David J. Cochran

The Effects of Handle Shape and Size on Exerted Forces

This research empirically evaluated the effects of handle shape and size on the hands ability to resist or exert force in six directions. Thirty-six handles of four sizes and nine shapes were tested for maximum force exertion by male and female subjects. The results show that subjects were able to generate higher forces with different sizes and shapes of handles, depending upon the direction of force exertion. This suggests that handles that are associated with high forces on particular directional tests are probably suited for tasks that incorporate that particular type of force or movement; they may not be appropriate for other tasks that do not incorporate such movement.

An Investigation of Preferred Shapes for Warning Labels

An examination was made of 19 different geometric shapes of warning labels, using the method of paired comparisons. Sixty-six college students viewed slides of all pairs of the shapes and each time selected the shape that was the preferred indicator of warning. An ordinal scaling method was used to evaluate the differences among the shapes. Results show that the triangle on its vertex was the preferred warning indicator among the shapes tested.

An Analysis of Grasp Force Degradation with Commercially Available Gloves

Excessive force has been indicated as a causal factor in repetitive motion related occupational diseases such as carpal tunnel syndrome, tendinitis, and tenosynovitis. When gloves are worn, grasp capabilities may be compromised causing more force to be exerted. The objective of this research was to evaluate several brands and styles of gloves. Specifically, this evaluation determined the degradation of maximum grasping force for each of the gloves tested versus a bare-handed grasp. The results of this study indicate how much each glove inhibits grasp and a hierarchy of gloves is formed using the percentage of grasp degradation for each glove when it is compared with a bare-handed grasp.

Design of assembly lines with the concurrent consideration of productivity and upper extremity musculoskeletal disorders using linear models

Work-related musculoskeletal disorders (WMSDs) are common occupational diseases among assembly workers due to repetitive motions or heavy workloads. The conventional approaches to decreasing WMSDs in assembly workers usually focus on individual assembly work at the station level. These approaches, however, do not pay enough attention to work allocation at the whole assembly line level such as balancing ergonomic burdens among workers by proper work assignment. This paper presents a methodology that can be used to integrate ergonomic measures of upper extremities into assembly line design problems. Linear models are developed to link work-worker assignment to the upper extremity ergonomic measures based on a guideline from American Conference of Governmental Industrial Hygienists. These linear models allow ergonomic and productivity measures to be integrated as a mixed-integer programming model. The case studies of this paper show the new model can effectively balance and control exposure levels in the upper extremity while not significantly decreasing line efficiency. This research shows the potential to reduce the need of numerous task adjustments for ergonomic improvement after initial assembly line design in conventional trial-and-error based assembly task adjustment. Furthermore, these linearization methods can be generalized in order to incorporate other ergonomic measures in tabulated forms into assembly line design problems.

Dexterity Performance and Reduced Ambient Temperature

This study examined the dexterity performance of 35 male and 35 female subjects at ambient temperatures of 1.7°C, 12.8°C, and 23.9°C. Subjects wore typical industrial worker apparel without gloves. The Purdue Pegboard, two pencil-point tapping tasks, an assembly task, and a fine manipulative task were used to measure the dexterity performance. Results indicate that after approximately 15 min of exposure there was no difference between performance at 12.8°C and 23.9°C, but there was a difference between performance at 1.7°C and 12.8°C as well as between performance at 1.7°C and 23.9°C. The decrement in performance at 1.7°C ranged from 0.3 to 15.7% when compared with performance at 23.9°C . In five of the 11 performance scores used, a difference due to gender was seen, with the males performing better on an assembly task and the females performing better on single-activity, repetitive tasks. At the higher ambient temperatures (12.8°C and 23.9°C), the average of the minimum finger skin temperature for males was from 0.57°C to 1.2°C higher than for females, whereas at 1.7°C there was no significant difference between the groups for minimum finger skin temperature.

