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Table of Content

• Material For Digger Bucket
• Hand Calculation for MINI DIGGER
• FEA Using SOLIDWORK

Materials For Digger Bucket

Pin

Excavator pins are commonly made of a AISI 4130 or 4140 steels. The AISI 4000 series of steels are chromium molybdenum steel. Chromium improves corrosion resistivity and its ability to be hardened, while molybdenum increases strength and hardenability, too. The 41 stands for its tensile strength: 283 MPa The last two digits are the percent carbon, as in 0.30% carbon for 4130 and 0.40% carbon for 4140.

The steel used will mostly heat treated using induction hardening. This heat treatment processes produces a hardened surface up to 58 to 63 Rockwell C for wear resistance with a internal ductile for toughness. Note that the bushings are often made of the same material as the pins. Some cheaper pins made of AISI 1045. This is a medium carbon steel that can hardened.

Bucket Side

The bucket sides and cutting edge are usually made of AR steel plate. The most popular grades are AR360 and AR400. AR 360 is a medium carbon, low alloy steel that is heat treated to provide excellent abrasion resistance and high impact strength. AR 400 has also been heat treated but it provides abrasion resistance and superior yield strength. Both of these steels have been carefully quenched and tempered to product qualities that are key to a good bucket. Note that the numbers after AR are the Brinell hardness of the steel.

Bucket Shell

The bucket shell is generally made of ASTM A572 Grade 50 (sometimes written A-572-50), a high strength, low alloy steel. This steels is alloyed with columbium and vanadium. Vanadium helps with maintaining the toughness of the steel. This grade of steel is desirable as a bucket shell material due to it provides excellent strength without weighing quite as much as comparable steels such as A36. It is also readily welded and formed.

Bucket Teeth

It is important to understand that there are two ways of manufacturing bucket teeths: casting and forging. Cast bucket teeth may be made of a low alloy steel with nickel and molybdenum as the primary alloying element. Molybdenum improves the harden ability and strength of the steel and can also help minimize some forms of pitting corrosion. Nickel increases strength, toughness, and also helps keep corrosion resistant. They may also be made from an austempered ductile iron that has undergone heat treatment for wear and impact strength. Forged bucket teeth are made from a heat treated alloy steel, but the type of steel varies with manufacturer. The heat treatment improves wear properties and increases impact strength.

2. Hand Calculation For MINI DIGGER

• Breakout force

Figure shows the measurement of bucket curling force FB, arm crowd force FS, the other terms in the figure dA, dB, dC, dD, dD1, dE, and dF shows the distances as shown in Figure.

According to SAE J1179: Maximum radial tooth force due to bucket cylinder (bucket curling force) FB is the digging force generated by the bucket cylinder and tangent to the arc of radius dD1. The bucket shall be positioned to obtain maximum output moment from the bucket cylinder and connecting linkage. FB becomes maximum when distance dA reaches maximum, because rest of the distances in the equation.

FB=Bucket cylinder force /dD( dA×dC)/dB

Where,
Bucket cylinder force = (Working pressure) × (End area of bucket cylinder)

If the end diameter of the bucket cylinder = DB (mm) and the working pressure is p (MPa) and other distances are in mm then the equation can be written as:

FB=p× π/4 ×DB2/dD×dA×dC/dB
FB= 18.996 KN

• Bucket static force analysis

Bellow Figure shows the free body diagram of the bucket Forces. As can be seen the reaction force on the bucket teeth at point A4 due to the breakout force 18.996 KN acts at the angle 38.23° for configuration of the maximum breakout force condition.

Fb= 18.996 KN
At Angle 38.23°

• Free body diagram of bucket

Static forces on joints can be calculated by considering the summation of force must be equal to zero (ΣF = 0) and summation of moment equal to zero (ΣM = 0) for equilibrium condition of the bucket, arm and boom respectively.

All the force in the Fig. Firstly the reaction force acting on the bucket teeth (at point A4) is resolved in the horizontal (X) and the vertical (Y) directions using following equations,

F4H=FB∙cosρ
F4V=FB∙sinρ

Where, ρ is the angle between the breakout force of bucket and the ground level as horizontal reference surface of 38.23º as shown in Fig.;

F4H=14.921KN
F4H=11.755KN

Now considering the bucket in equilibrium ΣM = 0, taking moment about the bucket hinge point A3,

F4∙l4−Fgb∙lgb=F11∙l11

Where, F4 is the force acting at bucket tool tip when the bucket approaches to the earth in the maximum breakout force condition as shown in Fig., which is equivalent to the bucket breakout force FB.

• l4 is the distance of the tool tip of the bucket from the bucket hinge point.
• lgb is the distance between the C.G. of the bucket to the bucket hinge point.
• l11 is the distance of the bucket hinge point to the 117
• idler link hinge point on bucket.
• Fgb is the gravitational force acting on bucket.
• F11 is the force acting on hinge point of the idler link on bucket which can be found by using equation

Acting at an angle β11 of 64º as shown in Fig. The force F11 can be resolved in horizontal (X) and the vertical (Y) directions by using the following equations.

F11H=F11∙cos β11
F11V=F11∙sin β11

Fgb= 0.10437 ( FOR A2 TOOL STEEL AS BUCKET MATERIAL )

• Resolving vertical Forces

With Upper Equation and Dimension from Above Fig.

