J. agric. Engng Res. (1998) 71, 373—383Article No. ag980336

Mechanical Load and its Effect on Bulb Onions due to Harvest and Post-harvestHandling

B. Herold; B. Oberbarnscheidt; M. Geyer

Institute of Agricultural Engineering Bornim, Max-Eyth-Allee 100, D-14469 Potsdam-Bornim, Germany

(Received 23 June 1997; accepted in revised form 26 June 1998)

Data records of an artificial fruit (PMS-60) showedthat bulb onions are subjected to a considerable numberof mechanical impacts under practical harvest and post-harvest handling conditions. The effect of number andintensity of impacts on onion storage losses was studiedin laboratory drop tests by using hard and soft impactsurfaces. After storage, the mass losses (due to transpira-tion and respiration losses, or rot and sprouting) weredetermined. The data from the artificial fruit (number ofimpacts, peak load and load integral of single impacts)recorded under the same drop test conditions were re-lated to the resulting onion mass losses. In this way,allowable load thresholds for onions were derived andexpressed in terms of PMS-60 data. These threshold datawere used to evaluate practical handling systems by usingthe artificial fruit, and to predict the risk of onion storagelosses based on measured impact data.

( 1998 Silsoe Research Institute

1. Introduction

During harvesting and post-harvest handling, agricul-tural products such as bulb onions are very frequentlysubjected to mechanical loading. Mechanical loadsabove the allowable level can cause onion damage con-sisting of surface lesions or internal bruising. The latterkind of damage can result in considerable quality lossesand decay during storage.

Previous engineering research has been directed todeveloping both objective methods for detection of dam-age sources1, 2 and also to determine allowable load thre-sholds in the case of a single load on the produce.3–5

Hyde et al.6 studied the impact behaviour of apples,onions and potato tubers during multiple constant im-pact loading on the same location each time. They foundthat the rebound height increased for each successive

0021-8634/98/120373#11 $30.00/0 373

impact and approached a constant asymptotic value.Based on this, they partitioned the impact energy intorebound, absorbed and bruising energy. They did notdiscuss damage accumulation due to multiple impacts inpractice. Studies by Herold and Habelt,7 and Molemaand Bouman8 in potato handling, showed that the de-signer of handling lines has to pay attention to thenumber of mechanical loads. Relatively low but veryfrequent mechanical loads were blamed for blackspotdamage in potato tubers. In a similar way, multiplemechanical loads could also cause quality losses of othercommodities such as onions. Usually, onion damage isnot immediately visible but it causes increased respir-ation and storage loss.

The objectives of this study were: (1) to determinethe respiration response and storage loss of bulb oniondue to multiple mechanical loads under laboratoryconditions; (2) to develop relationships between result-ing onion storage losses and mechanical loads mea-sured by means of the artificial fruit2 PMS-60 underequivalent conditions; and (3) to acquire data onmechanical loads during harvesting and handling forpredicting the risk of onion damage under practicalconditions.9

2. Materials and methods

2.1. PMS-60 equipment and techniques

The PMS-60 system used in this study has been de-scribed in detail elsewhere.2 It consisted of a pressuremeasuring sphere (PMS), an interface device for datatransfer from and to a personal computer, and the soft-ware to operate the system and process the recordeddata. The diameter of the pressure measuring sphere was62mm, and its mass was 180 g. The sphere consisted ofa rubber ball (rubber hardness: 80° Shore-A) filled with

( 1998 Silsoe Research Institute

374 B. HEROLD E¹ A¸ .

silicon oil, whose hydraulic pressure level changed inresponse to external mechanical load. An electronic dataacquisition unit was embedded inside the sphere contain-ing a stand-alone microcomputer, a fluid pressure sensor,and a rechargable battery. This unit was able to detectpressure caused by both static and dynamic mechanicalload events, if they exceeded a preset threshold. Themeasured data were stored, together with time from aninternal clock.

