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temp

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Shawn Liner

on 28 September 2012

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Transcript of temp

allowable = yeild / Safety Factor

For the FIRST competition I use a Static Safety Factor of 4.
While on the high side it allows for unknowns and dynamic loads
Haven’t had anything break yet! Allowable Stresses Elevator
Forklift
Four Bar
Scissors Types of Lifts Very Flexible
Can Right a Flipped Robot
Can Place Object in an Infinite Number of Positions Within Reach
Minimal Z - Great for going under things Relative Advantages of Arms Over Lifts An “Arm” is a device for grabbing and moving objects using members that rotate about their ends What is an “Arm”? A Lift is a device for grabbing and moving objects in a predominately vertical direction What is a “Lift?”
I don’t want to go there Scissors - Calculations
Do You Really Want to Do This?
Members Must Be Good in Bending and Torsion
Joints Must Only Move in One Direction
The greater the separation between pivot and actuator line of action the lower the initial load on actuator
Best if it is directly under load Scissors - Design Considerations Scissors
Pin Loadings can be very high
Watch for buckling in lower member
Counterbalance if you can
Keep CG aft Four Bar - Design Considerations
Advantages
Great For Fixed Heights
On/Off Control
Lift Can Be Counter-Balanced or Spring Loaded to Reduce the Load on Actuator
Good candidate for Pnuematic or Screw actuation
Disadvantages
Need Clearance in Front During Lift
Can’t Go Under Obstacles Lower Than Retracted Lift
Got to Watch CG
If Pnuematic, only two positions, Up and Down Four Bar - Advantages & Disadvantages Four Bar Should be powered down as well as up
Segments need to move freely
Need to be able to adjust cable length(s).
Two different ways to rig (see later slide)
MINIMIZE SLOP
Maximize segment overlap
Stiffness is as important as strength
Minimize weight, especially at the top Forklift - Design Considerations Advantages
Can reach higher than you want to go
On/Off Control
Can be rigid
Can be Actuated via Screw, Cable, or Pnuematics, though all involve some cabling
Disadvantages
Stability issues at extreme heights
Can’t Go Under Obstacles Lower Than Retracted Lift Forklift - Advantages & Disadvantages Usually simple to construct
Easy to control (don’t even need limit switches)
Maintain CG in a fixed XY location
Don’t Require Complex Gear Trains Relative Advantages of Lifts Over Arms Material Desig Temper Yield Tensile Shear Modulus
(ksi) (ksi) (ksi) (msi)
Alum 6061 O 8 18 12 10
Alum 6061 T6 40 45 30 10
Brass C36000 18-45 49-68 30-38 14
Copper C17000 135-165? 165-200? 19
Mild Steel 1015-22 HR 48 65 30
PVC Rigid 6-8 0.3-1 Here are some properties for typical robot materials Allowable Stresses
Advantages
Minimum retracted height
Disadvantages
Tends to be heavy
High CG
Doesn’t deal well with side loads
Must be built precisely Scissors - Advantages & Disadvantages Cable Goes Same Speed for Up and Down
Intermediate Sections Often Jam
Lowest Cable Tension
Tcable = Weight of Object + Weight of Lift Components Supported by Cable Forklift - Rigging -Continuos Cascade Continuos Forklift - Rigging Forklift di do hi bi ho c bo A, c and I for Rectangular and Circular Sections Stress Calculations (cont.) Slider
(Stage3) Stage2 Stage1 Base Tcable3 Tcable2 Tcable1 Upgoing and Downgoing Cables Have Different Speeds
Intermediate Sections Don’t Jam
Very Fast
Tcable3 = Weight of Object + Weight of Slider
Tcable2 = 2Tcable3 + Weight of Stage2
Tcable1 = 2Tcable2 + Weight of Stage1
Where n = number of moving stages
Different Cable Speeds Can be Handled with Different Drum Diameters or Multiple Pulleys Forklift - Rigging - Cascade Shear Tensile Bending Where:
 = Shear Stress
Fshear = Shear Force
A = Area of Section Where:
 = Tensile Stress
Ftens = Tensile Force
A = Area of Section Where:
 = Bending Stress
M = Moment (calculated earlier)
I = Moment of Inertia of Section
c = distance from Central Axis It all boils down to 3 equations: Stress Calculations cx2 cx1 h1 cy b2 h2 b1 Y X A, c and I for T-Sections Stress Calculations (cont.) Mlower Dlower/2 Hlower Flower1 Dlower Flower2 Fupper1 Dupper Fupper2 Hslider Fhit Dslider Dobject Mslider Fslider2 Fslider1 Fobject Mbase
Fobject = Weight of Object + Weight of Slider
Dobject = Distance of Object CG
Mslider = Fobject• Dobject
Fslider1 = - Fslider2 = Mslider / 2Dslider
Fhit = G value [I use .5] • (Weight of Object + Weight of Slider)
Mhitlower = Fhit•Hlower + [(Weight of Upper + Weight of Lower) • (Hlower / 2)]
Flower1 = - Flower2 = [Mslider + Mhitlower] / 2Dslider
Mhit = Fhit • Hslider + [(Weight of Lift • G value • Hslider ) / 2]
Mbase = Mslider + Mhit Forklift - Calculations Hupper Dupper/2 cx2 cx1 h1 cy b2 h2 b1 Y X A, c and I for C-Sections (Assumes Equal Legs) Stress Calculations (cont.) Under Construction Check Back Later Mlink Dlower/2 Flink1 Dlink Flink2 Hgripper Fhit Dgripper Dobject Mgripper Fgripper2 Fgripper1 Fobject Mbase Llink Four Bar - Calculations Should be powered down as well as up
Slider needs to move freely
Need to be able to adjust cable length. A turnbuckle works great
Cable can be a loop
Drum needs 3-5 turns of excess cable
Keep cables or other actuators well protected Elevator - Design Considerations Advantages
Simplest Structure
On/Off Control
VERY Rigid
Can be Actuated via Screw, Cable, or Pnuematics
Disadvantages
Lift Distance Limited to Max Robot Height
Can’t Go Under Obstacles Lower Than Max Lift Elevator - Advantages & Disadvantages cy2 cx2 cx1 h1 cy1 b2 h2 b1 Y X A, c and I for L-Angles Stress Calculations (cont.) Elevator Hslider Fhit Tcable Dslider Dobject Mbase Mslider Fpulley Fslider2 Fslider1 Fobject Fobject = Weight of Object + Weight of Slider
Dobject = Distance of Object CG
Tcable = Fobject
Mslider = Fobject• Dobject
Fslider1 = - Fslider2 = Mslider / 2Dslider
Fpulley = 2 Tcable
Fhit = (Weight of Object + Weight of Slider) • G value [I use .5]
Mhit = Fhit • Hslider
Mbase = Mslider + Mhit Elevator - Calculations
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