Design Process For 3 Wheeler Cart: CAD/CAM And Rapid Prototyping

Identification of product and description of new function that add value

Da Vinci was not only designer but also an artist and a painter and that also reflect in his work as well .His one of the best work was propelling cart and this cart was impossible of design at that time without necessary resources .This cart is a masterpiece having perfect use of gears .All the parts of cart were recreated using solidworks CAD software . Rendering and detailed drawing of all parts are prepared for manufacturing purpose . Analysis of wheel of cart is done and comparing it for prototyping  purpose and cnc manufacturing .

DESIGN PROCESS

The main focus of design process is to recreate the prototype of 3 wheeler cart effectively considering all design aspects .   

DESIGN CONCEPT/SKETCH

Wheel

Wheel provide the rolling motion to cart , the spokes present on its centre balance center of gravity of wheel .

• Under the design concept , a rough sketch  of 3 wheeler cart is obtained. This sketch gives us the basic idea about the product visualization. The above sketch shows that gears assembly is surrounded by frame .This frame not only kept all the parts together but also carry out all the load act upon it .¹
• 3 Wheeler cart should be designed similar to propelling cart and all the main components like spring , gears and frame should be used accordingly .
• 3 Wheels are used in this cart , front single wheels is used for direction purpose and backward two wheels provide the motion

DESIGN SPECIFICATION

Under design specification all the parts of 3 wheeler car should be specify accordingly . All the component are explained below .

• Truss/Frames

Truss/Frames are the structure members generally used to carry loads and coupled multiple parts in single unit for smooth operation of assembly. To carry the weight of multiple gear units , here two types of truss are used .

Truss/Frames used are:-

1. Bottom Truss/frame
2. Top Truss/frame
3. Bottom truss

According to its name suggest ,bottom truss present at the bottom of gear assembly .The load of all mechanical components are directly act on this truss .At the surface of bottom truss , multiple threaded holes are present .These holes are used for mounting and rotating  gear shafts . Other holes are used to fastened the bolts with other frame . Fillets are present at corners of truss for stress reduction.

Manufacturing process –

The shape of frame is non uniform , So all the linear parts are joined together with fasteners , at necessary places holes of required diameter are drilled . Fillets are provided by using grinding operation .

Major properties of Wood is shown below 

Property	Value	Units
Elastic Modulus	3000	N / mm^2
Poisson’s Ratio	0.29	N / A
Shear Modulus	300	N / mm^2
Mass Density	160	kg / m^3
Yield Strength	20	N / mm^2
Thermal Conductivity	0.05	W / (m·K)

Table 1.1 Material Properties 

1. Top truss/frame

Top truss is present at the top of gear arrangement and lock the linear motion of all gears . The top truss share the load of all components with bottom truss .At its surface, multiple threaded holes are present .These holes are used for mounting and rotating  gear shafts. Fillets are present at corners of truss for stress reduction.⁴

Manufacturing process –

The shape of top truss is similar to bottom truss as compare in area covered but it is also non uniform , So all the linear parts are joined together with fasteners , at necessary places holes of required diameter are drilled . Fillets are provided by using grinding operation

The stages of design process

Major properties of Wood is shown below 

Property	Value	Units
Elastic Modulus	3000	N / mm^2
Poisson’s Ratio	0.29	N / A
Shear Modulus	300	N / mm^2
Mass Density	160	kg / m^3
Yield Strength	20	N / mm^2
Thermal Conductivity	0.05	W / (m·K)

Table 1.2 Material Properties

• Wheel

There are 3 wheels in this vehicle , front wheel adjust itself according to direction of cart and two back wheels are assembled with the gear arrangement and provide motion to the cart .

