WEEK 4

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Mechanical_engineering

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May 3, 2024

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FACULTY OF HEALTH SCIENCES - HLSC 4676U Occupational Ergonomics and Work Disability Prevention - Winter, 2023 Sessional Lecturer Dr. Heather Matchett B.Kin (Hons), D.C. WEEK FOUR January 29, 2024 Outline Occupational Health Hazards Continued o Biomechanical NIOSH MHH Biomechanical Models MSK System and LBP Posture (Seated work and Chair Design) Cumulative Trauma Disorders/Repetitive Strain Injuries BIOMECHANICAL Revised NIOSH Lifting Equation – Assessing Relevant Handling Factor Revised in 1994 to improve upon prior 1981 lifting equation The determination of whether a lift is too heavy can be made by using the NIOSH lifting equation, which considers various factors in lifting tasks that contribute to the risk of injury. These factors include the weight of the load, as well as other variables such as the frequency of lifts and the distance of the load from the body. By applying this equation, a recommended weight limit (RWL) can be calculated, which takes into account these variables and helps determine if a lift is within safe limits. To calculate the recommended weight limit (RWL), you need to measure or assess several variables related to the lifting task. These variables include the horizontal distance (H) the load is lifted, the starting height of the hands from the ground (V), the vertical distance of lifting (D), the time between lifts or frequency of lifting (F), the angle of the load in relation to the body (A), and the quality of the grasp or handhold (C). Each variable is assigned a numerical value from reference tables, and the NIOSH equation uses these values to calculate the RWL.
Figure 1. F=Frequency, A=Angle of load, H=Horizontal, C=Coupling, D=Distance travelled, V=Vertical location Figure 2. Figure 2 illustrates the AM (Asymmetric Multiplier) and its body angle in relation to the load. The proximity of the multiplier factors to the pick up location indicates a Recommended Weight Limit (RWL) of approximately 23kg (or 51 lbs), which is called the load constant. On the other hand, if the multiplier factors deviate from 1, the RWL should be reduced. To determine the appropriate multiplier value, you will need to measure the distance in centimeters for each factor. For the horizontal multiplier, measure the distance from the person's ankles to their hands while holding the object. Record this measurement and refer to the "horizontal distance" chart in the Calculating Recommended Weight Limit (RWL) to find the corresponding multiplier factor. Use this factor in the lifting equation. Please note, if the measured value does not match any figures in the table, NIOSH allows for extrapolation or using the next higher value from the table for more protection. Repeat this process for the other five factors (vertical multiplier,
distance multiplier, frequency multiplier, asymmetric multiplier, and coupling multiplier). Once all of these values are obtained, you can use the Revised lifting equation calculator to determine the recommended weight limit. Finally, to assess the weight of the object, and thus the safety of the lifts, compare it with the recommended weight limit. If the actual weight exceeds the limit, identify the factor(s) posing the greatest risk and make adjustments to the lifting process. A special hint: The factors with the lowest multiplier values indicate the highest level of risk. The Revised NIOSH Lifting equation is not applicable in various scenarios. It does not apply when lifting with one hand, lifting for more than 8 hours, lifting while seated or kneeling, lifting in a restricted work space, lifting unstable objects like buckets or containers of liquids, lifting while pushing or pulling, lifting with wheelbarrows or shovels, lifting with high speed motion greater than 0.76m/s (or 30 inches/second), lifting extremely hot or cold objects or in extreme temperatures, or lifting with poor foot/floor coupling that poses a high risk of slip or fall. NIOSH Lifting Equation and its factors The equation is: RWL = LC x HM x VM x DM x AM x FM x CM LC, Load constant - A "load constant" (LC) of 23 kg (about 51 lb) was established by NIOSH as a load that, under ideal conditions, is safe for 75% of females and 90% of males to lift. AM, Asymmetric Multiplier Factor - Twisting angle of the body while lifting, measured in degrees A = Angle (degrees) AM Factor 90° 0.71 60° 0.81 45° 0.86 30° 0.90 0° 1.00 HM, Horizontal Distance Multiplier Factor - Horizontal distance in cm, from the midpoint between the ankles to the hands while holding an object H = Horizontal Distance (cm) HM Factor 25 or less 1.00 30 0.83 40 0.63 50 0.50 60 0.42
