P e r t a n i k a 9 ( l ) , 7 - 1 5 (1986)

A Width of Cut Analysis on the Performance ofa Rotary Strip Tiller

DESA AHMADDepartment of Power and Machinery Engineering,

Faculty of Engineering,Universiti Pertanian Malaysia,

43400 Serdang, Selangor, Malaysia.

Key words: Cutting width; Rotary tiller.

ABSTRAK

Kertas kerja ini menerangkan kajian tentang kesan lebar bilah terhadap prestasi bajak putar.Kajian ini a da la h sebahagian daripada penyelidikan yang dilakukan oleh pengarang mengenai pem-bajakan tanah dengan menggunakan bajak jenis putar. Dengan menggunakan satu piring bilah,bilahbilah yang direkabentuk telah diuji pada beberapa gabungan kelajuan antara piring bilah dantraktor dalara sebuah tangki tanah yang diubahsuai menyerupai batas semaian. Hasil kajian yangdiperolehi bagi setiap gabungan kelajuan berasingan menunjukkan peningkatan bererti dari segipenggunaan kuasa apabila bilah dilebarkan. Dari segi kuasa tentu, semakin lebar bilah yangdigunakan kuasa tentunya semakin menurun. Tahap kehancuran tanah tertinggi dicapai denganmenggunakan bilah terkecil pada jarak pemotongan yang paling pendek.

ABSTRACT

This paper describes a soil bin study on the effect of blade widths upon rotary tiller perfor-mance which forms part of the research undertaken by the author on rotary tillage. Blades used weredesigned and tested at several forward and rotor speed combinations using a single rotor flange.Results obtained for each combination of forward and rotor speed show significant increase in powerconsumption as cutting width increased. In terms of specific power, however, the wider the cuttingwidths the lower the specific power values. The highest degree of soil pulverization was caused by thesmallest width at the smallest bite length.

INTRODUCTION

One of the major problems associated withseedbed is the aggregate size which must be fineenough so that after germination the very youngcrops are able to grow through the soil to securenutrients and water and its top growth able tobreak through the crust on soil surface. For seed-beds, it is generally accepted that an aggregatesize of 10 mm is required (Russell, 1961). How-ever, it is still impossible to predict what tillageoperations are necessary to convert soil in a givencondition into a seedbed as the soil structure

produced by any given tillage implementdepends on a number of factors including thehistory of cropping and the moisture content(Ojeniyi, 1979).

The rotary cultivator is a tool that possiblyholds the answer to some of the problems asso-ciated with tillage of the soil around very youngcrops because of its shallow working operation.This is supported by the fact that while the intro-duction of the farm tractor has not greatly in-fluenced the design of the basic tillage imple-ments, few successful attempts at applying the

DESA AHMAD

available tractor power directly to the implementhave been made, the most successful being therotary cultivator (Hendrick, 1971). Hendrickfurther reported that if the power efficiency ofrotary tools is to be increased, a number ofrelationships that exist must be investigated.Among other things, he suggested that" fieldefficiency in seedbed preparation may be increas-ed through shallower operation at higher speeds orthrough preparing seedbed strips without an un-reasonable increase in power requirement ".Review of the previous investigations reveal thatno fundamental work carried out so far con-cluded on seedbed trip tilling and precisionshaped raised beds for planting crops (Ahmad,1980). Hence, Hendrick's suggestion has clearlystressed the need for a critical study on theimportant criteria for the design of cutting toolwhich has hardly been touched on. Consequent-ly, if the rotary cultivation is to be significantlyimproved as a strip tilling device, a closer lookmust be taken as its mode of operation and thefactors that contribute to its effectiveness.

Following from the above, the objectives ofthis investigation were;

i. To find the power requirement of thevarious cutting widths at different forwardand rotary speeds.

ii. To determine the effect of cutting width onthe resultant cold size.

It was anticipated that this work would leadto the discovery of an optimum width having lowpower requirement, high rate of work whilst atthe same time having the capacity to producefiner tilth approaching seedbed condition for apower driven tine in rotary tillage.

MATERIALS AND METHODS

Blade Design

As the criterion for the present work hasbeen the engineering aspect of investigating striptillage, blades of different cutting widths weredesigned and fabricated. Simplification in

design was sought as the objective of this preli-minary work was to obtain the fundamentalknowledge of blade performance. The complexthree dimensional shape of a rotary cultivatorblade has been defined (Beeny and Khoo, 1970)as shown in Fig. 1.

