Design and Fabrication of Test Rig for Static Pushing and Pulling Experiments

Muscular strength data associated with pushing and pulling forces are crucial for ergonomists to design tasks and equipment in manual materials handling. Usually, ergonomists measure pushing and pulling forces using a force gauge. Subjects participating in the pushing and pulling experiments need to grip and hold the force gauge. Additional weight from the force gauge can affect the muscular strength of the subjects, which impacts the validity and reliability of the pushing and pulling data. However, research on the design and fabrication of test rigs for facilitating pushing and pulling experiments seems to be scarce. This study aimed to design and fabricate a test rig to facilitate ergonomists and subjects in performing symmetric two-handed static pushing and pulling experiments. To develop the test rig, the researchers performed a maximum force measurement, a computer-aided design model, materials selection, a finite element analysis, a functionality test, and a reliability test. Seven subjects with a body mass of more than 120 kg participated in the validation of the developed test rig. Key findings of this study showed that the test rig could sustain the pushing and pulling forces up to 900 N, representing almost double the muscular strength of the subjects. This study concluded that the developed test rig was sturdy and helpful for facilitating ergonomists in quantifying the magnitude of pushing and pulling forces.


Introduction
Pushing and pulling actions are commonly found in daily activities.Pushing involves exerting force on an object to move it away from the person or to apply pressure.Pushing applies muscle exertion or external force to create a movement or a displacement.On the other hand, pulling refers to exerting force on an object to move it towards the person or to draw it closer.In other words, pulling is similar to pushing but in the opposite direction of pushing.There are many applications of pushing and pulling in daily activities, such as opening a car door, closing a drawer, and handling the shopping trolley cart.In industries, manual material handling involves pushing and pulling actions such as pushing and pulling a hand pallet jack.
Muscular strength data associated with pushing and pulling forces are crucial for ergonomists.They need to study the force exerted by the individuals involved in manual material handling tasks to prevent musculoskeletal disorders such as back pain.To prevent musculoskeletal disorders, manual material handling tasks should not exceed 63% of the individual's maximum voluntary effort [1].Based on the guidelines on ergonomics risk assessment 2017 published by the Department of Occupational Safety and Health of Malaysia [2], the recommended load weight for pushing and pulling for stopping or starting a load for male is 1000 kg (equivalent to 200 N pushing or pulling force) on smooth surface using well maintained handling aid.For female, the recommended weight is 750 kg (equivalent to 150N pushing or pulling force).Hence, it is crucial for ergonomists to quantify the maximum pushing and pulling forces exerted by a person to design material handling tasks and equipment to minimize the risk of musculoskeletal disorders.
To quantify the pushing and pulling forces, the researchers should conduct the experiment at the biomechanics laboratory or research centre.Most importantly, the pushing and pulling force data obtained from the experiment should be valid and reliable for further applications such as designing material handling tasks and equipment.Three factors determined the validity and reliability of the data produced by the experiment.Firstly, the instrumentation used to measure the data, such as force gauge as shown in Figure 1.The force gauge should be calibrated by an authorized institution so that the data obtained is accurate.Secondly, the validity and reliability of the pushing and pulling forces is influenced by the experimental procedures.A consistent and standardized procedure throughout the experiment is crucial to control the variables under study.A standardized procedure ensures the reliability and validity of the data.Lastly, the test rig that is used to attach the force gauge during the experiment.A test rig is a mechanical structure that can assist the experiment procedure to produce valid and reliable data.The structure of the test rig should be sturdy; otherwise, the test rig can deteriorate the validity and reliability of the data.Additionally, the test rig should be designed to be adjustable so that different heights of subjects can perform the experiment comfortably.A proper test rig design permits the subjects to grip the force gauge handles and does not require holding the weight of the force gauge.
There are several methods applied to measure the pushing and pulling forces.Table 1 shows some methods used by previous researchers, applications and their limitations in measuring pushing and pulling forces.Studies in [3] and [4] did not use any test rig to measure push and pull force in static conditions.Neither study considered the effects of the force gauge weight.In another study [5] the researchers applied a test rig; however, the finite element analysis (FEA) simulation is not presented, and mechanical strength such as total deformation, strain and safety factors remain unknown.Without the FEA simulation, a prototype or actual test rig should be constructed and tested by the destructive mechanical method to analyze its mechanical strength.This method makes fabricating and studying the test rig time-consuming and wastes construction materials and money.Studies in [6] applied a test rig; however, the study was conducted to measure pushing and pulling forces for moving box pallets, reflecting sustained or dynamic pushing and pulling forces.

