Vol.40, No.2, October/November 2011, pp.26-36 Backhoe ... · The purpose of this study, ... The...

V. Ramsahai et al.: Backhoe Bucket Mechanism for Ejecting Adhered Soil 26

ISSN 1000 7924The Journal of the Association of Professional Engineers of Trinidad and Tobago

Vol.40, No.2, October/November 2011, pp.26-36

Backhoe Bucket Mechanism for Ejecting Adhered Soil

Vivek Ramsahai a, April Bryanb Ψ, and Robert A. Birch c

Department of Mechanical & Manufacturing Engineering, The University of the West Indies, St Augustine, Trinidad and Tobago, West Indies

aE-mail: [email protected] bE-mail: [email protected]

cE-mail: [email protected]

Ψ Corresponding Author

(Received 13 July 2011; Revised 27 September 2011; Accepted 17 October 2011) Abstract: The clogging of backhoe buckets can be expensive as removal of clogged soil from backhoe buckets leads to downtime and the methods used to remove clogged soil e.g. jerking, can damage buckets. The purpose of this study, which was part of a final year capstone project in the Department of Mechanical and Manufacturing Engineering at The University of the West Indies, St. Augustine, was to develop an efficient method for ejecting clogged soil from backhoe buckets. The causes for soil-metal adhesion were first investigated. Then, a method for predicting adhesive forces in backhoe buckets was developed. This adhesive force prediction was used to determine the forces that an ejector mechanism needed to supply to remove adhered soil. The design of an ejector mechanism that supplied the force to counteract adhesion was then developed. A prototype mechanism, constructed from low cost EN19 steel, was used to test the effectiveness of the ejector mechanism. Field tests on clayey soil indicated that the ejector mechanism increased the speed of ejection by a factor of five, reduced the number of jerks by a factor of eight, and did not result in any soil being permanently lodged in the bucket. As the ejector mechanism is easily integrated into standard backhoe buckets, local operators can readily adopt this ejector mechanism. However, there are a couple of limitations to this study. Firstly, the ejector mechanism did not completely eliminate the need for jerking. Therefore, there was still some potential for actuator damage. In addition, although field test results indicated that the ejection mechanism was effective in releasing adhered soil, no tests were used to specifically validate the adhesive force prediction model.

Keywords: Backhoe bucket, Soil adhesion, Ejector mechanism

NOMENCLATURE Lower case letters Upper case letters (continued)

F shear forces required to overcome adhesion fcam force supplied by the cam L perpendicular distance from the point of application of a

force to the point about which moments are taken M bending moment N normal force on a tangent plane R radius of curvature V volume W weight Subscripts b bending si subscript indicating a discrete subsection i wj wall section j x cross section Greek Letters μ external friction angle ρ density of the soil σ normal stress σb bending stress τ shear stress required to overcome adhesive forces ρ density of the soil

a adhesion coefficient ac capilliary force as a result soil-metal interaction ae eletrostatic attraction between contcting bodies ag negative air pressure produced when the pores in the soil

close upon contact with another surface am molecular forces of attraction av viscous resistance of the liquid soil aw wedged pressure produced by potential chemical non-

equilibrium between the liquid film of area of contact and the void liquid

g acceleration due to gravity h height ℓ length r radius of an arbitrary surface in a curved plate

r radius of the centroidal axis of a curved plate ˆ r radius of neutral axis of the curved plate w width

e distance from the neutral axis to the point of application of a force

Upper case letters A area AR real contact area

V. Ramsahai et al.: Backhoe Bucket Mechanism for Ejecting Adhered Soil

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1. Introduction This paper presents the problem justification, mathematical modeling, prototype definition and testing that were undertaken to design, build and test an ejector mechanism for a backhoe bucket which is used on a JCB 1CX excavator. This project, which was initiated by a final year student through his association with the Rio Claro Construction Co. Ltd, was undertaken in order to satisfy the requirements of the capstone design course in the Department of Mechanical and Manufacturing Engineering at The University of the West Indies (UWI).

