Dynamic Hand Impact Tasks: How can you Measure Them?

According to the U.S. Bureau of Labor Statistics, there were 8,765 cases of non-fatal occupational hand and wrist injuries involving days away from work in Michigan alone in 2002.  In the manufacturing industry, workers are continually exposed to various hand intensive tasks requiring a combination of repetition, force, posture, contact stress, vibration and duration.  

In the manufacturing industry, ergonomic guidelines have been developed and continue to be developed to minimize the risk of injury to the worker. The type of tasks a worker performs can generally be broken into two categories:  Static Tasks and Dynamic Tasks.  Until recently, these tasks have been evaluated in exactly the same manner and have had the same ergonomic threshold values applied to them.  A Static Task requires minimal or no change in movement or posture to install the part correctly.  

Examples of Static Tasks
Installation of wiring along the floor pan	Weatherstrip installation

The traditional method of measuring static tasks is through use of a force gauge. The gauge is used to push and install the part, and the peak force during the installation process is recorded.

A Dynamic Hand Impact Task is any task that requires the worker to strike the object with their hand in order to install it.  It is often referred to as "using the hand as a hammer."  The worker uses a combination of force and acceleration to assist with installing the part.  Many of the interior trim parts in a vehicle are installed by an operator hitting the part with the palm of their hand.  Using the “hand as a hammer” is not the preferred method to install a part, but it is a reality in the manufacturing industry. Therefore it is important to establish guidelines for dynamic hand impact tasks to ensure that they do not increase the risk of injury to the worker.  Possible risk factors for dynamic hand impact tasks are overexertion, repetition, acute injury to the hand through contact stress and tissue compression.

Examples of Dynamic Tasks
Headliner installation	Door trim panel installation

The traditional method for measuring a dynamic hand impact task has been the same as the method used to measure a static task – using a force gauge.  The problem with using a force gauge to measure dynamic tasks is that the results are often:

• Inaccurate because it measures the task in a completely static manner and does not take into consideration the effect of momentum.
• Misleading as the tester will often bottom out the force gauge resulting is falsely high values.
• Highly dependant on the measurement technique of the tester.

From the above problems, it becomes obvious that measuring a dynamic task statically will not result in a true representation of the force required by the operator to install the part; therefore it must be evaluated in a different manner, by looking at the relationship:

Force = mass x acceleration

If we cannot measure Force, then the only other variable that can be measured is Acceleration. 

To measure acceleration a tool was developed called the Dynamic Impact Measurement Tool (DIMT).  This tool consists of an accelerometer that is attached to the workers hand and a PDA to collect the acceleration data.

To validate the tool three different types of tests were conducted:

1. Equipment capability testing was conducted to evaluate the repeatability of the 3-axis accelerometer.

2. University testing to establish maximum acceptable limits applicable to the manufacturing industry.

3. In plant testing to prove the real world capabilities of the measurement system.

University Testing (Potvin, J., Chiang, J., Mckean, C., Stephens, A. (2000))

It was determined that the best method to develop limits for a dynamic hand impact task was through a psychophysical study.  Psychophysical studies look at the relationship between sensations and their physical stimuli (Snook et al., 1970).  It assumes that individuals can identify working conditions that they perceive as having an acceptable level of stress for them.

Purpose:  To determine the acceptable ergonomic guideline for hand impact tasks by measuring the acceleration of the hand and the force applied during a simulated door trim panel installation task. 

Method: 

• 3-axis accelerometer was attached to the subjects hand.
• Subjects were trained for 8 hours prior to testing – skilled workers.
• Force and acceleration time histories were recorded for each subject.
• Force was measured using a force plate located behind the door trim panel that they were impacting.
• Subjects were instructed to impact the simulated trim panel as hard as they found acceptable for an 8 hour day without causing injury, numbness or pain.

Results: 

By using the force and acceleration data collected in the study, ergonomic guidelines were developed for maximum acceptable dynamic hand impact force based on the acceleration of the hand.  When evaluating a dynamic hand impact task, the ergonomist would only be required to measure the acceleration at the hand during the impacts and the number of hand impacts per minute.  This data can then be charted on the graph below to determine what population is capable of performing the task.

In a comparison between performing the same task statically versus dynamically, workers were found to typically use almost four times the amount of force than the minimal static force needed to complete a particular trim installation task.  This result alone justifies that dynamic tasks need to measured in a dynamic manner.  Occupations such as automotive assembly, carpentry, mechanics, construction, and floor installation have traditionally required workers to "use their hand as a hammer."  These occupations would benefit from using the Dynamic Impact Measurement Tool (DIMT) to evaluate the hand impact tasks they perform. The DIMT can tell the ergonomist if the forces that the workers are exerting are above or below the threshold, and therefore can assist in the design of tools to help reduce or eliminate these stresses.  

For more information on the DIMT (Dynamic Impact Measurement Tool) please refer to our Sandalwood website at http://www.sandalwood.com/products.html under the Products line item, or for inquires you can email Sandalwood at contactus@sandalwood.com . 

REFERENCES

Potvin, J.R., Chiang, J., Mckean, C., Stephens, A.  (2000)  A psychophysical study to determine acceptable limits for repetitive hand impact severity during automotive trim installation.  International Journal of Industrial Ergonomics.  26: 625-637.