Methods
Applications Limitations [3] In the pushing experiment, the force gauge is pushed towards a static wall.While for the pulling experiment, the force gauge is pulled from a static pole.
Static pushing and pulling.No test rig.
[4] A broom handle or a cylindrical object is taped onto a weighing scale.The cylindrical object is then used as a handle to push onto the weighing scale.
Static pushing and pulling.No test rig and no force gauge [5] The force gauge is attached to a test rig.
Static pushing and pulling.No study on finite element analysis of the test rig.[6] Force gauge attached to a test rig with roll box pallet.
Dynamic or sustained pushing and pulling.
The data obtained is not applicable for static pushing and pulling.
A previous study [3] applied a force gauge, a static wall and a static steel post.The subjects held the force gauge and gripped its handles firmly before exerting effort to push or pull the wall or the post.The weight of the force gauge and its handle is 0.7 kg (6.9 N).Due to the gravitational acceleration, the weight of the force gauge causes it to move downwards.To counteract the effects of the downward force, the subjects must contract the hand muscles to maintain the force gauge, limiting their maximal effort to produce a maximum force.For example, when a subject performing an experiment with the pushing or pulling height set at lower than elbow (e.g., waist height) or higher than shoulder level, he or she needs to pay extra effort to push or pull the force gauge.This can be seen in Figure 2 (A), whereby a subject holding the force gauge and stabilizing his body during a pushing experiment.Moreover, if the ergonomists and subjects use a steel post for pulling force experiments, the steel post has no proper attachment kit to place the force gauge, as illustrated in Figure 2 (B).This limitation causes the force gauge not to measure the maximum effort of the subjects during the pushing and pulling experiments.As a result, the forces data deteriorate in terms of validity and reliability.
In recognition of the importance of above-mentioned issues, the objective of this paper was to design and fabricate a test rig to hold the force gauge used in the procedures of isometric pushing and pulling (static exertion) activities.Furthermore, this study performed FEA, a functionality test, and a reliability test to ensure the test rig can facilitate the ergonomists and subjects to acquire valid and reliable data on pushing and pulling forces.The potential benefits of this study are the design and development of a specialized test rig for facilitating pushing and pulling experiments.The test rig can be stationed in the biomechanics laboratory or research centre to enable ergonomists/ researchers and subjects to produce accurate and reliable data for designing manual material handling tasks and equipment to prevent musculoskeletal disorders.

Methodology
Figure 3 shows the process flow to design and fabricate the test rig for measuring the pushing and pulling forces of the subjects participating in the experiment.The processes generally involved a maximum force measurement, developing the computer-aided design (CAD) and material selection, finite element analysis (FEA), and functionality and reliability tests.The following section describes the details of each design and development process.

Maximum force measurement
The main purpose of the maximum force measurement was to quantify the input data (maximum pushing and pulling forces exerted by subjects) for the FEA in ANSYS software.In addition, the data gathered from the maximum force measurement were used for a reliability test to determine any difference in terms of pushing and pulling forces when using the test rig and without the test rig.The inclusion criteria for the subjects participating in the maximum force measurement were healthy male adults, had no physical impairment, and the body mass should be greater than 120 kg to represent the 100 th percentile of the weight of Malaysian adults [7].The body mass was chosen as one of the inclusion criteria as previous studies [3,8,9] proved that it is strongly associated with the maximum pushing and pulling forces.
However, inviting subjects with a body mass greater than 120 kg within the university campus was challenging.Due to this constraint, the maximum force measurement was carried out by only seven male subjects (four undergraduate students and three academic staff) of a Malaysian university.The subjects were healthy, aged between 20 to 40 years old.The experimental procedure was referred to study in [3].The pushing and pulling heights were set at the elbow height.All subjects performed the pushing and pulling forces measurement in a standing staggered stance.Each subject was asked to push and pull the force gauge for three attempts, yielding a total of six attempts per subject.Each attempt was given a 5-minute interval break.The experimental procedures were reviewed and approved by the Research Ethics Committee of Universiti Teknikal Malaysia Melaka (Reference no.: UTeM.11.02/500-25/1/4(Jilid 2)(33).The force gauge was calibrated and certified according to the National Institute of Standards & Technology (NIST, USA) to ensure the validity and reliability of the data.

