Dynamic performanc eof self-controllinghydro-pick cutting rock

Dynamic performance of self-controlling hydro-pick cutting rockSongyong Liu a,b,Xiaohui Liu a,b,n,Weimin Cai a,b,Huifu Ji a,ba School of Mechatronic Engineering,China University of Mining and Technology,Xuzhou221116,Chinab Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment,China University of Mining and Technology,Xuzhou221116,Chinaa r t i c l e i n f oArticle history:Received25June2015Received in revised form25October2015Accepted1January2016Keywords:Self-controllingHydro-pickWater jetDynamic responseRock breakinga b s t r a c tA new self-controlling hydro-pick,which can control waterflow automatically,was designed based onhydrodynamic theory.Firstly,the balance of forces acting on the pick was shown to present the operatingprinciple of it,specifically,the force exerted by the water pressure acting over an annual area of the pick.A rock-breaking test bed was developed to study the frictional resistance,dynamic response,flow androck-breaking characteristics of the self-controlling hydro-pick.The results indicate that,the external andinternal diameter of the annual area on the pick should be40.6mm and40mm,respectively to ensurenormal shutdown of the self-controlling hydro-pick at a jet pressure40MPa.The opening times(timetaken to open the valve)increased with jet pressure and decreased with the cutting depth.For a jetpressure and cutting depth of40MPa and12mm,respectively,the opening time was0.035s.The waterflow of the self-controlling hydro-pick was less than that of the common hydro-pick when the diameterof the damping hole was0.5mm.The percentage waterflow reduction increased with the jet pressureand cutting depth.When the jet pressure and cutting depth are40MPa and12mm,respectively,thepercentage cutting force reduction was22.6%,the depth of the cutting slot increased by24.2%,and thevolume of rock breakage increased by29.3%.&2016Elsevier Ltd.All rights reserved.1.IntroductionConical picks are essential cutting tools that are widely used inengineering machinery.The pick that directly impacts rocks is in aharsh cutting environment.1–3During the excavation of hard rock,conical picks are subjected to high cutting forces,extensive wearand high temperatures,which result in a short life and low cuttingefficiency.4,5Many studies have been conducted,and the resultsshow that the cutting efficiency can be improved by changing thepick structure,although this does not solve the fundamentalproblems of extensive wear and high temperature.6–11In contrast,it has been shown that using a water jet in rock breaking couldeffectively assist the cutting tools and reduce the cutting force,wear,temperature and dust.12–17Thus,the performance of water jet was studiedfirstly in nu-merical and experimental.18–24It has been pointed out that thereare two main types of rock fragmentation via a water-jet,thecutting effect and extrusion effect.For cutting effect,the rock isinitially cut by the water jet,which results in a certain depth ofslotting and the formation of a free surface on the rock surface,followed by fragmentation via machine tools.This approach canimprove the rock fragmentation efficiency and reduce the cuttingforce of the machine tools.When the water jet acted via an ex-trusion effect,cracks are generated under the action of cutting viamachine tools,which extend and propagate via the water jet,leading to rock further breaking.Then,water jet technology has been widely used to assist toolsfor rock breaking,such as PDC cutters25–28and drilling tools.29Asexpected,it has been also applied in rock breaking by a conicalpick.The mechanism of rock breaking via a pick assisted by a high-pressure water jet was studied and found to be divided into threeprocesses:12(1)the hydraulic erosion process,when the water jeterodes the rock breaking zone;(2)the hydraulic fracturing pro-cess,when the expansion of micro cracks is accelerated by a waterjet;and(3)the pore pressure process,when a pressurized waterjet produces expansion,thus accelerating crack initiation andpropagation.At present,the arrangements of the water jet and pick are JCP(a water jet through the centre of the pick),JFP(a water jet placedin front of the pick),JSP(a water jet placed at the side of the pick)and JRP(a water jet placed behind the pick),as shown in Fig.1(a)–(d).30–32The JCP mode is proved the best,followed by the modesof JRP and JFP,and the worst mode is JSP.The decrease in the ratesof the pick force of the JCP,JRP,JFP,and JSP mode are up to30.96%,28.96%,28.17%,and25.42%,respectively,in experiments.For the JRP,the nozzle is easy to