Size, mass and elastic compliance of the PMS weresimilar to that of onions and potatoes,2 although thesphere’s average density (o"1)4 g/cm2) was higher thanthat of these commodities. It should also be noted thatthe spherical shape caused the PMS to roll more easily.Under static load conditions (compression between par-allel plates), the measuring range was 0—100N, and themeasuring error was approximately 5% (in the lowerrange, about 10%). Under dynamic loading (droppingthe sphere at random orientation vertically onto a hori-zontal surface), the measured pressure values showeda relatively strong dependence on the orientation angleduring impact application. For this reason the measuringerror was higher, and 10—15 measuring runs are requiredto obtain representative average results. To simplify mat-ters, a static calibration mode (between parallel plates)was performed. Presuming the existence of mechanicalsimilarity between PMS-60 and real onions, the PMS-60load data could be used to describe approximately theonion impact behaviour.

Fig. 1. Definition of parameters of a mech

The operation of the PMS was as follows. A five-pinconnector joined the sphere to the serial interface ofa personal computer. The sphere’s data acquisition unitwas controlled (including setting of data sampling rate,sampling threshold value, and other parameters) usingsoftware on a portable computer. In operation, thesphere was initialized, disconnected from the computerand passed with the commodities through the machin-ery. The sphere was then reconnected to the per-sonal computer to transfer recorded data for furtherprocessing.

From measured data of each mechanical load event,three parameters could be derived:

(1) duration of the load event,(2) peak force (i.e. the maximum value of the load event),

and(3) load integral (i.e. the area under the force—time curve

for the load event).

An example of an impact and definitions of load para-meters are given in Fig. 1.

The number of recorded mechanical loads or im-pacts, the peak force and the load integral of eachload event were used to evaluate the measured handlingprocess.

For data acquisition under laboratory drop tests aswell as under practical conditions, the sampling rate wasset at 3 kHz (sufficient to acquire both static loads andshort impacts), and the preset sampling threshold was

anical load event measured by PMS-60

MECHANICAL LOAD AND ITS EFFECT ON BULB ONIONS 375

20N. Experiences showed no significant damage ifonions are contacting flat surfaces under mechanical loadequivalent to force values below 20N.

2.2. Comparison with other devices

Since 1989, two types of instrumented spheres havebeen available that are able to record mechanical loaddata together with time from an internal clock. Thesespheres have been developed for basically the same pur-pose. They are used to mimic real fruit and to requireinformation on mechanical loads applied to fruit duringhandling. The main alternative to the PMS-60 is theimpact recording sphere1 IS100. The IS100 shell is madeof bee’s wax or plastic and is relatively rigid. The spherewas originally developed to detect causes of apple bruisesand is not very robust. Inside the sphere, accelerationsare measured with a triaxial acceleration sensor. TheIS100 is calibrated under dynamic load conditions usingdrops from different heights onto a horizontal surface. Itsmeasuring range is 0—500 g where g is the accelerationdue to gravity, and the measuring error is nearly 5%. TheIS100 measures impact deceleration, but unlike thePMS-60 it cannot detect static load events.

2.3. Bulb onion response to laboratory drop tests

Preliminary studies showed that most mechanicalloads were due to drops during transfers between suc-cessive components of machinery. Hence, laboratoryinvestigations were focussed on drop tests simulatingsuccessive transfers. Tests were performed in the period1994/95.

Onions of the cultivar ‘‘Rijnsburger Balstora’’ wereharvested carefully by hand. Onions ranging from 40 to80mm in diameter were selected and placed in a room todry for four days. Onions were divided at random intofifteen 35 kg samples. One sample was used as a control.Each of the other 35 kg onion sample was given one ofthe following treatments. The onions of each sample wereplaced one by one at random orientation onto a beltconveyor (belt speed 0)4m/s), and dropped from heightsof 17)5, 35, 70 and 105 cm onto a horizontal impactsurface. After bouncing off the impact surface, each onionwas caught to avoid a second impact. This procedure wasrepeated successively to give the desired number ofdrops. The desired numbers of drops were defined ac-cording to two different levels of impact kinetic energy(or of equivalent drop height sum), where the drop heightsum was the drop height multiplied by the number ofdrops. The values were 210 cm, calculable in four ways(12]17)5, 6]35, 3]70, 2]105 cm), and 630 cm

(36]17)5, 18]35, 9]70, 6]105 cm). Each sample wassubjected to one of these multiple drops onto eithera 10 mm thick steel plate (steel) or a PVC-conveyor belt(belt) stretched over 400mm spaced support rollers (theimpact was between the supports). The minimum dropheight of 17)5 cm was used only for drops onto the steelplate. Immediately after testing, the 15 samples weredivided into two groups. The first group was used forrespiration tests. From each sample two 1)5 kg sub-samples (about 20 onions in each) were selected atrandom and placed into two closed containers with con-trolled ventilation. These 30 containers were stored in-side a dark room at 12—14°C, with a relative humidity of60%, and a fresh airflow 10 1/kgh. Two infrared gasanalyzers (VTI Valtronics 2007 DHE) were used to re-cord the onion respiration (difference between the carbondioxide percentages of the incoming and the outgoing airflow) for each container. The onion respiration was re-corded for a period of about 11 weeks.

