Design and Calibration of Epicyclic Gear Transmission Dynamometer for Rural use

Transcription

1 Journal of Experimental Research. March 203, Vol No edi Received: Dec. 5, 202 Design and Calibration of Epicyclic Gear Transmission Dynamometer for Rural use Adisa A.F. Agricultural Engineering Department, University of Agriculture, Abeokuta, Ogun State, Nigeria. Accepted for publication Mar. 7, 203 Abstract Developing crop processing machinery for rural village use where less sophistication for both manufacturing and testing is required, simple and cheaper means of power demand assessment device is needed. A simple and locally produced epicyclic gear transmission laboratory dynamometer was developed to measure power demand of a rice dehusking machine for village use which was to be powered manually by pedalling. The instrument needs to be calibrated which must be consisted and systematic in the operating environment over reasonable period of time. Keywords: Calibration, Dynamometer, Demand, Epicyclic gear and Rural use. INTRODUCTION A dynamometer is essentially a device for measuring force, moment of force (torque), or power which tends to change the state of rest or of uniform motion of a body. Broadly, there are two main types which may be distinguished namely absorption dynamometers and transmission dynamometers. The power produced by an engine, motor or other rotating prime mover can be calculated by simultaneously measuring torque and rotational speed as reported by Elisha et al (200). Dynamometer can as well be used to determine the torque and power required to operate a driven machine like pump or stationary machine which is called motoring or driving dynamometer while the one that is driven is called absorption or passive dynamometer. The dynamometer that can either drive or absorb is called a universal or active dynamometer, Elisa et al (200). All monitoring instruments must be calibrated before they are used to measure environmental samples, Porfert and Sotolongo (200). Instrument or equipment calibration is to carry out measurement of some physical variables like force, temperature etc where its magnitude or strength cannot be detected or quantified directly but can be done through their effect(s) only. 'These effects, which can be measured by mechanical force gauges and digital force gauges, called signals, often need proper conditioning for easy, accurate and reliable detection and measurement. Studies have shown that many instrument application errors are due to improper calibration for example, centrifugal pump performance is very sensitive to speed and inlet pressure variations which may produce uneven pump output under some operating conditions, Hofman and Solseng (2004). When integrity of instrument calibration is compromised in a system, the links between scientific theory, engineering practice and mass production that measurement provides can be missing from the start on new work or eventually lost on old work, Wikipedia (20). Manarp oscillograph strain gauge can be used to measure the power supplied by pedalling, the torque input is non uniform. The non uniform power supply usually causes vibration and fluctuation in torquemeter reading, Adisa (987). In a power demand assessment study of a developed prototype rice dehusking machine for rural use, there was need to overcome problems that was being experienced in using old existing power measuring device. This required local production of simple and cheap means of power measurement. A new laboratory dynamometer was to be designed, constructed and calibrated by using a bicycle epicyclic gear transmission 6

2 Calibration process begins with the design of the measuring instrument that needs to be calibrated which must be able to hold a calibration through its measuring interval within engineering tolerance in stated environmental conditions over reasonable period of time as reported by Wikipedia (20) which was the aim of this study. MATERIALS AND METHODS adjustment of torquemeter to give the chains proper tensioning as shown in Figure. Development Approach This Equipment development was a combination of design, construction and calibration of dynamometer. A bicycle Sturmey Archer hub type epicyclic gear train was mounted between the source of power and machine which was designed to measure the power transmitted. This can be classified as a driving dynamometer. The epicyclic gear type was selected because of its following characteristics: Free running (where the power loss should be small) Appropriate load bearing capacity for this application Readily available in major town's market Easily adapted Conventional epicyclic gear advantages (compact, collinear input and output shafts e.t.c) Figure : Torquemeter and Idler Drawing The Sturmey Archer hub has four gear ratios, but :.3,: and.3: ratios were used in this study. The variable gear selection made it possible to use this torquemeter for power measurement on different machines with various speeds. Two sprockets of.3cm (0.5 inch) pitch were fixed to the hub, one of 8 teeth at the gear assembly power input side and 24 teeth at the output side of the gear assembly. Epicyclic gear shaft was mounted and coupled to the instrument frame through ball bearings. The torquemeter arm was fixed to measure the reaction torque on the central shaft at power input side of the torquemeter where the free arm end was connected to the spring balance. The length of chain for driving the torquerneter output and the fast speed rubber roller sprockets on the machine was 80 pitches as calculated. Slots were made on the frames for both vertical and horizontal Figure 2: Torquemeter assembly mounted on the machine The base frame was made of angle iron on which the torquemeter mainframe, spring balance frame and idler frame were fixed. An idler was made for transmitting power between the electric motor through belt and pulley to the torquemeter through chain and sprocket. The idler shaft was mounted 7

