ارزیابی تنظیمات ارگونومیک چرخ دستی های تهیه غذا برای کاهش هل دادن نیروهای خارجی

An existing standard catering cart was compared with two prototypes for pushbar and castor design. The first objective of this study was to find out which cart was accompanied with the lowest manually exerted external forces in pushing in a straight way and in pushing a 90° turn. The second objective was to explore effects of the pushbar and castor design of the carts. In the initial and ending phase, the prototypes were accompanied with higher exerted forces compared with the standard catering cart. In pushing straight, the reversed start position of the bigger castors of the prototypes hampered a fluent acceleration and caused higher initial forces. In decelerating, the lower rolling friction of the bigger castors required higher forces to stop the prototypes compared to the standard cart. During the sustained phase, the prototype carts were more favourable. Effects of pushbar and castor design were studied during a turn. The vertical pushbars of the prototypes resulted in lower time-integrated pushing forces. Providing an axis of rotation for turning activities by means of a fixed wheel was proven to be advantageous.

Manual materials handling has been identified as a potentially harmful physical work factor (Bernard, 1997; Hoogendoorn et al., 1999; Kuiper et al., 1999). Lifting loads is generally considered hazardous and has been extensively studied in relation to low back complaints. Handling materials by pushing and pulling has received only limited scientific attention, although it has been estimated that nearly half of the manual handling of materials consists of pushing and pulling (Baril-Gingras and Lortie, 1995; Kumar, 1995). Furthermore, pushing and pulling was found to be associated with musculoskeletal complaints, in particular low back, shoulder, and forearm complaints (Van der Beek et al., 1993; Hughes et al., 1997; Hoozemans et al., 1998; Macfarlane et al., 2000). To prevent the development of musculoskeletal complaints related to pushing and pulling at work, pushing and pulling tasks should be designed such that workers are enabled to work with a minimum of health risk, in the short and long term. Risk evaluation of pushing and pulling tasks is generally aimed at the assessment of exerted hand forces (Hoozemans et al., 1998; Van der Beek et al., 1999). As an increase in exerted hand forces is accompanied with an increase in mechanical stress on the musculoskeletal system, especially the shoulder and low back, it is plausible that the exerted forces are important contributors to the risk of musculoskeletal complaints. Maximum acceptable forces have been established for a variety of pushing and pulling tasks from a psychophysical point of view by Snook and co-workers (Snook, 1978; Snook and Ciriello, 1991; Ciriello et al., 1993). Exerted forces were distinguished into initial forces, required to accelerate an object, and sustained forces, required to keep an object at a more or less constant velocity. Maximum acceptable forces were determined for a variety of handle heights, frequencies, distances, and for both genders. These and several other aspects of pushing and pulling tasks have recently been reviewed in relation to the level of exerted forces (Hoozemans et al., 1998). Generally, handle height has received a lot of scientific attention. For pulling, maximum acceptable forces and maximum exerted forces were found to increase with lower handle heights (Warwick et al., 1980; Chaffin et al., 1983; Snook and Ciriello, 1991; Fothergill et al., 1992; Mital et al., 1997). For pushing, contradictory results were reported but, generally, the highest maximum acceptable forces were found at about 1 m (Snook and Ciriello, 1991; Mital et al., 1997). However, from studies that examined the mechanical load at the low back and shoulder it can be concluded that higher handle heights are preferable (Lee et al., 1991; Resnick and Chaffin, 1995; Van der Woude et al., 1995; De Looze et al., 2000, Hoozemans et al., 2000). Ergonomic interventions concerning handle height are often easy to accomplish and should be focussed on reducing the magnitude of the exerted forces. For instance, for pushing or pulling a 181 kg cart, Al-Eisawi et al. (1999a) reported lower exerted forces as handle height increased. Another ergonomic aspect influencing the exerted forces is castor design, recently studied by Al-Eisawi et al. (1999b). They investigated the effect of castor width, castor diameter, castor orientation, cart weight, and floor type. No effect was found for castor width. However, it has been found that the minimum required initial push forces increased with smaller wheel diameters and that these forces proportionally increased with cart weight. Although the influence of castor orientation on the minimum required initial push forces was not consistent, in general the magnitude of these forces were smallest with all wheels aligned in the forward direction. With regard to floor type, the lowest rolling friction has been found for pushing/pulling on smooth concrete and tile, and the highest rolling friction on asphalt and industrial carpet. The present study was based on a question arising from a Dutch catering service, whose employees are daily exposed to pushing catering carts. To reduce the musculoskeletal problems reported by the employees, the existing standard catering cart (SCC) was redesigned at the initiative of the catering service. Two new types of catering carts were developed by adjustments to the pushbar and castor design of the SCC. The catering service wanted the adjusted carts to be evaluated from an ergonomic point of view. Most of the studies mentioned above, concerning ergonomic aspects of pushing and pulling devices, are based on laboratory experiments. The present study was performed in the workplace environment to be able to draw more practical conclusions. The aim of this study was twofold: (i) to test the hypothesis that pushing two new types of catering carts under different conditions resembling normal working situations, involves lower manually exerted external forces than pushing the SCC and (ii) to explore effects of the pushbar and castor design of the carts and of the experimental conditions.