CAD Development, material selection, and FEA
In this stage, the design process starts with referring to the anthropometry data of Malaysian adult males [7] to decide the minimum and maximum height of the test rig.This is crucial to make sure the test rig is designed to match the subjects' anthropometry.The subsequence design process was the material selection of the test rig.The Ashby method [10] was used to select the most suitable material for the test rig.The selection criteria of the material considered the tensile and shear strengths so that the test rig could withstand the pushing and pulling forces with very minimal deformation on its structure.The CES Edupack software (ANSYS, USA) was used to screen the alternative materials and rank them based on the cost, availability, and machinability to select the best material.After selecting the most suitable material, a 3D CAD model of the test rig was designed using the CAD software Solidworks (DASSAULT SYSTEM, France), as shown in Figure 4.The FEA method simulated by ANSYS software was used for calculating the total deformation, strain, and safety factor to validate the test rig structural design.

Fabricating the test rig
The test rig was then developed using a modular design approach.This design approach will ease ergonomists to assemble and disassemble the test rig.In addition, the test rig allows the ergonomists to adjust the handle height so that they can set the experimental variables for shoulder height and elbow height easily.Moreover, this design feature can save time for the ergonomists to set the handle heights.Besides, this adjustable height promotes comfort to the test subjects.
The test rig is a trapezoidal structure consisting of a platform to place the force gauge, an aluminium 2000 series plate to hold a hook for the force gauge attachment, and a high-density foam back support.The back support is used to counter the reaction force during the pushing experiment.Additionally, it functions to prevent the subjects from falling backwards during the pulling experiment.The back support is adjustable so that different sizes of subjects can use the test rig without any interference.The back support is adjustable; for instance, the back support's pole can be retracted for larger subjects, while the back support pole can be extended for smaller subjects.There is sufficient clearance between the back support and the subjects' back during the pushing and pulling experiments.This gap is crucial to give space to the subjects to comfort their body posture so that the subject can exert a maximum pulling force.Another part of the platform to place the force gauge is smooth or frictionless to prevent frictional force between the platform and the force gauge.Four lockable wheels are attached to the base of the test rig to allow mobility.An aluminium 6000 series profile was selected for the structure of the test rig.All the test rig parts were assembled using bolts and nuts.Figure 5 shows the bill of materials of the test rig.After fabricating the test rig, this study performed functionality and reliability tests to ensure it helped the ergonomists and subjects in pushing and pulling experiments.Directly, these tests are useful to evaluate the rigidity of the test rig so that it can help ergonomists to attain accurate and reliable data in pushing and pulling forces experiments.
The functionality and reliability tests recruited the same subjects and experimental procedures for maximum force measurement.However, in this stage, the subjects performed pushing and pulling experiments with the aid of the test rig.The data obtained were compared to the maximum pushing and pulling forces performed without the test rig.Table 2 compares the pushing and pulling forces experiments with and without the test rig.

Table 2. Conditions of pushing and pulling forces experiments with and without the test rig
Test Rig Without Test Rig Pushing: A subject performing the pushing procedure using the test rig.
Pushing: A subject performing the pushing procedure without the test rig and using only the wall for the pushing procedure.
Pulling: A subject performing the pulling procedure using the test rig.
Pulling: A subject performing the pulling procedure without the test rig and using the pole to attach the force gauge.

Results
Figure 6 shows the complete assembly of the test rig.The force gauge (MARK 10, USA) is firmly attached to the platform of the test rig.

Maximum force measurement
Table 3 shows the magnitude of pushing and pulling forces (unit in Newton, N) exerted by the seven subjects who participated in the experiment.All subjects have performed three attempts of pushing and three attempts of pulling, which have yielded 21 points of data for each pushing and pulling force making a total of 42 sets of data.

Finite Element Analysis (FEA)
The FEA method was applied to simulate the mechanical strength of the test rig.This analysis calculated the total deformation, strain and safety factor.A parametric design study method was used to obtain the test rig's total deformation, total strain, and safety factor based on the maximum force measurement.This is crucial to ensure the test rig is sturdy and does not deform during the experiments.The parameters set were the force acting on the test rig, total deformation, strain, and safety factor.The parametric design study continued by increasing the force applied on the test rig iteratively until the allowable deformation, strain, and safety factor were reached.Figure 7 until Figure 9 illustrate the graphical representation of the test rig's total deformation, total strain and safety factor respectively.Table 4 shows the value of total deformation, total strain and safety factor subjected to forces ranging from 200 N to 900 N.

