Transmission characteristics of suspension seats in multi-axis vibration environments

The multi-axis vibration transmission characteristics of selected suspension seats were investigated in the laboratory. Subjects were exposed to a flat acceleration spectrum and two low frequency signals extracted from multi-axis acceleration data recorded at the floor of a passenger locomotive. Triaxial accelerations were measured at the floor of the vibration table and at the interfaces between the subject and mounted seat (seat pan and seat back). The transmission ratios between the overall seat pan and seat back accelerations and floor accelerations provided an effective tool for evaluating the effects of measurement site, vibration direction, and posture among the selected seating systems. The results showed that the system transfer matrix, estimated using a multiple-input/single-output model, would be less than ideal for predicting low frequency operational seat vibration when using suspension seats. The Seat Effective Amplitude Transmissibility (SEAT), estimated for the tested locomotive seats, was used to predict the weighted seat pan accelerations and Vibration Total Values for assessing a 1-h operational exposure in accordance with ISO 2631-1: 1997.

Relevance to industry

Multi-axis SEAT values can be estimated for seating systems tested in the laboratory using representative operational exposures. These values can be applied to monitored vehicle floor accelerations to target potentially harmful vibration in accordance with ISO 2631-1: 1997, assuming the operational exposures have similar frequency and magnitude characteristics. The transmission at the seat back should be considered when substantial low frequency multi-axis vibration is present.     

The effect of posture and vibration magnitude on the vertical vibration transmissibility of tractor suspension system

The efficiency of suspension seat can be influenced by several factors such as the input vibration, the dynamic characteristics of the seat and the dynamic characteristics of the human body. The objective of this paper is to study the effect of sitting postures and vibration magnitude on the vibration transmissibility of a suspension system of an agricultural tractor seat. Eleven (11) healthy male subjects participated in the study. All subjects were asked to sit on the suspension system. Four (4) different sitting postures were investigated – i) “relax”, ii) “slouch”, iii) “tense”, and iv) “with backrest support”. All subjects were exposed to random vertical vibration in the range of 1–20 Hz, at three vibration magnitudes - 0.5, 1.0 and 2.0 m/s² r.m.s for 60 s. The results showed that there were three pronounced peaks in the seat transmissibility, with the primary resonance was found at 1.75–2.5 Hz for every sitting postures. The “backrest” condition had the highest transmissibility resonance (1.46), while the “slouch” posture had the highest Seat Effective Amplitude Transmissibility (SEAT) values (64.7%). Changes in vibration magnitude for “relax” posture from 0.5 to 2.0 m/s² r.m.s resulted in greater reduction in the primary resonance frequency of seat transmissibility. The SEAT values decreased with increased vibration magnitude. It can be suggested that variations in posture and vibration magnitude affected the vibration transmission through the suspension system, indicating the non-linear effect on the interaction between the human body and the suspension system.Relevance to industry: Investigating the posture adopted during agricultural activities, and the effects of various magnitudes of vibration on the suspension system's performance are beneficial to the industry. The findings regarding their influence on the human body may be used to optimize the suspension system's performance.     

Reducing whole-body vibration and musculoskeletal injury with a new car seat design

A new car seat design, which allows the back part of the seat (BPS) to lower down while a protruded cushion supports the lumbar spine, was quantitatively tested to determine its effectiveness and potentials in reducing whole-body vibration (WBV) and musculoskeletal disorders in automobile drivers. Nine subjects were tested to drive with the seat in: 1) the conventional seating arrangement (Normal posture); and 2) the new seating design (without BPS (WO-BPS) posture). By reducing contact between the seat and the ischial tuberosities (ITs), the new seating design reduced both contact pressure and amplitude of vibrations transmitted through the body. Root-mean-squared values for acceleration along the z-axis at the lumbar spine and ITs significantly decreased 31.6% (p < 0.01) and 19.8% (p < 0.05), respectively, by using the WO-BPS posture. At the same time, vibration dose values significantly decreased along the z-axis of the lumbar spine and ITs by 43.0% (p < 0.05) and 34.5% (p < 0.01). This reduction in WBV allows more sustained driving than permitted by conventional seating devices, by several hours, before sustaining unacceptable WBV levels. Such seating devices, implemented in large trucks and other high-vibration vehicles, may reduce the risk of WBV-related musculoskeletal disorders among drivers.     

