Machine Elements Transmissions, Bearings, Shafts
TRANSMISSIONS Matching the torque and speed of the motor to the torque and speed of the task
Speed (rpm) Torque (oz-in) Motor torque-speed curve Stall No-load Load torque Operating point
Speed (rpm) Torque (oz-in) Changes with motor V Increasing V Load torque
Small DC Motors Plain motors: spin fast, low torque Gear motors: spin slow, high torque More voltage = more speed and more torque Less voltage = slows down, but torque down If spin too fast, bearings fail If too much current, coil heats up and melts
Speed (rpm) Torque (oz-in) Motor complaints Stall too wimpy! No-load spins too fast! Solution: A transmission gets you on a new T-S curve
Speed (rpm) Torque (oz-in) 5:1 Transmission Before: low torque, high speed After: high torque at low speed T2 = n*T1 v2 = (1/n)*v1
Same power!
Fast, weak motor Slow, strong turntable Belt Drive
Fast, weak motor Slow, strong turntable Rim Drive
Cam As a Transmission FOLLOWER CAM moparmusclemagazine.com
Da Vinci Cams
16 US
17
BEARINGS and SHAFTS Getting things to spin by transmitting rotary power
What is a bearing? A BEARING REDUCES DEGREES-OF-FREEDOM
SPECIFICATIONS Ability to constrain motion Friction Load Lifetime Fabrication/Mounting Cost
TO SLIDE OR TO ROLL? Rolling elements:Low friction, complex, expensive Sliding elements:Simple, high friction
timken.com Tapered Roller Bearing Angular contact roller bearing
Rulon bearings
Sliding bearings (prismatic joints) L D L/D = 6 to 10 BIND NO BIND
More sliding joints BINDNO BIND
KH Linear bushing Danahermotion.com alibaba.com linear bearings
Shaft
SUPPORTING SHAFTS YES! NO!
Bearing misalignment OFFSET ANGULAR
Bearing load radial thrust
JOINING SHAFTS ? Offset misalignment Angular misalignment
A classic binding problem one shaft, 4 supports = binding! ?
Coupling methods rigid coupler, press-fit or glue or set-screws flexible coupler
Or, just tape ‘em together electrical tape or fiber reinforced tape