interfere with rock in thecutting process,thus a new typed JRP was designed,shown inContents lists available at ScienceDirectjournal homepage:/locate/ijrmmsInternational Journal ofRock Mechanics&Mining Sciences/10.1016/j.ijrmms.2016.01.0041365-1609/&2016Elsevier Ltd.All rightsreserved.n Corresponding author at:School of Mechatronic Engineering,China Universityof Mining and Technology,Xuzhou221116,China.E-mail address:TB13050011@(X.Liu).International Journal of Rock Mechanics&Mining Sciences83(2016)14–23Fig.1(e).33Experiments on JFP,N-JRP and the combination of those two water jet con figuration modes were conducted.The combined mode is shown in Fig.1(f).The results showed that,the N-JRP is less effective than the JFP on decreasing cutting force.The variance of JFP is smaller compared with the N-JRP.The combination of those two water jet con figuration modes is effective for rock breaking.It can both decrease the cutting force and low down the fluctuation.However,the effect of the combined water jet con-figuration is not that of the direct adding of JFP and N-JRP.Thus,the combined water jet should not be applied due to much waterand energy waste.The fruitful achievements of the previous studies provide a reference for this paper,but there still exists room for improve-ment.On the one hand,during the cutting progress,picks work in severe conditions.To avoid damage to nozzles by broken rocks,nozzles are installed far away from picks,resulting in long stand-off distances and high energy losses.Moreover,due to installation errors,the impact points of the pick and water jet may not be the same,and thus,water wedging cannot be fully effective.Thus,water jet should flow through the centre of the pick.On theotherFig.1.Arrangements of water jet andpick.Fig.2.Structure of the self-controlling hydro-pick and carbide tip.S.Liu et al./International Journal of Rock Mechanics &Mining Sciences 83(2016)14–2315hand,sustainedflow from the nozzle can lead to large amounts ofwasted water and energy,even producing inundations in thetunnelled surface.A new self-controlling hydro-pick,which canadjust the outletflow automatically,is designed and studied tosolve the above problems.2.Self-controlling hydro-pick2.1.Structural designThe structure of the self-controlling hydro-pick is shown inFig.2.The pick shank(2)is installed in the pick holder(3)and canmove axially.The valve sleeve(5)is installed in the pick holder tocontrol the axial displacement of the pick shank.There is an an-nual area(6)on the pick shank.An O-ring(7)is installed in thesealing groove.Centreflow channel(9)is established in the pickshank,carbide tip and pick holder.The pick shank is equippedwith a damping hole(11)to reduce the pressure.The nozzle(8)and the centreflow channel(9)are set in thecarbide tip and the pick shank along the axial direction.The radialthrough-hole(10)and damping hole(11)are set at the root of thepick shank.The inlet hole(12)and the leaking hole are set on thepick-holder.The shape of the carbide tip is similar to that of acommon one,and the inside is shaped as a cylinder nozzle,asshown in Fig.2.The shrink angleδ1is13°,the diffusion angleδ2is 90°,the diameter of the nozzle inlet D is6.4mm and the diameterof the outlet d is0.6mm.The carbide tip was processed usingmould shaping.With powder metallurgy technology applied,asteel wire same to the nozzle in diameter was put in the centre ofthe pick carbide before its shaping.Then,the nozzle was formedby drawing the steel wire out after the shaping of pick carbide.The flow channel was grinded repeatedly to avoid disturbing the streamlines,pressurefluctuations and cavitations formed by the impact of a high pressure water jet.2.2.Operating principle of self-controlling hydro-pickThe opening and closing of a high-pressure water jet is con-trolled by the relative motion of the pick shank and pick-holder. There are two moving states for the pick shank.The opening and closing time,the key performance parameters of the self-con-trolling hydro-pick,are time taken to open and close the valve, respectively.The working principle of the self-controlling hydro-pick is shown in Fig.3.When cutting rock,the pick bears cutting re-sistances F a along the axis of the pick,frictional resistance F f1and annual area force F2on the annual area(part6in Fig.2or annual area in Fig.3),as shown in Fig.3(a).When the cutting resistance is greater than the resultant force of F f2and F2,the pick shank moves back.As a result,a high-pressure channel is opened,which is shown in Fig.2(a).Because the inner cavity is always full of water, theflow and impact kinetic energy increase