The capstone design course of the Department of Mechanical and Manufacturing Engineering at UWI forms a critical portion of the final year curriculum and is a requirement for graduation. It is structured as a project only course in which students work individually on projects. Although the type of projects undertaken may vary, the design, build and test is the most popular type of project. Not only because it gives students an opportunity to synthesize knowledge from a wide range of courses, but also because students often obtain a great sense of satisfaction from the hands on experience gained in building prototypes.

The course is evaluated in two phases; a preliminary design phase, and a final design phase. In the preliminary design phase, students submit a written project proposal and give a ten-minute formal presentation. They then receive feedback on the suitability of their topic and project scope. At the end of the final design phase, students submit a final report and give a formal presentation to a panel of faculty evaluators. The skills assessed at this phase include written and oral communication skills, project planning and scheduling, decision making, development of functional specifications, drawings, creativity, concept generation, analysis, prototyping and testing. The set of assessed skills satisfies the criteria for design experiences that have been established by the Institute for Mechanical Engineers (IMechE, 2009) and the Accreditation Board for Engineering and Technology (ABET, 2007; Howe, 2010). 2. Project Description The excavator is a versatile machine that is often used in construction because of its double bucket feature and numerous other attachments that give operators a wide range of capabilities. The backhoe bucket is the primary attachment of the excavator and has existed with it since its inception. It serves as a tool for digging soil and other landfill materials, which earned it the nickname “digger buckets” as a result. Typical backhoe buckets range in size from twelve inches (12”-305mm) up to sixty inches (60”-1,524mm). The largest buckets are used for digging and removing earth from trenches, while the smaller buckets are used for similar functions on a smaller scale (e.g., for digging box drains).

A standard backhoe bucket is comprised of teeth, a

bottom plate, a convex plate and side plates (Borinara (2002). The wedge-shaped teeth facilitate the penetration of the bucket into soil. The bottom plate, which is sometimes referred to as the separation plate, is used to provide shear forces that separate the body of the soil above from the soil below it. The bottom, convex, and two side plates form a container for holding the soil.

According to Sakaida (2008), the backhoe bucket performs three general operations: 1) digging, 2) transporting, and 3) emptying. At the beginning of the digging operation, the bucket is uncurled so that the opened end of the bucket is oriented with the teeth at an acute angle to the digging surface. The bucket then moves through the soil in some predefined trajectory performing the operations of penetrating, dragging, and curling (Singh, 1995). At the end of the digging operation, the bucket emerges from the soil in a curled orientation with the opened end almost parallel to the horizontal. The backhoe bucket is then lifted and transported to the dumping site. At the dumping site, the bucket is uncurled until the opened end of the bucket is perpendicular to the ground, thereby allowing for the release of the bucket’s contents.

During digging in certain soil media, e.g. clayey soils, the load sometimes becomes firmly lodged inside the backhoe bucket due to the adhesive forces that exist between the soil and the metal. In order to release clogged material from buckets, excavator operators either jerk the bucket with the hydraulic arm or hit the bucket against a solid object. The results of a survey of local operators indicate that clogging occurs more frequently with smaller buckets (Ramsahai, 2011). In addition, eight out of eleven operators preferred to release the clogged soil by hitting the bucket against a solid object. In order to accomplish this latter action, high actuator pressures are applied to increase the momentum at which the bucket hits the solid object. At impact, the teeth are the first to come into contact with the object. The combination of small contact area and large impact force, cause the teeth to become deformed and dulled. As a result of the worn and dulled teeth, increased hydraulic forces are required in order for the backhoe bucket to perform the standard amount of work (Singh, 1995). This action also leads to actuator damage.

Hitting the backhoe bucket against a solid object has another unintended result. This action results in the transmittance of higher than normal forces through the entire tool, which also increases the likelihood of shear pin wear at the bucket manipulator joint. Failure of the teeth, actuator and shear pin damage makes it necessary for excavator owners to prematurely replace their backhoe buckets. However, the backhoe bucket is not only an expensive attachment, but the replacement of buckets often results in downtime, which leads to project delays and increased costs. It has been observed that this adhesion phenomenon can reduce the efficiency of excavation by as much as 30-50% (Ren et al, 2001).


