Cordless Drill Take-Apart
April 24, 2015
Plan for Next Week
• Remote lectures Tues/Thurs on controller design
• Brian taking lead for remainder of class
– Guest lecture by Shawn Zhang on machine design
• No Monday lecture – work on MCM milestone
• No labs next week
• Thursday – Most Critical Module demonstrations
during lab time (we’d like to see them remotely)
• No Friday lecture (holiday)
Cordless Drill
Cordless Drill
Bearing 2
12V DC Motor
Planetary
Gearbox
Trigger
Chuck
Bearing 1
Switch
Sliding contact bearings
12V Battery
Which Fundamental Principles
1. Occam’s Razor
2. Newton’s Laws
3. Conservation of Energy
4. St Venant’s Principle
5. Abbe’s Principle
6. Self Help
7. Structural Loops
8. Sensitive Directions
9. Elastic Averaging
10.Exact Constraint
11.Preload
Battery
• Voltage – 12V
• Mili-Amp hour rating – 1 amp-hr
– How many Joules?
Battery
• Voltage – 12V
• Mili-Amp hour rating – 1 amp-hr
– How many Joules? 12V*1A-hr*3600s/hr = 43.2kJ
Trigger
What type of bearings?
Which fundamental principles?
3-Pole Switch
Which type of motor?
• Torque (at drill bit) 15Nm
• Speed(at drill bit) – 550 RPM
Bearing 1
Bearing 2
What is the purpose of these indentations?
What is the purpose of these?
What is this?
Planetary Gearbox
• Two reduction stages between motor and output
• Gear transmission ratio?
Planetary Gearbox
Why plastic here and metal here?
Motor side
Drill bit
side
Planetary Gearbox
• Gear transmission ratio?
19 teeth
9 teeth
19 teeth
9 teeth
(hidden)
45 teeth
Planetary Gearbox
• Gear transmission ratio?
19 teeth
(planet)
9 teeth
(sun)
19 teeth
(planet)
9 teeth
(sun, hidden)
45 teeth (ring)
Planetary Gears
Attain high gear reduction in small space
Planet
Carrier
Planet
Sun
Ring
19 teeth
(planet)
9 teeth
(sun)
Stage 1
19 teeth
(planet)
45 teeth (ring)
9 teeth
(sun, hidden)
Stage 2
#teeth on planet doesn’t matter
19 teeth
(planet)
9 teeth
(sun)
Stage 1
19 teeth
(planet)
45 teeth (ring)
9 teeth
(sun, hidden)
Stage 2
#teeth on planet doesn’t matter
TR_1 = (9+45)/9 = 6
TR_2 = (9+45)/9 = 6
TR_12 = 6*6 = 36
What is the motor torque, speed?
Drill Specs:
-Max output torque at drill bit = 15Nm
-Max drill bit speed = 550 RPM
η𝑇𝑚𝑜𝑡𝑜𝑟 𝑁 = 𝑇𝑑𝑟𝑖𝑙𝑙
η = 0.94 = 0.66
𝑇𝑚𝑜𝑡𝑜𝑟 =
η=efficiency
N=gear reduction=36
(four gear interfaces, assume each 90% efficient)
𝑇𝑑𝑟𝑖𝑙𝑙
15𝑁𝑚
=
= 0.63𝑁𝑚
𝜂𝑁
0.66 ∗ 36
(“motor stall torque”)
Max drill bit speed = 550 RPM = 57.6 rad/sec
Max motor speed = 36*57.6 rad/sec = 2074 rad/sec (“no load speed”)
For how much time can we drill at max power?
Assume we operate at half of max torque
Calculate speed and then power at drill bit
Divide by efficiency to get power at motor
τmax
Torque(τ)
Operating point
τmax/2
ω0 = (1/2)*ωmax
ω0 = 1000 rad/sec
τmax/2 = 0.6Nm/2 = 0.3Nm
ω0
ωmax
Speed (ω)
Power = τω = E/t
t = E/(τω) = 43.2 kJ/(0.3Nm*1000) = 144s
Bearings
• What type of forces do we expect to apply on
the chuck?
Chuck
Fradial
Radial
Tapered
roller
Faxial
Thrust
enginemechanics.tpub.com
Cordless Drill
12V DC Motor
Mystery
Bearing 2
Planetary
Gearbox
Trigger
Chuck
Mystery
Bearing 1
Amplifier
Sliding contact bearings
12V Battery
Bearing 1
Bearing 2
Bearing 2 – which type?
Retainer ring
Bearing 1 – which type?
Fundamental principle?
Which material?
Fradial
Radial
Tapered
roller
Faxial
Thrust
enginemechanics.tpub.com
Clutch
• Slips when torque is too high
– For example, when screw has reached the bottom
of a hole and no further rotation is necessary
Clutch
• Slips when torque is too high
Screw
Higher preload force on spring means higher slip torque
L- or R-handed thread?
?
Direction of rotation for drilling
Left-hand threads
Crank (red)
• Prevent loosening due to mechanical
precession
Pedal axle
(blue)
• Mechanical precession:
– Loosening due to a part rotating in a round hole
under a cyclic radial force
Left pedal
Left-hand thread
locations:
Right bottom bracket
http://en.wikipedia.org/wiki/Precession_%28mechanical%29
L- or R-handed thread?
Direction of rotation for drilling
L- or R-handed thread?
Left side of a bicycle
L-handed
Direction of rotation for drilling
L-handed
Fundamental Principles
•
•
•
•
St Venant – trigger bearing, shaft bearing
Structural Loop
Elastic Averaging – chuck gripping drill bit
Sensitive directions
For how much time can we drill at max power?
Assume we operate at half of max torque
Calculate speed and then power at drill bit
Divide by efficiency to get power at motor
τmax
Max drill bit speed = 550 RPM
= 57.6 rad/sec
Torque(τ)
Operating point
τmax/2
ω0
ωmax
Speed (ω)
𝑃𝑚𝑜𝑡𝑜𝑟
𝑃𝑑𝑟𝑖𝑙𝑙 𝐸
=
=
𝜂
𝜂𝑡
𝑡=
𝐸
𝐸
43.2𝑘𝐽
=
=
= 300𝑠𝑒𝑐
𝜂𝑃𝑑𝑟𝑖𝑙𝑙 𝜂𝜏𝜔 0.66 ∗ 7.5𝑁𝑚 ∗ 28.8𝑟𝑎𝑑/𝑠𝑒𝑐