Trigger-Pull Force Evaluation of Three Manual Tree-Marking Paint Guns
Hand-operated paint guns are routinely used in the U.S. Department of Agriculture (USDA) Forest Service for tree marking. The paint gun is designed to screw onto the top of a 1-quart can of tree-marking paint. Several brands of paint guns can be used with Forest Service tree-marking paint.
More than 170 USDA Forest Service ranger districts responded to a simple survey sent by San Dimas Technology and Development Center (SDTDC) requesting information on the types of paint guns used in the field and whether there have been any gun-related problems. Survey results revealed that the Nel-Spot (64%) is the most used manual paint gun. The Trecoder (25%) was second, followed by the Idico or Duz-All (3%). Powered guns comprise the remaining 8 percent. The most common problems associated with manual gun use were nozzle clogging (61%), hand/wrist pain (26%), poor cold-weather performance (11%), and hard-trigger pull (8%). See Tree Marking Tricks of the Trade Tech Tip (9724 1301—SDTDC) for additional information on the paint gun survey.
The three manual paint guns typically used by a paint crew are similarly designed. These guns are not powered and rely on the operator’s trigger squeeze to propel the paint. The Nelson has a short trigger for a two-finger pull; the other two models have a long trigger for a four-finger pull.
Figure 1—Trecoder, Nel-Spot, and Duz-All manual paint guns.
The objective of this study was to measure the pull force required to engage each of the three most frequently used manual paint guns in the USDA Forest Service. These guns are used to spray USDA Forest Service paint onto trees at shoulder level and at the base of the tree a few inches from the ground surface.
Figure 2—Chatillon DFGHS-R-100 digital-force gauge.
Measuring the actual trigger pull force on a manual gun is complicated because of the different trigger designs, rates of pull, and pressures required to propel the paint. The methods used in this test attempt to eliminate some of these variables. Each of the three guns (Trecoder, Nel-Spot, Duz-All) (figure 1) was tested mechanically for pull force with and without the paint can attached. A Chatillon DFGHS-R-100 digital-force gauge with a remote load-cell attached (figure 2) was used to measure the forces with and without the resistance of the paint (figure 3). The force gauge and the load cell were connected to a Pentium laptop computer through the RS-232 port. Data acquisition software, Data Stream, by Chatillon was utilized to collect the data both from the manual load-cell testing, as well as from the test stand.
Figure 3—Testing a Nel-Spot paint gun with remote-load cell while attached to paint can.
Problems and complaints about this human tool interaction are only partially explained by mechanical methods used to test the guns. The other part of the equation that must be examined is the human side. USDA Forest Service tree markers have complained about pain of the hands and wrists caused by what they feel is the excessive force required and the number of repetitions needed to accomplish the task. Whenever ergonomists determine the relationship of soft tissue complaints to a job, the standard equation (figure 4) is used.
Figure 4—Standard equation to determine cumulative trauma disorders.
How are the components of this equation determined? The methods described previously revealed the forces required to engage each gun; the repetitions can be calculated from data or viewed from video tapes taken in the field under actual working conditions. The postures are easily observed from the video or a simulation. The unknown that remains is what does the task/tool require of the muscles to elicit and maintain the postures?
Electromyography (EMG), a technique that allows measurement of muscle activity, was used to determine muscle activity in the muscles most important in the paint gun task; the wrist flexors, the wrist extensors, some hand muscles (interossei) and selected shoulder muscles.
EMG measures electrical activity in voluntary muscles in the same way electrocardiography measures electrical activity of heart muscle. EMG data were collected using a Noraxon surface EMG telemetry system (Noraxon™, Phoenix, AZ) with bipolar silver-silver chloride surface electrodes. Norquest software, also by Noraxon, was used to process the data. Surface EMG and associated signals were measured and processed in a belt-worn transmitter using technology that allows more artifacts free results. Telemetry allowed the data to be transmitted from the test subject via radio waves to a computer-connected receiver
Using this technology, the subject was able to move freely in a manner that resembled the movements of persons who use paint guns for the USDA Forest Service. One male subject whose maximum grip strength was 80.5 pounds performed the tests. The subject performed 9 to 11 trigger pulls at and below shoulder level using each of the three guns (figures 5 and 6).
