SAAST Robotics Program 2008

Mechanical Design Lab 1

Mechanism Synthesis, Prototyping and the World’s Strongest Truck

In this lab you will be introduced to 3 different linkage mechanisms and prototyping

techniques such as foam core mock-ups and laser cutting of acrylic parts. You will

design and build a “truck” to transport as much payload as possible up an inclined plane.

Read the entire lab first before starting.

Part 1: Mechanism Synthesis*

An assembly of n links has a total of 3n degrees of freedom before they are joined to

form a mechanism. The act of connecting the links with joints results in the loss of

degrees of freedom for the total system. Pin (or revolute) joints and slider joints both

only allow for one degree of relative motion for the links that they join. Therefore,

Gruebler has derived the following formula to determine the number of degrees of

freedom for a mechanism that is comprised of members attached with pin and/or slider

joints:

)(2)1(3# 21 jjnDOF +⋅−−⋅=

where: n = number of links/members

j1 = number of pin (revolute) joints

j2 = number of slider joints

For each mechanism/linkage identify:

• how many members comprise the mechanism (don’t forget ground)

• the types of joints present

• the number of degrees of freedom

Note: The number of pin joints at a common connection is j1 = m -1, where m is the

number of links joined by a single revolute joint.

In the report: Include completed Mechanism Synthesis Worksheets.

* Reference:

• Erdman, A., Sandor, G. Mechanism Design – Analysis and Synthesis, Volume 1,

2nd

ed. Prentice-Hall, 1994, Englewood Cliffs, NJ.

Part 2: Foam Core Prototype – Lifting Mechanism

Choose one of the first two lifting mechanisms presented and build a foam core prototype

of the device. Mount your 2D foam core mechanism to flat sheet of foam core to act as

your ground member.

Foam Core Basics:

• Cutting,

– Use a straight edge to make cuts

– Use a sharp blade

– Make two passes, the first half way as a guide, the second all the way

through.

• Glue with hot glue along the edges

– Be careful, the glue can be very hot, and can melt the foam

• Use supplied pins for pin joints

• May need to make spacers in order to avoid interference between some of the

links

In the report: Include picture of your prototyped lifting mechanism. Get

instructor/RTA sign-off indicating that prototype is functional.

Part 3: WORLD’S STRONGEST TRUCK

Part 3a: Gearbox Design

Each gearbox kit comes with enough gears to support 4 different speeds or gear ratios:

126:1; 441:1; 1543:1; 5402:1

The kit contains the following gears, with the number of gear teeth listed:

Calculate the 4 possible gear ratios for the 4 mounting scenarios pictured in the assembly

instructions. Use the numbers of gear teeth provided and the equation below. Use your

knowledge of the actual gear rations to check your answers.

Gear G4: N6= 36, N7 = 12

Gear G2 (x2): N1= 42, N2 = 12

N3 = 42, N4 = 12 Gear G3: N5= 42

Gear G5: N8= 42, N9 = 12

gearsdriveronteethofnumbersofproduct

gearsdrivenonteethofnumbersofproductRatioGear

driven

driver==

ω

ω

A worm gear is used to change the direction of motion of output shaft from the motor.

Assume that the worm gear is rotating at ωin = ωdriver = 7000 revolutions per minute

(rpm). Calculate the speed in rpms for the output shaft of the gear box for all 4 cases.

Choose the appropriate gear ratio for this application and assemble the gearbox.

In the report:

Include completed Gear Ration Calculations Worksheet for each gear ratio case. Identify

the drive and driven gears – i.e. driven gear: G3-N5. Show all calculations used to

determine gear ratios and output shaft speeds. Also, list choice for gear ratio and explain

the reasons for your choice.

Part 3b: Chassis and Wheel Design

Using the 0.25” thick acrylic material, design a truck bed that has inside dimensions of 5”

x 3” and walls that are 2” high. Be sure to include mounting holes for gearbox on bottom

of truck bed and one for a “caster” that will be used as a third support for your truck.