L v = Effective vertical length (mm)

L M = Blade cutting width (mm)

R = Curvature of a section through the blade

(mm)

r = Curvature between L v and L H (mm)

t = Thickness and sharpness of blade (mm)

w = blade span (mm)

a * sweepback angle (degrees)

(3 = clearance angle (degrees)

Fig. 1: A typical rotary cultivator blade showingdesign parameters (BEENY and KHOO,1970).

PERTANIKA VOL. 9 NO. 1, 1986

A WIDTH OF CUT ANALYSIS ON THE PERFORMANCE OF A ROTARY STRIP TILLER

One of the crucial factors considered in thedesign was the depth of tillage since it plays avital role in the disturbance and mixing of soil aswell as in the cutting of roots. As rotary tools arethree dimensional (because of intermittent ope-ration both along the path of movement of themachine and across the width of the machine)ridginess is unavoidable. The regions of unfilledsoil as shown in Fig. 2 can be overcome by havinga uniform depth. To achieve this the effective

SHAFT

FLANGE

Cutting edges oftwo blades invertical position

Ro = Radius of tiller (mm)

hp = Height of ridge (mm)

Rp = Ridge to centre of rotor shaft (mm)

Fig. 2: The ridginess created by rotary tiller bladesin the bottom of a furrow in the verticaltransverse plane (GILL and HENDRICK,

horizontal length was modified and made per-pendicular to the vertical length. The blade spanof 75 mm was selected (conventional blade spanranges from 60 - 90 mm). A sharpening angle of15° for minimum compressive stress (Spoor,1969) and a clearance angle of 20° (Sohne, 1957)for least power requirement were selected.

Experimental Design

The experiment carried out comprisedthree blade widths (56.8, 107.6 and 209.2 mm)and a single shank (6 mm) in combination withfive treatments varying in alpha ratios (ratio ofperipheral speed to forward speed) and threebite lengths. Three bite lengths of 25, 50 and 75mm were chosen as beyond these values, a majorportion of the clods produced would be eithertoo small in size for seedbed condition in the caseof bite lengths lower than 25 mm or too big insize in the case of bite lengths greater than 75mm. The shank was included in order to obtainthe comparison of power requirements as bladewidths were increased. The experimental workconsisted of sixty runs completed over threereplicated blocks. Each block comprised twentyruns whereby five treatments were applied toeach individual blade. The treatments appliedwere as given in Table 1.

Tests were carried out on the effect of bladecutting widths on the rotor power requirement aswell as the resultant soil aggregates at constantbite lengths (given by treatments T 2 , T^ and

1976).

Treatments

T,

T ,

T ,T 4

Forward speedV (mm/s)

66

66

150

133

150

T4) .

TABLE 1Treatments applied

Rotational speed,N (RPM)

80

40

90

80

60

Treatments T x,

Bite length

25

50

50

50

75

T,. T s and T 5 wer<

Alpha ratioX

38

19

19

19

13

PERTANIKA VOL. 9 NO. 1, 1986

DESA AHMAD

applied to find out the effect of changing thelength on rotor power requirement and the soilaggregates at constant forward speeds. Thesetreatments would also give information on theeffect of rotational speeds on rotor powerrequirements. It was also of interest to study theeffect of changing the bite length on power andsoil aggregates requirement at constant rotorspeed (by changing the forward speed) usingtreatments T J and T 4 . The bite length (or tillingpitch) can be calculated from

e = v_.6oN Z

or

where

VNZ

UR

machine forward speed (mm/s)blade rotational speed (RPM)number of blades which wouldcut identical paths if V = Oblade peripheral speed (mm/s)radius of tiller (mm)

Details of soil properties used are as givenin Table 2.

RESULTS AND DISCUSSION

Effect of Cutting Width on Mean Power Values

Referring to Fig. 3, the increments in powerfor increasing cutting widths would be attributedto the higher cutting force. For each combina-tion of forward and rotor speed, increasing thecutting width to 50 mm from the shank aloneinitially reduced the mean power requirement.Beyond this, the increment in cutting widthcaused the power to increase significantly.

Analysis of variance was carried out be-tween cutting widths for each treatment and be-tween treatments applied for each blade. Fromresults calculated at 5% significant level;

i. No significant difference has been notedbetween any of the block mean power valueswhich indicate the general uniformity of soilprepared.

ii. The effect of cutting width is significant onthe mean power values for each treatmentapplied.

iii. For each cutting width, the treatmentsapplied were significant on the mean powervalues at 5% significant level.