3 and coupled to the idler frame at both ends through ball bearings as shown in figure. The chain length that drives the idler and torquemeter input sprockets were also 80 pitches as calculated. Slots were made on the idler frames for both horizontal and vertical adjustment of both chain and belt tensioning. Figure 2 shows torquemeter and idler coupled to the machine in readness to take measurement. I n order to achieve a smooth power supply to the torquemeter to limit vbrational effect on the torquemeter reading for easy calibration, a.lkw electric motor was used to supply the power instead of manual pedalling. The frame Electric Motor on which the electric motor was fixed was welded to the machine frame base. Slots were made to make provision for the horizontal movement of motor and hence belt tensioning. The prony brake pad (shoe) was made of a 20mm x 20mm x 45mm cube of wood, with 70mm diameter hole drilled at the centre which was splitted into two equal parts to house the brake drum which were held together with two 0mm diameter bolts and nuts. This brake assembly was then fixed to the other end of the fast speed rubber roller shaft of a rice dehusking machine as shown in figure 3. The free end of the spring balance was coupled to the free end of the prony brake arm. Prony brake was used for loading and calibrating the torquemeter for testing power transmission efficiency. This was used for the testing because the rubber rollers could not be fed efficiently at high feeding rate without some quantity of paddy rice spilling away from the rollers undehusked as shown in figure 4 in readiness to commence measurement. Figure 4: Prony brake arrangement on the machine. Preparation for the Testing Programme After several trials, ratio.3: was finally selected for running this test and sprocket sizes were selected to obtain the right speeds on the rollers. The electric motor was checked and was confirmed to be in good condition before it was installed. The required chain pitches, sizes and length was used for the power transmission as well as belt and pulley. Dynamometer Calibration Procedure In order to carry out the dynamometer calibration, the speed of electric motor, idler and fast roller shafts were measured by a digital tachometer. Corresponding force readings on the spring balances of the torquemeter and prony brake were noted when the reading was fairly steady on the spring balances. These were repeated at different loading of prony brake by systematically tightening of the two pairs of bolts and nuts that held the two pieces of the wood (prony brake pad) on the brake drum. Whenever the prony brake was too hot, the machine was stopped to cool down before further readings were taken. This was done until the readings on the spring balances were not increasing appreciably. The torquemeter and prony brake arm lengths were measured. Later with grain metering equipment, paddy rice was metered at different rates while the prony brake was disconnected to measure corresponding speeds and forces in order to determine dehusking power requirement at this various feeding rates. Figure 3: Prony brake assembly. Dynamometer Testing There were three possible positions on the set up of 8

4 the rubber roller rice dehusking machine with the T 8 = Sprocket torque at input, Nm hω = Speed of dynamometer where power can be measured. These sprocket at input, rad/s are the power input by the electric motor at various Th=hub and output sprocket torgue, speeds, the input power transmitted by the Nm hω = speed of hub and output sprocket, rad/s torquemeter from the electric motor to the machine and finally the power output from the machine on the T h= torque of axle, Nm aω= speed of axle, rad/s prony brake all as shown in Figures 2 and 4. T = F x r.. (3) Electric Motor Power Determination A 3 phase electric motor of. KW power rating at 420 rpm was used. The electric motor power was calculated at various speeds as follows: Power supplied for each rev./minute drop 2nnPP-=... () (Kurma and Gupta, 2008). Where: At zero load (idle speed), motor speed, n =500rpm At full load, motor speed, n, = 420 rpm Corresponding power at full load, p =.Kw Hence Power supplied for each rev./min. drop, p = 3.75 w / rev Making use of equation (), the power supplied by the electric motor for the speed drop of 500 rev /min at zero load to 494 rev./min. was found to be 83 watts. In the same way the rest of results for electric motor power supplied were calculated. Dynamometer (Torquemeter) power transmitted Figure 5 is showing forces and speeds of sprockets and shaft on the torquemeter. Under ideal condition, there was no power loss. The axle torque was resisted by the hub which was the torquemeter power to be measured and calculated as shown below: Fig. 5: Forces and speeds on Sprockets and shaft of epicyclic gear. Power transmitted by sprocket is expressed using the equation: T sωs, = sshhat ωωωω)(-.. (2) (Green wood, 962). Where: a where: F is force reading on the torquemeter spring balance (N) r = torquemeter arm, (m). For example, from testing result, = 92.32rad/ s ω=68.4 rad/s hω s F =3N r = 0.0m. Hence power transmitted T ss ω = 78 watts from equation (2). In the same way the rest of the results for torquemeter power transmitted were calculated. Prony Brake Power Output power on the prony brake was measured and calculated at various speed as follows: π260 nfrp =. (4) (Kepner et al, 2005) where: P = Power on the prony brake, output power, W T = prony brake torque, Nm Y = prony brake torque arm, m n = speed of prony brake drum, rpmw= angular speed of prony brake, rad/ s F = force on the prony brake spring balance N. For example, a ready from test result, F = 2N, r = 0.285m, n = 008 rev./min. Hence prony brake power P = 60 watts from equation (4). The rest of the prony brake power results were calculated as above. Electric Motor and Dynamometer Power Transmission Efficiency The results obtained from the prony brake power applied, dynamometer power transmitted and electric motor power supplied in Table, the graphs in figure 6 were plotted by the use of linear regression method. Except for the behaviour of the 9

