1. Introduction
Welders comprise a large occupational group that works long hours in forced postures [
1]. Maintaining forced posture can cause early muscle fatigue [
2], while it can lead to work-related musculoskeletal disorders (WMSDs) in the long-term or in severe cases [
3]. Moreover, prolonged forced postures can lead to occupational injuries to workers [
4], which will cause long-term physiological and psychological harm [
5]. A survey of forest workers in New Zealand have found that physical fatigue might constitute a significant risk factor for accidents and injury [
6]. Meanwhile, WMSDs are the most common occupational injury worldwide and the most common cause of long-term pain and disability in workers [
5]. It is undeniable that full mechanization would be the best approach towards minimizing worker fatigue and injury, but, because of the high cost, ergonomic interventions for workers are still necessary and effective for smaller scale businesses [
7]. Therefore, on the basis of the current situation regarding serious occupational hazards and safety, ergonomics research focused on welding posture in China can effectively protect people’s physical and mental health and prevent accidents.
From an ergonomic perspective, the evaluation of workers in different fields has already provided mature theories and a large number of research results. Some scholars have conducted a questionnaire survey on the health status of primary and secondary school teachers in Hong Kong, and found that, in addition to mental stress, work-related musculoskeletal disorders should also be valued [
8]. On the basis of the Nordic Musculoskeletal Disorder Questionnaire, the results of ergonomics related to nurses have shown that poor working postures tend to cause WMSDs, and measures to prevent WMSDs in nurses have been proposed [
9]. Other scholars have proposed starting from a training and management perspective to overcome or reduce the adverse effects of welding on the upper limbs by developing a training program for welders [
1]. In addition, Francisco C and Edwin T analyzed the stress on the upper limbs during the work of the auto assembly welder. They proposed regularly adjusting the welding work site, which encourages the welder to frequently change posture and welding torch, for the purpose of reducing any occupational hazards [
10], this program has had a positive effect on improving common occupational injuries. Similarly, the ergonomic study of office workers in the United States shows that, whether standing or sitting, for a long time, it will have an adverse effect on the lower back, and relaxation seems to be more effective in avoiding injury [
11]. In 2017, Goncn et al. used computer software to conduct ergonomic studies on the working posture of wheeled mowers as well as evaluating the performance of the wheeled mowers [
12]. Moreover, in terms of ergonomic visibility, Qiu Shiguang’s team evaluated the ergonomics of maintenance workers’ hand tool repair operations by writing a programming language to check whether there is any obstacle between the line of sight and the target part to determine whether it is visible or not [
13]. Recently, Brazilian researchers conducted ergonomic evaluations of workers carrying two types of beer kegs and proposed ways for optimizing this [
14]. It is not difficult to see that the study of ergonomics is roughly divided into two parts: the improvement of equipment and the improvement of working posture. Therefore, welding posture and welding torch performance in accordance with safety ergonomic requirements can increase welder productivity, reduce injuries and accidents, and enhance the economics of a business.
In the literature, most scholars use questionnaires or observations to explore and evaluate ergonomics. However, there are fewer studies that make accurate quantitative evaluations of human hazards. Some scholars have used simulation tools to quantitatively study a certain part of the human body, but this is not comprehensive. At present, there are few ergonomic studies on welders, and some of the models that were established in the related research have been oversimplified. Many research conclusions based on European body data do not reflect the true working state of Chinese welders. Therefore, further research is needed to break through the limitations of traditional research methods and validate and supplement existing results. Herein, the Jack digital human body simulation software is used to evaluate the posture of Chinese welders. In this study, a common standing welding posture was selected; this was done while using a safety ergonomic analysis method and using digital human body simulation technology to simulate the manual welding operation of welders. By studying the weight of the welding torch, the upper limb force of the welder, and the line of sight of the welder during the welding operation, an adjustment strategy related to the welding torch weight, the welder’s upper limb posture, and the welder’s neck posture are proposed. The results can improve the working conditions and working methods of Chinese welders and improve the health and safety conditions.
2. Research Methods
2.1. Digital Human Body Modeling
This study mainly used three-dimensional (3D) simulation technology to analyze the ergonomics of the standing welding operation while using a hand-held welding torch. The researchers selected the ergonomics analysis software Jack version 8.3 as a tool to model and import the digital human and welding torch, build the overall simulation scene of the welding operation, simulate the dynamic welding operation, and realize the operation and analysis of the action example. Four evaluation analysis modules for Lower Back Analysis (LBA), Ovako Working Posture Analysis (OWAS), Rapid Upper Limb Assessment (RULA), and Comfort Assessment (CA) in the Jack software were used to qualitatively and quantitatively analyze the weight of the welding torch and the welder’s posture during welding. The best welding torch weight and welding posture for maximum comfort and better working conditions were selected on the basis of the results of the evaluation.
