Efficiency Differences Between Resonant Oscillating Tools and Those ...

Efficiency Differences Between Resonant Oscillating Tools and Those Driven by Inertial Propulsion W. J. Chancellor MEMBER ASAE

by a stirrup connected to the tractor assembly via rubber shock mounts (Fig. 3). The tool system was instrumented to measure: (a) net draft, (b) longitudinal tool displacement relative to the tractor, (c) longitudinal blade acceleration, (d) hammer displacement from the anvil, (e) average forward speed of the tractor, and (f) oscillator frequency and driving torque. These quantities were recorded on magnetic tape during the tests of the original tool system and of the inertially propelled tool, and later were reproduced on a multi-channel strip-chart recorder. During tests of the resonant oscillating tool only items a, e, and f were measured, and these values were recorded directly on a strip-chart recorder.

N field tests, a tool originally designed as an inertially propelled cable layer also performed as a resonant oscillating tool (Lawyer et al. 1973). Further analysis indicated that such tools offer advantages in terms of simplicity of construction, but that their operation is subject to certain efficiency limitations (Choa and Chancellor 1972). An investigation was conducted to determine quantitatively, efficiency and performance characteristics of both a resonant oscillating tool and an inertially propelled tool so that these characteristics could be compared.

I

APPARATUS The equipment used was the tool system tested previously (Lawyer et al. 1973), and two modifications of that system. The only aspects of the original system considered here were those which applied when behavior was resonant due to the fact that the bias spring pressure (see Fig. 1) was so great that the hammer and anvil (Fig. 1) could not separate, causing the tool to move back and forth in the previously cut soil channel. In its first modified form, the system was converted to produce true resonant oscillation of the tool by removing the bias springs and bolting the hammer and anvil (Fig. 2) together as a single unit. This system then functioned as a tuning fork, with the tool attached to

Article was submitted for publication in December 1973; reviewed and approved for publication by the Power and Machinery Division of ASAE in March 1974. Presented as ASAE Paper No. 73-1540. The author is: W. J. CHANCELLOR, Professor, Agricultural Engineering Dept., University of California, Davis. Acknowledgments: The authors wished to acknowledge the contributions of A. G. Bodine, Bodine Soundrive Co., Van Nuys, Calif., who supplied the initial design of the equipment used; James A. Joy, Senior Mechanician, University of California, Davis, who executed the construction and modification of the unit; and Dr. Gajendra Singh, G. B. Pant, University of Agriculture and Technology, Pantnagar, U.P. India, who assisted with field tests and data processing.

1974 - TRANSACTIONS of the ASAE

TEST CONDITIONS FIG. 1 Original resonant tool system. Note springs which biased the hammer on the tuning fork assembly against the anvil on the tool with such force that the hammer seldom left the anvil, causing the blade to oscillate back and forth in the soil [1].

one leg and a mechanically driven oscillator attached to the other. In its second modified form, the system was altered by separating the tool from the tuning fork in order to produce inertial propulsion of the tool by allowing a hammer on the fork to strike repeatedly an anvil attached to the tool, thus driving the tool forward through the soil (Fig. 3). In order that the tuning fork assembly powering the hammer not become permanently separated from the tool as a result of the impulse generated during the hammer blows, it was necessary to exert a continuous force on this assembly to bias the hammer against the anvil. This was done by applying a torque to the tuning fork assembly via a bar loaded in torsion by a hydraulic cylinder (Fig. 4). The tool itself (when not in contact with the hammer) was free to move back and forth over a 2-in. range relative to the tractor. At the ends of this range it was restricted

The soil in which the tests were conducted was the same as used for

FIG. 2 Modified resonant tool system. Bias springs were removed and hammer was permanently bolted to the anvil. 593

level was applied to the oscillator drive, and the forward speed of the tractor was increased from about 0.5 to about 3.0 ft per sec by incrementally increasing the setting of the tractor governor. Characteristics of the Inertial Propulsion System

FIG. 3 Tool system modified for inertial propulsion. Bias springs were removed and blade [A] was allowed to swing free within the limits imposed by the stirrup [B], which was attached to the tractor assembly by elastic rubber shock mounts |C|. Both blade and tuning fork assembly [D] pivoted relative to the tractor around an axis through the upper mounting points [E]. Hammer [F] was biased against the anvil [G] by a torque [H] applied at flexible coupling [I].