Transmission of vertical vibration through a seat: Effect of thickness of foam cushions at the seat pan and the backrest

The perception of vehicle ride comfort is influenced by the dynamic performance of full-depth foam used in many vehicle seats. The effects of the thickness of foam on the dynamic stiffness (i.e., stiffness and damping as a function of frequency) of foam cushions with three thicknesses (60, 80, and 100 mm), and the vibration transmitted through these cushions at the seat pan and the backrest were measured with 12 subjects (6 males and 6 females). With increasing thickness, the stiffness and the damping of the foam decreased. With increasing thickness of foam at the seat pan, the resonance frequencies around 4 Hz in the vertical in-line and fore-and-aft cross-axis transmissibilities of the seat pan cushion and the backrest cushion decreased. For the conditions investigated, it is concluded that the thickness of foam at a vertical backrest has little effect on the vertical in-line or fore-and-aft cross-axis transmissibilities of the foam at either the seat pan or the backrest. The frequencies of the primary resonances around 4 Hz in the vertical in-line transmissibility and the fore-and-aft cross-axis transmissibility of foam at the seat pan were highly correlated. Compared to sitting on a rigid seat pan with a foam backrest, sitting with foam at both the seat pan and the backrest reduced the resonance frequency in the vertical in-line transmissibility of the backrest foam and increased the associated transmissibility at resonance, while the fore-and-aft cross-axis transmissibility of the backrest was little affected. Compared to sitting without a backrest, sitting with a rigid vertical backrest increased the resonance frequency of the fore-and-aft cross-axis transmissibility of the seat pan cushion and increased the transmissibility at resonance.Relevance to industryThe transmissibility of a seat is determined by the dynamic properties of the occupant of the seat and the dynamic properties of the seat. This study shows how the thicknesses of foam at a seat pan and foam at a backrest affect the in-line and cross-axis transmissibilities of the foams at the seat pan and the backrest. The findings have application to the design of vehicle seats to minimise the transmission of vibration to the body.     

Effect of the seating condition on the transmission of vibration through the seat pan and backrest

Changes in the seating condition may change the body posture which could affect the transmission of vibration through a vehicle seat. This study investigates the effect of different seating conditions on the transmission of vibration through a car seat. Ten male subjects sat on the passenger seat of a sedan car driven at 60 km/h adopting one of six conditions at a time. The VDV was measured on the seat and backrest. Backrest contact affected the VDV measured on the seat pan in the z- and y-axis only. Increasing the backrest angle increased the VDV at the backrest in the x-direction and reduced the VDV at the backrest in the z-direction. With the increase in the backrest angle, the total VDV at the backrest became higher than the total VDV on the seat pan. The study showed no effect of foot position and contact with a headrest on the VDVs.Relevance to industryThis research presents the effect of the seating condition on the transmission of vibration through the seat pan and backrest of a car seat. Research of this kind may help seat manufacturers recommend seating conditions that reduce discomfort caused by whole-body vibration.     

Whole-body vibration transmitted to the framesaw operator

At the moment there are more than 40 framesaws in use in Croatia, and many framesaw operators have a serious spine problem. Complex measurements have been carried out in order to determine the vibration level on the operator's seat. This paper reports and analyzes the measurement results of the whole-body vibration transmitted to a framesaw operator during an ordinary working day. Vibrations were measured at all operations performed during the normal framesaw working cycle. For all the measurements frequency spectra were obtained and the results graphically represented according to the ISO 2631-1-1986 recommendations. The weighted r.m.s. acceleration was also calculated and the duration of each single framesaw operation was measured. The energy-equivalent vibration level, corresponding to the total duration of exposure was calculated, too. The values so obtained were compared with the daily exposure limits according to the ISO 2631-1-1997. The measurement results showed that the framesaw operator, under the given measuring conditions, is exposed to a higher vibration level than the given daily exposure limits. Also, the energy-equivalent vibration level calculated according to the guidelines given in the new ISO 2631-1-1997 is higher than the limits recommended for an effective 4-h daily exposure.     