The second group was used for a 5 month storage test.In a similar way, two 15kg subsamples from each samplewere placed in separate cylindrical containers with per-meable steel walls and stored in an airy shed. These 30containers were covered by a tent-cloth, and an electricradiator was switched on if the air temperature fell below0°C. This way, the storage temperature changed between#15°C and !1°C, and the relative humidity was nearthe open air humidity. On half of these containers (one ofeach sample), a static load equivalent to an onion pile 5 mhigh was applied by means of a flat plate. The differencebetween initial mass (before storage) and final mass (afterstorage) was attributed to transpiration and respirationlosses. Other loss portions after storage were determinedin connection with EC quality standards by grading thestored onions into marketable (sound), rotted andsprouted, and other unsaleable components (separatedskins, dirt).

2.4. Relationship between mechanical load input andresulting onion storage losses

The pressure measuring sphere PMS-60 was subjectedto the same laboratory drop test conditions as applied tobulb onions, and the resulting load data were recorded.Only the parameters peak force and load integral of thefirst impact of each drop were selected, so that a singleimpact event could be graphically represented as a datapoint on a peak force versus load integral diagram. Theparameters for multiple drops (impacts) were obtained byadding the parameters of single impacts. The followingparameters were used as load input for further calcu-lation: the number of drops, the peak force, and the loadintegral of the single drop, the sum of peak forces, and the

376 B. HEROLD E¹ A¸ .

sum of load integrals of multiple drops. The data foronion storage losses (mass and quality losses) and thePMS-60 laboratory test data were processed by means ofregression analysis in order to create a mathematicalrelationship between mechanical load applied and onionstorage losses.

2.4. ¸oad data acquisition under practical conditions

Mechanical loads were measured during all practicalhandling steps from harvest to storage. Measuring runscovered lifting, windrowing to a swath, harvesting fromthe swath and loading onto a truck, unloading froma truck, separation of under-sized onions and dirt, anddelivery to storage. The measuring runs were repeated10—15 times, because a single run might not be represen-tative of the possible diversity of paths through the ma-chinery. Multiple runs also reduced the measuring errorsof the PMS-60 occurring in single runs (see Section 2.1).

3. Results and discussion

3.1. Bulb onion response to laboratory drop tests

Onion respiration rates for onions dropped onto steel,drop height sum 630 cm, and for the non-loaded control

Fig. 2. Respiration rate of dropped and non-dropped (control) onio

are shown in Fig. 2. Initially, the respiration rates in-creased strongly with time, including the control. Onionsdropped from 105 cm reached the highest value of26mg/kgh CO

2. Then a gradual decrease occurred. By

the end of 12 d, the respiration rates seemed to reach anapproximately constant level. After 16 d, a further de-crease was observed. Thereafter, the respiration rates didnot show substantial changes except the samples sub-jected to 105 and 70 cm drop height. After the 19th day,the values for the sample with 17)5 cm drop height andthe control reached the same final value of about8mg/kg h CO

2.

Table 1 lists the onion respiration and transpirationrates on the 19th day, the accumulated respiration andtranspiration (over 19 d), and the mass loss (19th day) fordrops from 105, 35, and 17)5 cm onto both steel and belt(drop height sums of 630 and 210 cm). The greatest onionresponse was due to 105 cm drop height at 630 cm dropheight sum onto steel (first line in Table 1). Under thatcondition, the respiration and transpiration rates of theonion showed the highest values (17mg/kgh CO

