*> v^-> v-»v • v^v v^y - v$?v *< 5r*°- /^t% ^*$teS /sM&S> '*ti&k- ° /<•&% / /\^,\ y^^X y^jSkX //^* °o 1°* . n* A «7» %«* v ■fr*^ ^1* '* •"• • r. ++* :/ V * / a5^ -1 %• * ^ v a5°«* * «? W - °o /-^^\ Aci&r% /^I%\ c^ife: °o /,^^\ .r>» . •< * ^o^ ^o^ «5°^ -3 V .»*^L'* <^ ^ *••"' /\. ^ A .<* *' *o^ *o.T* A <^V V'^V V^^\/ %?!si-^ \*^^\/ %'^^\o »°^K »«»' <$ « 4.^ o_ ^6* o * V *i*°* <^ /^VWVV; *bF "lottos?* * v +*k!l'* * •*? => ^ .earner* .♦ ^ y ^ ^ ^ •lii:-.^ >°.iite.>*. ^.:i-i:.X j?*.ja»t.^ ^ .:k^, %. ./.-^ic->. JT)§'L~7 BUREAU OF MINES INFORMATION CIRCULAR/1989 Development and Testing of a Pneumatic Scraper Blade for Conveyor Belt Cleaning By C. A. Rhoades, S. G. Grannes, and T. L. Hebble UNITED STATES DEPARTMENT OF THE INTERIOR Mission: As the Nation's principal conservation agency, the Department of the Interior has respon- sibility for most of our nationally-owned public lands and natural and cultural resources. This includes fostering wise use of our land and water resources, protecting our fish and wildlife, pre- serving the environmental and cultural values of our national parks and historical places, and pro- viding for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also promotes the goals of the Take Pride in America campaign by encouraging stewardship and citizen responsibil- ity for the public lands and promoting citizen par- ticipation in their care. The Department also has a major responsibility for American Indian reser- vation communities and for people who live in Island Territories under U.S. Administration. ft^J^^ . BMfy^^^^) Information Circular 9234 Development and Testing of a Pneumatic Scraper Blade for Conveyor Belt Cleaning By C. A. Rhoades, S. G. Grannes, and T. L. Hebble UNITED STATES DEPARTMENT OF THE INTERIOR Manuel Lujan, Jr., Secretary BUREAU OF MINES T S Ary, Director Library of Congress Cataloging in Publication Data: Rhoades, C. A. (Charles A.) Development and testing of a pneumatic scraper blade for conveyor belt cleaning. (Bureau of Mines information circular; 9234) Supt. of Docs, no.: I 28.27:9234. 1. Conveyor belts-Cleaning. 2. Blades-Testing. I. Grannes, S. G. (Steven G.) II. Hebble, T. L. (Terry L.) III. Title. IV. Series: Information circular (United States. Bureau of Mines); 9234. TN295.U4 [TN335] 622 s [622\66] 89-600122 CONTENTS Page Abstract 1 Introduction 2 Experimental procedure 3 Pneumatic cleaning blade mechanisms 12 Conclusions 12 ILLUSTRATIONS 1. Uneven edge wear on blade-type belt cleaners 2 2. Generic pneumatic blade design 3 3. Air distribution system 4 4. Manifold pressure versus time for initial pneumatic blade tests 5 5. Solid and pneumatic blade edge wear over time 5 6. Carryback versus manifold pressure 6 7. Carryback versus contact pressure 7 8. Pneumatic blade edge wear over time for notched blade 8 9. Manifold pressure versus time for notched blade 9 10. Edge wear over time for solid blade 10 11. Pneumatic and solid cleaner blade service life versus carryback and wear rate 10 12. Wear rate versus time 11 13. Dust level measurements 11 14. Blade wear mechanisms 12 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT cfm cubic foot per minute mil/yr mil per year ft foot fim micrometer g/ft 2 gram per square foot pet percent h hour psi pound per square inch in inch s second mg/m 3 milligram per cubic meter DEVELOPMENT AND TESTING OF A PNEUMATIC SCRAPER BLADE FOR CONVEYOR BELT CLEANING By C. A. Rhoades, 1 S. G. Grannes, 2 and T. L Hebble' ABSTRACT A major contributor to the problem of short life expectancy for blade-type conveyor belt cleaners is uneven wear along the blade edge. Uneven wear results in the formation of channels in the blade edge, which allow material to be carried back between the blade and the belt. In an effort to reduce the uneven wear problem, the U.S. Bureau of Mines has studied the mechanisms responsible for effective belt cleaning. From this study emerged a design for a cleaner blade that would greatly reduce uneven edge wear. The design consists of a standard cleaner blade incorporating air passages that allow for the expulsion of air along that part of the blade edge in contact with the conveyor belt surface. The results of 18-h tests indicated that the expulsion of air on the blade edge prevents scratches from developing into deep grooves. These tests showed that effective blade cleaning life can be extended 25 times using the pneumatic cleaning blade, compared with solid metal cleaning blades. Mining engineer. 2 General engineer. Metallurgist. Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN. INTRODUCTION Conveyors are used throughout the mining and mineral processing industry for transporting high volumes of material over relatively short distances. Conveyors offer several advantages, including continuous material handling, good transport energy efficiency, low labor requirements, and high material-handling capacities. However, several problems are inherent to conveyor transport. These include conveyor adjustment (i.e., belt tracking), system reliability, and system spillage. As part of its health and safety program, the U.S. Bureau of Mines has undertaken a basic study of mechanisms responsible for effective conveyor belt cleaning. Improved cleaner blade design can reduce exposure of mine personnel to dangerous situa- tions, such as explosion, respirable dust, and pinch-point- type hazards, resulting from spillage accumulation on or beneath troughed conveyor belts. The problem of system spillage was recently examined as part of a 3-year research effort to determine the mech- anisms involved in the operation of blade-type belt clean- ers. Significant conclusions drawn in this research were that all types of conveyor blade material tend to wear unevenly and that wear preferentially occurs in regions of the blade not in direct contact with the belt. 4 These Rhoades, C. A., T. L. Hebble, and S. G. Grannes. Basic Parameters of Conveyor Belt Cleaning. BuMines RI 9221, 1989, 19 pp. regions of wear are called wear channels. Wear channels occur on numerous scraper blade types, including poly- urethane, steel with ceramic inserts, tool steel, and mild steel (fig. 1). This research demonstrated that the blade wearout phenomena could be slowed by maint ainin g proper blade-belt pressures, by eliminating belt irregular- ities such as metal splices or recessed belt logos, and by increasing blade hardness. With these suggested improve- ments, blade cleaning life could be approximately doubled, but maximum effective blade life was still generally less than 1 day under laboratory conditions. The possibility of extending blade life even more led to the consideration of the benefits of fluid flow through the wear channels be- fore carryback becomes a problem. The fluid would exit through the minor wear channels until the surrounding blade topography matched the wear channel. Thus, the blade would be self-healing. Fluid flow would prevent particles from corrupting the blade's cleaning edge. Several pneumatic blades were designed and tested, with various combinations of slots or holes in the cleaning blade edge. Slot widths ranged from 0.016 to 0.250 in. The number of holes used in the edge ranged from 6 to 53, with diameters from 0.016 to 0.125 in. This report de- scribes the results of tests with the slotted pneumatic blades compared with solid metal cleaning blades, unless otherwise stated. Polyurethane Ceramic Insert Tool Steel Mild Steel Smi Figure 1. -Uneven edge wear on blade-type belt cleaners. Mild steel blade is 6 in long. EXPERIMENTAL PROCEDURE A series of experiments were designed to test the effec- tiveness of the pneumatic blade concept. The pneumatic blade consisted of a standard cleaner blade incorporating passages that allowed for the expulsion of water or air along that part of the blade edge in contact with the conveyor belt surface. These experiments were designed to evaluate optimum fluid flow rates and pressures and to develop a theoretical model for the pneumatic blade performance. For a description of the test conveyor belt and the conveyed test mixture, the reader is referred to the previous report. 5 The test procedures used caused highly accelerated blade and component wear. Preliminary Testing Preliminary tests were conducted using a mild (AISI- SAE Type 1045) steel blade with a pressurization slot cut into the blade surface. Water was selected as the fluid because of its low cost, good momentum effects, and lubricating properties. The results from these tests were favorable. The blade remained flat for 8 h of testing with no signs of uneven wear. This was a better result than had been achieved with any solid blade previously tested. However, water was determined to be impractical for this purpose because of serious handling problems in cold climates. For this reason, pressurized air was selected as the fluid for subsequent tests. Testing with air as the fluid was performed using blades similar to the water blade (fig. 2). Air was delivered to ^ork cited in footnote 4. Conveyor return belt ^CT~^ "Airflow Figure 2.-Generic pneumatic blade design. each pneumatic blade separately by plastic tubing con- nected to an air distribution system. The air distribution system consisted of a mass-flow meter, four rotameters, and four pressure-dampening reservoirs attached to the laboratory's 80-psi air line. The flow and pressure could be controlled on each blade. The system is shown in figure 3. Initial tests were conducted at high blade-belt pressures (16 psi) to assure air manifold sealing. Carryback of mate- rial was negligible for the duration of these tests, averag- ing 0.2 g/ft . Figure 4 shows the manifold pressure of the blade for the duration of the test. The initial large in- crease in pressure during the first half hour indicates the "wearing in" period, in which the blade conformed more closely to the belt, resulting in a tighter seal