Anstotz, Benjamin

Matthiesen, Sven

Zimprich, Sebastian

English

KITopen

Human Interaction and Emerging Technologies (IHIET 2022), Vol. 68, 2022, 683–689https://doi.org/10.54941/ahfe1002793Hydro-Mechanical Dampers for TestingHigh Cyclic Shock Loads in PreventiveHealth ProtectionSebastian Zimprich, Benjamin Anstotz, and Sven MatthiesenInstitute of Product Engineering, Karlsruhe Institute of Technology, 76131 Karlsruhe,GermanyABSTRACTConstruction workers are exposed to harmful vibration emissions in the usage ofpower tools. For preventive health protection, the vibration emission is therefore regu-lated in the EN ISO 28927, as well as tested and optimized in the development ofnew power tools. The vibration emission of hammer drills and other impact drivenpower tools can be tested by using a steel ball energy absorber as a substitute work-piece, which reduces statistical variance and expenses in laboratory tests. The steelball energy absorber, as the current used damper in EN ISO 28927, is not adjustable inits properties and has no variable influence to the power tools vibration emission. It isunknown, whether an adjustable damper could affect the power tools vibration emis-sion. Therefore, a new adjustable hydro-mechanical damper for testing high cyclicshock loads was designed and tested in this study. The tests revealed that hammerdrills vibration emissions can be influenced by an adjustable hydro-mechanical dam-per. In field tests, the used tool, tool wear or the machined workpiece influences thehammer drills vibration emissions. The new adjustable damper can improve the pre-ventive health protection of workers, as these factors now can be taken into accountin laboratory vibration testing.Keywords: Human-machine system, Vibration, Preventive health protection, Power tool, Relia-bility, TestingINTRODUCTIONHand-arm vibrations increase the probability of vascular and neurologi-cal diseases. Vibration white finger is such a known long-term conditionand acknowledged occupational disease (Nilsson et al., 2017). Constructionworkers are exposed to harmful vibration emissions in the usage of powertools. The maximum daily working time with power tools is therefore limi-ted by considering their vibration emission (EN ISO 5349-1:2001). Everypower tools vibration emission is specified with the ahv value, which weightsharmful frequency components (EN ISO 5349-1:2001). A smaller vibrationemission results in a lower ahv value and allows a longer daily working time.Manufacturers optimize and test power tools in their development thereforeto a low vibration emission.© 2022. Published by AHFE Open Access. All rights reserved. 683684 Zimprich et al.Scattering concrete properties lead to a high statistical variance in thesetests. For hammer drill tests, the concrete can therefore be replaced by themechanical particle damper “Dynaload” as a substitute workpiece in labora-tory tests (EN ISO 28927-10:2011). With this particle damper, the hammerdrills tool strikes in a container with more than 3000 bearing balls. Thesebearing balls convert impact energy into heat by friction. The consistent pro-perties reduce the statistical variance in the tests and leads to measurementsthat are more comparable. McDowell et al. compared the vibration emissionin tests with this mechanical particle damper to field tests in working con-ditions. These tests revealed large differences in tests between the vibrationmeasured with the particle damper and the field (Mc Dowell et al., 2012).This lowers the tests reliability, as test results derive from the real application(Hewitt et al., 2011). In the field, the vibration emission of hammer drillscan depend on many different factors, such as bit diameter, bit wear or theconcrete type (Antonucci et al., 2017; Coggins et al., 2010). These influe-ncing factors to the vibration emission are not yet considered in laboratoryvibration tests.Due to their nature, mechanical particle dampers are not adjustable in theirproperties (Sánchez et al., 2012). Hydro-mechanical dampers can providevariable stiffness and damping parameters (Jugulkar et al., 2016). However,it is unknown whether variable stiffness and damping parameters have aninfluence to the hammer drills vibration emission.With an adjustable damperfor high cycle