Major properties of Acrylic is shown below 

Property	Value	Units
Elastic Modulus	3000000000	N / m^2
Poisson’s Ratio	0.35	N A
Shear Modulus	890000000	N / m^2
Mass Density	1200	kg / m^3
Tensile Strength	73000000	N / m^2
Yield Strength	45000000	N / m^2
Thermal Conductivity	0.21	W / (m·K)
Specific Heat	1500	J/(kg·K)

Table 1.3 material properties⁵

Manufacturing process

All 3 wheels are similar and have uniform cross-section so it is manufactured by machining process .Back wheels have taper tube on its inner face which is joined separately to it .

⁵Wingerter, R., and Lebossiere, (Feb 2011) P., ME 354, Mechanics of Materials Laboratory: Structures, University of Washington

Major properties of Acrylic is shown below 

Property	Value	Units
Elastic Modulus	3000000000	N / m^2
Poisson’s Ratio	0.35	N A
Shear Modulus	890000000	N / m^2
Mass Density	1200	kg / m^3
Tensile Strength	73000000	N / m^2
Yield Strength	45000000	N / m^2
Thermal Conductivity	0.21	W / (m·K)
Specific Heat	1500	J/(kg·K)

Table 1.4 material properties

• Gear arrangement

The gear arrangement shows the combination of different gears , shafts , winding unit , hub connecting in such a way to obtain desired output .

This gear arrangement having following parts explained below –

1. WINDING UNIT

The winding unit is input link in which a key is present .This unit is connected with the spring .As the key is rotated manually or by external means then this rotational motion stored in spring and then further transfer to gears .

Major properties of Acrylic is shown below 

Property	Value	Units
Elastic Modulus	3000000000	N / m^2
Poisson’s Ratio	0.35	N A
Shear Modulus	890000000	N / m^2
Mass Density	1200	kg / m^3
Tensile Strength	73000000	N / m^2
Yield Strength	45000000	N / m^2
Thermal Conductivity	0.21	W / (m·K)
Specific Heat	1500	J/(kg·K)

Table 1.5 material properties

1. INPUT GEAR A

         The winding unit is directly connected to the input gear a and both are having common shaft .⁶

Major properties of Acrylic is shown below 

Property	Value	Units
Elastic Modulus	3000000000	N / m^2
Poisson’s Ratio	0.35	N A
Shear Modulus	890000000	N / m^2
Mass Density	1200	kg / m^3
Tensile Strength	73000000	N / m^2
Yield Strength	45000000	N / m^2
Thermal Conductivity	0.21	W / (m·K)
Specific Heat	1500	J/(kg·K)

Table 1.6 material properties

1. STEPPED GEAR SHAFT AND CAM GEAR

The input gear A is connected with stepped gear shaft which is further connect with wheel brake hub , other part of gear A is connected Cam gear which help in completing the circuit and provide motion to other gear.

Major properties of Acrylic is shown below 

Property	Value	Units
Elastic Modulus	3000000000	N / m^2
Poisson’s Ratio	0.35	N A
Shear Modulus	890000000	N / m^2
Mass Density	1200	kg / m^3
Tensile Strength	73000000	N / m^2
Yield Strength	45000000	N / m^2
Thermal Conductivity	0.21	W / (m·K)
Specific Heat	1500	J/(kg·K)

Table 1.7 material properties

1. CLUTCH SHAFT

The clutch shaft is directly connected with wheels tube which help the 3 wheeler assembly to stop .

Major properties of Acrylic is shown below 

Property	Value	Units
Elastic Modulus	3000000000	N / m^2
Poisson’s Ratio	0.35	N A
Shear Modulus	890000000	N / m^2
Mass Density	1200	kg / m^3
Tensile Strength	73000000	N / m^2
Yield Strength	45000000	N / m^2
Thermal Conductivity	0.21	W / (m·K)
Specific Heat	1500	J/(kg·K)

Table 1.8 material properties

DESIGN EVAUATION

As a prototype , consider the cart having speed of 10 m/second .