This paper presents a method for predicting the adhesive forces in backhoe buckets and the design of an ejector mechanism to automatically release the clogged dirt from small, 305mm (12”), backhoe buckets. The ejector mechanism, which is attached directly to the bucket, provides a great advantage to excavator owners as it aids in the safe and easy release of material, saving them the cost of new buckets and lost earnings due to downtime. Results from the testing of a prototype ejector mechanism are also presented. 3. Review of Relevant Literature 3.1 Adhesion Forces Adhesive forces are the bonding forces that exist between soil and another material surface. These adhesive forces develop when the forces of attraction between the soil and the surface exceed the separation forces due to sliding and the soil’s weight. The main factors affecting the strength of adhesive forces are soil type, real contact area, level of soil compaction, soil density, moisture content, and acidity of the soils (Ren et al., 2001; Khan, 2010).

Adhesion is greatest in clayey soils, which have an extremely small grain size, usually <2µm (ISO 14688-1, 2002, ASTM 2006). Adhesive forces increase as the real contact area between the soil and the metal surface increases. Hence the reason why it is more pronounced in smaller buckets, in soils with a greater level of compaction (Hazrat, 2010) and in more dense soils. As the parallelism of clay micro platelets to metal surfaces increases, the adhesive forces increase (see Figure 1). This indicates that increases in the real contact area between soil and a metal interface, even on a micro scale, results in increases in adhesive forces.

Figure 1. Relationship between real contact area of the soil-metal interface and the adhesion value

Source: Abstracted from Kooistra et al. (1998) Generally, adhesive forces are highest when the

moisture content is between the plastic and liquid limits. Zhang (1985) proved that a particular soil would have its

highest value of adhesion at moisture contents between 22% and 32%. Figure 2 shows results of an experiment performed by Khan et al. (2010) that support the findings of Zhang (1985). Finally, acidic soils are less adhesive than soil containing fresh rotten material.

Figure 2. Relationship between the soil moisture content

and adhesion Source: Based on Khan et al. (2010)

One accepted model for characterising the strength of

adhesive forces in soils is the Mohr-Coulomb equation (Kooistra et al., 1998), which defines adhesion in terms of the shear stress that needs to be applied to release soil from a metal surface. It is defined as in Equation 1. τ = a + σ tan μ …….. Eq.1

where τ is the shear stress required to overcome the forces of adhesion, a is the adhesion coefficient which is defined as in Eq. 2, and σ tanμ is the shear stress due to friction on an inclined plane. The latter term consists of σ, the normal stress due to the soil’s weight, and μ, the external friction angle. Figure 1 shows the relationship between τ with σ.

Khan et al. (2010) proposed that the total adhesive force, a, is determined by summing the effect of six individual forces as shown in Equation 2. These forces are am, the net molecular forces that result when soil contacts a metal, ae, the electrostatic force of attraction between the soil and the metal surface, ac, the capillary force that develops when soil contacts a metal, av, the force due to the viscosity of the liquid phase of the soil at the interface, aw, the wedge pressure that results from the pressure difference between voids in the solid soil matrix and the liquid soil pressure (Fecan et al., 1999), and lastly, ag, the negative air pressure that develops when soil pores close upon contact with a metal.

a = am + ae + ac + av + aw + ag ……..Eq.2

The Bowden and Tabor (1964) equation defines the normal stress on the friction plane as follows:

N = σ AR …….Eq.3

where N is the normal force due to the weight of the soil or other clogged material on the shear plane, and AR is the


real contact area between the soil and metal. 3.2 Approaches to Overcoming Adhesion Several approaches have been introduced to overcome the effects of adhesive forces between soil and the metal surfaces of backhoe buckets. These include liquid injection, soil scouring by vibration, surface modification, electro osmosis, and mechanical methods (Khan et al., 2010; Ren et al., 2001). Mechanical approaches to overcoming adhesion typically involve the use of devices that supply sufficiently high forces to overcome the forces of attraction between the soil and the metal surface. They tend to be preferred because the devices are usually cheap, effective, operator friendly, and reliable. Therefore, a mechanical approach is being pursued in this study.