Functionality test
Table 5 shows the results of seven subjects participating in the pushing and pulling experiments using the test rig.A total of 42 points of data have been yielded for this experiment.All participants have performed three attempts of pushing and three attempts of pulling, which have yielded 21 points of data for each pushing and pulling force.Based on the results, this study found that the test rig was able to sustain a maximum force of 512 N without any noticeable deformation on its structure.

Reliability Test
As shown in Table 6, the subjects produced greater pushing and pulling forces while using the test rig than the forces without using the test rig.A paired two-sample t-test was performed to compare the pushing and pulling forces without the test rig and with the test rig.There was a significant difference in pushing force between without the test rig (Mean = 357.86,StDev = 29.10)and with the test rig (Mean = 377.67,StDev = 16.65;T-Value = 2.71, DF = 31, p = 0.011).Meanwhile, there was a significant difference in pulling force between without the test rig (Mean = 455.62,StDev = 43.36)and with the test rig (Mean = 477.05,StDev = 33.4);T value = -1.79,DF = 37, p = 0.0081.After notifying a significant difference between the test rig and without the test rig, the accuracy and reliability of the data while using the test rig were identified.The descriptive statistics method was used to identify the minimum, mean, maximum, standard deviation, kurtosis, and skewness of the data.Table 6 shows the results of descriptive statistics for pushing and pulling forces while using the test rig and without using the test rig.Figure 10 and Figure 11 visualize the normality graphs and line of best fit for pushing and pulling experiments while using the test rig.

Finite Element Analysis (FEA)
Based on the findings in Table 4, the FEA simulated that the safety factor was 1 when 900 N of force was applied.A safety factor lesser than one is deemed unstable and unsafe to use [11].A past study stated that only 20% of the length of the structure is allowed to deform [12].Based on the results in Table 4, the maximum deformation of the test rig's structure was only 0.007 m, which is negligible.The permissible strain is 20% from the original structure, according to an earlier study in [13].
Through the FEA simulation, the results of the strain calculation are still within the allowable strain.Since the deformation, strain, and safety factor complied with the allowable limit, the test rig is safe to use when pushing and pulling forces reaching 900 N.

Functionality test
In the functionality test, pushing and pulling experiments using the test rig were performed by seven male subjects with a body mass greater than 120 kg.Each experiment was repeated three times for each pushing and pulling; hence the test rig was used 30 times.This study found that the test rig was capable of facilitating the ergonomists and subjects to produce pushing and pulling force data.Interestingly, no noticeable deformation or bending was detected on the test rig's structure.
In the experiments, the body mass of Malaysian young men in the 100 th percentile as the main criterion of the subjects.This is due to the fact that body mass greatly influences the force of pulling and pushing forces [3].In other words, greater body mass results in greater pushing and pulling forces.This information demonstrates that the test rig was able to retain pushing and pulling forces with neglected deformation after being used by the subjects with greater body mass.One of the key factors that contributed to very minimal deformation on the test rig structure was the material used, the aluminium 6000 series.The mechanical strength properties of this material are yield tensile strength of 40.0 to 517 MPa, shear modulus of 25.8 to 53.8 GPa, and shear strength of 60.0 to 331 MPa [14].As the aluminium 6000 series has good mechanical properties, this material is effective at retaining high forces, such as those frequently applied in manufacturing crash components for the automobile sector [15].
Since the forces have been repeatedly applied to the test rig, the tendency of the test rig to rotate about a specific point or axis might appear.In other words, the test rig is subjected to rotation (effects of the moment).To prevent the test rig from the effects of the moment, two support bars were assembled, as shown in Figure 12.These two support bars countered the moment when a force is applied and thus prevented the test rig from rotating.As pointed out in this paper's introduction, the handle height for pushing and pulling exertions is designed to be adjustable so subjects with different heights can use the test rig.In addition, it saves time for ergonomists to set up the handle height according to the subjects' shoulder height, elbow height, and knuckle height.It only takes one minute to adjust the handle because Allen key bolts were used instead of hexagonal nuts and bolts to set the desired heights.In addition, the Allen key bolts were fitted into the aluminium profile slot, making the platform of the force gauge easy to move up or down along the slot, as shown in Figure 13 (A).Furthermore, during the pushing experiment, the test rig is positioned against a static wall, shown in Figure 13 (B), so that the wall halts the force exerted by the subjects.Thanks to the lockable wheel, the test rig can be turned opposite of the wall to counter the effects of pulling experiments.