4.1. External forces in pushing the prototypes compared to the SCC In accelerating the carts, results were inconclusive with respect to the exerted peak forces. However, it seems obvious that both prototypes are preferable with respect to the sustained forces and the integrated pushing force. Concerning the latter variable, the Hupfer cart was advantageous over both the SCC and the Animo cart, for pushing in a straight way as well as pushing a turn. The deceleration of the prototypes involved higher integrated pulling forces than the SCC. Briefly, the prototype carts did not involve lower manually exerted forces than handling the SCC for all effect measures. It can be concluded that the prototype carts, and especially the Hupfer cart, are only preferable in long-distance pushing or pushing a turn. However, to assess the impact of these carts to the exposure to pushing forces in the catering profession it is recommended to perform additional task analysis to gain insight in the frequency and duration of pushing activities (Van der Beek et al., 1999). Minimum cart pull forces as reported by Al-Eisawi et al. (1999b) where generally lower in comparison to the initial forces found in the present study for handling carts of comparable weight. Differences may be explained by the working technique. The minimum pull forces of Al-Eisawi et al. were assessed by ‘increasing the force slowly and steadily without jerking’. In the present study, the participants were explicitly instructed to work at a normal work pace. As a result, the catering carts were probably accelerated at a higher rate which caused relatively higher initial forces. Furthermore, part of the differences may be explained by the fact that the minimum cart pull forces of Al-Eisawi et al. were differently assessed in comparison to the initial forces in the present study. Recently, the push and pull forces assessed using continuous 3D measurements were compared to the push and pull forces using a hand-held force gauge (Hoozemans et al., 2001). It was concluded that in certain situations, especially for pushing and dynamical work situations, the assessment of push or pull forces using the hand-held force gauge could not be considered valid. Therefore, it was decided to use a more accurate and precise measurement device. External resultant forces were used to compare catering carts and to explore the effects of some ergonomic adaptations. It is assumed that the level of exerted forces is indicative as to the mechanical load of, for instance, low back and shoulders during pushing and pulling. Differences in the level of exerted forces would therefore point to differences in mechanical loading. However, next to the level of exerted forces also the direction of the exerted forces and, more specific, the direction with respect to joint centres determines the actual mechanical loading of the worker. Estimating mechanical load during the pushing of the catering cart would have required advanced 3D analyses of forces and movements at the workplace. However, most of the studies on the ergonomics of pushing and pulling have been aimed at the external forces (Al-Eisawi et al., 1999b; Hoozemans et al., 1998), which can be easily assessed at the workplace (Van der Beek and Frings-Dresen, 1998). To evaluate the potential hazardous activities or catering carts, the exerted forces found in the present study for pushing in a straight way can be compared to maximum acceptable forces (Snook and Ciriello, 1991; Mital et al., 1997). When pushing two-handed at a frequency of once every 5 min, over a distance of 7.6 m, and at a handle height of 89–135 cm, female industrial workers are recommended not to exceed initial, sustained and ending force levels of 176.4–186.2, 78.4–88.2, and −166.6 to −186.2 N, respectively (Mital et al., 1997). However, although all carts were loaded with a reasonably heavy load (58.8 kg), none of the measured forces exceeded these recommended levels. 4.2. Effects of pushbar, castor design, and floor type In the present study, several adaptations of the carts were compared. Since not all combinations of pushbar and castor design were available on each cart, exploring effects of the pushbar and castor design of the carts was partly hampered from a methodological point of view. Despite this limitation, significant differences were found between the experimental carts that might yield indications for cart design in general. 4.2.1. Pushing straight Although carts with vertical pushbars and an adapted castor design caused lower sustained forces, this concept offered no advantage in accelerating or decelerating. An explanation for the fact that initial forces did not differ between carts can be found in castor design. In general, a larger wheel is associated with lower rolling friction. The findings of Al-Eisawi et al. (1999b) underline this statement. They found lower minimum (initial) forces needed to manually push a cart from a stationary position when using a greater castor diameter. However, in the present study the subjects had to start pushing while the castors were in reversed direction of movement. Although Al-Eisawi et al. (1999b) also have taken castor orientation into account, their conclusions were contradictory to the present study: accelerating required equal peak forces for all carts in this practical setting. Briefly, the reversed start position of the bigger castors hampered a fluent acceleration and offered no benefit when compared to an unadapted cart. An advantage of vertical pushbars and an adapted castor design in decelerating a cart was not confirmed. The significantly higher integrated pulling forces in decelerating the prototypes even indicated the opposite. This result can be explained by the lower rolling friction of the bigger castors of the prototypes. Consequently, higher forces were required to stop the prototypes in comparison with the SCC. The higher weight levels of the prototype carts additionally contributed to this result. 