Laboratory study as basis of the development for a seat testing procedure in horizontal directions

Horizontal whole-body vibration (WBV) can have an influence on health and comfort of drivers. Driver seats with additional suspensions in x- and/or y-axis should reduce the influence of vibration on the driver. No standards or test procedures are available for testing seats with horizontal suspensions. A standard test procedure for seats with horizontal suspensions could be developed from a test protocol used in a recent inter-laboratory study. To reduce the variability of the results obtained by different laboratories or repetitions, the test procedures should carefully consider methodological aspects with relevance to the results. Specifications of typical signals obtained in practice, or well-defined random time series as a basis for generation of test signals, with defined signs of acceleration would enable identical excitations in time domain and could reduce the variability of results caused by the non-linearity of the seat–human system. Nine healthy subjects volunteered for the study to test a seat with suspensions in three directions. They were selected with regard to three groups of body mass (52.4–54.6, 75.5–77.1, 98–100.7 kg). A random signal and two signals obtained in practice (tractor and truck) were selected for excitations in x- and y-directions by a six DOF electro-hydraulic simulator (hexapod). The errors between the desired and measured accelerations were calculated. The error depends on the signal, the direction and the subject. In the time domain, the absolute value of the error ranged from 1.8% to 42.4%. The SEAT value generally depends on the body mass, but in some cases the SEAT value of a lighter subject was lower than the SEAT value of a heavy one. This indicates that only one heavy and one light subject are not suited to calculate reliable parameters for an evaluation and comparison of seats. The non-symmetric displacements of the seat frame in x-direction indicate the non-linear behaviour of the seat–subject system.

Relevance to industry

Driver seats with suspensions in horizontal directions can reduce the influence of WBV on the health-risk of drivers. Laboratory test procedures are helpful for the evaluation and design of seats, if the results obtained by different laboratories are comparable. The paper describes methodological aspects that affect the test results and their variability.     

The effects of the duration on the subjective discomfort of a rigid seat and a cushioned automobile seat

The subjective discomfort caused by the seat would affect the judgements of discomfort for the seated subjects. However, there have been few studies concerned with the discomfort on the rigid seat in static states, especially for a relatively long duration. This paper investigated the subjective discomfort caused by a rigid seat and a cushioned automobile seat for an hour. Twelve students (eight males and four females) rated the overall discomfort on a category-ratio scale and the local body discomfort on a 6-point rating scale every 10 min caused by two seats in two separate days. The static discomfort increased with increasing time, and the rigid seat caused greater discomfort than the cushioned seat. The local discomfort on the back dominated on the automobile seat, whereas the local discomfort on the buttock area dominated on the rigid seat. We established the empirical equations to predict the relations between the discomfort and duration of the two types of seats, for benchmark in the future studies on vibration and noise discomfort.     

Study of simultaneous shock and vibration control by a fore-and-aft suspension system of a driver’s seat

A single degree-of-freedom simulation model of a seat equipped with a non-linear fore-and-aft suspension system was developed. The performance of a typical fore-and-aft suspension system was analysed using typical vibration and shock acceleration signals measured in loaders while travelling on off-road terrain and while driving the bucket into a heap of soil. Damper modification was suggested to improve the suspension performance in the control of both vibration and shocks. Suspension system performance was analysed using the simulation approach for variations in both mass loading and excitation intensity. A discussion of results obtained and practical suggestions for improvement of the fore-and-aft driver’s seat suspension system concludes this contribution.