Reliability Test
Two statistical analyses for the reliability test have been conducted.The first was descriptive statistics.The accuracy and reproducibility of the data were examined in this phase.A low standard deviation means the data produced are reliable because of high repeatability [16].Results in Table 5 show that the standard deviations of pushing and pulling forces with the aid of test rig were lower than without the test rig.This demonstrates that the test rig was able to facilitate the ergonomists and subjects to produce reliable data.In addition, the kurtosis and skewness were computed to justify the normality of the data.According to [17][18][19], data were deemed normal if the skewness and kurtosis were between -2 and +2 and -7 and +7, respectively.Table 5 shows that the skewness and kurtosis are within the recommended range for normal data distribution.Based on the statistical analysis, the data were proven to be normally distributed and consistent (evidenced by Figure 10 and Figure 11).Subsequently, the t-test was used to identify if there were any significant differences [20] in terms of pushing and pulling forces with the aid of the test rig and without the test rig.The t-test results showed a substantial difference in pushing force with or without the aid of the test rig, as shown by a P-value < 0.05 [21].
The results mentioned above were achieved with the help of the force gauge platform equipped in the test rig.The subjects do not need to exert muscular strength to hold the force gauge.Instead, the force gauge was positioned on the platform to counter the effects of the gravitational acceleration.The advantage of this setting is that the subjects were able to exert their maximum muscular strength consistently.Moreover, the test rig was able to assist the subjects in complying with the standardized procedures that the ergonomists had set.

Conclusion
The aim of this study was to design and fabricate a test rig to hold a force gauge for facilitating the experimental procedures in measuring the static pushing and pulling forces.This study performed a FEA simulation and found that the test rig was able to withstand a maximum force of 900 N. The FEA simulation detected a very minimal deformation (0.007 m) on the test rig structure subjected to the maximum force.The factors contributing to minimal deformation are high tensile strength aluminium for the test rig structure, a rigid structural design, and the counter-moment support bars.
In the functionality test, the maximum force exerted by the subjects was 512 N, which is 56.9% of the maximum force the test rig can withstand.Based on the statistical analysis, the kurtosis was -1.3 and -1.7 for pushing and pulling forces data, respectively.Meanwhile, the skewness was 0.8 for both pushing and pulling forces data.These findings showed that the data of pushing and pulling forces generated with the assistance of the test rig were normally distributed and consistent in terms of repeatability.The force gauge platform attached in the test rig facilitated the subjects to exert maximum muscular strength during the pushing and pulling experiments.This study concluded that the test rig was able to facilitate the ergonomists and subjects to acquire valid and reliable data in pushing and pulling experiments.
In the future study, the subjects representing the 5 th , 50 th , and 95 th percentiles should be recruited to further examine the rigidity and reliability of the test rig.

Acknowledgement
The Faculty of Manufacturing Engineering, the Centre for Research and Innovation Management (CRIM), and the Ministry of Higher Education of Malaysia under grant number (FRGS/1/2022/TK10/UTEM/02/4) are all thanked by the author.Universiti Teknikal Malaysia Melaka is also acknowledged.The author also would like to acknowledge Mr. Tuan Syed Faiq Eizahan bin Tuan Azman, Mr. Akmal Harith bin Ibrahim, Mr. Muhammad Sufi, Mr. Mohd Hanafiah bin Mohd Isa, and Mr. Mohd Taufik bin Abd Aziz for their participation in data collection.

Figure 2 .
Figure 2. The subjects grip and hold a force gauge in pushing (A) and pulling (B) experiments

Figure 3 .
Figure 3. Flow chart to design, develop, and test the test rig

Figure 4 .
Figure 4. 3D CAD model of the test rig

Figure 6 .
Figure 6.Complete assembly of the test rig

Figure 9 .
Figure 9. Simulated safety factor of the test rig

Figure 10 .Figure 11 .
Figure 10.Normal distribution graph and line of best fit for pushing force data (with test rig)

Figure 12 .
Figure 12.The support bars of the test rig to prevent the effects of moment

Figure 13 .
Figure 13.(A) Bolts were fitted into the slot of the aluminium profile (B) The backside of the test rig against the wall for pulling experiment

Table 1 .
Method, applications, and limitations of past pushing and pulling experiments

Table 3 .
Pulling and pushing forces without using the test rig

Table 4 .
Finite element analysis results

Table 5 .
Pulling and pushing forces while using the test rig

Table 6 .
Descriptive statistics of pushing and pulling forces (N)