2, and

46mg/kgh H2O), while the accumulated transpiration

and the mass loss were nearly the same as for 35 cm dropheight (drop height sum 630 cm, steel). The respirationvalues ranked clearly according to drop height, dropheight sum, and impact surface. The ratio of accumulatedrespirations between the highest (105 cm, 630 cm, steel)and the lowest (35 cm, 210 cm, belt) mechanical input was

ns during an 11 week test period for given drop height sum in cm

Table 1Onion response to different treatments after 19 d storage (averages of two 1 · 5 kg subsamples)

Drop Respiration Transpiration Accumulated AccumulatedDrop height rate rate respiration transpiration Mass loss

Impact Number height, sum, (at 19th day), (at 19th day), (19 days), (19 days), (19 days),surface of drops cm cm mg/kg h mg/kg h g/kg g/kg g/kg

Steel plate 6 105 630 17)0 46)0 9)5 27)5 34)1Steel plate 18 35 630 10)7 42)8 6)3 27)8 34)5Steel plate 36 17)5 630 7)8 35)8 4)9 24)4 29)4Steel plate 2 105 210 9)6 34)9 5)6 23)5 27)1Steel plate 6 35 210 9)2 36)8 4)9 22)7 25)7Steel plate 12 17)5 210 7)6 28)9 4)0 21)0 28)7Conveyor belt 6 105 630 9)2 33)7 4)5 21)4 24)6Conveyor belt 18 35 630 6)4 27)6 3)7 21)1 25)9Conveyor belt 2 105 210 6)8 28)2 3)9 19)9 21)1Conveyor belt 6 35 210 6)7 30)2 3)6 19)8 23)4Not dropped 0 0 0 7)0 32)6 3)7 19)8 21)7

MECHANICAL LOAD AND ITS EFFECT ON BULB ONIONS 377

approximately 2)6. This was the most sensitive indicatorof mechanical stress. On the other hand, the values oftranspiration and mass loss were not so clearly ranked,and the corresponding ratios of accumulated transpira-tions and mass loss were lower, in both cases about 1)4.Both accumulated respiration and transpiration oflowest mechanical input (35 cm, 210 cm, belt) did notdiffer from non-dropped control, and the mass losseswere only slightly different. Evidently, the respirationrate was influenced by random variations between theonion samples.

For drops onto hard steel, the accumulated respirationdepended more strongly on drop height than on numberof drops. This result was not found for drops onto the

TablPercentages of onion mass losses after 5 mont

Storage Impactheight surface 0 210 210 210(staticload),

m 0 17)5 35 70

0 12 6 3

5 Steel 13)4 18)4 16)9 18)60 Steel 17)5 NA 14)4 NA5 Belt 13)4 NA 12)8 NA0 Belt 17)5 NA 15)6 NA

NA"not available.

soft belt. In that case, the accumulated respiration causedby the different mechanical inputs did not substantiallydiffer from that of the control (except for the case of the105 cm drop height and 630 cm drop height sum). Thiscan be attributed to the different deformation stress in-duced by hard and soft impact surfaces on the ‘‘medium’’hard bulb onion. Impacts onto the soft belt were assumedto cause only effects on small volumes of onion cell tissue.In contrast, an increased respiration seemed to reflectphysiological response of large volumes of cell tissue dueto hard impacts.

Table 2 shows the results of long-term storage testswith 15 kg subsamples. Herein, additional results areincluded from drop test onto the belt from a height of

e 2h storage (data from each 15 kg subsample)

Drop height sum, cm210 280 630 630 630 630 700

Drop height, cm105 140 17)5 35 70 105 140

Number of drops2 2 36 18 9 6 5

Total loss, %

25)0 NA 17)2 24)9 23)5 34)0 NA24)3 NA NA 19)2 NA 26)0 NA15)2 15)9 NA 21)8 22)0 21)1 16)211)9 NA NA 19)7 NA 17)3 NA

378 B. HEROLD E¹ A¸ .

140 cm (drop height sum 280 and 700 cm). Total lossparameters are given, i.e. the difference between initialmass of subsample as above and its final mass afterremoving the unsaleable components. Two static loadconditions during storage are considered: with static load(equivalent to 5 m height of storage pile), and withoutstatic load (equivalent to 0m height). These results agreewith those from respiration tests. The ‘not dropped’ con-trols had total losses of 13)4% and 17)5%, respectively.The highest total loss, 34%, was found for six drops ontosteel from 105 cm (with static load). However for thedrops onto the belt, the highest total loss, 22%, was fornine drops from 70 cm (with static load). Most of thesamples stored without static load showed lower totallosses.