shock loads, these tests could be carried out with more vali-dity, as the influencing factors to the vibration emission could thus be takeninto account. The research question in this paper is therefore formulated asfollows:Have variable stiffness and damping parameters an influence to a hammerdrills vibration emission?MATERIALS AND METHODSA new designed hydro-mechanical damper with variable stiffness and dam-ping parameters is presented in this chapter. This damper is used for vibrationtests, according to EN ISO 28927, to answer the research question.The Adjustable Hydro-Mechanical DamperIn order to answer the research question, a new adjustable damper for highcycle shock loads was designed. As particle dampers are not adjustable due totheir nature, a hydro-mechanical damper concept was selected. The scope ofapplication is analogous to the particle damper used in the EN ISO 28927-10: To be used as a substitute workpiece to replace concrete in laboratoryhammer drilling tests. The new damper compared to the particle damper“Dynaload” is shown on the top in Figure 1.The impacts of the hammer drill are transmitted into the piston via the rod,causing the piston to move. The piston is pivoted by coil springs. Differentcoil springs can be used to vary the stiffness. The piston is designed as athrottle. The movement of the piston causes oil to flow through the piston,which dissipates the energy introduced by the impacts.Hydro-Mechanical Dampers for Testing High Cyclic Shock Loads 685Figure 1: Sketches of the new adjustable hydro-mechanical damper in comparison tothe particle damper and the two different damping principles.The piston was designed as two throttle variants, one with a high dampingand one with a low damping parameter. In the concept “high damping”, thepiston is sealed to the cylindrical wall and the oil can flow through boreholes.In the concept “low damping”, the oil can flow through a circular ring gap.Both damping concepts are shown on the bottom of Figure 1.Experimental SetupTo answer research question, whether adjustable stiffness and damping para-meters damper have an effect to hammer drills vibration emissions, the newhydro-mechanical damper was loaded with high cycle shock loads and the-refore tested as a substitute workpiece according to the test procedure in ENISO 28927-10. A Hilti TE 7-C is used as a hammer drill, which has an impactfrequency of 67 Hz at a single impact energy of 2.6 J. This requires a dampingperformance of 174 W.To obtain stable und reproducible test conditions, the drilling tests areperformed in a drilling test rig, replacing the human operator. A pneumaticcylinder provides a constant feed force of 300 N. For valid vibration emis-sions, the hammer drill is clamped in a hand-arm model (Jahn and Hesse,1986).The vibration emission of the hammer drill is measured via accelerationsensors PCB 356A02 in drilling direction. The acceleration sensor is atta-ched to the hammer drill at the proposed location, according to EN ISO28927. The vibration emission is evaluated in the drilling direction, as theused hand-armmodel is designed and valid for this direction (Jahn andHesse,1986). The mean ahv value over a test run is calculated from the acceleration686 Zimprich et al.Figure 2: Experimental setup to test the influence of variable stiffness and dampingparameters to hammer drills vibration emission.Table 1. Factor levels for stiffness and damping.Factor Levels Stiffness Factor Levels DampingS1 535 N/mm High Sealed piston with boreholes throttlesS2 1250 N/mm Low Throttle as a ring gapS3 1455 N/mmS4 3132 N/mmsignal according to EN ISO5349-1:2001. This method weights harmful fre-quency components higher than non-harmful ones. The higher the resultingvalue, the more harmful the frequency and the shorter the daily exposuretime allowed. The experimental setup is shown in Figure 2.Experimental PlanIn the tests, the stiffness and damping parameters in the hydro-mechanicaldamper were varied. To vary the factor stiffness, four different coil springswere used as factor levels. The levels S2 and S3 were used, as the stiffness isnear the theoretical concretes stiffness value 1670 N/mm in hammer drilling(Jahn and Hesse, 1986). S1 and S4 were selected as a yielding and stifferfactor level, compared to the