VM, Vertical Distance Multiplier Factor - Vertical distance in cm, of the hands from the ground at the start of the lift V = Starting Height (cm) VM Factor 0 0.78 30 0.87 50 0.93 70 0.99 100 0.93 150 0.78 175 0.70 >175 0.00 CM, Coupling Multiplier Factor - Quality of grasp (classified as good, fair, or poor), and depends on the body position (standing or stooping). C = Grasp CM Factor: Standing Stooping Good (handles) 1.00 1.00 Fair 1.00 0.95 Poor 0.90 0.90 FM, Frequency Multiplier Factor - Frequency of lifts and the duration of lifting (in minutes or seconds) over a given workshift F = Time Between Lifts FM Factor Lifting While Standing: OR Lifting While Stooping: One Hour or Less Over One Hour One Hour or Less Over One Hour 5 min 1.00 0.85 1.00 0.85 1 min 0.94 0.75 0.94 0.75 30 sec 0.91 0.65 0.91 0.65 15 sec 0.84 0.45 0.84 0.45 10 sec 0.75 0.27 0.75 0.27 6 sec 0.45 0.13 0.45 - 5 sec 0.37 - 0.37 -
DM, Distance Multiplier Factor - Vertical distance in cm, that the load travels D = Lifting Distance (cm) DM Factor 25 or less 1.00 40 0.93 55 0.90 100 0.87 145 0.85 175 0.85 >175 0.00 RWL, Recommended Weight Limit Canadian Centre for Occupational Health and Safety EXAMPLE 1 - considering HM Factor: Determine whether this person is lifting under or over their RWL for this particular task. A worker lifts 13 kg boxes from the table to the shelf five times for one hour or less. In this example, there is a barrier between the worker and the box. The revised NIOSH Lifting Equation is: RWL = 23 Kg x HM x VM x DM x AM x FM x CM H (Horizontal Distance) - 50 cm V (Vertical Distance) - 70 cm D (Lifting/Carrying Distance) - 40 cm F (Frequency) - 12 min A (Angle) - 0° C (Coupling/quality of grip) – fair, standing Look up the values for HM, VM, DM, AM, FM, CM in respective charts above and calculate RWL. 23 Kg x 0.50 x 0.99 x 0.93 x 1.00 x 1.00 x 1.00 = 10.59 kg Therefore, they are over the RWL and are at risk for a lifting related injury. Workspace design revisions: Remove the barrier so the worker can get closer to the box they are lifting. Now the horizontal distance will be changed to 30cm. The revised NIOSH Lifting Equation is: RWL = 23 Kg x HM x VM x DM x AM x FM x CM 23 kg x 0.83 x 0.99 x 0.93 x 1.00 x 1.00 x 1.00 = 17.58 kg Compare weight of the load against Weight Limit in new workspace. The weight of the load at 13 kg is now lower than the recommended weight limit of 17.58 kg. Therefore, most workers can safely perform the task.
Canadian Centre for Occupational Health and Safety EXAMPLE 2 - considering VM Factor: A worker lifts an 18 kg load from piled pieces of metal from the floor to the table five times in an hour. H (Horizontal Distance) - 30 cm V (Vertical Distance) - 0 cm D (Lifting/Carrying Distance) - 115 cm F (Frequency) - 12 min A (Angle) - 0° C (Coupling/quality of grip) – poor The revised NIOSH Lifting Equation is: RWL = 23 Kg x HM x VM x DM x AM x FM x CM 23 Kg x 0.83 x 0.78 x 0.85 x 1.00 x 1.00 x 0.90 = 11.39 kg ** Please note: * The distance was 115 cm. By looking at the reference tables, the distance factor was greater than 100cm, so the value of 0.85 was instead selected in order to be more protective. Since the weight of the load is 18 kg, it is significantly higher than the calculated RWL. This task will therefore likely increase the chance of a lifting related injury. Workspace design revisions: If the metal collection area (starting height) is raised off the floor, D (lifting distance) is decreased from 115 cm to 40 cm, while increasing V (vertical distance) from 0 cm to 75 cm. The revised NIOSH Lifting Equation is: RWL = 23 Kg x HM x VM x DM x AM x FM x CM 23 kg x 0.83 x 0.93* x 0.93 x 1.00 x 1.00 x 0.90 = 14.86 kg * Please note that the vertical distance is 75 cm. Since VM was greater than 70 cm, 0.93 was selected as the value in order to be more protective. In this example, the load at 18 kg is still higher than the new recommended weight limit of 14.86 kg. Therefore, the worker is still at risk for a lifting related injury, and more workspace design revisions must be made. Further revisions could include supplying the working materials in containers with handles, or by reducing the weight of the lift to lower than 14.86 kg. Canadian Centre for Occupational Health and Safety EXAMPLE 3 - considering Frequency Factor: A worker lifts 6 kg boxes from the conveyor to the cart ten times every minute for two-hours. H (Horizontal Distance)- 20 cm V (Vertical Distance) - 75 cm D (Lifting/ carrying Distance) - 0 cm F (Frequency) - 6 sec over 1 hour, standing A (Angle) - 90° C (Coupling/quality of grip) - fair, standing The revised NIOSH Lifting Equation is: RWL = 23 Kg x HM x VM x DM x AM x FM x CM 23 kg x 1.00 x 0.93 x 1.00 x 0.71 x 0.13 x 1.00 = 1.97 kg
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