rapidly.When the rock is broken,there is no cutting resistance F a,and the pick moves forward when F2is greater than F f2.As a result,the high-pressure channel closes,as shown in Fig.2(b).High-pressure water is then released as lower-pressure water through the damping hole to reduce waste.At the same time,the inner cavity of the pick is full of water,which can prevent dust from entering into the pick and blocking the nozzle.3.Experiments on self-controlling hydro-pickThe test self-controlling schematic is shown in Fig.4,which consists of a cutting system,a hydraulic system,an electrical control system,high-pressure water system and a test system.A high-pressure plunger pump was used to provide high-pressure water with a maximum pressure of80MPa and a maximumflow rate12L/min.A speed control valve was used to control the re-ciprocating speed of the self-controlling hydro-pick.A pushing cylinder was integrated to linearly reciprocate the pick.The oil pressure and water pressure were measured by a pressure trans-ducer and recorded using an NI data acquisition card.3.1.Frictional resistance characteristicsThe self-controlling hydro-pick suffers internal frictional re-sistance while working.Thus,it is necessary to study its frictional resistance characteristics to improve its structural design.For that forces loaded on the pick is complex on the cutting condition,the resultant force is not completely along the axis of the pick.Thus, the radial of pick also bears great force,and the opening frictional resistance F f1is not easy to test.While there is no load on the radial of pick in the unload condition,it is easy to test the closing frictional resistance F f2.In this section,it mainly presents the method to test the closing frictional resistance F f2.The test sche-matic to evaluate the frictional resistance characteristics ofself-Fig.3.Working principle of self-controlling hydro-pick.S.Liu et al./International Journal of Rock Mechanics&Mining Sciences83(2016)14–2316controlling hydro-pick is shown in Fig.5.The self-controlling hydro-pick was installed on the test bench,and high-pressure water was pumped in ensure that the pick was in the unloaded condition.The tensile-compressive force sensor was fixed on the front plate of the test bench.The water pressure from the water pump was set to 0MPa,10MPa,20MPa,30MPa,and 40MPa.The advancing speed of the pick was set to 0.005m/s by the speed control valve and veri fied by the displacement sensor.At a water jet pressure of 40MPa,the force variation measured by the tension-compression sensor is shown in Fig.6As shown in Fig.6,the force was initially nearly 0,which indicates that the pick was not in contact with the sensor from 0to 4.5s,while at 4.5–4.6s,the force increased greatly to a local peak corresponding to the maximum static friction.At this time,static frictional re-sistance formed between the pick shank and pick holder,but the force measured by the sensor was less than the static frictional resistance.From 4.6to 5.3s,the force remained stable after a slight fall,which indicates that stable dynamic frictional resistance occurred between the pick shank and pick holder and that the dynamic value was smaller than the static value.Finally,the force increased gradually and remained stable,indicating that the self-controlling hydro-pick opened completely and the pick shank re-mained relatively motionless with respect to the pick holder.Ignoring the dynamics of the pick,a force balance analysis ofthe pick was conducted,as shown in Eq.(1).Eq.(2)is used to calculate the force acting on the annual area.=+()F F F 1a f 22Fig.4.Rock breaking test bench for self-controllinghydro-pick.Fig.5.Test schematic for frictionalresistance.Fig.6.Force variation from tension-compression force sensor.S.Liu et al./International Journal of Rock Mechanics &Mining Sciences 83(2016)14–2317π=(−)()F pD d 42222where F f 2is the closing frictional resistance (N),F a is the force variation from the tension-compression sensor (N),F 2is the force acting on the annual area (N),p is the water jet pressure (MPa),D is the external diameter of the annual area (mm),and d is the internal diameter of the annual area (mm).The forces on the self-controlling hydro-pick under different water pressures are shown in Table 1.It can be seen that the frictional resistance increased with water jet pressure.During the closing process of the self-controlling hydro-pick,the pick was subjected to the annual area force F 2and closing frictional resistance F f 2.To ensure normal shutdown of the high pressure channel for the unloaded condition,the relation between F 2and F f 2should be satis fied as follows:>()F F 3f 22The internal diameter of the annual area was set to 40mm,and the external diameter was varied to change the load acting on the annual area.The annual area force and frictional resistance during the closing process varied with the external diameter under dif-ferent water jet pressures,as shown in Fig.7.The annual area force was calculated according to the Eq.