Several mechanical devices for overcoming adhesive forces in backhoe buckets have been proposed. These devices typically involve the use of an integrated flat plate to dislodge the soil (Long, 1970; Gene, 2004; King, 2004; Campbell, 1973). A major flaw with the use of flat plates is that they reduce the volume of the bucket, thereby reducing the digging efficiency of the bucket. Manufacturers of earthmoving equipment such as Liebherr (n.d.), install hydraulically operated ejector mechanisms into their backhoe buckets. The mechanisms involve the use of a steel plate that passes a fraction of an inch away from the bucket’s surface. The plates typically match the geometry of the bucket. This approach is quite useful and effective. However, these buckets are expensive and limited to the companies’ respective machineries. Local distributors find excavators equipped with these ejector mechanisms to be too expensive and as a result, they do not stock nor sell them. Therefore, this study presents the development of a new inexpensive, effective, and reliable device for ejecting soil from a backhoe bucket. 3.3 Adhesive Forces The design of a mechanism that is effective in removing clogged soil from a backhoe bucket requires the determination of the forces required to overcome the existing adhesive forces between the soil and the backhoe bucket. The net adhesive force in a backhoe bucket is the sum of the adhesive forces on the individual surfaces, viz. the bottom and sidewalls. As previously indicated, the Mohr-Coulomb Equation in Equation 1, which characterises the adhesive force in terms of the shear stress required to remove the soil from a metal surface, is used to determine the strength of the adhesive forces at the soil-metal interface. Recall that this equation has two main components, the adhesion coefficient and the normal stress that is computed according to Equation 3 (Bowden and Tabor, 1964). Equation 3 is not directly applicable for the computation of adhesive forces in a backhoe bucket as this equation was originally developed for flat surfaces and the backhoe bucket has a complex curved geometry.

Therefore, in this study, a new procedure is introduced for predicting adhesive forces in a backhoe bucket.

In this procedure, the interior of the backhoe bucket is first divided into six Sections (A, B1, B2, C, D and E) as shown Figure 3. Within each discrete section, the curved surface of the bucket’s bottom is approximated to a flat plane by constructing a tangent plane to the curved surface. The normal force acting on tangent surface i, Nsi, is the component of the soil’s weight in section i, Wsi, that acts perpendicular to the tangent surface as shown in Eq. 4.

Nsi = Wsi cosμsi ………Eq.4 where

Wsi = ρg(ℓsi hsi wsi) …….. Eq.5

In Equation 4, μsi is the angle between the weight in the discrete section and the perpendicular to the tangent surface. In Equation 5, ρ is the density of the soil, and g is the acceleration due to gravity. ℓsi, hsi, and wsi are the length, height and width of the discrete section, respectively.

Figure 3. Backhoe bucket sections

It is observed that the approximated tangent plane is

analogous to the shear plane on which the adhesive forces act, justifying the use of Equation 1 for determining the magnitude of the adhesive forces in the backhoe bucket. Substituting Nsi and Wsi into Equation 1, and considering Fsi to be the shear force required to overcome the adhesive forces in a given section of the bucket, Fsi can be computed as follows:

Fsi = a +Wsi cosμsi

Asi

⎛

⎝ ⎜

⎞

⎠ ⎟ Asi

……..Eq.6

where Asi is the real contact area. It is observed that the shear force required to remove

the soil from the bucket’s walls, Fwj, differs from that required in the previous sections. This is due to the fact that the component of weight on the walls is negligible. As a result, Equation 6 reduces to Equation 7 at the


bucket’s walls. Fwj = aAwj ……..Eq.7

4. Ejector Mechanism Design The proposed backhoe bucket ejector mechanism is shown in Figure 4. It is designed for a 305mm (12”) JCB 1 CX backhoe bucket. It consists of a plate with a compound geometry, an extrusion with a cam surface and a spring. One section of the compound plate is flat while the other section of the plate is curved. The free end of the curved plate applies the forces necessary to shear the soil from the bucket. At the opposite end, the flat plate is pivoted to the bucket. A cam, positioned at approximately at the center of the flat plate, supplies the compound plate with the necessary ejection forces. A spring at the middle of the plate returns the plate to its original location at the end of the ejection operation. The materials and geometry for the ejector mechanism are given in Table 1.

Figure 4. Backhoe bucket with ejector mechanism

Table 1. Ejector Mechanism Materials

Component Material Cross-section Plate

EN19 6.35mm x 304.8mm (1/4" x 1')

Extruded rod (aka Extrusion)

EN3b 38.1mm (1-1/2") Dia.