previous tests (Lawyer et al. 1973). For tests with the original tool system and inertially propelled tool system, moisture content of the Yolo loam soil was approximately 11 to 12 percent; for tests with the resonant oscillating tool, moisture content was about 18 to 20 percent. Draft of the nonoscillating tool at the lower moisture content ranged from 638 to 2190 lb during tests of original tool system, and from 1688 to 3222 lb during tests of the inertially propelled tool. The range during tests of the resonant tool system (high soil moisture) was from 726 to 1274 lb. The 0.5-in. wide tool was operated at a depth of 10.75 in. In one test procedure, the tool was placed in the soil without oscillation, and the draft was measured under these conditions. Then, with the tractor moving forward at the same fixed governor setting as used without oscillation, the drive to the oscillator was engaged. Incremental increases of oscillator drive torque were applied, followed by incremental decreases. The oscillator drive was then disengaged, allowing a final measurement of conditions without oscillation. A second test procedure was similar to the first except that a fixed torque 594

Details of tool motion with the original tool system operating in the resonant mode are given by Lawyer et al. (1973). However, the details of tool motion during inertial propulsion (hammering) reported in this reference were quite different from those for the tool system modified to achieve full-time inertial propulsion. When the tool was undergoing full-time inertial propulsion there appeared to be two modes of operation. In the first of these the power transmitted to the tool by the hammer was insufficient to meet draft power requirements. Consequently, the tool moved to the rear of its restraining stirrup, allowing forces intermittently applied by the tractor through the elastic stirrup mounting to make up the power deficiency. Under these circumstances only limited draft reduction was achieved. In the second and more desirable mode of operation, the power transmitted to the tool by the hammer was more than adequate to meet the draft power requirement. Under these circumstances, the tool moved to the front of its stirrup and transmitted forward-directed forces to the tractor through the elastic stirrup mounting. In this mode, draft reduction was great; and in approximately 8 percent of the test points evaluated, average tool draft while oscillating was only 5 percent or less of the draft required without oscillation. Details of the motion of system components were obtained by operating the strip-chart recorder at high chart speeds. Motions during one cycle of inertial propulsion in the second mode (tool near front of stirrup) are illustrated in Fig. 5. As the hammer struck the anvil, the tool was driven forward until it struck the front of the stirrup. It was then promply decelerated by the resistance of the soil to the movement of the tool and by the force between the tool and the front of the elastically mounted stirrup. This stirrup force then caused a slight but prompt rearward motion of the tool relative to the soil. Once this rearward relative motion had stopped, the tool remained in place in

FIG. 4 Hydraulic cylinder and torsion bar assembly used in the inertial propulsion system to apply and transmit bias torque to the tuning fork assembly. Flexible coupling and self-aligning pillow block bearing allowed torque to be independent of tool system motion. Hydraulic pressure was applied by a charged hydraulic accumulator so that torque would not vary significantly with rotational movements of the tool assembly about a lateral axis.

the soil (shown in Fig. 5 as moving rearward relative to the tractor at a speed equal to tractor forward speed) until the tool was again struck by the hammer. Under the circumstances described in Fig. 5, the tractor does not exert any force directly on the tool. However, application of the bias torqu.e to the tuning fork assembly (Fig. 3 and 4) is balanced by a couple consisting of a rearward force on the tractor assembly at the pivot point at which the tuning fork is mounted, and a forward force exerted on the tool by the hammer and transmitted to the tractor via the stirrup. When the inertial propulsion system is operating, there is a considerable portion of time during each cycle that the tool is not in contact with the stirrup, and the couple then consists of a forward force used to accelerate the hammer and a rearward force acting at the tuning fork pivot point. It is the average impulse caused by this latter force (net of any forward impulse through the stirrup) which results in some average draft force being applied to the tractor. In field tests, various levels of bias torque were applied to the tuning fork assembly. Although a torque providing an approximate bias force at the TRANSACTIONS of the ASAE - 1974

Inertial Propulsion System Forward speed = I 0 9 ft/sec Oscillating frequency = 31 cycles/sec Oscillating draft = 2 5 percent of nonoscillating d r a f t Modified Resonont Tool System

,. Original Resonont Tool Sy:

OSCILLATOR POWER / NONOSCILLATING DRAFT POWER

FIG. 6 Average values of [oscillating draft]/[nonoscillating draft] for field test data sets categorized according to ratios of [oscillator power]/[nonoscillating draft power].