TABLE2Mechanical Properties of Experimental Soil

Angle of Angle ofCohesion shearing Adhesion Soil MetalC (kPa) resistance a (kPa) friction

Soil Bulk Density

(3)(1) (2)

Penetrating Resistance (MPa)

at 75 mm depth at 150 mm depth

(1) (2) (3) (1) (2) (3)

18 22C 0.7 18° 1989 1973 1963 1.32 1.39 1.35 1.17 1.19 1.18

Position (1) and (3)

Position (2)

Soil type

Moisture content

No. of passes

No. of experiments

Pressure of compacting ram

0.5 m from ends of tank

centre of tank

sandy clay loam

12.2%

2

20

6 MPa

10 PERTANIKA VOL. 9 NO. 1. 1986

A WIDTH OF CUT ANALYSIS ON THE PERFORMANCE OF A ROTARY STRIP TILLER

70

60

8. so

30

20

10

Minimum disturbance

Maximum disturbance

Treatments

50 100 150

Cutting width, mm

200 250

Fig. 3: Relation of specific power to cutting widthfor various treatments

Although the power requirement increasedwith blade cutting width, when the volume ofsoil disturbed and specific power were con-sidered, the results show that the larger the cutt-ing widths, the lower the specific power (Fig. 4).The high power consumed by the shank alonecan be explained by the higher cutting anddraught forces encountered due to difference insoil failure laterally around the shank. Thisphenomenon had been observed by otherresearchers notably Godwin (1974) and Godwinand Spoor (1977).

Effect of Bite Length on Mean Power Values

The results obtained from mean powervalues show that at constant forward speed, in-

600

300

150

D- o T

, . T ,

N^

\\

50 100 150

Cutting width (mm)

200 250

Fig. 4: Relation of mean power values to cuttingwidth for various treatments

creasing bite length would reduce the power con-sumption. The main reason for the power reduc-tion appears to be due to the lower rotor speed(given by treatments T ] - T^and T 3 - T&asshown in Fig. 5). The results of Harral (1977)however, indicate that power increases with bitelength. This contrasting effect couhi be at-tributed to the somewhat larger diameter ofrotor used which tended to increase the length ofcutting path and hence increased the powerrequirement. Harral, however, attributed thishigh loss largely on the moisture content beinggreatest in wet conditions presumably because ofsoil sticking to the blade as large clods whichthen accelerated and carried over the rotor.

Fig. 5 also shows that at constant rotorspeed, the increase in power requirement seemed

PERTANIKAVOL. 9 NO. 1, 1986 11

DESA AHMAD

V(mm/2) N(RPM) £(mm)

600 r t ,

_ 400

200

6666

1331 5 0

8040908060

2550505075

25 SO

Bite length^, (mm)

(constant forwardspeed V (mm/s)reducingrotor speed)(constant rotorspeed N (RPM)increasingforward speed)

CUTTING WIDTH

o 6 mm

• 56.8 mm

o 107.6 mm

* 209.1 mm

7S

Fig. 5: Effect of increasing bite length on meanpower requirement at constant rotor andforward speeds for various cutting widths

600

400

Icflj

i 200

CUTTING WIDTHo o 6 mm

• • 56.8 mm

o D 107.6 mm

^ — * 209.2 mm

0 50 100 150 200

(Forward speed V (mm/s)

4 0 Rotor speed, N (RPM) 8 0 9 0

Fig. 6: Effect of rotor and forward speeds on meanpower requirement (at constant bite length)

to be due to higher forward speed as bite lengthincreased (treatments T ( - T4) . Similar resultswere also observed by Chamen and Cope (1979).Perdok and Burema (1977), Wolfe (1958) andHendrick and Gill (1971).

Effect of Rotor and Forward Speeds on MeanPower Values

Referring to Fig. 6, the results obtainedclearly indicate a marked influence of rotor andforward speeds on power requirement. Theeffect clearly justifies the trend towards low rotorand forward speed for lower power values.

In contrast to the above Beeny and Khoo(1970) who studied blade performance insaturated clay observed that at constant bitelength there seemed to be an optimum speedbeyond which excessive slipping between soil andblade occurred. The relationship obtained washyperbolic rather than linear. As regards thetine alone, increasing rotor and forward speedscaused a significant increase in power consumedwith marked difference shown at higher speedswhich can be explained by the rapid incrementin cutting force with speed.