Relevance to industry

The simulations are of use to seat manufacturers in developing improved fore-and-aft seat suspension for wheel loaders. It is shown that a damper with an optimised progressive damping characteristic provides better shock mitigation than is available with either a seat without any suspension or a suspended commercial seat as presently available on the market.     

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.     

Effects of aircraft seat pitch on interface pressure and passenger discomfort

Seat pitch, defined as the distance from a point on the back of one seat to the same point on the seat in front, is one of the most important factors influencing aircraft seating comfort. This study assessed the influence of different airline seat pitches on subjective ratings of discomfort and body-seat interface contact pressures. This was a laboratory within-subjects study using an aircraft interior mock up to vary seat pitch. Twelve participants completed 1 h of sitting in each of five different seat pitches (28inches, 30inches, 32inches, 34inches, and 36inches). Interface pressure mats measured seat and backrest pressure distribution, subjective rating scales were used to measure overall and local body region discomfort. The results showed that overall body and local body region discomfort ratings tend to be lower when the seat pitch increased from 28 inches to 36 inches (p < 0.05). For pressure variables, the upper back average contact area, upper/lower back average contact pressure, upper/lower back average peak contact pressure, right buttock average contact area, left/right thigh buttock average peak contact pressure, and left buttock average peak contact pressure were significantly affected by seat pitch(p < 0.05). Separate analyses support that seat pitch was more strongly correlated with backrest interface pressure than with seat pan pressure. In conclusion, seat pitch was found to be an important factor associated with body-seat contact pressure and discomfort ratings.     

Exposure of agricultural pilots to occupational whole-body vibration: The effects of runway maintenance and the stages of flight

BackgroundAgricultural pilots fly inside rural areas, usually, in runways without proper maintenance. Neglected runway maintenance contributes to increased exposure to shocks and vibrations. On the other hand, during cruise and spraying, the pilot is not exposed to the effect of the runway surface, and thus the exposure may be different.ObjectiveThe aim of this study is to investigate the factors that may increase occupational exposure to WBV in agricultural pilots.MethodThe methodology used in this study consisted of evaluating the exposure of WBV according to the runway surface quality and the stages of flight for a sample of four different agricultural aircraft models.ResultsThe results confirmed that the magnitude of the vibration is significantly influenced by the runways surface quality and the stages of flight. This suggests the possibility to decrease the exposure to WBV improving runway maintenance and managing flight time.Relevance for the industryBased on the results of the present study, it was possible to define strategies to reduce whole-body vibration exposure and thus improve the working conditions of agricultural pilots.     

Assessing the discomfort of the whole-body multi-axis vibration: laboratory and field experiments

Laboratory and field experiments were conducted to determine the best procedure for predicting the discomfort caused by multi-axis vibration. In the laboratory experiment, 11 seated subjects compared single-axis vibration in one axis to single-axis vibration in another axis, and compared dual-axis vibration to single-axis vibration. In the field experiment, 22 lorry drivers rated the discomfort of 16 different rides. The results show that the best procedure for predicting the discomfort is to combine the vibration inputs by taking the square root of the sum of squares of the weighted r.m.s. values of the vibration in each axis.     

Modelling and simulation of a fore-and-aft driver''s seat suspension system with road excitation

This paper describes a simplified simulation of two configurations of the fore-and-aft seat suspension system. A fore-and-aft suspension system model was proposed based on the laboratory measurements of the seat vibration isolation performance. Friction was identified as an important parameter, so different approaches to simulating the suspension friction were investigated. Predicted seat vibration mitigation properties were compared with those measured in the laboratory in response to the recordings of the fore-and-aft vibration measured at the base of the driver's seat in an on-road tractor–trailer combination (articulated truck). Optimisation of the suspension elements parameters was then performed to identify the maximum attainable attenuation. A solution incorporating supercritical suspension damping predicted to give an improvement of the order of 10% in the x-direction mitigation properties as compared to a fixed (locked) horizontal suspension system.

Relevance to the industry

Simulations conducted in this study are of use to seat manufacturers in developing the fore-and-aft seat suspension systems with improved vibration mitigation properties and for predicting its dynamic performance. The optimisation study shows the attainable vibration mitigation limits for a horizontal suspension system.     