4.2.2. Pushing a turn Next to the sustained forces in pushing straight, the advantages of the vertical pushbars combined with an adapted castor design were also evident in pushing a turn. In the first place, significant higher integrated pushing forces were observed while handling the SCC when compared to the Hupfer cart. Secondly, the favourable effect of providing an axis of rotation for turning activities by means of a fixed wheel was proven by the significantly higher integrated pushing forces in moving the Animo cart in free condition when compared to the same cart in fixed condition. The regression analysis further showed the importance of the different properties of the carts separately with regard to the integrated pushing forces. The benefit of vertical pushbars combined with an adapted castor design was confirmed. However, the higher weight of the prototype carts partly neutralised the effect of these properties due to the result that higher weight levels are accompanied by higher integrated pushing forces. In pushing the carts, all turns were performed to the right. Since the fixed wheel of the Animo cart (fixed) was localised in the front on the left of the cart, this may have affected the results in the turning conditions. However, both concepts of a fixed wheel (Hupfer and Animo (fixed)) resulted in lower integrated pushing force despite the location of the wheel. Briefly, to equip a cart with a fixed wheel seems advantageous in pushing a turn, although the optimum location of the wheel has not yet been established. Concerning the integrated pulling force which represents the deceleration of carts, neither a benefit of a vertical pushbar design nor of an adapted castor design was found. This result is probably due to similar factors as described before for pushing straight: the lower rolling friction of the bigger castors and the higher mass of the prototypes requires higher forces to decelerate the cart. This combination may have smoothed the favourable effects of the adaptations. 4.2.3. Linoleum compared to carpet The analysis of variance showed a significant main effect of floor type for all variables except for the decelerating peak force, indicating that using a cart on carpet is disadvantageous under almost all circumstances. No interaction effect of cart and floor was found, suggesting that differences between carts are independent of floor type. Interactions between wheel and floor, and between floor and weight only existed for the integrated pulling force in pushing a turn. In the final phase of a turn, an adapted castor system as well as a higher weight of the cart did increase the level of the integrated pulling force more on linoleum than on carpet. This is probably caused by the fact that linoleum is associated with lower rolling friction and, therefore, requires higher decelerating forces. This leads to more explicit differences between carts on linoleum, since carpet causes higher rolling friction and decelerating a cart on this surface consequently requires less active force exertion. 4.3. Practical implications and recommendations for future designs The simultaneous application of a vertical pushbar design and larger castors reduces the required exerted hand forces when compared to carts equipped with horizontal pushbars and small castor wheels. However, this recommendation does not hold true for all situations. When at the start of the pushing action, the larger castors are in reversed direction of movement, higher initial forces are needed to accelerate the cart. Furthermore, because the larger castors are accompanied with lower rolling friction, relatively higher pulling forces are required to decelerate or stop the cart. The latter problem can be prevented by the application of other ergonomic adaptations such as hand brakes. However, the design of the actual pushing or pulling task at the workplace would determine the beneficial effect of using larger castors. When pushing or pulling is a relatively frequent activity, when only short distances are covered, and when the cart has to be turned regularly, it is not recommendable to apply large castors. Larger castors are most advantageous when carts are pushed or pulled over large distances in a straight way. In most cases, it would be disadvantageous to use carts with only free turning wheels (Al-Eisawi et al., 1999b). When carts have to be moved in small areas at slow speeds and when carts also have to be moved sidewards the application of only free turning wheels may be beneficial. Otherwise, it should be recommended that carts are equipped with a fixed wheel. The choice between applying one or two fixed wheels would also depend on the task design. When pushing or pulling turns are relatively common it would probably be beneficial to apply only one fixed wheel. Studies on mechanical load at the low back and shoulder indicated that higher handle heights are to be preferred. In this light, the relatively low horizontal pushbar is not recommendable. Workers should be able to push or pull at a preferable height, which for the type of carts used in the present study would be around shoulder height. Vertical pushbar designs enable each worker to push or pull at their preferable height. However, some discussion may arise as to the width between the two vertical pushbars. Two fixed vertical bars restrain the workers from grasping at a preferable width. Although from a mechanical point of view, pushing or pulling at shoulder width would be recommendable, this aspect of the design of catering carts remains a topic for future research.