the range of 5%–58%, primarily depending on the specific tool and its vibration spectra in the three directions. The two AV gloves were not more effective than the other gloves with some of the tools considered in this study. The implications of the results are discussed.Relevance to industryHand-transmitted vibration exposure may cause hand-arm vibration syndrome. Vibration-reducing gloves are considered as an alternative approach to reduce the vibration exposure. This study provides useful information on the effectiveness of the gloves when used with many tools for reducing the vibration transmitted to the palm in three directions. The results can aid in the appropriate selection and use of these gloves.     

Validity and reliability of the IAT: Measuring gender and ethnic stereotypes

The Implicit Association Test (IAT) was developed in response to reports of low validity of explicit (self-report) measures of attitudes, stereotypes, and prejudices. Usually, people are unwilling to report what they think and feel about other races, groups, and nationalities. The IAT has been written about in many books, newspapers, journal articles, websites, and has been featured frequently on radio and television many times; its web site has now reached a peak of 5 million visits. However, despite this popularity its validity and particularly its reliability is under question. This article reports on the validity and reliability of the IAT. Four different experiments were conducted on 150 students at California State University, Long Beach to investigate the temporal reliability of IAT. Also students’ opinion (trust) about the validity and reliability of the test was evaluated. The results showed that while there are numerous reports of moderate validity of the test, its reliability as measured in this study, particularly for the first time users, is relatively low. Familiarity with similar tests, however, improves its reliability.     

Causes of dissociation between subjective workload measures and performance ; Caveats for the use of subjective assessments

Dissociations between subjective workload assessments and performance were investigated. The difficulty of a Sternberg memory search task was manipulated by varying stimulus presentation rate, stimulus discernibility, value of good performance, and automaticity of performance. All Sternberg task conditions were performed both alone and concurrently with a tracking task. Bipolar subjective workload assessments were collected. Dissociations between workload and performance were found related to automaticity, presentation rate and motivation. The results were interpreted as supporting the hypothesis that the specific cognitive processes responsible for subjective assessments can differ from those responsible for performance. The potential contamination these dissociations could inflict on operational assessments was discussed.     

Hand-arm and whole-body vibrations induced in cross motorcycle and bicycle drivers

Generally, and particularly at sports, the human body is constantly exposed to physical requests and to tests in many different situations. Although the practice of sports is considered a healthy act, there are limits and, when these limits are reached, the benefits of sport can turn into problems. Thus, the biodynamic response method is increasingly being used to study the human injuries induced by external vibrations. Moreover, the European Directive 2002/44/EC on the minimum health and safety requirements, regarding worker exposure to risks from physical agents (e.g. vibration), limit the exposure to vibrations. The aim of this study is to analyze the exposure level of cross motorcycle and of cycling drivers to hand-arm vibration (HAV) and to whole-body vibration (WBV). For this research, vibration levels of a common 200 cc cross motorcycle were experimentally measured and the maximum driving time that could be safely used in a stone road was established. Moreover, bicycle vibration measurements were performed using two different bicycles: a road cycling bike; a bike for track cycling. The road bike was evaluated at three road scenarios: asphalt; paved; and stone road pavement. The track bike was evaluated in track cycling and rollers. In the case of cycling the results indicate that impacts and transient vibrations lead to a higher musculoskeletal request particularly in what concerns shoulders, arms, wrists, knees and spine.     

Development of a non-expert risk assessment method for hand-arm related tasks (HARM)

To support health and safety practitioners in their obligation of risk assessment the ‘Hand Arm Risk Assessment Method’ (HARM) was developed. This tool can be used by any type of company for risk assessment of developing arm, neck or shoulders symptoms (pain) resulting from light manual tasks.