TimeMilii3econds

FIG. 5 Details of hammering process during one cycle of operation of the inertial propulsion system. Negative values of hammer-anvil separation were due to transducer overshoot during hammer impact. Rapid reversal of the tool after reaching maximum forward position was due to rebound of the elastically mounted stirrup with which the tool was momentarily in contact. Note that after rebound the tool moved rearward at a speed equal to tractor forward speed, indicating that the tool was stationary with respect to the soil.

hammer of 580 lb seemed to give good general performance, the system worked satisfactorily at forces ranging from 290 lb to 870 lb. The level of this force did not appear to have much effect on draft, as at high force levels greater average forward impulses were applied to the tractor through the stirrup. When the bias force was extremely low (approximately 290 lb), the hammer tended to miss alternate blows because the rearward impulse during contact with the anvil was greater than the forward impulse provided by the bias system. At excessively high bias forces, (approximately 870 lb) hammer blows became less intense, and sometimes failed to occur, because forces transmitted to the hammer by the oscillator system were insufficient to overcome the bias forces. When low levels of bias force were used, power requirements of the oscillator did not vary appreciably with major changes in draft power reductions caused by oscillation. However, when high bias forces were used, oscillator power requirements tended to increase noticeably with an increase in draft power reductions due to oscillation. During operation of the inertial propulsion tool system, there were fatigue failures of bolts clamping the anvil to the tool, of bolts holding the hammer to the tuning fork, and of the tuning fork leg holding the oscillator. It appeared that these failures could, 1 9 7 4 - TRANSACTIONS of the A S A E

in the future, be avoided by minor changes in the design and construction of the system. Critical parts such as the hammer, anvil, oscillator bearings, and tuning fork clamping assembly performed satisfactorily. RESULTS System Performance Comparisons

Test results were evaluated by choosing numerous points along the multi-channel strip-chart recordings and evaluating tool system test parameters at each of these points. The parameters used were: 1 (Oscillating draft)/(nonoscillating draft). This measured the proportion of draft reduction achieved by the tool oscillation system. Power efficiency =

power was computed from the product of nonoscillating draft and the speed of the tractor at any instant during the test run. Each of the sets of these three parameters was categorized twice. Once was according to the value of the operator-controlled input parameter of (oscillator power)/(nonoscillating draft power) to show the effects of either applying various levels of oscillator power at a given tractor speed, or of using various levels of tractor speed for a given application of oscillator power. The second categorization was done according to the value of the output performance parameter (oscillating draft)/(nonoscillating draft). Mean values for the three parameters in each categorical division constitute the data reported here. From the point of view of the system operator, the parameter (oscillator power)/(nonoscillating draft power) relates the factors over which he has control. Figs. 6 and 7 thus illustrate the sort of system performance that can be achieved at various levels of this parameter as might be selected by the operator. In general, it appears that at any given level of (oscillator power) / (nonoscillating draft power) greater than 1.0, the inertial propul-

(draft power without oscillation) — (draft power with oscillation) oscillator power input This indicated the efficiency with which power supplied to the oscillator was able to replace draft power. 3 (Oscillator power)/(nonoscillating draft power). This provided a measure of the intensity of tool oscillation relative to the draft power that would otherwise be required. Nonoscillating draft was determined by interpolating between values measured at the beginning and end of each test run. Nonoscillating draft

sion system is capable of achieving greater draft reduction and slightly higher power efficiency than is the resonant tool system. Below the level of 1.0, differences between the two system types appear negligible. The higher level of draft reduction achieved by the inertial propulsion system is believed due to the fact that the proportion of the period of each cycle during which soil cutting is actually taking place (''contact ratio") (Smith et al. 1972) tends to be smaller in the inertial propulsion system than 595