(2),and the frictional re-sistance is according to Table 1.In Fig.7,the annual area force shows a linear relationship with the larger diameter.Additionally,the higher the water jet pressure is,the more rapidly the annual area force increased with the diameter.When the force F 2is greater than the closing frictional resistance F f 2,the high pressure channel can shut down normally.As the load on the pick is complex in reality,the annual area should be designed to ensure normal opening and shutdown.The annual area parameters under different water jet pressures are shown in Table 2,where it can be seen that the external dia-meter increases with the water jet pressure,while the internal one remains unchanged.When the water jet pressure is 10MPa,the optimum external diameter is 40.6mm to ensure normalshutdown and rapid opening of self-controlling hydro-pick.3.2.Dynamic response performanceAs the rotating cycle of the cutting head is short,the self-controlling hydro-pick should have a rapid dynamic response to ensure its effectiveness.Dynamic response performance experi-ments were conducted on a test bed,and the test principle is shown in Fig.8(a)and (b).An acceleration sensor was fixed to the bottom of the pick,and a cylindrical hole was processed into the bottom of the pick holder.The length of the cylindrical hole was 0.5mm smaller than that of the acceleration sensor.When the self-controlling hydro-pick cutting rock,the oil pressure increased,the pick moved along the x-direction,and time was recorded as t 1.When the relative dis-placement of the pick and pick holder reached 3mm,the accel-eration sensor contacted the pick holder,and the time was re-corded as t 2.The opening time was de fined as t ¼t 2Àt 1.The external and internal diameters for optimum design were 40.6mm and 40mm,respectively,to ensure normal shutdown and rapid opening of the hydro-pick.The in fluences of cutting speed,water jet pressure and cutting depth on the dynamic re-sponse performance were studied.The cutting speed was set to 0.05m/s,0.1m/s,0.15m/s,0.2m/s,0.25m/s and 0.3m/s.The water jet pressure was set to 20MPa and 40MPa.The cutting depth was set to 3mm,6mm,9mm and 12mm.All of the listed cutting speed and cutting depths were tested at each of the jet pressures.When the cutting depth and speed were 3mm and 0.3m/s,respectively,the dynamic response was as shown in Fig.8(c)and (d).From Fig.8(c),the oil pressure and the pick acceleration were nearly 0,indicating that the pick was in the unloaded condition from 0to 1.967s;At 1.967–2.159s,the oil pressure increased gradually,but the acceleration remained near 0.The pick began to contact the rock and move backwards relative to the pick holder,and the valve port began to open gradually during the opening process.From 2.159to 3.223s,the oil pressure fluctuated stably,while the acceleration increased rapidly at first and then stabilized.While cutting the rock,the pick remained in the opening process.The acceleration sensor maintained contact with the bottom of the pick holder,and the fluctuations re flect the breakage of the rock.After 3.223s,the oil pressure and pick acceleration decreased ra-pidly,signifying that the pick was in the unloaded condition,and the valve port closed while the pick moved forward relative to the pick-holder,pressed by high pressure water.Similarly,from Fig.8(d),the dynamic response of self-control-ling hydro-pick includes three processes,opening process,cutting process and unload process.However,the opening time of 40MPa water jet is longer than the one of 20MPa water jet.Moreover,the maximum speed set in the hydraulic system of the test bed was 0.3m/s,less than the real cutting speed of the pick,which made it challenging to test the dynamic response under real conditions.Thus,it was necessary to study the relation between the dynamic response performance and cutting speed.Table 1Force of self-controlling hydro-pick under different water pressures.p /MPa F 2(N)F a (N)F f 2(N)001541541063610163802012721903631301909285494540254538811336Fig.7.Annual area force and frictional resistance during closing process.Table 2Design parameters of annual area.Water jet pressure (MPa)External diameter of an-nual