What do you have to know to calculate the recommended weight limit (RWL)?
First, you have to measure or assess several variables related to the lifting task. The six variables that are considered in determining the recommended weight limit (RWL) are:
Each of these variables is then assigned a numerical value (multiplier factor) from look-up charts. The NIOSH equation includes six multiplier factors to calculate the recommended weight limit (RWL):
where LC is the load constant and other factors in the equation are:
Figure 1: F=Frequency, A=Angle of load, H=Horizontal, C=Coupling, D=Distance travelled, V=Vertical location
Figure 2: AM=Asymmetric Multiplier, the angle of the body in relation to the load
The closer the multiplier factors are to 1, the better the conditions are for a Recommended Weight Limit (RWL) close to 23kg (or about 51 lbs). When the conditions are not within the recommended ranges, (multiplier factor farther from 1), the weight limit must be reduced accordingly.
How do I figure out which multiplier value to use?
To figure out which multiplier value to use, you must measure the distance in centimetres for each factor. For example, to determine horizontal multiplier, measure the distance from in between the person's ankles to their hands when holding the object. Write down this number. Next, look up the number on the "horizontal distance" chart (see Calculating Recommended Weight Limit (RWL)) and find the matching multiplier factor. Use this factor in the lifting equation. NIOSH does allow for extrapolation for values that do not match the figures presented in their tables, or you can opt to use the next higher value (more protective) from the table.
Do the same for the other five factors.
Once you have all these values, you can use the Revised lifting equation calculator to determine a recommended weight limit.
Compare this value with the actual weight of the object. If the recommended weight limit is lower than the actual object, you will have to determine which factor(s) is contributing the highest risk and modify the lift. (The factors that are contributing the highest risk will have the lowest multiplier values).
- context Vertical Multiplier is the distance is measured as the starting point of the lift and is the distance of the hands up from the ground. Measure this distance and use the number to determine which value to use on the chart (see Calculation Recommended Weight Limit).
- context Distance Multiplier is the number of centimetres the load travels up (or down) from the starting position. Again, measure this distance and use the number to determine which value to use on the chart (see Calculation Recommended Weight Limit).
- context Frequency Multiplier is how often the lift is repeated within a certain time period. You need to determine if the lift is done while standing or stooping, for more or less than one hour (in total time for the shift), and how much time there is for rest between lifts.
- context Asymmetric Multiplier measures if the body must twist or turn during the lift. This measurement is done in degrees (with 360 being one complete circle).
- context Coupling multiplier determines the "coupling" or type of grasp the person has on the container. It rates the type of handles as good (handles), fair (make-shift cut outs in cardboard boxes) or poor. You also need to know if the lift is done in a standing or stooping position.
Can I use this equation in all situations?
No. The Revised NIOSH Lifting equation only applies in certain situations. It does not apply in situations where a person is lifting (or lowering):
- context with one hand,
- context for over 8 hours,
- context while seated or kneeling,
- context in a restricted work space,
- context objects that are unstable (such as buckets or containers of liquids),
- context while pushing or pulling,
- context with wheelbarrows or shovels,
- context with high speed motion (faster than about 0.76m/s (or 30 inches/second),
- context extremely hot or cold objects or in extreme temperatures, or
- context with poor foot/floor coupling (high risk of a slip or fall).
So, when do I use this equation?
This recommendation applies to most workers for:
Calculation of recommended lifting limit using this formula indicates which of the six components of the task contribute most to the risk. The lower the multiplier factor, the more it contributes to the risk.
- context two-handed lifting,
- context comfortable lifting postures, and
- context comfortable environments and non-slip floorings.
Why is this equation called "revised"?
NIOSH published their first lifting equation in 1981. In 1994, a new "revised" equation was published which took into account new research findings and other variables that were not used in the first equation. This "revised" equation can be used in a wider range of lifting situations than the first equation.
- context Fact sheet confirmed current: 2016-07-04