Cam (head) EN3b 76.2mm (3") Dia.

Plate connector (pivot) EN19 76.2mm (3") sq.

Dog bone

EN3b 76.2mm (3") Dia. 152.4mm (6") EN3b

When the bucket is in the digging or transporting

orientations, with the opened end of the bucket parallel to the horizontal, the ejector mechanism remains aligned with the surface of the bucket. The mechanism becomes activated during uncurling. During uncurling, the cam travels on the angled arm of the excavator causing the compound plate to rotate outwards from the bucket’s

surface. This cam action supplies the shear forces at the end of the curved plate that results in ejection of the soil.

The ejector mechanism is integrated into the bucket by simply creating a hole at the top of the bucket to accommodate the extruded rod and cam and welding the flat portion of the plate to the pivot. This device is therefore, much lower in cost than the purchase of an ejector bucket and can be easily added and removed as needed. The use of the low cost materials indicated in Table 1 and the simplicity of manufacture ensure that the cost of the device remains low enough to remain within the reach of local manufacturers.

The successful design of an ejector mechanism requires the supply of sufficiently high forces by the ejector plate, sufficiently stiff plates to withstand the reaction loads of the soil, a strong enough spring to return the displaced mechanism to its original position, and a small enough modification to the bucket so that the capability of the bucket to withstand digging forces is not affected by the presence of the hole. Explanations of how the ejector mechanism’s design meets these requirements are given in the following four subsections. The last subsection describes the construction of a prototype bucket. 4.1 Forces Supplied by the Backhoe Bucket Ejector

Mechanism The backhoe bucket ejector mechanism unclogs a bucket by the application of a shear force at the end of the curved section of the compound plate that is greater than the adhesive forces between the soil and metal at the point of application of the force. As the bucket rotates from the curled position to the emptying position, the cam rolls on the excavator’s arm, thereby increasing the forces supplied to the plate. The equation for computing the shear force that needs to be supplied to the soil by the ejector plate was given in Equation 7 and the computation of the cam force required to accomplish this action is given in Equation 8.

fcam =Fsi

i∑ Lsi

Lcam

……Eq.8

For the 305mm (12”) excavator used in this study, the parameters for computation of Nsi according to Equation 4 are given in Table 2, while the parameters used for computation of Fsi are given in Table 3. It is noted that the value of Lcam is 13.5 cm. Section A, which has the greatest adhesive forces due to its large volume, is the primary point at which the shear forces supplied by the ejector mechanism begin to act. As uncurling progresses, the ejector mechanism then supplies shear forces to the other Sections B2, C, E and the interior base of the bucket. In Sections B1, D and E. the soil is not in contact with the shear forces supplied by the plate and tends to adhere to the ejector mechanism’s surface as shown in Figure 5.


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Table 2. Parameters for computation of the normal force

Section i αsi Lsi (m) Asi (m2) Vsi (m3) Wsi (N) µsi Nsi (N) A 0 0.092 0.025 0.007 180 32 -153 B1 30 0.084 0.023 0.006 150 2 -150 B2 30 0.084 0.023 0.006 150 62 -71 C 60 0.040 0.011 0.002 54 88 2 D 60 0.041 0.011 0.002 55 28 -49 E 80 0.023 0.006 0.001 14 68 5 Int. Base 90 0.023 0.006 0.001 14 58 8 Int. Top 90 0.041 0.011 0.001 25 58 -13

Table 3. Parameters for computation of the shear force required to overcome adhesion

Section Lsi (m) Asi (m2) Nsi (N) σsi (Pa) τsi (Pa) Fsi (N) A 0.093 0.025 -153 -6061 12794 323 B1 0.099 0.027 -150 -5586 12967 349 B2 0.099 0.027 -70 -2624 14045 378 C 0.079 0.022 2 88 15032 325 D 0.076 0.021 -49 -2373 14136 293 E 0.120 0.033 5 164 15060 494 Int. Base 0.038 0.010 8 727 15264 159 Int. Top 0.160 0.044 -13 -307 14888 651

4.2 Plate Stiffness Therefore, the removal of the soil from these surfaces is dependent on the soil’s weight being greater than the adhesive forces. The stresses are given in Table 3. The adhesion coefficient used for the computation of shear force was taken to be 150,000Pa.