V =10 m/s

Cart Weight = 1000 Grams

Formula

Kinetic energy = ½ x mass of cart x velocity^2

KE =  ½ * 1000/1000 * 10*10  

K.E= 50 Joules

So this cart need 50J to reach speed of 10 m/s

But spring energy is equal to kinetic energy

S.E = K .E

½ * k *x  = 50

k = spring constant= 800 N/m

Apply values we get

x =125 mm

Dia meter of coil = 12 mm

Detail drawings, assembly drawing, and parts list

So its Perimeter = 12 x pi= 12*3.14

                    =37.7 mm

No . of rotations = 125 /37.7

                               = 4

So maximum 4 rotations needed to reach the cart to 10 m /second .

DFMA PRINCIPLES & ANALYSIS

DFMA refers to design for manufacture and assembly .DMFA is technique that focus on efficiency in manufacturing and assembly of a product by simplifying all process include under it .It main motives to manufacture and assembly the product in minimum time and at least cost .

Main principles of DFMA

• Simplify the automation process by reducing number of parts in assembly .The 3 wheeler assembly is completely automated .
• Encourages to use standard parts that are easily available rather than complex manufacturing that increase cost of product .In 3 wheeler assembly ,standard parts like gear ,spring , shaft hub are used .
• Reduce the use of flexible components like gaskets, seal ,rubber . As assembly of these parts are more difficult .No flexible part used in 3 wheeler assembly .
• Use adhesive and snap type fitting for assembly rather than fasteners .
• To simplify manufacturing , parts design should be simple and unnecessary features on parts should be avoided .

Advantages of DFMA

• Cost of assembly and manufacturing becomes low .
• Time required for assembly and manufacturing is less .
• As numbers of parts reduces then chances of parts failure also reduces therefore reliability increases .
• High quality of product is achieved .

⁸Boothroyd, G., Dewhurst, P. and Knight, W., “Product Design for Manufacture and Assembly, 2nd Edition”, Marcel Dekker, New York, 2012.

DESIGN VALIDATION

Consider the factor of safety is 2 and design the wheel of cart , considering its weight of 7000 grams.

Weight on wheels = 7*9.81 N = 70 N ,To maintain factor of safety is 2 , load applied should be 70 x 2 = 140 N

As Applied load is 140 N on wheels centre and check the maximum stress and deflection produced by the wheels .Diameter of shaft is 3 mm.

Solid part	Properties
Revolve1	Mass:0.4 kg Volume:6.e-007 m^3 Density:1200 kg/m^3 Weight:4 N

Model Reference	Properties
	Name: Acrylic (Medium-high impact) Model type: Isotropic Default failure criterion: von Mises Stress Yield strength: 45 MPA

Loading

As discussed below , applied load is 140 N while considering the factor of safety of 2 .Applied load is the total weight of cart acting on the wheels , so this load is applied at centre of wheel .

Mesh type	Solid Mesh
Mesher Used:	Standard mesh
Jacobian points	4 Points
Element Size	2 mm
Tolerance	0.01 mm
Mesh Quality	High

Total Nodes	33998
Total Elements	18454
Maximum Aspect Ratio	14
% of elements with Aspect Ratio < 3	18.1
% of elements with Aspect Ratio > 10	0.001
% of distorted elements(Jacobian)	0

RESULT

As all the input parts are completed now the run the analysis and obtained the output result .

Stress

STRESS

Values of stress is given below

Minimum :-  0 MPa

Maximum :- 31.39 MPa

Deflection

DEFLECTION

Value of deflection obtain is given below

Minimum :-  0.00 mm

Maximum :- 0.166 mm

FACTOR OF SAFETY

Factor of safety is the ratio of yield stress to maximum stress generated while applying load. Yield stress is maximum permissible stress for a particular material after which deformation takes plate. Here Yield stress is 45MPa and maximum stress generated is 31.39 MPa .Although this design is safe but factor of safety value of 2 is not achieved

F.O.S = 45/31.39 =1.43

So changes need to be done to reduce the maximum stress value and achieve the factor of safety to 2

CHANGES IN DESIGN

To maintain the factor of safety of 2 and above , necessary changes need to be done accordingly .Earlier the shaft and hub of wheels having diameter of 3 mm .

