Keywords

1 Introduction

Both the shape and position of the gripping structure for heavier objects directly affect the efficiency of manual handling operations. Prolonged and frequent use of improper shape and position of the gripping structure will lead to work-related acute or chronic injuries to the operator, especially when manually handling heavy loads. Tooley [1] considered participatory ergonomics activities, with the appropriate ergonomics infrastructure, should reduce work-related musculoskeletal disorders. Alzuheri, Atiya [2] proposed a framework that allows for the consideration of multiple ergonomics measures to assess ergonomics stresses resulting from work postures in manual assembly work. In order to study the manual operator comfort, some scholars performed human motion modeling and analysis related to arm and finger movements. Chaffin [3] developed a set of human motion prediction models to resolve the dynamics of the motions of people reaching and moving which were measured by a motion capture system. Bae [4] established a model describing human finger motion for simulation of reach and grasp for selected objects and tasks. Dickerson [5] defined the quantitative relationship between external dynamic shoulder joint torques and calibrated perceived muscular effort levels for load delivery tasks. Jung [6] implemented a two-handed reach prediction model that the human upper body was modeled as a seven-link system with thirteen degrees of freedom, being regarded as a redundant open kinematic chain with two end-effectors.

In order to facilitate the operational efficiency and reduce the operator fatigue, many ergonomics experts performed the man-machine interface adaptation regarding products or equipment by using experimental study or virtual simulation analysis. Mallam [7] optimized the design and the layout of ship engine control room utilizing human factors and ergonomics knowledge. Määttä [8] evaluated the impact of Virtual Environments on safety analysis and participatory ergonomics. Ekandem [9] assessed the ergonomics of BCI (brain computer interface) devices for research and experimentation. Otten [10] provided thumb reach envelopes to help guide the placement of controls on handheld devices and useful methods to gather and analyze thumb reach data. Freeman [11] determined optimal pedal positioning for automobiles using Jack modeling software. Carey [12] identified ergonomics hazards using the Jack software system at the very early design stages of new vehicle programs. Freeman [11] determined simulations of pedal positioning, using Jack modeling software for 1st percentile female to 99th percentile male, the optimal fore/aft positioning of the accelerator pedal for joint comfort, and the resulting consequences on strength and comfort for the using of brake pedal. Lu [13] introduced a new method which combined grasping plan with posture prediction to realize grasping strategy for ergonomics simulation. Clift [14] discussed the appraisal of the fit between operating theatre tables and the surgical staff who have to use them.

Focusing on the grip structure and location of a product or device to facilitate manual handling operation, the ergonomics theory was applied through samples testing via the human motion capture system VICON and ergonomics analysis platform JACK. In this study, a 30 kg rectangular box was used to simulate a heavy object, in which six pairs of slots on both sides of the box were treated as the installation locations of the gripper structures. These structures with fixed opening width, seven depths, and five tilt angles were designed, and the gripping structure samples were manufactured. Two types of experimental studies were conducted for this study by performing the following key steps. First, two samples of the same size were inserted into the slots on both sides of the box. Secondly, the study subjects lifted the box and evaluated their operating comfort levels regarding the slot depth and the tilt angle of the gripping structures, respectively. Thirdly, actions were repeated using different sized samples with the corresponding test data being recorded. Finally, repeated actions using the same box with different slot positions lead to the operating position data which allowed subjects to feel more comfortable while being recorded. By analyzing the experimental data, reasonable geometry shape parameters, position, and size, range of the gripping structure were obtained.

2 Methods

In this study, virtual simulation and sample testing were applied, as shown in Fig. 1.

The sample testing method was a specific process. First, corresponding series of hand groove samples with different size were developed by analyzing the factors of structural dimensions that affect the efficiency of the use of hand-clasping in actual use. After considering the characteristics of the user groups, subjects were recruited to perform the operational task with analog samples. Finally, the results of the experimental study were analyze, and geometry values of gripping hand groove structure that subjects felt operation convenient and comfortable were obtained.

Virtual simulation was completed by analyzing manual operation of the man-machine system, the factors of shape, size, and position of gripping groove affecting the efficiency of manual operation and comfort were extracted. Subjects performed specific operating actions similar to the actual work scene. The experimental action data were recorded via the human motion capture system VICON. The size and location of the manual device for the comfort of operation were obtained through a comprehensive analysis of human joint activity satisfaction and muscle tension force via the ergonomics analysis platform JACK.

Fig. 1.
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Technical route of this study

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2.1 Experimental Study of the Shape for the Gripping Structure

Twenty-four male participants ranging from ages 21 to 40 years old without physical defects or diseases, or healthy, contributed to this experiment. The test procedure contained many steps. First, the main factors affecting the operation efficiency and comfort levels of the manual handling of heavy objects were analyzed which included opening width, depth and tilt angle of the gripping structure of the box. Secondly, the opening width of grasping slot was generally greater than 25 mm according to the middle finger thickness of the 95th percentile male and experimental data, Thirdly, samples of the buckle groove of different depths were designed based on the finger length of the 95th percentile male (See Fig. 1). The specific depth values were 25 mm, 35 mm, 45 mm, 55 mm, 65 mm, 75 mm, and 85 mm, respectively. Fourthly, five samples were manufactured according to the actual common tilt angles of gripping structures (See Fig. 2) with the increased value of the tilt angle being 15°. The specific values of the tilt angles were 15°, 30°, 45°, 60°, and 75°, respectively as shown in Fig. 2. Finally, twenty young men with similar careers were recruited as study subjects whose feedback was recorded. The size range of the grip structure in ergonomic design was achieved by analyzing the experimental data (Fig. 3).

Fig. 2.
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Simulation samples of groove depth of grasping structure

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Fig. 3.
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Simulation samples of groove inclination angle of grasping structure

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2.2 Experimental Study of the Position for the Gripping Structure

Twenty-four male participants ranging from ages 21 to 40 years old without physical defects or diseases participated in this experiment.

An experimental cube sample with a mass of 30 kg was designed and built, representing a heavy object of 775 mm length, 540 mm width, and 240 mm height as shown in Fig. 4. The experimental positions of the gripping slot were located on both sides of the cube sample. There were seven heights measured from the top face of the cube sample to the center of the gripping slot incremented by 30 mm. Specific values of the heights were 30 mm, 60 mm, 90 mm, 120 mm, 150 mm, and 180 mm, respectively. After inserting buckle samples into the slots on both sides of the cube sample, the comfort assessment for the height test related to manual lifting was performed. Then, a randomly selected study subject and an assistant lifted the cube sample using the grip structure and walked five steps forward. The action of the two operators carrying the cube sample was recorded by the human motions capture system VICON. The experimental scene was shown in Fig. 5. Finally, the motion capture data was imported into the JACK ergonomics simulation platform and the change in waist forces of the movement for the subjects was analyzed. By analyzing the experimental data, the appropriate height location range of the ergonomic design for the gripping structure was attained.

Fig. 4.
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Prototype for heavy objects and experimental location set of grasping structure

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Fig. 5.
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Experimental scene

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3 Results

3.1 Ergonomic Structure Size of Grip Groove

(1) Gripping groove depth. Of the twenty-four healthy male subjects participating in this experiment, twenty subjects selected 55 mm as the optimal groove depth, three subjects thought 45 mm groove depth was comfortable and one subject preferred 65 mm. Eighteen of these healthy male subjects thought that a groove depth of 25 mm was an acceptable minimum limit. However, the results showed that 55 mm was the optimum grove depth and 25 mm was the worst. The proportion of subjects who selected groove depth was shown in Fig. 6.

Fig. 6.
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Representation of subjects and their preferred groove depth

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By analyzing the correlation between the above test results and the dimensions of the human hands, groove depth was described as: the middle finger distal phalanx Length + 0.5 * middle of the second knuckle lengths of the 95th percentile male population, as shown in Fig. 7. Therefore, the recommended groove depth was at most 45 mm. The minimum groove depth should not have been less than 30 mm.

Fig. 7.
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The length of the middle finger distal and proximal knuckle to fingertip length

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(2) Inclination angle of gripping groove. Based on the twenty-four healthy male subjects who participated in this experiment, 2 % preferred a groove angle of 60° and 56 °% preferred an angle of 45°. A 30° inclination angle was the acceptable minimum value. Most of the subjects considered 15° too low because they were easily slipped from the user’s hands. Figure 8 showed results of the proportion of subjects who selected the optimal groove inclination angle.

Fig. 8.
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Proportion of subjects that selected the preferred groove inclination angle

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According to the experimental test results, ergonomic evaluation criteria of groove inclination angles were identified as the bend angles of the second knuckle relative to the first knuckle of four fingers. On the basis of the experiment results, the ergonomic recommended range of gripping groove angle was greater than 45°.

3.2 Position Height of Grasping Groove

There were eighteen sets available out of twenty-four healthy male subjects who participated in this experiment. Test data showed that when subjects lifted the test bench with their right hands in different height positions as shown in Fig. 4, their waist strengths gradually increased with the decrease of the position of the gripping groove. At the fourth experiment height position, waist strengths of subjects increased significantly. By analyzing the objective and subjective data of all subjects, 59 % considered the fourth position as easy to operate, while 91 % considered the fifth position as difficult to operate. Therefore, the recommended height for gripping grooves was less than or equal to 150 mm (about position 4). The acceptable height limit of the gripping slot was position 5. The maximum height of the gripping slot should not have been greater than 180 mm.

4 Discussions and Conclusion

By conducting these experimental studies, the appropriate groove depth, angle, and position of the gripping structure could be determined. The empirical results were summarized. First, the recommended groove depth was between 30 mm and 45 mm. Second, the ergonomic recommended range of gripping groove angle was greater than 45°. Third, the acceptable height of the gripping slot was position 5. However, the maximum height of the gripping slot was 180 mm.

The research methods and results had some implication and guidance for the adaptation of the shape and location for gripping structures. Therefore, they could be used to prevent acute or chronic injuries to the operator in the work environment. Follow-up experiments should consider how to reasonably determine the step size of sample geometry sizes based on experience with actual usage, which would lead to an increase in the accuracy of the experimental results.