theoretical concretes stiffness value.To vary the damping principle, the two different pistons were used as quali-tative factors with a hydraulic oil of viscosity 22 cSt. Both damping principleshad a theoretical damping, which is high enough to damp the previous impactat the time the next impact occurs. The factor level “low damping” had apiston with a ring gap to the cylindrical wall as a throttle, resulting in alower damping coefficient, whereas the level “higher” had a piston whichwas sealed to the cylindrical wall and had four boreholes as throttles, resul-ting in a higher damping coefficient. The values for the factor levels are givenin Table 1.Hydro-Mechanical Dampers for Testing High Cyclic Shock Loads 687Figure 3: Ahv values for the stiffness and damping levels.Each factor combination was tested for with the hammer drill for30 seconds and was repeated for six times. The four factor levels for stif-fness, the two factor levels for damping principle and six test runs for eachcombination result in 48 test runs in total for the full factorial test design.The test runs were partially randomized to reduce trend effects.To compare the vibration emission of tests with the new hydro-mechanicaldamper to tests according to EN ISO 28928, six test runs were performedwith the particle damper “Dynaload” in the in the here presented test setup.These tests were carried out at the same feed force of 300 N and wereevaluated regarding the ahv value analogous to the 48 tests runs.RESULTSThe ahv values for the 48 test runs with the new hydro-mechanical damperand the six tests runs with the particle damper “Dynaload” are shown inFigure 3. The Mann-Whitney U-Test observed, that tests of the factor level“low damping” had a significant higher ahv value than tests with the factorlevel “high damping” (U = 8.000, Z = −5.774, p < 0.001, n = 48).The six tests with the particle damper “Dynaload”had a mean ahv value of6.9 m/s2. This mean value is marked as the black line in Figure 3. Comparedto the tests with the hydro-mechanical damper, this value is higher than mosttests with the factor level “low damping”, but higher than all ahv values oftests with the factor level “high damping”.For tests with the factor level “high damping”, the ahv value decreasedwith an increasing stiffness value. For tests with the factor level “low dam-ping”, this decreasing trend with an increasing stiffness value is not thatclear.The highest effect for a decreasing ahv value with a higher stiffness at thesame damping concept was observed for test with the stiffness “S1” compa-red to test runs with the stiffness “S4” (S1: 9.4 m/s2 vs. S4: 7.4 m/s2). Thehighest effect the ahv value was observed between for test with the factor688 Zimprich et al.combination “S1/Low damping” to “S4/High damping” (S1/Low damping:9.4 m/s2 vs. S4/High damping: 5.7 m/s2).The ahv values standard deviation for the six tests runs with the particledamper “Dynaload” was 0.48 m/s2. This standard deviation is bigger thanthe standard deviation for tests runs with the level “low damping” at thehighest or lowest stiffness (S1: σ = 0.23 m/s2 vs. S4: σ = 0.22 m/s2).DISCUSSIONIt is possible to carry out vibration tests for preventive healthcare, accordingto EN ISO 28927, with the new adjustable hydro-mechanical damper. In thetests performed, the ahv values are comparable to tests with the existing par-ticle damper “Dynaload”. The tests revealed that the ahv value is dependedby the dampers stiffness and damping parameters. Therefore, the researchquestion “Have variable stiffness and damping parameters an influence to ahammer drills vibration emission?” can be answered with yes.Although both damping principles provided valid ahv values, the adju-stability of the vibration emission via the varied stiffness is larger for thedamping principle with the throttle as a ring gap than the sealed piston withboreholes. This indicates that the concept with the throttle as a ring gap is bet-ter suited for future tests, since a greater adjustability is reached. With this, awider range of effects to the vibration emission can be mapped in laboratoryvibration tests.As only one hammer drill was used in this study, the effect of stiffness anddamping parameters to the vibration emission should be tested with varioushammer