between the air chamber and the belt. The blade wear that did occur was relatively even and appeared to be the result of fine particles polishing the surface. Development of wear channels would have resulted in a pressure decrease. The pressure increase and stabilization over time showed the tendency of the blades to wear flat (conforming to the belt). Figure 5 compares the cleaning edge of the pneu- matic blade after 30 h of service with solid metal cleaner blades after 18 h on the test conveyor. All blades were made from AISI-SAE Type 1045 carbon steel. The solid blades were removed from service because the carryback had increased to over 5 g/ft 2 . The pneumatic blade carry- back was less than 0.6 g/ft 2 when the blade was removed from service. Although some smooth contours existed at the ends, the general shape was not consistent with chan- nel wear. After 30 h of run time in the preliminary test, no evi- dence of channel formation was apparent, negligible carry- back was observed, and manifold pressures were holding at a high level. It appeared as if the test would continue indefinitely until the slot depth was consumed by the flat polishing wear that was occurring. A subsequent test was designed to determine whether existing wear channels could be flattened by the pneumatic blade. In this test, artificial wear channels were machined across the blade edge. Five severe channels were machined to a depth of 0.010 in and a width of 0.125 in. A 1-h series of prelimi- nary setup tests were performed to determine airflow effectiveness limits and blade-belt contact pressure on cleaning. Figure 6 shows the effectiveness of air in reduc- ing carryback at constant blade-belt pressures of 35 and 21 psi. Notice that carryback was significant for both trials with no airflow but was reduced as airflow rates were in- creased. Figure 7 shows the effect of contact pressure on carryback. Carryback was essentially zero (immeasurable) for contact pressures of 20 to 25 psi. Long-Term Wear Testing Based on these tests, a long-term wear test to deter- mine the pneumatic blade's ability to correct the artificial wear channels was designed. The blade-belt pressure for this test was set at 20 psi, and the manifold pressure was set at 20 psi. Figure 8 shows the profile of blade 2 during the 21-h test. Figure 9 shows the manifold pressures over the test interval. Notice the gradual smoothing of the blade and the corresponding increase in manifold pres- sures. Figure 10 shows a typical solid cleaner blade with time. The troughs form and widen as time goes on. The solid blades were removed from service between 12 and 18 h because the carryback increased to over 5 g/ft 2 . The pneumatic blade did not develop the wear pattern of the solid blade. The air being forced through the blade pre- vented abrading and eroding particles from localized attack on the blade. This is shown in figure 8 by the even wear of the blade near the air slot. The comparison of the two blades in figures 8 and 10 shows the "self-healing" nature of the pneumatic blade by the gradual total surface wear and the elimination of the starting notches. The pneumatic blade experiment was stopped at 21 h with no indications of uneven wear. The carryback was less than 0.6 g/ft 2 . Because of flat wear, the service life of the pneumatic blade is significantly greater than that of conventional solid blades. Figure 11 contrasts the carryback amounts and wear rates for conventional solid blades and the pneumatic blade. Wear channels in the solid blade cause a significant carryback amount of 2 g/ft 2 after 5 h and 6 g/ft 2 in 24 h. These wear channels are illustrated in figure 10. The pneumatic blade carryback remained stable at 0.5 g/ft 2 - one-tenth the carryback of the solid blade at the conclu- sion of the test interval. Although the pneumatic blade has a 33-pct higher average wear rate, the wear pattern is flat (figs. 5, 8). This evidence suggests that the service life of the pneumatic blade is limited only by the air chamber depth and would be about 500 h under the accelerated test conditions. Since wear rates can be decreased by using harder steels, this service life could be further extended. The pneumatic blade wear rate is affected by the total airflow through the system and through each individual blade. The wear data in figure 12 show the decreasing wear rate for increased airflow. The airflow was not in- creased beyond 34 cfm because of the dust created and to prevent the lifting of the blade's cleaning edge away from the belt. A balance of airflow, surface treatment, and de- sign should optimize the cleaner blade for each individual conveyor and the material conveyed. Figure 3.-Air distribution system. Figure 4.-Manifold pressure versus time for initial pneumatic blade tests. b m Figure S.-Solid and pneumatic blade edge wear over time. A, Solid blade, 18 h; B, pneumatic blade, 30 h. KEY Contact pressure 21 psi 35 psi 5 10 15 MANIFOLD PRESSURE, psi 20 Figure 6.