As a result of transverse reaction forces from the soil, the ejector mechanism’s compound plate would be subjected to bending forces during soil ejection. Therefore, it is necessary that plates are sufficiently stiff to overcome these forces. Figure 6 shows that the location of the maximum bending force is at the joint between the flat and curved sections of the plates.

Using the values in Tables 2 and 3 and Eq. 6, the force required at the end of the compound plate to shear the soil was found to be 8,449N. This led to a total required cam force of 6,294N.

Figure 6. Ejector mechanism bending loads Therefore, the system is modeled as a curved cantilever beam and the curved beam equation is used to determine the maximum stresses experienced in the plate, Equation 9 (Barber, 2001). Figure 5. Backhoe bucket showing cam force and moment arm


σb =MAx

1−

ˆ r r

⎛ ⎝ ⎜

⎞ ⎠ ⎟

r − ˆ r ( ) …….Eq.9

where for a rectangular cross-section,

ˆ r = hxwx

lnr + hx 2r − hx 2

⎛

⎝ ⎜

⎞

⎠ ⎟

…….Eq.10

From Equation 9, the maximum stress experienced in a plate with the assumed geometry was found to be 183 MPa. Recalling that the ejector mechanism was constructed from EN19 steel, this led to a factor of safety of 2.7 for the plates. 4.3 Spring Design A spring is used to return the deflected plate to its original position after the mechanism has completed the emptying operation. The spring has to be sufficiently strong to overcome the weight of the compound plate, but soft enough to transmit the applied cam forces to the plate. The bucket with the spring is shown in Figure 7.

Figure 7. Spring force diagram

The weight of the compound plate required for

computation of the spring force is assumed to consist of the weight of the plate, the extruded rod and cam head. The combined weight of the plate, extruded rod, and cam was found to be 96.51 kg. This weight was found to act where the flat section of the plate was joined to the curved section of the plate. Using the above descriptions, the required spring force was found to be 750N. 4.4 Impact of Modification to Bucket It is necessary to locate a hole at the top of the bucket to accommodate the cam and extruded rod. However, this hole then becomes a point of stress concentration in the bucket. As indicated earlier, the backhoe bucket is subjected to significant stresses due to the dragging operation and this stress concentration point makes it

vulnerable to failure. Therefore, a FEM analysis was performed to

determine the impact of the hole on the excavator bucket. The FEM analysis was performed in SolidWorks Express. The joint between the bucket and arm was considered to be a fixed boundary condition for the FEM analysis. The results of the FEM analysis in Figure 8 indicate that the maximum expected Von Mises stress of 103MPa was well below the yield strength of the ASTM A36 material from which the bucket was constructed, leading to a factor of safety of 2.4 for the bucket.

Figure 8. FEM analysis of ejector mechanism

5. Testing and Results Three main tests were performed; a field test, a shear test, and a sieve analysis. The field test, which was performed in Duncan Village in Trinidad and Tobago, was used to determine the effectiveness of the ejector mechanism in releasing soil from the bucket. The shear test and sieve analysis were used to determine the properties of the soil used for field tests at the test site. These latter two tests are especially important in assuring the consistency of the properties of the soil used for testing as four sets of soil were dug from two separate pits at the test site. Soils C and D originated from Pit #1 while Soils A and B originated from Pit #2. 5.1 Field Tests The equipment used for field tests included a JCB 1CX excavator, a backhoe bucket with the ejector mechanism, a backhoe bucket without the ejector mechanism, and a scale. Ten replications of the following experiment were performed for the backhoe bucket with the ejector mechanism and without the ejector mechanism.

The backhoe bucket was used to dig soil from a hole and to transport the soil to an emptying location as shown in Figure 9a and 9b. When the backhoe bucket arrived at the empting location, a timer was started. The timer was stopped when all the soil was ejected from the backhoe bucket or at the end of 60 seconds, which ever came first. The number of jerks required to remove soil from the backhoe bucket was recorded.


