[Occupational exposure levels of hand-transmitted vibration and vibration reduction measures in handheld workpiece grinding operations]

Objective: To study the occupational exposure levels of hand-transmitted vibration in handheld workpiece grinding operations and the vibration reduction measures, in order to provide scientific basis for reducing hand-transmitted vibration hazards in handheld workpiece grinding operations. Methods: From April to December 2022, 34 handheld workpiece grinding operation positions in a metal manufacturing enterprise in Guangdong Province were selected using the judgment sampling method for hand-transmitted vibration measurement. The frequency-weighted vibration acceleration values (a(hwx), a(hwy), a(hwz)) along the X, Y, and Z axes of each operation position were compared using the Friedman test. Four experiments were conducted: dynamic balance improvement of the grinding machine's driving wheel, relative angle correction between the driving wheel and driven wheel, vibration reduction improvement of the machine base, and testing of different grinding belts. Variance analysis or linear mixed models were employed to analyze and compare the differences in the total vibration values (a(hv)) of hand-transmitted vibration frequency-weighted root mean square acceleration of different improvement measures. Results: For the handheld workpiece grinding operation positions, the M (P(0), P(100)) of a(hwx) was 3.15 (1.19, 9.24) m/s(2), a(hwy) was 3.01 (1.47, 11.70) m/s(2), both of which were significantly higher than a(hwz)[2.58 (0.89, 8.56) m/s(2)] (P<0.05). The M (P(0), P(100)) of the 8-hour energy-equivalent frequency-weighted vibration acceleration[A (8) ] values of the positions was 4.93 (2.51, 14.54) m/s(2) with 17 operation posts (50.0%) exceeding 5 m/s(2). Across five eccentric mass groups (with eccentric masses of 3, 10, 15, 22, and 32 g), the a(hv) values were (1.94±0.22), (4.51±0.51), (3.92±0.48), (7.34±0.62), and (11.65±2.18) m/s(2), respectively. The higher eccentric mass of the driving wheel correlated with a larger a(hv) (P<0.001). Under different relative positions of the front and rear wheels (angles of <1° and 5°), the a(hv) values were (3.01±0.34) and (3.07±0.37) m/s(2), showing no statistically significant difference (P=0.628). The a(hv) when using a double-layer rubber seat pad for operation was (7.22±0.58) m/s(2), which was lower than that of a single-layer rubber seat pad[ (7.77±0.50) m/s(2)] (P=0.012). When grinding the same number of workpieces, the a(hv) of 50 mesh belt[2.98 (2.05,7.68) m/s(2)] was lower than that of 60 mesh belt[4.29 (2.17, 8.38) m/s(2)] (P=0.023) . Conclusion: The hand-transmitted vibration level in handheld workpiece grinding operations is relatively high. By correcting the eccentric mass of the driving wheel of the grinding machine, installing double-layer rubber pads, and selecting appropriate grinding sand belts, the hand-transmitted vibration exposure level can be reduced.