The relations between different load factors and stor-age losses were studied by several simple linear regressionanalyses. The analyses were done separately for bothdrops onto steel and drops onto belt, including on theone side the number of drops x

1, drop height x

2, drop

height sum x3

and static load during storage x4, and on

the other, direct mass losses y1, total mass losses y

2, loss

due to rotting y3, and loss due to sprouting y

4:

y1,2,4

"b0#b

1x1#b

2x2#b

3x3#b

4x4

(1)

The different load conditions have a significant effect ondifferent portions of onion storage losses which is ex-pressed by the amount and the sign of regression coeffi-cients b

i(Table 3).

Table 3Results of regression analyses based on data of onion storage

losses (level of significance for probability p595%)

Regression coefficient bi

Regressand Regressor Drop onto steel Drop ontobelt

x1

NS 0)088y1

x2

0)011 NSDirect mass loss x

30)0027 NS

x4

!0)26 !0)15x1

NS NSy2

x2

0)094 !0)029Total mass loss x

30)0074 0)0113

x4

NS NSx1

NS NSy3

x2

0)061 NSRotting loss x

3NS 0)0068

x4

0)89 0)49x1

NS NSy4

x2

0)033 NSSprouting loss x

3NS NS

x4

!0)33 NS

NS"not significant parameter.

The direct mass loss, i.e. the difference between initialand final mass due to respiration and transpiration, in-creased gradually with increasing both drop height anddrop height sum for drop onto steel. For drops onto thebelt, a strongly increasing effect was found with increas-ing number of drops. For drops onto both steel and belt,the direct mass loss decreased with increased static loadduring storage.

The total mass loss increased strongly with increasingdrop height for drops onto steel. However for drops ontothe belt, a gradual decrease was found. For both impactsurfaces, an increasing drop height sum led to graduallyincreasing total mass loss.

The loss due to rotting increased strongly when a staticload was applied for drops onto both steel and belt.Increasing drop height was found to cause increasingrotting loss for drops onto steel, while for drops onto thebelt, the drop height sum had increasing effect.

The loss due to sprouting was not significantly affectedby drops onto the belt. For the drops onto steel, anincreasing effect of drop height was found, while thestatic load caused decreased sprouting loss.

3.2. Relationship between mechanical load input andresulting onion storage losses

Summarizing these results, very different relationswere found between load input and onion storagelosses for the two impact surfaces. In order to explainthis behaviour, the PMS-60 load data were used to devel-op a clearer model of impacts against hard and softsurfaces.

The data of PMS-60 measurements under equivalentdrop conditions are shown in the peak force versus loadintegral diagram (Fig. 3). This diagram is useful to dis-criminate between static and dynamic load events, andamong the latter between ‘hard’ and ‘soft’ impacts. Theplotted points represent the impact loads for 330 dropsfrom heights between 10 and 110 cm onto steel, and for420 drops from heights between 10 and 140 cm onto thebelt. Separate regression lines were calculated for dropsonto steel (line A), and for drops onto the belt (line B),respectively, and entered in the diagram. From thesedata, it could be concluded that the value of load integraldid not exceed 0)9Ns under impact conditions. There-fore, the region C with higher values of load integral wasseparated from the region of the graph where impactsoccurred.

Based on the storage loss data and on the averagedimpact data from PMS-60 measurements, a multiplelinear regression analysis for total storage losses S¸ usingthe number of drops (impacts) n, the peak force F

max, the

sum of peak forces n Fmax

, the load integral ¸I, and the

Fig. 3. Peak force versus load integral diagram with the impact data from PMS-60 drop tests onto steel and onto belt, and with thedamage threshold lines calculated according to resulting 5% additional onion storage losses for different numbers of drops

MECHANICAL LOAD AND ITS EFFECT ON BULB ONIONS 379

sum of load integrals n¸I, gave the following equation:

S¸"13)9!0)566n#0)054Fmax

!0)0027nFmax

!9)8¸I#3)31n¸I (2)

where r2"0)71 (r"correlation coefficient) and N"25(N"number of data points).

This regression model reflected satisfactorily the basicrelations between mechanical load measured by means ofPMS-60 and resulting total mass losses for impactsagainst both hard and soft impact surfaces (Fig. 4 ).