An Evaluation of Knife Handle Guarding

This research evaluated two variables affecting the performance of tang guards in preventing injury due to the hand slipping forward on a knife handle: the radius of transition from the handle to the blocking portion of the tang and the height of the tang. For the radii evaluated, there was no effect for either men or women. The guard height, however, did have a significant effect for both genders. There is a height, different for men and women, that appears to be critical. Guard heights exceeding this critical height do not enhance the capability of the guard to prevent the hand from slipping. Guard heights less than this critical value are less capable of preventing slipping.

An Evaluation of the Strengths, Weaknesses and Uses of Voice Input Devices

This paper examines the function of systems which are now available for voice input into computer memory. Strengths of present systems are discussed along with their weaknesses. Situations in which a voice data entry system is appropriate are explored as well as those system requirements which make voice input device systems feasible. One voice input device is tested in an industrial situation. The system is compared to two other methods of data entry on the basis of data entry time, total task time, and on error rate. The voice system shows longer entry time when compared with standard keyboarding but with higher accuracy. When the voice system is allowed to function to its potential by eliminating intermediate steps in the process, it is more efficient and more accurate. Tests conducted have shown that the maximum number of entries per minute over a very short duration (10 sec.) was about fifty-five. In an eight-minute sustained test, rates of about forty entries per minute were possible, but in long-term, the rate dropped to about 26 entries per minute. With proper coding and use on tasks where some components can be shortened or eliminated, the voice can be better than the keyboard.

An Investigation of Shapes for Warning Labels

An examination was made of nineteen different geometric shapes of warning labels, using the method of paired comparisons. Sixty-six college students viewed slides of all pairs of the shapes and each time selected the shape that was the better indicator of warning. The binomial test was used to statistically test the difference between the shapes. Results showed that of the shapes tested, the triangle on its vertex (yield sign shape, ∇) was the best warning indicator.

Perceived and Actual Grasp Forces on Cylindrical Handles

Objective: To compare the grasp force and the perceived grasp force, as a percentage of the maximum voluntary contraction (MVC), on cylindrical handles and describe a functional relationship between the two. Background: Repeated forceful exertions during work are associated with musculoskeletal disorders, and direct measurement of those forces is often difficult. Estimates are frequently made based on the judgments of force relative to the maximum grasp force capability of the individual. Method: Participants exerted grasp forces on five sizes of cylindrical handles. Pressure values at 16 locations (12 on the fingers and 4 on the distal ends of the metacarpal bones) were measured. Participants were asked to exert what they perceived to be specific percentages (in 10% increments) of their maximum grasp force on each of the five handles. Results: A linear relationship between perceived and actual grasp force was found up to the point of about 80% of perceived MVC. Above that point, the relationship became quadratic. A piecewise regression model was developed to fit the entire range of perceived grasp forces in one model. Conclusion: Grasp force is linear and consistent up until the perceived percentage MVC reaches 80%. After that point the relationship becomes quadratic. Application: In actual application, for grasp on cylindrical handles, practitioners can use a linear relationship between perceived percentage MVC and actual percentage MVC for perceived percentages of 80% or less. Above 80%, the piecewise quadratic relationship should be used.

Hand-Handle Orientation and Maximum Force

Many industries today have very high incidence rates for cumulative trauma disorders (CTD). Several factors are known to contribute to the onset of CTDs including the application of high forces. A reasonable inference is that a tool and/or task design that requires less grasp muscle effort will be less likely to contribute to CTD. The findings of a study by Jorgensen et al. (1989) indicated that the direction of power grasp force relative to the hand appears have a significant effect on the magnitude of the force exerted. This study was conducted to confirm and quantify these force magnitude differences for four orientations of opposition forces to the hand in a power grasp for two friction conditions and two handle shapes. The results showed that the orientations can be placed in two groups, one containing a single orientation and the other grouping containing the remaining three orientations. The orientation found to be superior to the other three was the pull resisting a force at the bottom of the hand (nearest digit 5). In this orientation, the hand is better able to oppose forces acting at the hand-handle interface than in two of the other orientations. The results of this research indicate that avoidance of the inferior orientations (those acting to ulnarly deviate the wrist) will be beneficial. Because current jobs are designed without considering the force direction, workers are exposed to unnecessary stresses. This study will give work task designers one more piece of information to help reduce the stresses on the hand and wrist.