Original Resonant Tool Sysf«m Modified Rttonont Tool

OSCILLATOR POWER / NONOSCILLATING DRAFT POWER

FIG. 7 Average values of power efficiency for field test data sets categorized according to ratios of [oscillator power]/[nonoscillating draft power]. Theoretical maximum value is that which would pertain if oscillating draft were zero.

in the resonant system because of the very high tool velocities achieved by hammer impact. The ratio of oscillating draft to nonoscillating draft is directly related to this "contact ratio", and the two ratios are theoretically equal in the case in which soil-cutting resistance does not change with tool velocity (Smith et al. 1972). The lower efficiency of the resonant tool system is probably related to the fact that this system requires considerable movement of the tool back and forth in the previously cut soil channel, whereas inertial propulsion has no such requirement. The frictional energy required for this movement appears to have been greater than the energy lost by the inertial propulsion system during impacts between the hammer and the anvil. The draft reduction performance of the two oscillating tool systems may be approached from the analytical point of view by determining parameter levels associated with various ratios of (oscillating draft)/(nonoscillating draft). Figs. 8 and 9 show average values when sets of test data were categorized according to this latter performance parameter. For all ratios of (oscillating draft)/(nonoscillating draft) less than 0.35, the inertial propulsion system was more advantageous than the resonant tool system both from the standpoint of requiring lower ratios of (oscillator power)/ (nonoscillating draft power), and from the standpoint of improved power efficiency. These advantages increased in magnitude as ratios of (oscillating draft)/(nonoscillating draft) became smaller. Of particular note in Figs. 8 and 9 are the low ratios of (oscillating draft)/(nonoscillating draft) achieved by the inertial propulsion system (0.05), and the moderately high power efficiencies (50 percent) with which they were accomplished. 596

OSCILLATING DRAFT / NONOSCILLATING DRAFT

FIG. 8 Average values of [oscillator power]/[nonoscillating draft power] for field test data sets categorized according to ratios of [oscillating draft]/[nonoscillating draft].

The modified resonant tool system driven by a resonant oscillating syst em was somewhat superior to the original resonant tool system (Figs. 8 and 9). was compared to that of one driven by This may have been due to the fact an inertial propulsion system. 1 The inertial propulsion system that modifications prevented any was more complex in construcill-timed relative motion between the tion and more subject to fatigue hammer and anvil. failures, but indications were that The main purpose for including such a system is workable. results from the original resonant tool 2 Under circumstances in which system was to determine the effects of levels of oscillator power applied soil moisture and draft resistance on were equal to or higher than the system resonse. Although tests with nonoscillating draft power otherthe original system were conducted wise required, and/or circumprincipally at high levels of oscillator stances in which draft with oscilpower input, when data were categorlation was reduced to 35 percent ized according to the parameter or less of the nonoscillating draft, (oscillator power)/(nonoscillating the inertial propulsion system was draft power) the responses from both superior to the resonant tool systhe original and modified resonant tem in terms of draft reduction, tool systems were similar (Figs. 6 and efficiency, and relative oscillator 7) despite difference in soil resistance. power requirements. When data were categorized according to the parameter (oscillating draft) / (nonoscillating draft), there References were major differences between the 1 Choa, S. L. and W. J. Chancellor. 1972. performances of the original resonant Optimum design and operation parameters for tool system and those of the modified a resonant oscillating subsoiler. TRANSACresonant tool system. It is not clear TIONS of the ASAE 16(6): 1200-1208. 2 Lawyer, J. N., W. J. Chancellor and S. L. whether these differences were related Choa. 1973. Performance of a subsoiler to differences in soil resistance, tool incorporating repeating inertial propulsion construction, or test operating condi- drive. TRANSACTIONS of the ASAE 16(2): tions. 208-213. SUMMARY OF RESULTS Performance of a simple tillage tool

3 Smith, J. L., J. L. Dias and A. M. Flikke. 1972. Theoretical analysis of vibratory tillage. TRANSACTIONS of the ASAE 15(5):831-833.

Modified Rttonont Tool Syttem ,

OSCILLATING DRAFT/ NONOSCILLATING DRAFT

FIG. 9 Average values of power efficiency for field test data sets categorized according to ratios of [oscillating draft] / [nonoscillating draft]. TRANSACTIONS of the A S A E -

1974