5 electric motor curve near the origin, the graph is the result obtained from the test was linear. Using figure 6, the amount of power loss between electric motor and prony brake during transmission was calculated at point x as shown on the graph: environmental conditions. From this result, it was possible to conclude that the maximum power input required to dehusk at 49kg/h rate was 0watts. i.e Prony brake output power = 00 = 83% Electric motor input power Hence 83% of power supplied by the electric motor was available for paddy rice dehusking while 7% was lost in the transmission from the electric motor via the dynamometer to the machine and prony brake. Also the transmission efficiency after the dynamometer was calculated at point x as shown below: Transmission efficiency = Prony brake output power = 00 = 82% Dynamometer power transmitted that is, 92% of the power transmitted from the dynamometer was available for the paddy rice dehusking process. 8 % was lost between the dynamometer to the machine and prony brake. Figure 5: Electric motor power against speed Figure 6: Torquemeter power against speed RESULTS AND DISCUSSION Power Demand for Paddy Rice Dehusking Test Result The marked point X on Figure---- graph was the testing result reading when the prony brake was disconnected from the system and the grain handling equipment was used for feeding uniformly the paddy into the rollers at 49kg/h rate. It was found that total amount of power supplied by the electric motor to dehusk 49kg/h of parboiled paddy rice was 0watts and 05 watts was transmitted by the torquemeter. Only 5watts got lost within the instrument, which is just 4.5% out of the total power supplied. Power required to drive the empty dehusking machine plus the bearings' internal friction without any load was 69watts as indicated at point Y on Figure-----graph. Net power absorbed by the machine for dehusking 49kg/h of paddy rice was 4 watts (i.e.0.8w /kg). The hydraulic torquemeter that was abandoned before producing this one, absorbed 64watts (net power) to dehusk 60kg/h of paddy rice (i.e.. w /kg) when used on the same prototype rice dehusking machine under the same operating and Figure 7: Prony brake power against speed REFERENCES Adisa A. F Assessment of the Power Demand of a Small Capacity Rubber Roller Rice Dehusking Machine. Unpublished M.sC. Thesis, Cranfield Institute of Technology, Silsoe College, Bedford, U.K. Elisha F., John D. and Winther M. P Dynamometer, Wikipedia, free encyclopedia, Wikimedia foundation, Inc. Greenwood D. C Mechanical Power Transmission, McGraw- Hill, New York. Hofman V. and Solseng E Spray Equipment and Calibration. Agricultural and Biosystems Engineering. AE-73 (revised), U. S.A Kepner R.A., Bainer R. and Barger E. L Principles of Farm Machinery. CBS Publishers & Distributors, third edition. New Delhi, India. Kurmi R. S. and Gupta J. K Theory of Machines. Eurasia Publishing House (PVT) Ltd, fourteenth edition, New Delhi, India. Portfert C. and Sotolongo G Standard Operating Procedure Calibration of field Instruments. Quality Assurance Unit, U. S. Environmental Protection Agency, North Chelmsford. Wikipedia. 20. Calibration. Free encyclopedia. Wikimedia foundation, Inc. 20