You need to include mounting hole for on/off switch on one side of truck bed. The

switch requires a ¼” diameter hole. Include through hole on one wall where wires to

battery can be routed.

Choose the appropriate wheel diameter for this application and explain your choice.

Design the wheel for high traction when operating on the test platform – hint: completely

circular wheels may slip. Include mounting holes in wheels so that you can mount it to

drive shaft coming from gearbox. Choose length of caster to in conjunction with wheel

diameter choice.

Optional: You can alter the supplied drive shaft or design new ones to mount to the

wheels if you so desire. You can use the supplied connectors or mount directly to the

drive shaft.

Use the laser cutter to manufacture the parts.

In the report:

Include print-out of CAD file of part designs used for laser cutting. Describe reasons for

choice of wheel diameter.

Part 3c: Assembly

Assemble truck bed components – use acrylic glue in press-fit joints of acrylic parts.

Mount gearbox assembly to truck bed and wheels to gearbox.

Mount switch to truck bed and wire them to 2 AA 1.5V batteries that are in series -

motors operate at 3V. You can mount the batteries to one side wall of the truck bed with

tape or choose to design additional mount. Note: the additional mount will add more

weight to your design.

In the report:

Include a picture of your assembled design.

Part 3d: Testing

Use scale in GM lab to weigh payload of miscellaneous GM Lab hardware.

Load payload into truck bed and position truck on inclined plane.

Turn on truck to see if it can transport payload up inclined plane. Start with a small

payload and gradually increase it to see the limits of your design. Be careful not to burn

out motors when stalling occurs. More than a few seconds of stalling the motor can burn

it out.

In the report:

Include the maximum payload that your design could move up the incline plane.

Describe what worked and didn’t work in your design. How would you change it to

make it better?

Sample Component Design Schematic:

Wheels

Caster

Truck Bed Bottom

Side Wall

Side Wall

Side Wall

Side Wall

Lab Worksheets

Name: ______________________________

Part 1: Mechanism Synthesis – Worksheet

Number of members (label in figure): n = ________

Number of joints (label in figure): j1 = ________ j2 = _______

Degrees of Freedom Calculations:

Number of members (label in figure): n = ________

Number of joints (label in figure): j1 = ________ j2 = _______

Degrees of Freedom Calculations:

Name: ______________________________

Part 1: Mechanism Synthesis – Worksheet

Number of members (label in figure): n = ________

Number of joints (label in figure): j1 = ________ j2 = _______

Degrees of Freedom Calculations:

Use cab as

ground link (1)

Hydraulic piston-cylinder

Hydraulic piston-cylinder

Name: _______________________________

Part 3a: Gear Ratio Calculations Worksheet

gearsdriveronteethofnumbersofproduct

gearsdrivenonteethofnumbersofproductRatioGear

driven

driver==

ω

ω

N1=N3=N5=N8=42 N2=N4=N7=N9=12 N6=36

Case I: 126:1

Driven gears:

Drive gears:

Gear Ratio Calcs:

Output Shaft Speed:

Case II: 441:1

Driven gears:

Drive gears:

Gear Ratio Calcs:

Output Shaft Speed:

Name: _______________________________

Part 3a: Gear Ratio Calculations Worksheet

gearsdriveronteethofnumbersofproduct

gearsdrivenonteethofnumbersofproductRatioGear

driven

driver==

ω

ω

N1=N3=N5=N8=42 N2=N4=N7=N9=12 N6=36

Case III: 1543:1

Driven gears:

Drive gears:

Gear Ratio Calcs:

Output Shaft Speed:

Case IV: 5402:1

Driven gears:

Drive gears:

Gear Ratio Calcs:

Output Shaft Speed:

--------------------------------------------------------------------------------------------------------

Gear Ratio Choice:

Reasons for choice: