ASPECTS OF VIBRATIONS OPERATING ON HUMAN BODY AT WORK POST OF WOOD WORKING MACHINES. Angela REPANOVICI, Ioan CURTU, Vasile CIOFOAIA

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1 UNIVERSITATEA TRANSILVANIA DIN BRA!OV Catedra Design de Produs &i Robotic) Simpozionul na3ional cu participare interna3ional4 PRoiectarea ASIstat4 de Calculator P R A S I C ' 0 Vol. II - Organe de ma&ini. Transmisii mecanice 7-8 Noiembrie Braov, România ISBN ASPECTS OF VIBRATIONS OPERATING ON HUMAN BODY AT WORK POST OF WOOD WORKING MACHINES Angela REPANOVICI, Ioan CURTU, Vasile CIOFOAIA Universitatea Transilvania din Braov Abstract: This paper presents the actors which inluence the human body vibrations as result o vibrations produced by the wood-working machines. Here are presented varied types o dampers used at rames, carcasses and boards design. We can see the accurate results o experiments concerning the vibrations eect on human body lied in dierent positions: standing on a lot o actors, inclined, normal, tensed and relaxed. The natural requencies dangerous/noxious or the human body can be ound into the Hz o scale interval. Keywords: vibrations, dampers, requency dangerous, mechanical impedance, relaxation, resonance. At work posts o wood-working machines, wanting protection against vibrations, can be generated local or global vibrations which may much exceed the admitted level. The vibrations at work-post depend on a lot o actors like: machine exploitation duration; the dynamic conditions o working aggregates; the number and type o vibration sources which run. As with a machine duration exploitation, the vibration at work-post could increase almost twice. The vibrations are propagated to the human organism either to the entirely body or on certain body s parts only, aecting its health by physiopathological eects which o them are: nervous system stimulation; the appearance o some tactile thermic and painul sensitiveness agitations; the appearance o some drowsiness and tiredness conditions which are maniested by diminution o vigilance and visual acuity, leading at working ability decrease; ear conditions appearance; the appearance o thoracic, epigastric aches accompanied by nausea,inappetence, vomitings, headaches, a high pulse and also blood pressure; changes o respiratory apparatus unction; the provocation o both osteoarticular lesions and muscular tendons. These also are worsen both o noises existence and an environment ull o dust particles. By the manner o application and in dependance on the magnitude o vibrations kinematic parameters (displacement, speed, acceleration) neuromuscular and sensorial anxieties such as: visual acuity agitations correlated with indications reading o machines control-panel or inacurracies o operations eectuation in the technological processes; unctional agitations o upper and lower limbs lead to the loss o ine and accuracy controls execution ability. Among mechanical harms brought about vibrations are: brain s and lungs lesions o cardiac nature, aroused by vibrations with a requency higher than 30 Hz; lesions o bowels internal wall, when the requencies surpass 5 Hz; the labby tissues tearing, the tendons and joints stretching. The research made by Dieckmann [] have established certain criteria concerning the way in which the human organism perceives the vibrations, the physiological and vegetative changes, the subjective sensations and working ability. The research has been carried out or the harmonious vibrations, on perpendicular and horizontal direction.

2 Thus, a vibrations strength coeicient k that takes into consideration the simultaneous inluence o requency and o vibrations amplitude x 0 is proposed. The expression o k is: or requencies lower than 5 Hz: k = x 0 ; () or requencies between Hz: k = 5x 0 ; () or requencies between Hz: k = 00x0. (3) The values o coeicient k could be established analytic using the ollowing relations: k = ae ; 0 + k = ve ; 0 + k = xe, (4) + 0 where: ae is the eective acceleration mm/s ; v e the eective speed mm/s; x e the eective displacement mm; the requency Hz and 46 = 8 s / mm; = 0. s / mm; = 0.7s / mm; = 0 Hz. Knowing the values o vibratory stress coeicient k, some estimates about vibrations inluence method on work conditions, could be made (table ). Imposing k, the reckonings may be done viceversa/conversely, too; knowing the kinematic parameters x0, v 0, a 0 and, the coeicients values 0, 0 and 0 can be established. To realize a protection against vibrations i.e. their diminution (at source vibroprotection active and at the operator-passive vibroprotection) detached actors or groups o elastic elements which are constituted in the shape o antivibrating structural systems, are used. Modern examples o such systems used at carcasse s (astening) and rames are presented in Figure. The indicator o vibroisolation eiciency is the transmissibility coeicient T, which represents the percentage o dynamic orce propagated rom the vibrations source to oundation or rom the surace that vibrates to the operator. A simple expression o T is done below (the abrasion is neglected) T =, (5) p where: p is the body s natural requency and is the requency o strained vibrations; = n / 60, Hz in the case o rotative motion; n is the rotative speed, in rot/min. Table. The values o vibrations strength coeicients and the work conditions type: Coeicient k Sensitiveness Estimates on work 0. Unperceptible Unhampered A little perceptible Unhampered Perceptible ater a ew hours, a A little uncomortable little unpleasant; bearable Well perceptible, ater a ew Uncomortable, but still possible hours becomes unpleasant, but still tolerable Unpleasant, ater an hour becomes unbearable Violently uncomortable, but possible, yet 0 30 Very unpleasant ater /6 hour; it Too little possibhle needs activity s interruption Unpleasant in the highest degree; Impossible to work ater one minute the work has to be interrupted Over 00 Unbearable Impossible to run

3 47 The insulation is realized when / p >. Practically, the values o this ratio are included between.5 and 5.0, and the natural vibrations requency is 5 p = Hz, (6) x where x is the static deormation (measured in centimeters), o vibroinsulators under vibrating mass action. The vibroinsulation eiciency increases as the natural vibrations requency decreases, so, as the static deormation grows. The rubber endering and rubber-metal are used or the suspension o operator s compartment and o strength aggregates. Their utilities are: elastic and damping properties, they also realize the dying-out sound vibrations and they have a simple structure design, too. These ones have the drawbark that they change their Fig.. Compartments rames (a, b) and boards (c) dampers: layer; damper; 3 astening; 4 rubber hushing; 5 - clamp; 6 rubber element; 7 external strap. elastic properties in the long run. On the other hand, it came out that, owing to mighty strain, the rubber elements which work at shearing provide or a value o.5 times lower than natural requency, comparatively with those ones which run at compression. The solution o problems concerning the human subjects protection against vibrations needs the human body modelling. In the most global case, such a model should be a non-steady non-linear system having an ininity o liberty degrees. The dynamic properties o such system are very complex, because they are aected by organism's structure, the position during the work, the tiredness degree and the global psychological and mental state. Because not all these actors are taken into consideration or modelling, usually, the number o reedom degrees, non-linearity and non-uniormity are reerred to. The dynamic properties o human body considered as an visco(us) elastic linear mechanical system, can be determined/as certained in dependace on the mechanical input resistance Z ( p), as ratio between the magnitude o orce amplitude transmitted to human body in the stimulus point and the speed amplitude in the some point. The argument o input resistance represents the phase dierence between orce and speed. In the Figures -7 are presented the measurement results o human body mechanical impedances in varied states and or dierent points o vibrating signals application.

4 48 Fig.. Mechanical impedance and the impedance argument or normal position o human body Fig. 3. Argument s and impedance s variation with the requency: tensed position; relaxed position Fig. 4. The variation o impedance and argument with the requency: the legs prop up on a dead abutment; without abutment or legs Fig. 5. The variation o impedance Z and o argument s when the body is inclined to back

5 49 Fig. 6. The impedance s Z and argument impedance when the body is inclined ahead Fig. 7. The variation o impedance Z and o argument or standing position Fig. 8. The impedance s Z and argument s variation or standing position in the case o an additional mass existence at lumbar section level o the spine Fig. 9. The variation o impedance Z and o argument or the case when the legs are bent Fig. 0. The variation o Z impedance and o argument or the case in which the subject sits on heels Fig.. The variation o impedance Z and o argument or the case when the subject sits on tiptoes