drills from different manufacturers to secure the findings. A specificmapping of influencing factors to the vibration emission, like the used tool,tool wear or the machined workpiece, in laboratory vibration tests is withthe findings of this study not yet possible. One the one hand, the quantita-tive effect of these factors to the ahv value is unknown, on the other hand,the precise adjustment of the vibration emission by different stiffness anddamping parameters is unknown. For this, the determination of stiffness anddamping effects to the ahv value needs further research. This would enable tomap the influencing factors in laboratory tests and would therefore lead tolaboratory vibration tests with a higher degree of validity. With further resea-rch, it would be conceivable to integrate the hydro-mechanical damper to thestandard EN ISO 28927 for laboratory vibration tests. With this, power toolmanufacturers can estimate the ahv values more reliable. This protects theuser in a preventive manner, as he has a better estimation and more preciseinformation about the vibration emission affecting him.CONCLUSIONA new adjustable hydro-mechanical damper for testing high cyclic shockloads was designed and tested as a substitute workpiece for hammer drills inlaboratory vibration tests. The tests revealed, that a hammer drills vibrationemission can be influenced by an adjustable stiffness and damping parame-ters. This can be used in future tests with hammer drills to map different useHydro-Mechanical Dampers for Testing High Cyclic Shock Loads 689cases in laboratory vibration tests. The findings in this study help to considerrelevant influencing factors to the ahv value in laboratory tests. Vibrationtests with hammer drill can thus be carried out more reliable. This protectsthe user in a preventive manner, as he would have a better estimation andmore precise information about the vibration emission affecting him.REFERENCESAntonucci, A., Barr, A., Martin, B. and Rempel, D. (2017) ‘Effect of bit wear onhammer drill handle vibration and productivity’, Journal of occupational andenvironmental hygiene, vol. 14, no. 8, pp. 642–651.Coggins, M. A., van Lente, E., McCallig, M., Paddan, G. and Moore, K. (2010)‘Evaluation of hand-arm and whole-body vibrations in construction and propertymanagement’, The Annals of occupational hygiene, vol. 54, no. 8, pp. 904–914.EN ISO 28927-10:2011 Hand-held portable power tools – Test methods for evalu-ation of vibration emission – Part 10: Percussive drills, hammers and breakers:European committee for standardization (2011), Berlin: Beuth Verlag.EN ISO 5349-1:2001Mechanical vibration –Measurement and evaluation of humanexposure to hand-transmitted vibration: European committee for standardization(2001), Berlin: Beuth Verlag GmbH.Hewitt, S., Shanks, E., Heaton, R. andMole, M. (2011) Evaluation of EN 60745 testcodes: BS EN 60745-2-6:2003+A2:2009 concerning the vibration of hand-heldelectric hammers.Jahn, R. and Hesse, M. (1986) ‘Applications of hand-arm models in the investiga-tion of the interaction between man and machine’, Scandinavian journal of work,environment & health, vol. 12, 4 Spec No, pp. 343–346.Jugulkar, L.M., Singh, S. and Sawant, S.M. (2016) ‘Analysis of suspension with vari-able stiffness and variable damping force for automotive applications’, Advancesin Mechanical Engineering, vol. 8, no. 5, 168781401664863.Mc Dowell, T. W., WARREN, C., WELCOME, D. E. and DONG, R. G.E.N. (2012)‘Laboratory and field measurements and evaluations of vibration at the hand-les of riveting hammers’, The Annals of occupational hygiene, vol. 56, no. 8,pp. 911–924.Nilsson, T., Wahlström, J. and Burström, L. (2017) ‘Hand-arm vibration and the riskof vascular and neurological diseases—A systematic review and meta-analysis’,PLoS ONE, vol. 12, no. 7.Sánchez, M., Rosenthal, G. and Pugnaloni, L. A. (2012) ‘Universal response of opti-mal granular damping devices’, Journal of Sound and Vibration, vol. 331, no. 20,pp. 4389–4394.

Hydro-mechanical Dampers for Testing high Cyclic Shock Loads in Preventive Health Protection

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Hydro-mechanical Dampers for Testing high Cyclic Shock Loads in Preventive Health Protection

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