3.3. Application of relationship for damage prediction

Equation (2) could be used to describe the potential foronion storage losses, based on PMS-60 data of mechan-ical loads during handling.

To define the allowable mechanical load thresholds ordamage thresholds, a mechanical load was assumed to bedamaging if the storage losses of the dropped onionsdiffered significantly (p"95%) from those of not drop-ped onions. The damage threshold lines for onions undermultiple drop conditions were constructed by using theregression equation, Eqn (2). Transposing this equation,data pairs of the peak force and the load integral can becalculated for given (assumed) values of number of im-

pact loads and total mass losses. The value of total masslosses was assumed according to 5% additional masslosses (added to calculated mass loss of the ‘not dropped’control). The number of impacts were assumed as shownin Fig. 3. The calculation of two appropriately selecteddata pairs (points in the peak force versus load integraldiagram) is sufficient for each of the given values. Thenthe line joining each of the two points represents thecorresponding damage threshold line in this diagram.

The direction of these threshold lines indicated whichparameters determined the risk of onion storage losses. Ifonly a few impacts occurred, then the peak force wasmore important than the load integral. However, in thecase of numerous impacts, the load integral had greatereffect than the peak force.

By means of the lines A and B, combined with damagethreshold lines as defined above, the peak force versusload integral diagram was divided into eight classes ofdifferent mechanical load regions (see partition of data inFig. 6). These load regions were defined as follows:

¸oad region 1description: weak impacts;effect: negligible effect (additional mass losses

(1%).

¸oad regions 2 and 3description: mechanical loads due to very soft or light

impacts (region 2) or small quasistatic load

Fig. 4. Total mass losses of dropped onions after a 5 month storage period in comparison with that of not loaded onions (data calculatedby regression analysis)

380 B. HEROLD E¹ A¸ .

and scraping (region 3), e.g. during flowout of bunker, or permanent load in bulkstore up to 5 m height;

effect: insignificant effect.

¸oad regions 4, 5 and 7description: medium and strong impacts;effect: non-allowable load levels particularly in

regions 7 and 5, i.e. technical improvementis required (addition mass losses '5%);mostly allowable load level in region 4, ifonly a few impacts occur;

¸oad regions 6 and 8description: medium and strong quasistatic load and

scraping, e.g. on shed;effect: preliminary estimated: partly allowable

in region 6; non-allowable load level inregion 8, i.e. technical improvement isrequired.

It should be noted that the threshold boundaries arenot sharp. The definition of load regions was an ap-proach to evaluate the strength as well as the number ofload events. That means the method is strictly valid onlyfor multiple impacts of the same level as applied for theabove described laboratory conditions.

3.4. PMS-60 load data and estimation of practicalhandling damage

The mechanical load sequence detected by the PMS-60 during passage through a windrower is shown inFig. 5. The run was recorded with a sampling frequencyof 3)33 kHz over a period of 6 s and with a load thresholdof 15)7N. All load events above a threshold of 20N arepresented. The first is shown with an expanded time axis(top, left). This load event had a duration of 27)4 ms dueto a quasistatic compressive load. The expandedforce—time diagram is also shown for a typical impactevent with a duration of 5)5ms (top, right).

Average results of 20 measuring runs through thewindrower are presented in Table 4. Most of detectedload events were impacts with small values of load inte-gral and of peak force. The probability of an impact withpeak force above 100N during a run was 20%. A quasi-static load was found to occur in 60% of runs.

The raw data of the occurrence of different mechanicalloads could be useful for direct comparison of differenttechnologies as well as of different operation modes ofhandling machinery. However, this data alone does notenable an estimate of the damaging effects on produce.Therefore, the practical load data measured by means ofthe PMS-60 were added to the peak force versus loadintegral diagram. The intention was to use the same

Fig. 5. Force-time diagram of a PMS-60 measuring run through the windrower (diagrams of a quasistatic, and an impact load,respectively, with expanded time axis including the sampling points are shown on the top)

