trigger pull force - us forest service · trigger pull force evaluation of three ... guns used in...

Trigger Pull ForceEvaluation of Three

Manual TreeMarking Paint

Guns

United StatesDepartment ofAgriculture

Forest Service

Technology &DevelopmentProgram

2400—Forest ManagementApril 19999924 1202—SDTDC

FOREST SERVICE

DE P A RTMENT OF AGRICULT U RE

Trigger Pull Force Evaluation 29

Trigger Pull ForceEvaluation of

Three ManualTree Marking

Paint Guns

by

Ralph Gonzales—Mechanical Engineer

San Dimas Technology & Development Center

April 1999

Information contained in this document has been developed forthe guidance of employees of the Forest Service, USDA, itscontractors, and cooperating Federal and State agencies, TheDepartment of Agriculture assumes no responsibility for theinterpretation or use of this information by other than its ownemployees. The use of trade, firm, or corporation names is for theinformation and convenience of the reader. Such use does notconstitute an official evaluation, conclusion, recommendation,endorsement, or approval of any product or service to the exclusionof others that may be suitable.

The US Department of Agriculture (USDA) prohibits discriminationin all its programs and activities on the basis of race, color, nationalorigin, gender, religion, age, disability, political beliefs, sexualorientation, and marital or family status. (Not all prohibited basesapply to all programs.) Persons with disabilities who requirealternative means for communication of program information (Braille,large print, audiotape, etc.) should contact USDA’s TARGET Centerat 202-720-2600 (voice and TDD).

To file a complaint of discrimination, wrote USDA, Director, Officeof Civil Rights, Room 326-W, Whitten Building, 14th andIndependence Avenue, SW, Washington, DC 20250-9410 or call202-720-5964 (voice or TDD). USDA is an equal opportunity providerand employer.

28 Trigger Pull Force Evaluation

ACKNOWLEDGEMENTS

Industrial Biomechanics, Inc., Greensboro, NC, conductedthe test under contract with the USDA Forest Service. In-dustrial Biomechanics also provided a report of their test-ing and supporting data that was used in the preparation ofthis document.

A special thanks also goes to the following people. Withouttheir contributions this report could not have been written.

Don Hunt Business Unit Director, Force Measurement Sys-tems, John Chatillon & Sons, Inc. for the test stand andassistance in the mechanical testing of the three guns.

Mary Bergeron, MS, L.P.T.; Jeff Mahoney, R.P.T.; PatrickHuff, Engineering Student; all from The Therapy Center,Knoxville, TN for the electromyography testing and reports.


BACKGROUNDHand operated paint guns are routinely used in the ForestService for tree marking. The paint gun is designed to screwonto the top of a one-quart can of tree marking paint. Severalbrands of paint guns can be used with Forest Service treemarking paint.

More than 170 Forest Service ranger districts responded toa simple survey sent by San Dimas Technology andDevelopment Center asking for information on the types ofguns used in the field and if there have been any paint gunrelated problems. The results of the survey revealed thatthe Nel-Spot (64%) is the most used manual paint gun. TheTrecoder (25%) was second, followed by the Idico or Duz-All (3%). Powered guns comprise the remaining eight-percent. The most common problems associated withmanual gun use were nozzle clogging (61%), hand/wristpain (26%), poor cold weather performance (11%), and hardtrigger pull (8%). Additional information on the paint gunsurvey can be obtained from the Tree Marking Tricks of theTrade Tech Tip (9724 1301—SDTDC).

The three manual paint guns typically used by a paint creware similarly designed. These guns are not powered andrely on the operator’s trigger squeeze to propel the paint.The Nelson has a short trigger for a two-finger pull; theother two models have a long trigger for a four-finger pull.

OBJECTIVEThe objective of this study was to measure the pull forcerequired to engage each of the three most frequently usedmanual paint guns in the Forest Service. These guns areused to spray USDA Forest Service paint onto trees atshoulder level and at the base of the tree a few inches fromthe ground surface.

METHODSPaint Gun TestingMeasuring the actual trigger pull force on a manual gun iscomplicated because of the different trigger designs, ratesof pull, and pressures required to propel the paint. Themethods used in this test attempt to eliminate some of thesevariables. Each of the three guns (Trecoder, Nel-Spot, Duz-All) shown in figure 1 was tested mechanically for pull forcewith and without the paint can attached. A ChatillonDFGHS-R-100 digital force gauge with a remote load cellattached, figure 2, was used to measure the forces with andwithout the resistance of the paint, figure 3. The force gaugeand the load cell were connected to a Pentium laptopcomputer through the RS-232 port. Data acquisitionsoftware, Data Stream, by Chatillon was utilized to collectthe data both from the manual load cell testing, as well asthe test stand.

Figure 1—Trecoder, Nel-Spot, and Duz-All manual paint guns.

Figure 2—Chatillon DFGHS-R-100 digital force gauge.


Figure 3—Testing a Nel-Spot paint gun with remote load cellwhile attached to paint can.

Methods PerspectiveProblems and complaints about this human tool interactionare only partially explained by mechanical methods usedto test the guns. The other part of the equation that must beexamined is the human side. Forest Service tree markershave complained about pain of the hands and wrists causedby what they feel is the excessive force required and thenumber of repetitions needed to accomplish the task.Whenever ergonomists determine the relationship of softtissue complaints to a job the standard equation, as shownin figure 4, is used.

Figure 4—Standard equation to determine cumulativetrauma disorders.