Force Applied Without Paint Resistance
The Duz-All gun required less force when each gun was affixed to a ring stand and manual force was measured with a remote load cell on a specially adapted handle. The table below shows peak force by gun type without a paint can attached. The raw data for this test are contained in appendix 1.
Table 1—Manual trigger-pull force without paint.
Peak Force in Pounds
17 to 20
Force Applied at a Constant Rate
When the force is applied at a constant rate, the test results shown in table 2 confirm the previous readings of the trigger pull force without paint. The trigger-pull rate in this test was held constant to eliminate one of the variables. The Duz-All consistently registered the lowest trigger-pull force. The test data are included in appendix 2.
Table 2—Trigger-pull force at a constant pull rate without paint.
Peak Force in Pounds
12 to 17
15 to 17
The electromyographic tests of muscle activity corroborated the mechanical tests; the Duz-All required less force. EMG test results are shown in table 3. Variances from the general observation that the Duz-All required less trigger force are mostly explained by the muscle activity when the gun is pointed downward, to keep a protrusion on the back of the gun from digging into the hand. Duz-All’s requirement of less force at shoulder level gun engagement became somewhat of a liability when the subject lowered the gun, even though the forces remained generally lower than the forces with the other two guns. When the gun is lowered the “softer” spring mechanism does not provide sufficient counter force to the force of gravity on the can of paint. Due to the inadequate counterbalance, the subject found the small metal protrusion on the back of the Duz-All dug into the thumb web-space as the gun was lowered (dramatic on the video tape) to simulate marking at the base of the tree. This is not a difficult design problem to correct in light of the other attributes of the gun: light in weight, four-finger trigger design, wide trigger to allow wide force distribution. The Nel-Spot data are not included in the table because the gun malfunctioned and would not spray the paint.
Table 3—Muscle activity with gun use at and below shoulder level averaged peaks of activity in microvolts.
Force Applied With Paint Resistance
During these tests the Nel-Spot gun, which malfunctioned in the EMG test, was replaced. The trigger mechanism of the replacement Nel-Spot gun malfunctioned during this test with the paint can attached. The raw data for tests with the paint can attached are contained in appendix 3. These data clearly show the trigger malfunction. Peak force data with paint can attached and filled with USDA Forest Service Type III low-volatile, organic-compounds paint are shown in table 4.
Table 4—Manual trigger-pull force with USDA Forest Service Type III paint.
Peak Force in Pounds
40 to 45
28 to 38
The Duz-All requires less force than the Trecoder or the Nel-Spot based on the data from all of the tests listed and discussed. These three guns were tested using Type III paint. Approximately 20 percent increase in grip force should be added when gloves are used with any of the guns. This additional grip force is an accepted rule of thumb in ergonomics. The Duz-All is less stressful to the wrist and hand from force required. Intuitively, using the same guns with water-cleanup paint would require less pull force; however, the relative spread of values should be similar.
APPENDIX 1 -- Manual Application of Force to Triggers Without Paint Force Peak Data
APPENDIX 2 -- Test-Stand Application of Force to Triggers Without Paint Force Peak Data
APPENDIX 3 -- Manual Application of Force to Triggers With Type III Tree-Marking Paint Force Peak Data
Industrial Biomechanics, Inc., Greensboro, NC, conducted the test under contract with the USDA Forest Service. Industrial Biomechanics also provided a report of their testing and supporting data that was used in the preparation of this document.
A special thanks also goes to the following people. Without their contributions this report could not have been written.
Don Hunt, Business Unit Director, Force Measurement Systems, John Chatillon & Sons, Inc. for the test stand and assistance in the mechanical testing of the three guns.
Mary Bergeron, MS, L.P.T.; Jeff Mahoney, R.P.T.; Patrick Huff, Engineering Student; all from The Therapy Center, Knoxville, TN for the electromyography testing and reports.
Forest Management Program Leader
San Dimas Technology & Development Center
444 East Bonita Avenue, San Dimas CA 91773-3198
Phone 909-599-1267; TDD: 909-599-2357; FAX: 909-592-2309
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