MECHANICAL LOAD AND ITS EFFECT ON BULB ONIONS 381

relation found under laboratory conditions, in order toevaluate the measured practical load data. Figure 6shows data collected by the PMS-60 during 20 measur-ing runs through an onion harvester and the followingtransfer onto a truck. Several hundreds of load eventswere recorded. Most of them were impact loads, i.e. thevalues of their load integral did not exceed 0)9N s. Themajority of impacts were low (peak force below 100 N),while load integral values were spread over a wide range.Using the procedure to classify the load events into theeight load regions, the resulting data are shown in

TablClassification of mechanical loads detected by means of PMS-60

integral (averaged number

Peak force, N 0—0)2 0)2—0)5

20—60 45)9 4)660—100 1)4 2)4100—140 0)0 0)2140—180 0)0 0)0

Sum 47)3 7)2

Table 5. The data from the windrower, from harvestingincluding transfer to the truck, and from postharvesthandling (including unloading the truck and transport tothe storage pile), are shown in rows 1—3, respectively. Thegreat majority of impact loads were classified into loadregion 1, while about 15 impacts into load regions 4,5 and 7 would exceed the allowable threshold. Harvest-ing from the windrow, and in particular the transfer fromthe harvester onto the truck were found to cause severemechanical loads. Some of these impacts had high forcesrelated to low levels of load integral, which corresponded

e 4during windrowing of onions according to peak force and load

s of loads from 20 runs)

Number of loads

Load integral, N s0)5—0)9 0)9—5 '5 Sum

0)3 0)2 0)3 51)30)1 0)1 0)0 4)00)0 0)0 0)0 0)20)0 0)0 0)0 0)0

0)4 0)3 0)3 55)5

Fig. 6. Peak forces versus load integral diagram with the different load regions shown, together with data points of load events recordedduring a PMS-60 run through an onion harvester (*) and the transfer onto truck (]) (data collected during 20 measuring runs through

machinery)

382 B. HEROLD E¹ A¸ .

to dropping from about 1 m onto steel. Small impactsoccurred during windrowing. The handling steps from un-loading the truck to storage also caused numerous impactsof medium and small levels at the many transfer points.

These data can be applied to identify where improve-ments in the handling technique are required. It shouldbe noted that no results exist on the effect of multipleimpacts at different (mixed) levels. Unfortunately, inpractical production, multiple drops of different levelsoccur, and so this evaluation method could be consideredto be a preliminary approach.

Nevertheless, the method identifies the most importantfactors which affect the onion damage and resulting stor-age losses.

TablClassification of mechanical loads detected by means of PMS-60 du

regions (averaged number

1 2 3

Windrowing 49)3 2)1 0)6Harvesting 85)3 7)4 2)6Postharvest handling 47)6 1)7 2)4Sum 182)2 11)2 5)6

4. Conclusions

Multiple mechanical loads had a significant effect ononion respiration rate as well as on additional masslosses after a five month storage period. Therefore, theseparameters are more appropriate for determining onionresponse to excessive mechanical stress than parametersof visible damage that are evaluated immediately aftermechanical loading.

Results of laboratory drop tests at two kinetic energylevels (according to number of drops multiplied by dropheight) indicated a complex relation between mass lossesafter five months storage period, and drop height, num-ber of drops and elastic compliance of impact surface. In

e 5ring harvest and postharvest handling according to the eight load

s of loads from 21 runs)

Number of loads

Load region4 5 6 7 8 Sum

3)4 0)1 0)0 0)0 0)0 55)56)3 0)2 0)2 0)0 0)0 102)05)0 0)1 0)0 0)1 0)0 56)9

14)7 0)4 0)2 0)1 0)0 214)4

MECHANICAL LOAD AND ITS EFFECT ON BULB ONIONS 383

the case of dropping onto a hard surface (steel plate), themass losses increased predominantly with increasingdrop height. If the onions were dropped onto a softimpact surface (e.g. a freely stretched conveyor belt), then,at the same energy level, the mass losses correspondedpredominantly to the number of drops.

From laboratory multiple drop test results, a regres-sion equation was derived, that described the onion stor-age losses as a function of load parameters (number ofdrops, peak force, load integral) acquired by using thePMS-60 under equivalent conditions. Based on this re-gression function, a procedure was developed to evaluatethe damage risk caused by handling techniques. Furtherresearch is required to explore the results to mixed loadlevel conditions.

Two kinds of critical load sources emerged from prac-tice: too many loads due to insufficiently optimized oper-ating system, and excessively high peak forces due torough transfers.

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