How are the components of this equation determined? Themethods described previously revealed the forces requiredto engage each gun; the repetitions can be calculated fromdata or viewed from video taken in the field under actualworking conditions. The postures are easily observed fromthe video or a simulation. The unknown that remains is whatdoes the task/tool require of the muscles to elicit andmaintain the postures?

Electromyography (EMG), a technique that allowsmeasurement of muscle activity, was used to determinemuscle activity in the muscles most important in the paintgun task; the wrist flexors, the wrist extensors, some handmuscles (interossei) and selected shoulder muscles.

Figure 5

EMG measures electrical activity in voluntary muscles inthe same way electrocardiography measures electricalactivity of heart muscle. EMG data were collected using aNoraxon surface EMG telemetry system (Noraxon™,Phoenix, AZ) with bipolar silver-silver chloride surfaceelectrodes. Norquest software, also by Noraxon, was usedto process the data. Surface EMG and associated signalswere measured and processed in a belt-worn transmitterusing technology that allows more artifacts free results.Telem-etry allowed the data to be transmitted from the testsubject via radio waves to a computer-connected receiver.

Figure 6


Table 2—Trigger pull force at a constant pull ratewithout paint.

Gun Type Peak Force in Pounds (N)

Nel-Spot 12-17 (53-76)

Trecoder 15-17 (67-76)Duz-All* 8 (36)

ElectromyographyThe electromyographic tests of muscle activity corroboratedthe mechanical tests; the Duz-All required less force. EMGtest results are shown in table 3. Variances from the generalobservation that the Duz-All required less trigger force aremostly explained by the muscle activity when the gun ispointed downward, to keep a protrusion on the back of thegun from digging into the hand. Duz-All’s requirement ofless force at shoulder level gun engagement becamesomewhat of a liability when the subject lowered the gun,even though the forces remained generally lower than theforces with the other two guns. When the gun is loweredthe “softer” spring mechanism does not provide sufficientcounter force to the force of gravity on the can of paint.Due to the inadequate counterbalance, the subject foundthe small metal protrusion on the back of the Duz-All duginto the thumb web-space as the gun was lowered (dramaticon the video) to simulate marking at the base of the tree.This is not a difficult design problem to correct in light ofthe other attributes of the gun: light in weight, four-fingertrigger design, wide trigger to allow wide force distribution.The Nel-Spot data are not included in the table because thegun malfunctioned and would not spray the paint.

Using this technology, the subject was able to move freelyin a manner that resembled the movements of persons whouse paint guns for the Forest Service. One male subjectwhose maximum grip strength was 80.5 pounds (358 N)performed the tests. The subject performed 9-11 trigger pullsat both shoulder and below shoulder level using each of thethree guns. See figures 5 and 6.

RESULTSForce Applied Without Paint ResistanceThe Duz-All gun required less force when each gun wasaffixed to a ring stand and manual force was measured witha remote load cell on a specially adapted handle. The tablebelow shows peak force by gun type without a paint canattached. The raw data for this test are contained in appendix 1.

Table 1—Manual trigger pull force without paint.


Nel-Spot 17-20 (76-89)Trecoder 18 (80)

Duz-All* < 10 (<44)

Force Applied at a Constant RateWhen the force is applied at a constant rate, the test resultsshown in table 2 confirm the previous readings of the triggerpull force without paint. The trigger pull rate in this testwas held constant to eliminate one of the variables. TheDuz-All consistently registered the lowest trigger pull force.The test data are included in appendix 2.


Table 3—Muscle activity with gun use at shoulder and below shoulder level averaged peaks of activity in microvolts.

BelowAt Shoulder Shoulder

Level Duz-All* Trecoder Level Duz-All* Trecoder

Wrist 461 550 Wrist 410 547

Extensors ExtensorsWrist 435 777 Wrist 588 634

Flexors FlexorsAnt. Deltoid 402 305 Ant. Deltoid 175 390

Triceps 59 62 Triceps 73 71Upper 94 23 Upper 71 155

Trapezius TrapeziusLower 41 36 Lower 31 41

Trapezius TrapeziusPectoralis 272 211 Pectoralis 175 316

Interossei 235 444 Interossei 235 403

Force Applied With Paint ResistanceDuring these tests the Nel-Spot gun which malfunctionedin the EMG test was replaced. The trigger mechanism ofthe replacement Nel-Spot gun malfunctioned during thistest with the paint can attached. The raw data for tests withthe paint can attached are contained in appendix 3. Thesedata clearly show the trigger malfunction. Peak force datawith paint can attached and filled with Forest ServiceType III low volatile organic compounds paint are shownin table 4.

Table 4—Manual trigger pull force with Forest ServiceType III paint.


Nel-Spot 40-45 (178-200)Trecoder 28-38 (125-169)

Duz-All* 15 (67)

CONCLUSIONThe Duz-All requires less force than the Trecoder or theNel-Spot based on the data from all of the tests listed anddiscussed. These three guns were tested using Type III paint.Additionally, approximately 20 percent increase in gripforce should be added when gloves are used with any of theguns. This additional grip force is an accepted rule of thumbin ergonomics. The Duz-All is less stressful to the wristand hand from force required. Using the same guns withwater clean-up paint would intuitively require less pull force;however, the relative spread of values should be similar.


APPENDIX 1

Manual Application of Forceto Triggers Without Paint

Force Peak Data


APPENDIX 2

Test Stand Application ofForce to Triggers Without

Paint Force Peak Data


APPENDIX 3

Manual Application of Forceto Triggers With Type III

Tree Marking PaintForce Peak Data

trigger pull force - us forest service · trigger pull force evaluation of three ... guns used in...

Documents