Effect on Soil Aggregates

After each treatment, a representativesample of the selected clods was lifted by a spadeand later air-dried with minimum disturbance.A nest of square mesh sieves with aperture rang-ing from 4 to 16 mm was used through'which thedried clods were poured and gently shaken byhand. From the values obtained in percentage byweight, comparison was made on the degrees ofpulverization for clods below 10—11 mm for allcutting widths. This value was selected as a basisfor evaluating the generally accepted aggregatesize range approaching seedbed condition(Russell, 1961).

Analysis of variance carried out at 5% signi-ficance level on soil pulverization show that:

a. At constant bite length of 50 mm, changingrotor and forward speeds had a significantbreaking effect on the degree of pulveriza-tion both on the larger cutting widths of107.6 and 209.2 mm but not on 56 mmwidth.

b. At constant rotor speed, increasing bitelength by increasing forward speed has a

12 PERTANIKA VOL. 9 NO. 1, 1986

A WIDTH OF CUT ANALYSIS ON THE PERFORMANCE OF A ROTARY STRIP TILLER

significant breaking effect using 56.8 mmand 107.6 mm cutting widths but is notsignificant on the 209.2 mm width.

c. At constant forward speed, changing bitelength by reducing rotor speed, showedsignificant difference on 107.6 mm and209.2 mm cutting widths at both lower andhigher bite lengths. However, the effect on56.8 mm cutting width on the degree ofpulverisation was only significant at thehigher bite length.

d. Effect of treatments T1 ? T 2 and T 4 weresignificant for all cutting widths whereastreatments T 3 and T 5 showed no significantdifference on the degree of pulverisationbetween cutting widths. Fig. 7 illustrates theeffect of treatments on soil aggregates below11 mm for all cutting widths.

From results obtained, it can be shown thatfor all cutting widths, greater breakdown of soilaggregates was achieved at lower bite length.The highest percentage was given by the smallestcutting width at the smallest bite length of 25mm. A related observation was also noted byGrinchuk and Mat Yashin (1969) who found thatthere was an appreciable trend towards smalleraggregates as bite lengths were reduced (Table3).

BLADE CUTTING WIDTH (mm)

56.8i 107.6

209.2

n

V(mm/$) N(RPM)6666

150133150

8040908060

X2 T3 K

Applied Treatments

£(mm)2550505075

T5

Fig. 7: Effect of treatments on aggregates below11 mm at various cutting widths

TABLE 3Clod size diameter as affected by rotor diameter and bite length

(GRINCHUK and MAT YASHIN, after GILL and HENDRICK 1971 p. I l l )

Content of soil in fractions %

Diameter ofrotor (mm)

Bitecm 50 mm 50 - 25 mm 25 - 10 mm Less than 10 mm

150

240

320

4.87.0

11.8

4.87.0

1L8

4.87.0

11.8

19.821.544.0

23.036.827.6

13.029.743.4

15.917.012.8

20.219.319.6

20.417.012.5

17.517.512.8

20.612.216.6

23.515.313.2

46.844.030.4

36.228.736.2

43.138.030.9

PERTANIKA VOL. 9 NO. 1, 1986 IS

DESA AHMAD

From an overall standpoint, the resultsclearly illustrate the trend towards using smallercutting widths for a finer degree of soil pulveri-zation and agreement with the hypothesispostulated that smaller widths, smaller bitelengths and higher rotor speeds would increasethe degree of soil breakdown. However, in termsof energy requirement, small cutting widths con-sumed a significantly higher value compared tolarger cutting widths which tended to increasesoil disturbance area. This increase in width,however, was not compatible with the degree ofsoil breakdown required and gave a comparative-ly poorer percentage of aggregates formed. Inorder to minimize the energy requirements for agiven operation, the following suggestions couldbe made: —

i. Blade cutting width used, should be as smallas possible.

ii. Bite length should conform with tilthrequirements.

iii. Rotor speeds should be as slow as possible.

CONCLUSIONS

1. For each combination of forward and rotorspeed, the power requirement increasedalmost linearly with cutting width.

2. In terms of specific power, the wider thecutting width, the lower were the specificpower values.

3. Reducing rotational speed at a constantforward speed, (therefore increasing the bitelength) significantly reduced the powerrequirement for all cutting widths.