6 50 Fig.. Z impedance s and o argument s variation or the case in which the subject stands on a oot Fig. 3. The variation o impedance Z and o argument or the case when the subject sits on a dead abutment, and the vibrations are applied in eet s zone Fig. 4. The variation o impedance Z and o argument or the case when the subject props up on a dead abutment, and the vivrations are applied on tightened arm Analysing these results, the ollowing conclusions appear: the resonance properties o human body are indicated in requencies lower than 60 Hz; secondary orces applied to human body generate maximum amplitude variations o impedance at high requencies; the impedance appreciably changes in the case o inclined body; the maximum are about 5 Hz, owing to transverse vibrations o spine; body's impedance is inluenced by the position o stood person; the rotation angle lessening o knees' joints diminishes the vibration energy absorbed by human body; Fig. 5. The variation o impedance Z and o argument or the case when the subject props up on a dead abutment, and the vivrations are applied on arm bent rom the elbow in right angle in the case o human body excitation on vertical direction (z), the main resonant requencies are between Hz. (Figure 8 and 9). Vibrations acceleration variation (z) o person whose requency is in respect o his settlement position (-natural; strained / tensed position; 3 - relaxed position) is represented in Figure 8, a and b). We can notice that the accelerations at system level related at pelvis' zone are very low in the case o relaxed position (curve 3), high in the case o tensed position (curve ) and very high at natural position (curve 3).

7 5 The knowledge o data involved in this paper, allows the establishment o optimum operating conditions or their attendants, by design, conception and accomplishment Fig. 6. The variation o impedance Z and o argument or the case when the subject sits on a dead abutment and the vibrations are applied on Fig. 9. Resonance requencies o human body eye: -7 Hz; neck: 6-7 Hz; 3 chest: - Hz; 4 legs, hands: -8 Hz; 5 head: 8-7 Hz; 6 ace and mouth: 4-7 Hz; 7 lumbar area: 4-4 Hz; 8 belly: 4- Hz Reerences both arms Fig. 7. Frequency-amplitude characteristic in the case o sitting and standing position. Bratu, P. Sisteme elastice de rezemare pentru ma+ini +i utilaje. Editura Tehnic4. Bucureti, Bratu, P. Vibra.iile sistemelor elastice. Editura Tehnic4. Bucureti, Bratu, P., Mihalcea, A. Procedur0 de aplicare a nivelului de vibra.ii transmise omului în cazul ma+inilor +i utilajelor de construc.ii. În Buletinul celui de-al VI-lea Simpozion Na3ional de utilaje pentru Construc3ii. Bucureti, 6-7 iunie Buzdugan, Gh. Izolarea antivibratorie a ma+inilor. Editura Academiei. Bucureti, Gai3eanu, M..a. Vibra.ii +i zgomote. Ed. Junimea. Iai, Goldsmith, W. Impact. The theory and physical behaviour o colliding solids. E. A. London, Harris, C. M., Crede, Ch. E. 3ocuri +i vibra.ii, vol. I, II, III. Editura Tehnic4. Bucureti, Mihalcea, A. Analiza condi.iilor de securitate ca urmare a vibra.iilor transmise organismului uman în cazul ma+inilor de construc.ii, Tez4 de doctorat, Universitatea "Dun4rea de jos", Gala3i, 00.

8 5 Fig. 8. Acceleration amplitude-requency characteristic in dependance on subject s position manner: a sitting; b standing; - normal position; - stretched position; 3 relaxed position 9. Munteanu, M. Introducere în dinamica ma+inilor vibratoare. Editura Academiei. Bucureti, Pugna, L..a. Rela.ia om ma+in0 mediu. Editura Facla. Timioara, Radu, A., Curtu, I. Dinamica ma+inilor pentru prelucrarea lemnului. Editura Tehnic4. Bucureti, Sila, Gh..a. Studiul izol0rii vibra.iilor produse de un agregat ac.ionat cu motor electric. În: Lucr4rile Conerin3ei Vibra3ii în construc3ia de maini, Timioara, Voinea, R., Stroe, I. Dinamica sistemelor. Univeristatea "Politehnica". Bucureti, ***: Colec3ia revistelor Industria Lemnului, *** : Colec3ia revistelor Holz als Roh und Werlesto, Holzindustrie, Wood Industry, Forest Product Journal,