A new framework for evaluating potential risk of back disorders due to whole body vibration and repeated mechanical shock ∗

Abstract

Die Arbeitsaufgabon in der Landwirtachaft werden von dem biologischen Rhythmus von Boden, Pflanze und Tier vorgeschrieben, deren Ablauf durch die menschliche Arbeit so unterstiitzt werden soil, dass eine optimale Leistung zu erwarten iHt. Das bedeutet:

Fast alle Arbeiten miissen nach Beobachtung und Überlegung geplant werden—eine mtelloktuolle Aufgabe des Arbeitseinsatzes.

Der Landwirt muss über ein grosses Wissen und gutes Gedachtnis fur Arbeitsver- fahren verfügen—eine Aufgabe der Arbeitspadagogik

Schwere und Zeitanspruch der Arbeiten wecheeln raach, zeitweilige Uber-lastungen konnen nicht vermieden werden

Maschinen sind nur Mittel zum Zweek, sie müsaen daher einfach und überaichtlich konstruiert sein und dürfen weder die Aufmerksamkeit des Führers von der eigentlichen Aufgabe der Arbeit ablenken, noch dürfen sie die Gesundheit beeinträchtigen.     

Apparent mass matrix of standing subjects exposed to multi-axial whole-body vibration

This paper describes the experimental characterisation of the apparent mass matrix of eight male subjects in standing position and the identification of nonlinearities under both mono-axial and dual-axis whole-body vibration. The nonlinear behaviour of the response was studied using the conditioned response techniques considering models of increasing complexity. Results showed that the cross-axis terms are comparable to the diagonal terms. The contribution of the nonlinear effects are minor and can be endorsed to the change of modal parameters during the tests. The nonlinearity generated by the vibration magnitude is more evident in the subject response, since magnitude-dependent effects in the population are overlaid by the scatter in the subjects’ biometric data. The biodynamic response is influenced by the addition of a secondary vibration axis and, in case of dual-axis vibrations, the overall magnitude has a marginal contribution.

Practitioner Summary: We have measured both the diagonal and cross-axis elements of the apparent mass matrix. The effect of nonlinearities and the simultaneous presence of vibration along two axes are smaller than the inter-subject variability.     

The impact of different seats and whole-body vibration exposures on truck driver vigilance and discomfort

Laboratory studies have shown that exposure to whole-body vibration (WBV) increases physical and mental fatigue, which are common issues professional drivers face. The objective of this study was to determine whether altering WBV exposures had any effect on driver vigilance and discomfort. A repeated measures crossover design of five truck drivers with regular 10-h routes was used. Active and passive suspension truck seats were evaluated. For each seat, WBV exposures were measured. Participants completed a discomfort questionnaire and a reaction time task before and after their shift for two weeks, one week per seat. Compared with the passive seat, the active seat significantly reduced WBV exposures, decrements in the optimal and mean reaction times (p = 0.02, 0.047, respectively), and discomfort in the lower back and wrist(s)/forearm(s) (p < 0.01, 0.01, respectively). Study results indicated that reducing WBV helps reduce discomfort and maintain vigilance, which may improve drivers’ health and reduce the risk of truck collisions.

Practitioner Summary: The active suspension seat used in this study reduced truck drivers’ exposure to whole-body vibration (WBV) by over 33% in relation to their current industry standard passive suspension seat. This study demonstrated that reducing truck drivers’ exposure to WBV reduced fatigue and discomfort development over a workday.     

Whole-body vibration: exposure time and acute effects — experimental assessment of discomfort

The method of cross-modality matching was used to study the development of discomfort during a 1 hour exposure to whole-body vibration. The subject's task was to adjust a broadband noise to the level where it gave rise to the same degree of discomfort as a vibration. Random vertical vibrations were used with a resonance of either 3·1 or 6·3 Hz. The sound settings were transformed into vibration levels by means of sound-vibration functions determined for each subject.