2.2. Welding Environment Setting
Three sets of standing welding action modules for walking, raising arm, and contracting arm were designed while using the Jack software to simulate the welding operation posture. The design process of the experimental simulation is as follows:
- (1)
the different percentiles of Chinese localized welder’s body size, as shown in
Table 1 (the body size parameters are based on the Asian human body database in the Jack software, with reference to GB10000-88 Chinese Adult Body Size and GB/T13547-92 Workspace Human Body Size in anthropometric data), were used to create a Chinese localized welder body model;
- (2)
the welding torch model was introduced into the working environment, as shown in
Figure 1. According to the welding torch positioning technology, the program written on the Jack Script secondary development platform was used to achieve the fit of the welding torch, the palm, and the solder joint;
- (3)
after the fit was completed, the human body control window was used to adjust the static posture of the person, including the hand, arm, shoulder posture, etc. The palm shafts of both hands add the weight and load of the welding torch to the hand of the model; and,
- (4)
the static posture was sequence-adjusted, spliced into dynamic behavior, a welding operation animation was created for the welding process, and the animation was classified into three motion modules. Data were collected for LBA, CA, OWAS, and RULA while completing the experimental animation.
2.3. Welding Torch Weight Setting
The welder’s process of holding a welding torch for welding operations is essentially that of a person lifting heavy objects. The Jack software can be used to evaluate the standing welding posture and analyze the body’s force through real-time observations when lifting the lower arm in the working environment. In this study, the lifting arm mainly evaluated the following dimensions:
2.4. Upper Limb Posture Setting
During welding work, the welder might have to raise the arm, bend over, lean forward, etc., due to the difference in the position of the welding point and different weights of upper limb load, which might easily lead to discomfort and injury.
When evaluating the upper limb posture, in addition to the LBA, as mentioned earlier, RULA was also used. RULA assesses the risk of upper limb injury based on posture, muscle use, the weight of loads, and task duration and frequency. RULA gives a value that indicates the degree of intervention that is required to reduce the risk of an upper limb injury. Specifically, Level 1: Acceptable posture if not maintained or repeated for long periods (grand score 1–2); Level 2: Further investigation needed, may require changes (grand score 3–4); Level 3: Investigation, changes required soon (grand score 5–6); and, Level 4: Investigation, changes required immediately (grand score > 6) [
7]. In short, the higher the RULA score, the stronger the discomfort of the upper limbs [
21].
The arm (hand) function radius of rotation and the arm (hand) comfort zone height can describe and define the horizontal distance and vertical heights of the upper limb operations, respectively. The electric power industry standard that China issued in 1999: The ergonomic principles for the design of control centers, part 3, hand reach and zones of control (DLT 575.3-1999) stipulate the range of arm operation for sitting and standing positions. Arm (hand) function rotation radius, arm (hand) comfort operation area height are determined based on the standard of the male 5th percentile of body size. An additional evaluation of the arm (hand) function radius of rotation and the arm (hand) comfort zone height was required to measure and evaluate the corresponding stress in the lower back and upper limbs in order to study the comfortable operation space of the digital human with different percentiles and genders.
According to the DLT 575.3-1999, the 5th percentile of body size male arm (hand) has a minimum functional radius of rotation of 321 mm, the arm (hand) has a maximum functional radius of 610 mm, and the comfortable operating zone height ranges from 1050 mm to 1400 mm. Again, in males, the difference in arm length between the 5th, 50th, and 95th percentile is about 50mm; in the same percentile, the difference in arm length between different genders is approximately 50mm. Therefore,
Table 2 and
Table 3 show the calculation results of the arm (hand) function rotation radius range and the comfort operation area height range of the six different percentiles and different genders.
2.5. Neck Posture Setting
The welder has to adjust the angle of his neck joints in order to be able to see the solder joints. In the Jack software, the Gilbert and Johnson collision method is used [
22]. The collision detection technology emits a line of sight particle simulation to the solder joint according to the position of the human body viewpoint. As shown in
Figure 2, it determines whether the target is occluded on the basis of particle and environmental collision [
23].
• Field of view
Welding is a precise manual operation, wherein the welder’s eyes need to properly capture the exact position of the weld for the operation to be performed [
24]. The best horizontal direct field of view (−15°–15°) and vertical direct field of view (−45°–15°) from the standard of vision and viewport division of the Chinese Control Center ergonomics design guidelines were used as the vision parameters to construct the field of view of this welding operation.
• Visual angle calculation
The quality of the line of sight was evaluated and the bending angle of the neck joint of the welder was obtained to obtain the adjustment strategy for the human neck posture. In this study, the best viewing angle (middle field of view) criterion was used, with the optimal top viewing angle θ0 as the scope of sight, θ0 is 45°. The angle θ1, θ2 were calculated as the horizontal plane viewing angle and the vertical plane viewing angle, respectively. On the basis of the collision detection technology, the particle collision range in the viewing angle calculation program written in the Jack Script programming language was converted into the values of the viewing angles θ1 and θ2, and the visibility was evaluated by comparing θ1, θ2, and θ0.
4. Discussion
When compared with traditional ergonomics evaluation methods, the Jack software simulation analysis that was used in this study combined with actual human body verification during operation provides a more comprehensive ergonomic evaluation method, allowing for users to create various types of welding environments. The Jack software uses a modeled digital human body, meaning that the participation of real welders is not required, which greatly reduces the cost of testing. The experimental results that were generated by the software avoid the complex environmental variables associated with the experiment and also avoid the interference and influence of the behavior of the welder before and after the operation. Therefore, the research on safety ergonomics that is based on digital human body modeling using Jack software is economic and scientific.
Research on the weight of the welding torch shows that the weight of the welding torch should not exceed 6 kg. Cao W et al. showed that, in the evaluation of the ergonomics of the lower limbs in hospital nurses, when an empty stretcher (less than or equal to 6 kg) was lifted, the pressure of each part of the human body was within the prescribed allowable stress range; however, when a patient is lifted (greater than 6 kg), the pressure exceeds the limit [
26]. Similarly, Vieira used a questionnaire to review injury records, assessing the work-related lower back injuries of 64 welders, and comparing the discomfort scores and visual analogues with one-way ANOVA and Fisher’s least significant difference post-test. The results show that the average weight of the welder’s manual operation is 6 kg [
27]. The findings of this are similar to those of the safety ergonomics study in this paper.
The upper limb posture study shows that, according to the percentile of body size of Chinese welders, the best operating distances for males in the 5th, 50th, and 95th percentile are 321 mm, 371 mm, and 421 mm, respectively, and the optimal operating heights are 1050 mm, 1100 mm, and 1150 mm, respectively; for females in the 5th, 50th, and 95th percentile, the optimal operating distances are 271 mm, 321 mm, and 371 mm, respectively, and the optimal operating heights are 1000 mm, 1050 mm, and 1100 mm, respectively. It can be seen from
Table 4 that, during operation, the higher the percentile, the higher the comfort value, with the comfort value being greater than 1 with the increase in the percentile. This means that, the larger the person, the greater the stress on the limbs and joints, and the greater the discomfort.
Figure 3,
Figure 4,
Figure 5 and
Figure 6 indicate that, the higher the percentile, the greater the lower back pressure.
Table 5 and
Table 6 from the RULA score also indicate that taller people seem to be more likely to suffer from WMSDs. This is because taller workers do the same welding work, but their posture adjustment is larger and their discomfort is enhanced. This is consistent with previous research results, which state that the higher the body mass index (BMI), the greater the risk of developing WMSDs [
28].
According to
Figure 4 and
Figure 5, when comparing the standing raising arm posture with the standing contracting arm posture, the latter exerts more force on the lower back at the same operating distance. Moreover, by comparing the six digital humans in the standing contracting arm operation of
Figure 5, it was found that, when the operating distance is the same, the upper limbs are longer and the lower back is more stressed. This also proves that the curved upper limb posture causes greater lower back force when the operating distance is same. This conclusion confirms the results of using CATIA software to improve the research of hand-held dental devices [
29].
Our study of neck posture cleverly applied visualization techniques. The rotation of the eyeball translates into the rotation of the neck under direct vision. The neck will feel uncomfortable when the horizontal rotation angle exceeds 15° and the neck vertical rotation angle exceeds 8.7°, according to our force analysis of the neck.