area (mm)Internal diameter of annual area (mm)1040.60402040.50403040.50404040.5340S.Liu et al./International Journal of Rock Mechanics &Mining Sciences 83(2016)14–2318The dynamic response under real conditions was predicted ac-cording to experiments.The relationship between the opening time and the cutting velocity (cutting depth 3mm)is shown in Fig.9(a).When the cutting velocity was less than 0.2m/s,the opening time decreased with the cutting velocity,and when the cutting velocity was larger than 0.2m/s,the opening time remained stable.Therefore,it would be reasonable to use 0.3m/s to simulate a real situation.The relationship between opening time and cut-ting depth (cutting velocity 0.3m/s)is shown in Fig.9(b).It can be seen that the opening time decreases linearly with the cutting depth.Therefore,the self-controlling hydro-pick is more suitable for cutting rock to larger cutting depths.Moreover,the opening time increased with the water jet pressure,and the smaller the cutting depth is,the greater is the in fluence of water jet pressure on opening time.When the cutting depth is 3mm,the opening time for a water pressure of 40MPa is two times that at 20MPa.When cutting depth and pressure are 12mm and 40MPa,re-spectively,the opening time is 0.035s.Thus,the water jet should be set according to the cutting depth for rapid dynamic response.3.3.Flow performanceTo demonstrate that the self-controlling hydro-pick can pro-duce high-pressure flow during cutting and low-pressure flow when unloaded,a flow performance experiment was conducted.Two kinds of picks were tested,including a common hydro-pick and a self-controlling one.The common hydro-pick is a standard pick with a centre flow channel,through which is not self-controlling so that water flows all the time.The damping hole of the self-controlling hydro-pick was 0.5mm.The common hy-dro-pick and the self-controlling one had the same inlet and outlet nozzle diameters to ensure the same flow during the opening process and different flows in the closing process.The test schematic for the flow performance experiments is shown in Fig.10(a).The common hydro-pick and the self-con-trolling one were axially fixed into their respective pick holders.Water at 10MPa,20MPa,30MPa,and 40MPa was pumped into each pick.A water storage tank was used to collect water from the picks after achieving stable flow.An electronic scale was used to weigh the water in the storage tank after 10min.The flow from the common hydro-pick and self-controlling hydro-pick in the unload condition is shown in Fig.10(b).The flows in the unload condition increased with the water jet pressure,and the trends for both picks were the same.It can be seen that the flow of the self-controlling hydro-pick was much less than that of the common one.Therefore,the self-controlling hy-dro-pick meets the design requirements and operates normally.The flow test above was in the unload condition.However,the pick impacts the rock during half the time in realcuttingFig.8.Test principle of dynamic response characteristics and dynamic responses of self-controlling hydro-pick.S.Liu et al./International Journal of Rock Mechanics &Mining Sciences 83(2016)14–2319processes.To further consider the in fluence of the closing and opening processes on the flow performance,a percentage flow reduction η1was introduced ,which can be calculated byη=−(−+)+(−+)()⎡⎣⎤⎦q q t t q t t t q /4t t 1121212210100where q 1is the flow of common hydro-pick (L/min),q is the flowof the self-controlling hydro-pick in the unload condition (L/min),t 1is the opening time (s),t 2is the closing time (s),and t 0is the cutting cycle time (s).The percentage flow reduction of the self-controlling hydro-pick for different water jet pressures and cutting depths is shown in Fig.10(c).As seen in Fig.10(c),the percentage flow reduction of the self-controlling hydro-pick was approximately 30%.Under the same water jet pressure,the percentage increased with the cutting depth.The increases were small for pressures of 10MPa and 20MPa,while they were greater for 30MPa and 40MPa,as a re-sult of the close relationship between the dynamic response and the cutting depth.The higher the water jet pressure is,the greater is the in fluence of cutting depth on the closing and opening times.Under the same cutting depth,the percentage flow reduction decreased with the water jet pressure;the larger the cutting depth is,the less the decrease is.However,the in fluence of the cutting depth on the percentage flow reduction was not large (within 5%),as the closing and opening times were shorter than the cutting cycle time.Thus,the