Figure 9a. Backhoe bucket in field test showing Cam Head

Figure 9b. Backhoe bucket in field test showing Compound Plate

The frequency of jerks to release clogged soil and the time required to complete ejection are given in Figures 10 and 11. These results clearly indicate that the backhoe bucket with the ejector mechanism is much more effective at ejecting clayey soil than the backhoe bucket without the mechanism. Furthermore, the results indicate that soil was dislodged from the backhoe bucket without the mechanism in an average of 33 sec and 40 jerks, while the bucket with the mechanism ejected the soil on an average of 7 sec and 5 jerks. It is noted that the highest number of jerks for the bucket without the mechanism was 90, while the highest number of jerks required for the bucket with the mechanism was 17. Soil became firmly clogged in the backhoe without the ejector mechanism two times and no number of shakes was able to remove this soil. The operator had to eventually hit the bucket against the ground to dislodge the dirt. The bucket with the ejector mechanism had no occurrences of clogging.

These results indicate that, on average, the backhoe bucket with the ejector mechanism prohibits soil from becoming firmly lodged in the bucket, works five times as fast as the bucket without the mechanism and requires eight times less jerking than the backhoe bucket without the mechanism. By extension, the bucket with the mechanism will be eight times less likely to damage the actuators.

Figure 10. Frequency of jerks required to release clogged soil

Figure 11. Time (s) to Eject Soil from the Bucket

On average, 74N of soil was ejected from the bucket

without the mechanism and 75N from the bucket with the mechanism. No correlation was found between the weight of soil ejected and the number of jerks and ejection time. Since number of jerks required to release adhered soil and the ejection time are measures of ejection effort, it is observed that there was not a strong correlation between the weight of soil ejected and the ejection effort. 5.2 Sieve Analysis The purpose of the sieve analysis was to classify the soil used for field tests and to determine the similarity between the soils removed from the two pits. The moisture content and the sieve analyses were performed according to ASTM D421 and D422 standards (ASTM, 2007a, b). The ASTM D2487 (ASTM, 2006) soil classification standard was then used to classify the soil.

The results indicated that the soil used for field testing can be classified as clayey as its moisture content was in the range of 22 ± 2 % and its grain size was fine with 68% of the particles passing through a No. 200


(75mm) sieve. Recall that Soils A and B originated from Pit #2, while Soils C and D originated from Pit #1. The results in Figure 12 confirm the small particle size of the soil and indicate that the particles have a generally low coefficient of uniformity (Cu) in the range of 4.29-6.67. The coefficient of concavity for the sieve analysis was in the range of 0.42-0.69 indicating that some grain sizes were missing. Furthermore, the curves are very similar for soils from the two pits indicating that the properties of the soils were very similar.

Figure 12. Sieve Analysis of Soil at Test Site in Duncan Village

5.3 Shear Tests The purpose of the shear test was to obtain a comparison between the theoretical adhesion coefficient value that was used to design the mechanism to the actual value of the soil used in the field test. In addition, this test was used to determine the similarity in the frictional values of the soils removed from the two pits as this gives an indication of adhesiveness. Therefore, the tests described in the remainder of this section were performed for soils from each of the two pits. A soil hardness test was conducted prior to the shear test in order to ensure that the soil was sufficiently compact for the shear test.

The test apparatus and equipment consisted of a 160mm x 60mm x 150mm metal container filled with soil, a EN19 steel plate of dimensions 130mm x 40mm x 6mm, a penetrometer, and a Hounsfield H2-OK-W tensometer. The soil was first solidly compacted into the metal container. The soil strength was then determined with the cone penetrometer.

The container with soil was then placed in the moving jaw of the tensometer while the flat EN19 steel plate was placed in the stationary jaw on the opposite end so that the bottom of the plate was level with the top of the soil. Initially, the plate and the soil were not in contact and the plate was unloaded. The container was then moved at a rate of 10mm/min so that the top of the soil slid under the bottom side of the plate. The test was repeated for each of the two soil samples with weights of 14.4N, 54.4N, and

104.4N added to the plate. Readings of the tangential force required to keep the container moving under the plate at a constant velocity were recorded for the various normal loads.