4. Increasing the bite length at constant rotorspeed by increasing the forward speed, signi-ficantly increased the power requirement.

5. For a constant bite length of 50 mm, in-creasing rotor and forward speeds increasedthe power requirements.

6. For minimum power requirements, rotorspeed should be as low as possible operatingat the maximum bite length conforming tothe tilth desired.

7. At constant bite length, increasing rotor andforward speeds increased soil pulverizationfor the wider cutting widths but no signifi-cant change was observed for the smallestcutting width.

8. At constant forward speed, increasing thebite length by lowering the rotor speedreduced the degree of soil pulverization forall cutting widths.

9. At constant rotor speed, increasing bitelength by increasing forward speed, reducedthe degree of soil breakdown for all cuttingwidth.

10. For all cutting widths, greater fragmenta-tion of soil aggregates was achieved at lowerbite length, the most being caused by thesmallest cutting width (56.8 mm) at a bitelength of 25 mm.

ACKNOWLEDGEMENT

The work was carried out under thesupervision of Dr. R.J. Godwin of NCAE, Silsoein fulfilment for an M.Sc. degree. The author isalso grateful to the Director of NIAE and Dr.J.V. Stafford for the facilities provided and toUPM for the financial assistance.

REFERENCES

AHMAD. D.B. (1980): An investigation into themechanics of rotary tillage. Unpublished M.Sc.thesis, NCAE, Silsoe, U.K.

BEENYJ.M. and KHOO, D.C.P. (1970): Preliminaryinvestigations into the performance of differentshaped blades for the rotary tillage of wet ricesoil.J. Agric. Eng. Res. 15(1): 2 7 - 3 3 .

CHAMEN, W.C.T. COPE, R.E. (1979): Developmentand performance of new high speed rotarydigger. 8th Conference of the International SoilTillage Research, Organization, ISTRO, Bun-dersrepublik, Deutschland.

GILL, W.R. and VANDEN BERG, G.E. (1967): Soildynamics in tillage and traction, Agric. Hand-book No. 316, ARS, USDA, pp. 269-279.

GILL, W.R. and HENDRICK, J.G. (1976): The irregula-rity of soil disturbance depth by circular androtating tools. Trans. ASAE 19(2): 230 - 233.

GODWIN, R.J. (1974): An investigation into themechanics of narrow tines in frictional soils.Ph.D. Thesis, University of Reading.

14 PERTANIKA VOL. 9 NO. I, 1986

A WIDTH OF CUT ANALYSIS ON THE PERFORMANCE OF A ROTARY STRIP TILLER

GODWIN, R.J. and SPOOR, G. (1977): Soil failure withnarrow tines/ Agric. Eng. Res. 22(4): 213.

GRINCHUK, I.M. and MAT YASHIN, YU. I. (1968):Systems of operation of soil rotary tillers. MESK26(6): 7 - 9 . (NIAE trans. GILL/107).

HARRAL, B.B. (1977): The effects of some rotarydigger parameters on the power requirement.NIAE Departmental Note DN/CU/757/1270.

HENDRICK, J.G. (1971): Utilizing rotary tillers withhigher horsepower tractors SAE National FarmConstruction and Industrial Machinery MeetingMilwaukee, Wisconsin. Sept. 13-16. Paper No.710682.

HENDRICK, J.G. and GILL, W.R. (1971): Rotary tillerdesign parameters Part (1) Direction of rotation,Part (2) Depth of tillage and Part (3) Ratio ofperipheral and forward velocities. Trans. ASAE14(4): 669-683.

OJENIYI, S.O. and DEXTER, A.R. (1979): Soil factorsaffecting the macrostructures produced bytillage. Trans, of the ASAE 1979.

PURDOK, U.D. and BUREMA, HJ . (1977): Powerrequirements of rotary tiller blades in clay andsandy soils. Research report 7 7 - 1 . Wageningen,Netherland.

RUSSEL, E. W. (1961): Soil conditions and plant growthLongmans Green and Co. London.

SOHNE, W. (1957): Influence of shape and arrange-ment of tools on torque of rotary blades. DLK (9):69-87.

SPOOR, G. (1969): Design of soil engaging imple-ments. Farm machine design engineering Sept.1969 pp. 22 - 2 5 and Dec. 1969 pp. 14 - 19.

WOLFE, J.S. (1958): Rotary cultivator. Farm mechani-zation 10(110): 294.

(Received 7 February, 1985)

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