-Carryback versus manifold pressure. 0.4 *f o < CD > cr a: < o 8 12 16 20 CONTACT PRESSURE, psi Figure 7.-Carryback versus contact pressure. B Figure 8. -Pneumatic blade edge wear over time for notched blade. A, h; B, 6 h; C, 1 1 h; D, 21 h. Q. hi en z> (/) (/) LJ cr a. Figure 9.-Manifold pressure versus time for notched blade. 10 5 - V 4 - o < CD > cc rr < 3 - 2 - I - E LU < rr LU CC < 14 12 - k 10 - 8 - 4 - 2 - 1 B i 1 1 1 y^m - £ □ ■ 5 -/ ■ yb KEY /D ■ Pneumatic blade _ /a D Solid blade i i i I i 8 12 16 SERVICE TIME, h 20 24 Figure 10.-Edge wear over time for solid blade. A, 3 h; B, 5 h; C, 10 h; D, 15 h; E, 18 h. Figure 11. -Pneumatic and solid cleaner blade service life versus carryback and wear rate. 6 dust The Dust Generation One additional parameter noted during the testing was dust generation. The research conveyor was housed in a 50- by 75- by 15-ft building. A GCA RAM-l monitor was used to measure respirable dust levels, operation and performance of this instrument have been described in the literature. 7 The instrument can be oper- ated in three concentration ranges of to 2.0, to 20, and Reference to specific products does not imply endorsement by the U.S. Bureau of Mines. Lilienfeld, P. Improved Light Scattering Dust Monitor (contract HO377092, GCA Corp.). BuMines OFR 90-79, 1979, 48 pp.; NTIS PB 299-938. Williams, K. L., and R. J. Timko. Performance Evaluation of a Real-Time Aerosol Monitor. BuMines IC 8968, 1984, 20 pp. to 200 mg/m 3 , and in four measurement time constants of 0.5, 2, 8, and 32 s. The RAM-l was operated with a cyclone precollector for particle size selection, which permitted particles less than 10 /im in diameter. Calibra- tion was done with an Arizona road dust. The monitor was calibrated before and after each run. A time constant of 8 s and a range of to 20 mg/m 3 were used. The mon- itor was placed 10 ft in front of the head pulley and 10 ft above the floor. Figure 13 shows the respirable dust levels during a 3-h test run with a solid blade and three pneu- matic blades. The pneumatic blades had either six 0.125- in-diameter holes, fifty-two 0.016-in-diameter holes, or a 0.026-in continuous slot. The dust levels increased to a maximum of 2.5 mg/m 3 during the 3-h run. These levels are well below the maximum level of 5 mg/m 3 allowed by the U.S. Mine Safety and Health Administration and the U.S. Occupational Safety and Health Administration. 11 15 13 - £ II < cr cr. < LU 9 - 1 1 1 1 1 ▲ _A______- "*" ▲ - A A A A / KEY Airflow ▲ 27 cfm A 34 cfm - A-. — — 1 1 I 1 1 10 15 SERVICE TIME, h 20 25 30 Figure 12.-Wear rate versus time. ro E \ o> E _T UJ > UJ _i CO o £.a l I I KEY i i 2.0 Pneumatic blades: A 52 holes □ 6 holes A Slot i I 1. 5 ■ Solid blade / - I.O - .5 - i i i ■ i i 10 15 20 TOTAL AIRFLOW, cfm 25 30 Figure 13.-Dust level measurements. 12 PNEUMATIC CLEANING BLADE MECHANISMS Two mechanisms were found to contribute to uneven blade wear: particle wedging and viscous material flow. These mechanisms contribute to a blade-belt contact interface that could be described as being in a state of unstable equilibrium. The blade-belt interface will remain flat as long as no external disturbance occurs, such as a random particle gouge or a belt imperfection that causes a wear channel to begin. The concept of the pneumatic blade originated as a method of bringing the contact interface into a state of stable equilibrium. The idea was that a fluid emanating from the blade-belt interface would stabilize the contact interface by preventing preferential particle wedging and viscous material flow. The momen- tum and pressure of the fluid should preferentially restrict abrasive particles from entering possible wear channel growth areas. The fluid can be thought of as stabilizing the scraper blade cleaning process but not doing the actual cleaning. These concepts are illustrated in figures 2 and 14. If the pressure of the air within the manifold is kept within certain limits relative to the blade-to-belt contact pressure, air will be expelled along scratches, preventing further entrapment of abrasive particles. The parameters critical to the efficient operation of the pneumatic cleaner blade include (1) the blade-to-belt contact pressure, (2) the air pressure used in the blade, and (3) the shape and size of the air exit hole in the blade edge. The blade-to-belt contact pressure must be kept sig- nificantly above the critical pressure 8 to avoid intermittent loss of cleaning efficiency because of fluctuations in air pressure and irregularities in the drum and belt surface. If the contact pressure is near the critical pressure, fluctua- tions in pressure and belt and drum surface could be sufficient to allow enough separation of the blade from the belt to result in the loss of significant quantities of air. Conveyor return belt 5 o -n ■n 3 5-m <2. co c (0 o "5 o 3D O > o =? 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