According to formula below , stress is inversely proportional to Moment of inertia(I) .

Assembly sequence and manufacturing processes

For shaft

I =3.14 *D^4 /32

This shows that to reduce the value of maximum stress ,diameter of shaft need to be increased . Now in new design diameter of shaft is increased from 3 mm to 4 mm due to which stress value will reduce .

The below picture shows that diameter of shaft change to 4mm

Now kept all the procedure same i.e fixtures , loading and run the result with new design.

RESULT

As all the input parts are completed now the run the analysis and new results are obtained shown below

STRESS

Values of stress is given below

Minimum :-  0 MPa

Maximum :- 20.65 MPa

Deflection

DEFLECTION

Value of deflection obtain is given below

Minimum :-  0.00 mm

Maximum :- 0.21 mm

FACTOR OF SAFETY

Here Yield stress is 45MPa and maximum stress generated in new design is 20.65 MPa . Although this design is safe and also factor of safety value of 2 is also achieved .

F.O.S = 45/20.65 =2.17

So changes done are effective to achieve the factor of safety to 2 or more .

Conclusion

In 1^st case when the diameter of shaft is 3 mm and applied load is 140 N then although the design is safe but factor of safety of 2 need to be achieved .To increase the value of factor of safety , maximum stress occurs due to applied load need to be reduced . So in new design diameter of shaft is increased from 3 mm to 4 mm so that its moment of inertia get increased and maximum stress reduced . So after increasing the diameter of shaft to 4 mm the maximum stress value reduced to20.65 MPa and factor of safety of 2 is successfully achieved . So overall design is safe .⁹

^References

Lynch, Mike (2010-01-18), “When programmers should know G code”, Modern Machine Shop (online ed.).

Davinci, Leonardo (2011). The Notebooks of Leonardo Da Vinci. Lulu. p. 736. 

Fernando (2010). Technological Concepts and Mathematical Models in the Evolution of Modern Engineering Systems.Birkhäuser. ISBN 978-3-7643-6940-8.

Plesha, Michael E (2013). Engineering Mechanics: Statics (2nd ed.). New York: McGraw-Hill Companies Inc. pp. 364–407. ISBN 0-07-338029

Jacob; Papadopoulos (Oct 2016). Introduction to Solid Mechanics: An Integrated Approach. Springer. ISBN 9783319188782.

Wingerter, R., and Lebossiere, (Feb 2011) P., ME 354, Mechanics of Materials Laboratory: Structures, University of Washington

Record, Samuel, 2010. The Mechanical Properties of Wood. J. Wiley & Sons. p. 165. ASIN B000863N3W.

Sclater, Neil. (2011). “Gears: devices, drives and mechanisms.” Mechanisms and Mechanical Devices Sourcebook. 5th ed. New York: McGraw Hill. pp. 131–174. ISBN 9780071704427. Drawings and designs of various gearings.

Khurmi R S, (2014), ‘A text book of machine design’, Eurasia publishing house(P) ltd., New-Delhi, ISBN 9788121925372

Mahadevan K and Reddy K.Balaveera, (2015), ‘Design data hand book’, CBS publishers and Distributors (P) ltd., New-Delhi, ISBN 9788123923154

 Boothroyd, G., Dewhurst, P. and Knight, W., “Product Design for Manufacture and Assembly, 2nd Edition”, Marcel Dekker, New York, 2012.

Smid, Peter (2010), CNC Control Setup for Milling and Turning, New York: Industrial Press, ISBN 978-0831133504, LCCN 2010007023.

Lynch, Mike (2010-01-18), “When programmers should know G code”, Modern Machine Shop (online ed.).

 Fridtjov Irgens (2010), “Continuum Mechanics”. Springer. ISBN 3-540-74297-2

Ramsay, Angus. “Stress Trajectories”. Ramsay Maunder Associates. Retrieved April 2017.