Figure 13 is a plot of the tangential versus normal load for the soil tested with the mechanism (Soil A) and without the mechanism (Soil B). The penetrometer results indicate that the soils were of comparable hardness. The adhesion coefficients were relatively small and could be considered as negligible. The coefficient of friction of soil B is 13% higher than that of soil A, indicating that Soil B has a slightly higher tendency for sticking. Recall that the assumed adhesion coefficient for computation of adhesive force was 150,000 Pa. Therefore, the designed shear force applied by the mechanism would be much greater than the actual shear force required to remove the adhered soil.

Figure 13. Tangential force on Plate against Normal force for soils

A and B

The results of the sieve analysis and shear test

indicate that the soils used for field tests with the mechanism and without the mechanism from the various pits are sufficiently similar so as not to lead to a discrepancy in testing. 6. Conclusions The clogging of dirt in backhoe buckets is undesirable as it results in the need for bucket replacement and a loss in efficiency due to unproductive downtime. Adhesion at the soil-metal interface is the main cause of soils sticking in buckets. These adhesive forces are more pronounced in clayey soils with large real contact area.

In this paper, the design of a mechanical device for ejecting clogged soil from a backhoe bucket was presented. The device can be easily integrated into the bucket and activates automatically as the bucket is manipulated. Although the device was developed for a 305mm (12”) JCB excavator, it can be easily replicated


for backhoe buckets of any size. Results of field tests indicate that although backhoe buckets with the device still need to be jerked to release adhered material, the buckets with the ejector mechanism are five times faster at ejecting adhered soil and require eight times less jerking.

This project demonstrated the working principle of the mechanism for ejecting adhered soil from the excavator bucket. Future work is required to validate the model presented for predicting the ejection forces and to further develop this product for commercialisation. In addition, additional testing is required to demonstrate the feasibility of the device for larger excavator buckets.

The student undertaking this project developed skills in problem definition and justification, mathematical modeling of a real system, synthesis of a problem to working solution and testing. In addition to technical skills, the student developed soft skills as the students’ written communication skills, oral presentation skills, psychomotor skills and the ability to network were enhanced over the course of this project. Therefore, this capstone design project included many opportunities for the student to get training in preparation for industry. Acknowledgements: The authors would like to thank the Rio Claro Construction Co. Ltd. for its support of this project through the loan of excavators, backhoe buckets and personnel to conduct field tests. In addition, the authors are sincerely grateful to the personnel in the Mechanical Engineering Workshop and the Civil Engineering Laboratory at The University of the West Indies for their support in building and testing this prototype device. Special acknowledgements go to the assistance from Mr. Albert Tom Pack, Mr. Oswald Lawrence, Mr. Steve Ramoutar, Mr. Anthony La Croix and Mr. Christian.

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Authors’ Biographical Notes: Vivek Ramsahai is to graduate from The University of the West Indies in 2011 with a B.Sc. in Mechanical Engineering, upper seconds level. Mr. Ramsahai is currently employed under the Rio Claro Construction Co. Ltd, where he seeks to gain mechanical experience regarding machinery installation, and onsite management. He is also a part time employee at Trintech Chrome Ltd where he serves as a design analyst. His interests are product design, product assembly, manufacturing processes and site management. April Bryan graduated from The University of Michigan, Ann Arbor in 2008 with a Ph.D. in Mechanical Engineering and from Tuskegee University in 2000 with a B.Sc. in Aerospace Science Engineering. Dr. Bryan gained industrial experience as a Design Engineer at John Deere. Her interests are product development, product design, manufacturing system design, manufacturing processes, assembly,

product architecture, variation analysis, root cause failure analysis, metrology, and pedagogy. Robert A. Birch is an Instructor in the Department of Mechanical and Manufacturing Engineering at The University of the West Indies, St Augustine, Trinidad and Tobago. He is a registered professional engineer (R.Eng) and project management professional (PMP) with over sixteen years of industrial and teaching experience. He has a BSc. (Eng) and MPhil. in Agricultural Engineering from The University of the West Indies and is presently pursuing a PhD in mechanical engineering. Mr. Birch is a member of the Institution of Agricultural Engineers (UK) and also a member of Latin American and Caribbean Association of Agricultural Engineers (ALIA). His interests are in Field Machinery and Heavy Equipment Design, Fluid Power Technology and Soil-Machine interactions.

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