طراحی های مجدد ارگونومیک و ارزیابی بستن دسته ابزار

The handle of a commercial bar clamp was redesigned using ergonomic principles and then compared with an original clamp. Ten male and ten female students participated in simulated clamping tasks under various conditions, including different clamping heights, clamping methods, and handle-gripping methods, with respect to the dependent variables of clamping and handle-squeezing forces. The results showed that the redesigned clamp produced larger clamping force with lower handle-squeezing forces than the original clamp. As expected, males exerted more force than females in both clamping and squeezing forces. A pistol grip method was superior to an upside-down handle-gripping method. Two-handed operation was recommended for this type of clamp by simultaneously pulling the trigger and sliding the bar in order to initially tighten around objects. This study shows that the application of ergonomic guidelines increases the efficiency and usability of manual handtools.

Although many facilities have been automated, many tasks still require a manual handtool and new tools are still being developed for better assistance to workers. However, just as new technology can be worse than no technology if it is not done correctly, ergonomic research has clearly demonstrated the relationship between injury risk and poorly designed handtools. Guidelines have been suggested on handtool design to alleviate this problem (Aghazadeh and Mital, 1987; Mital and Kilbom, 1992b). As an example of such handtool design, user-centered ergonomic redesign of the manual handtool handle was initiated on the Quick-Grip® bar clamp (standard version, 6″/150 mm) manufactured by American Tool Companies, Inc. This manual clamping tool consists of front and rear jaws, a trigger, and a handle seen in Fig. 1. There are two ways to use this tool; first, for one-handed operation, the handle is repeatedly squeezed to move the front jaw closer to the rear until the desired positions of jaws are reached so that an object is gripped; second, for two-handed operation, the desired positions of jaws can be adjusted by sliding the bar with one hand while pulling the trigger with the other hand and then gripping an object by squeezing the handle once at the end of the clamping movement.

From the statistical analyses, it was found that clamp, clamping method, gripping method, and gender all had significant effects on both peak clamping forces and peak handle-squeezing forces. Objects were more tightly clamped with the redesigned clamp. The clamping forces when using the original clamp were only about 83% of those when using the redesigned clamp in the ending phase. With the original clamp, additionally, clamping forces decreased more with the upside-down gripping method due to its flaring handle at the end, since participants could not get a good grip with their stronger digits. The upside-down gripping method reduced clamping forces especially at shoulder height because this caused severe ulnar deviation of the wrist to be observed by the experimenter. It was surprising that the redesigned clamp caused smaller handle-squeezing forces than the original clamp because clamping forces in the ending phase was larger. This situation may be explained that hand comfort with the redesigned clamp allowed participants to better control handle-squeezing activities with lower forces. During the starting phase, lower handle-squeezing forces are desirable because the main role of squeezing the handle in this phase is only to make the bar slide in order to reduce the distance between jaws without actual clamping action. Therefore, excessive handle-squeezing forces can be avoided with the redesigned clamp in the starting phase and this may result in larger clamping forces in the ending phase. For better understanding how both clamps work, the ratios of clamping forces in the ending phase to handle-squeezing forces in the clamping phase were calculated, which defined how much handle-squeezing forces were transferred to clamping forces. The ratios for both redesigned and original clamps were approximately 0.55 and 0.34, respectively. Therefore, it could be concluded that the redesigned clamp was more efficient and caused larger resultant clamping forces, especially in pistol-type clamp usage. When clamping objects with the handle-squeezing method, approximately seven squeezes of the handle were needed during the starting phase to reduce the distance between jaws to the 3.8 cm wooden block width, while these repetitive squeezes were not required with the bar-sliding method. The decreased handle-squeezing and clamping forces while using the handle-squeezing method, presumably due to the forearm fatigue that resulted from this repetitive squeezing movement. It was frequently observed that participants grasped the tip of the handles when clamping with the upside-down gripping method. This grasping position increased the length of lever arm so that handle-squeezing forces in the starting phase were larger with the upside-down gripping method than with the pistol gripping method, especially at elbow and shoulder heights. However, this behavior was not found during the clamping phase and was not significant for clamping forces because participants controlled both handle-squeezing and clamping forces to reach a force level where they felt the wooden block was gripped tightly enough during the clamping phase regardless of gripping method. Males produced larger clamping and handle-squeezing forces than females. Females exerted about 52% clamping force of males in the ending phase and about 66% handle-squeezing force in the clamping phase. These findings more expected due to physiological and biomechanical difference between genders (Hallbeck and McMullin, 1993; Zellers and Hallbeck, 1995). In conclusion, a bar clamp handle was redesigned and the redesigned clamp compared to the original clamp in this study. When using the redesigned clamp, objects could be clamped more tightly and also this redesigned clamp had better efficiency with less forearm fatigue. Based on these results, guidelines for the use of this type of clamp can be suggested. It should be used with both hands at or below the elbow height of operators, using the pistol gripping method. Through this study, it was shown that the application of ergonomic guidelines for handtool design could increase tool efficiency and usability.