A3v = 9.9827 KN	Vertical Upward
A3h = 27.93 KN	Horizontal
A11v =26.676 KN	Vertical Downward
A11h = 13.009 KN	Horizontal

A3 = 29.66260 KN

• Bucket pin design

The resultant force of these forces is acting at the bucket pin which is higher than that of the force acting on bucket pin for dumping cycle. Therefore, the maximum force is considered for checking the design of bucket pin. The bucket pin is checked for bearing, shear and bending failure.

first check the bucket pin for bearing failure. The total length of the bucket pin is 165mm. But for the calculations, the effective length of the bucket pin (the part of bucket pin which is in paired with the arm) is considered.

Let,

Diameter of bucket pin, db=30 mm
Shear stress of bucket pin material (IS 1403), τ =56 MPa
Bearing pressure of bucket pin material (IS 2062), pb =40 MPa
Total length of the bucket pin, lpb=165 mm
Effective length of the bucket pin, lpbe=165 mm
Horizontal component of force acting at bucket hinge point A3, F3H= 27.93 N
Vertical component of force acting at bucket hinge point A3, F3V= 9.9827 N
ymax=db/2 = 15 mm

The resultant force acting on the bucket pin is,
Fpb=F3= 29662.60 N

Also, the bearing force acting on the bucket pin is,
Fpb=db*lpbe*pb

∴pb=5.99 MPa < pb =40 MPa

Here, the bearing pressure induced in the bucket pint is very less compared to the allowable bearing pressure of bucket pin material AISI4130 as 40MPa. So, the bucket pin is safe in bearing.

Considering bucket pin is in double shear and the shear area of the bucket pin is,
Apb=2*π/4*(db)2

Now, shear force acting on the bucket pin is,
Fpbfs=2*π/4* db 2*τshear
we get the shear stresses developed in the bucket pin is,
Fpb=Fpbfs
Fpb=2*π/4*(db)2*ηshear
ηshear=20.99 MPa<  =56 MPa

Here, the shear stresses developed in the bucket pin is very less compared to allowable shear stress of bucket pin material ASIS 4130 as 42 MPa. So, the bucket pin is safe in shear.

Consider bucket pin as simply supported beam with uniformly distributed loading condition and now let us check bucket pin for bending failure.

Now, the uniformly distributed load acting on bucket pin is, wb=Fpb*lpbe
wb=179.77 N/mm

Maximum bending moment occurs at the bucket pin is,
Mb=Wb*lpbe²/8
Mb=6.11×10⁵ N∙mm

Now, the moment of inertia of the bucket pin is,
Ib=π/64(db)⁴
Ib=3.98×10⁴ mm⁴

The maximum bending stress developed in the bucket pin is,
Ζmax/ymax= Mb/Ib
ζmax=σb=230.56 MPa< σb =500 MPa

The maximum bending stress developed in the bucket pin is within the stress limit of the bucket pin material, therefore the design of bucket pin is safe in bending.

3.3  FEA USING SOLIDWORK

From the FEA Analysis the BUCKET PIN is safe and minimum factor of shefty is 2.7.

It is near about to hand calculation.

The simulation report generated by solidworks is below;

Model Information

Model name: Pin
Current Configuration: with split

Study Properties

Study name	165Pin
Analysis type	Static
Mesh type	Solid Mesh
Thermal Effect:	On
Thermal option	Include temperature loads
Zero strain temperature	298 Kelvin
Include fluid pressure effects from SOLIDWORKS	Off
Flow Simulation
Solver type	FFEPlus
Inplane Effect:	Off
Soft Spring:	Off
Inertial Relief:	Off
Incompatible bounding options	Automatic
Large displacement	Off
Compute free body forces	On
Friction	Off
Use Adaptive Method:	Off

 

Units

Unit system:	SI (MKS)
Length/Displacement	mm
Temperature	Kelvin
Angular velocity	Rad/sec
Pressure/Stress	N/m^2

 

Material Properties

Load and Fixtures

Mesh information

Mesh type	Solid Mesh
Mesher Used:	Standard mesh
Automatic Transition:	Off
Include Mesh Auto Loops:	Off
Jacobian points	4 Points
Element Size	4.88731 mm
Tolerance	0.244365 mm
Mesh Quality Plot	High

 

Mesh information – Details

Total Nodes	10004
Total Elements	6415
Maximum Aspect Ratio	3.0954
% of elements with Aspect Ratio < 3	100
% of elements with Aspect Ratio > 10	0
% of distorted elements(Jacobian)	0
Time to complete mesh(hh;mm;ss):	00:00:00
Computer name:

Resultant Forces

Reaction forces

Selection set	Units	Sum X	Sum Y	Sum Z	Resultant
Entire Model	N	0.27388	29662.6	-0.0462036	29662.6

 

Reaction Moments

Selection set	Units	Sum X	Sum Y	Sum Z	Resultant
Entire Model	N.m	0	0	0	0

 

Study Results

Name	Type	MinX	Max
Stress1	VON: von Mises Stress	1.051e+006N/m^2 Node: 785	1.723e+008N/m^2 Node: 6863

Pin-165Pin-Stress-Stress1

 

Name	Type	MinX	Max
Displacement	URES:  Resultant Displacement	0.000e+000mm Node: 1	3.197e-002mm Node: 7787

 

Name	Type	MinX	Max
Strain1	ESTRN: Equivalent Strain	3.932e-006 Element: 391	6.232e-004 Element: 2043

Pin-165Pin-Strain-Strain1

 

Name	Type	MinX	Max
Factor of Safety1	Automatic	2.669e+000 Node: 6863	4.375e+002 Node: 785

Pin-165Pin-Factor of Safety-Factor of Safety1

Conclusion

• From the Analytic and FEA Simulation model it is prove that the BUCKET PIN is Safe against 15 MPa working pressure of bucket cylinder and this configuration of maximum Digging and break out force.

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