in fluence of the cutting depth could beignored if the dynamic response of the self-controlling hydro-pick was suf ficient.3.4.Rock breaking performanceRock breaking experiments for the common pick and the self-controlling hydro-pick were conducted on the rock breaking test bench (Fig.4).The common pick is a standard pick with no water jet assistance.The advancing speed of the pick was set to 0.25m/s by a speed control valve and veri fied by a displacement sensor.TheFig.9.Relationship between opening time and cuttingparameters.Fig.10.Test principle and results of flow characteristics.S.Liu et al./International Journal of Rock Mechanics &Mining Sciences 83(2016)14–2320water jet pressure was set to 10MPa,20MPa,30MPa and 40MPa.The cutting depth was set to 3mm,6mm,9mm and 12mm.All of the listed cutting depths were tested at each of the jet pressures.The rock is arti ficial,con figured by cement,sand and gypsum.Its compression strength is 15.5MPa,its density is 2456kg/m 3,and its size is 600Â500Â200mm 3.During the experiments,the rock was placed on the propulsion unit and the cutting depth could be increased by placing a steel sheet under the rock sample.The rock could be fixed by the clamping cylinder to ensure the stability of the cutting process.After cutting,the pick was moved backwards and the rock was released by retracting the clampingcylinder.Then,the propulsion unit moved the rock sample to a new position for the next cutting.The cutting resistances of the common pick and the self-con-trolling hydro-pick (cutting depth 12mm and speed 0.25m/s)are shown in Fig.11.The force fluctuations of the common pick were greater than those of the self-controlling hydro-pick.Moreover,the fluctuations became smaller and steadier with increasing water jet pressure,which could reduce the wear and increase the life of the pick.It can be concluded through analysis that the rock broke under the common pick mainly from crack propagation and energy ac-cumulation and that this process took a long time.In contrast,by cutting with the self-controlling hydro-pick,the water jet en-hanced crack propagation,swept away the rock fragments im-mediately,and decreased the cutting force,and it took less time for the rock to break.Moreover,the water jet was more effective at higher pressures.However,bene fits of jet assistance are much lower than re-ported by other researchers.There are mainly two reasons.On one hand,the valve in self-controlling hydro-pick will not open only if the cutting force is great enough,thus,there will not be bene fits of jet assistance when cutting force is small.On the other hand,ac-cording to the research above,the higher the water jet pressure,the longer the opening time,then,the less effective the jet assis-tance.Thus,there will not be obvious bene fits of jet assistance with the increase of water jet pressure.The forces of the common pick and the self-controlling hydro-pick (cutting depth 12mm and speed 0.25m/s)were analysed statistically,as shown in Fig.12.The common pick operated under a water pressure of 0MPa,while the self-controlling hydro-pick operated under pressures of 10MPa,20MPa,30MPa and 40MPa.It can be seen that the average,maximum,peak average and the standard deviation of the cutting resistance decreases with the water jet pressure,and the downtrend is almost the same.To analyse the rock breaking performance,a water jet-assisted percentage statistics force reduction η2was introduced.η=−()F F F 5p z p2where F p is the force of common pick (kN)and F z is the force of the self-controlling hydro-pick (kN).Then,the percentage statistics force reduction of the self-con-trolling hydro-pick are calculated according to the data in Fig.12,shown in Fig.13.The percentage reduction of average,maximum,peak average and the standard deviation of the cutting resistance with water pressure were calculated.The degree of reduction was small from 20to 30MPa and larger from 0to 20MPa and from 30to 40MPa.It can be expected that there exists a critical pressure and an optimum crushing effect near this critical pressure.The average force reduction rate was 22.6%at a water jet pressure of 40MPa and a cutting depth of 12mm.Moreover,it should be noted that the standard deviation of the results is much more than the mean at different water jet pressure.It can be concluded that,for self-controlling hydro-pick,it is easier to reduce fluctuationofFig.11.Force of common pick and self-controllinghydro-pick.Fig.12.Force statistics for common pick and self-controlling hydro-pick.S.Liu et al./International Journal of Rock Mechanics &Mining Sciences 83(2016)14–2321。