The sound-level settings increased as a function of exposure time and control measurements showed that this could not have been the effect of a lowered sensitivity to the noise. The increase, expressed as log acceleration, was a linear function of log exposure time. It is therefore concluded that results from studies of shorter exposure times might be extrapolated to exposure periods of at least 1 hour. Although the method employed probably led to an underestimation of the increase in discomfort over time, the experiment does imply that the increase is overestimated by ISO standard 2631.     

Subjective response of standing persons exposed to fore-aft,lateral and vertical whole-body vibration

The aims of this study were to propose multiply scale factors for evaluation of discomfort of standing persons and to investigate whether there exist differences between multiplying factors used for evaluation of discomfort of standing persons and those of seated persons exposed to WBV. Twelve male subjects were exposed to twenty-seven stimuli that comprise three acceleration magnitudes (0.2, 0.4, and 0.8 m/s² r.m.s.) along fore-aft (x), lateral (y) or vertical (z) direction. The subjects with seated or standing posture on the platform of the vibration test rig rated the subjective discomfort for each stimulus that has frequency contents ranging from 1.0 Hz to 20 Hz with a constant power spectrum density. The order of presentation of the test stimuli was fully randomized and each stimulus was repeated three times. The subjective scale for discomfort was calculated by using the category judgment method. The best combinations of multiplying factors were determined by calculating correlation coefficients of regression curves in-between subjective ratings and vibration magnitudes. In all the directions, body posture significantly influenced on subjective discomfort scales. Particularly in the fore-aft and lateral direction, the upper limit of all the categories for the standing posture resulted in higher vibration acceleration magnitudes than those for the seated posture. In contrast, in the vertical direction, only the upper limit of category “1: Not uncomfortable” for standing posture was observed to be higher than that for seated posture. The best agreement for ISO-weighted vibration acceleration occurred at x factor of 1.8 and y factor of 1.8 in the standing posture and x factor of 2.8 and y factor of 1.8 in the seated posture. The results suggest that seated people respond more sensitively and severely in perception of discomfort to fore-aft and lateral vibration than standing people do while standing people respond more sensitively and severely to vertical vibration than seated people do. Thus the effects of body postures on multiplying factors should be considered in evaluation of discomfort caused by whole-body vibration.Relevance to industryThis study reports differences in subjective response of standing persons to fore-aft, lateral and vertical whole-body vibration. The results obtained in this study propose the fundamental data on the sensitivity to whole-body vibration exposed with standing posture.     

Objective and subjective responses of seated subjects while reading Hindi newspaper under multi axis whole-body vibration

Train passengers often read newspapers while traveling. Vibration is one of the key factors that may occasionally inhibit this activity. An experimental study was, therefore, conducted to investigate the extent of interference perceived in reading task by seated subjects in two postures under random vibration. 30 healthy male subjects were exposed to vibration magnitudes of 0.4, 0.8 and 1.2 m/s² in mono, dual and multi axis in the low frequency range 1–20 Hz. The task required subjects to read a given paragraph of Hindi national newspaper, in two seated postures (lap posture with backrest support and table posture with leaning over the table). The reading performance was evaluated by both degradation in performance in terms of time required to complete the task and subjective rating using Borg CR10 scale. Both the methods of reading performance evaluation exhibit progressive increase with an increase in vibration magnitude for both the subject postures in all the direction of vibration and are found to be higher in lateral and vertical direction among mono axes. The effects of multi axis vibration on perceived difficulty have been found to be similar to dual axes vibration and greater than mono axes vibration; however degradation in reading performance in multi axis vibration was also found to be similar to that for lateral direction. A comparison of the effect of postures by both evaluation methods revealed that the reading performance was adversely affected for table posture in all direction of vibration, however for lap posture, only the X-axis vibration effect was more severe.

Relevance to industry

Available ride comfort standards for vehicles do not include the effects of vibrations on passenger activities. Assessment of activity discomfort would be useful for vehicle design optimization to facilitate activity comfort.