1

Beach Cities RoboticsFRC Team 294 Summer Academy

Motors and Gears

July 20, 2008

Rick Wagner, Mentor, Beach Cities Robotics

FIRST Team 294

2

Motors and Gears Outline

Concepts:

• Motors

• Construction

• Operation

• Power and speed characteristics

• Gears and Drive Trains

• Overview

• Examples

• Gears

• Roller chain

• Power, torque, and speed calculation

3

DC Motor with Permanent Magnet and Brushes

4

DC Motor Operation

Current flows through the armature coils.

The armature magnetic field pulls the armature toward the permanent magnet pole and repels from the opposite pole.

The comutator switches the current and the cycle starts again.

Rapid motion through the magnetic field creates a counter-voltage that reduces the current as the motor speed increases.

5

Motor Operation

At zero speed, motor torque is maximum because there is no counter-EMF.

At full speed, torque is zero. The motor will not accelerate because the counter-EMF is equal to the supplied voltage (minus some small amount due to friction).

6

DC Motor Characteristics

7

Motor Example

Atwood Chiaphua Motor Motor Performance

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

% of Stall Torque

% o

f F

ree

Spe

ed,

Sta

ll C

urre

nt,

Max

Pow

er,

Eff

icie

ncy

Speed (% of Free) Current Draw (% of Stall) Mechanical Power (% of Max)

Efficiency (%) Design Condition

8

Drive Trains

9

Overview of Gears and Roller Chain Drives

Gears provide the most efficient power transmission with the greatest power density (power to weight ratio). Most automotive internal combustion engines utilize gears for the transmission of power from the engine to the wheels. However, gears require precision manufacturing to maintain critical dimensions of gear mesh. Adjustable gear mesh can be provided, but in practice, gear adjustment is not generally used.

Roller chains and sprockets provide light duty flexible power transmission. As chains wear, however, they change in length, so some adjustment mechanism is usually provided.

10

Drive Train Example

• BCR summer 2007 prototype base, 4-speed transmission, 6-wheel drive, 2-motors each side

• The BCR 2008 robot drive base was developed from this base:•4-inch wheels, not 6-inch•2-speed transmission

11

Gear Examples

• AndyMark 2-speed shifter

• Shift Style

Dog gears, shift-on-the-fly, without clutch, driven by pneumatic cylinder (not included)

• Weight: 3.6 lbs.

• Hardware

ball bearings

4140 steel gears, 20 dp, 14.5 degree pressure angle

3/32" thick steel side plates, plated

2 - 12 tooth, 35 series sprockets, located on output shaft

12

Double Reduction Gear Box Example

• Gear box for BCR’s 2008 robot windup drum for the forklift• Fisher-Price motor with 19-tooth 32-pitch pinion gear• 72-tooth brass spur gear on keyed shaft turns 20-pitch pinion that drives output

spur gear to turn the windup drum (not shown)• 20-pitch gears are represented as wheels of the gear pitch diameter• Total speed reduction (torque increase) is 20:1

13

Spur Gear Details

• A gear's pitch is its number of teeth divided by its pitch diameter. For example, a gear with 16 teeth and a pitch diameter of 1/2" will have a pitch of 32

14

Planetary Gears

• BaneBots P80 Gearmotor: Standard Shaft, CIM Motor, 3:1, Price: $111.50

15

Gears and Sprockets

• This is how team 1501 (Indiana) lightens their steel gears and sprockets

16

Roller Chain

Chain drives are light weight and flexible, avoiding the precision alignment that gears require.

An idler sprocket, either adjustable or on a spring tensioner arm, is often used to keep the chain properly tensioned.

In use, a chain should be loose enough to avoid binding of the roller pins, but tight enough to prevent tooth skipping. If there’s no free play at all, it’s too tight.

Roller chain should be lubricated with a heavy oil and then wiped clean. Unlubricated chain can wear out and break.

17

Power, Torque, and Speed Calculation

For multiple motors on one drive, use the ratio of the motor free speeds for the motor gear ratios

Percents of free speeds don’t have to match exactly (close is good enough)

Maximum power occurs at about one half of motor free speed, so design for a torque that is slightly lower than that for margin

Output torque is motor torque times the gear ratio Use the half-speed torque Reduce output torque by 5% for each stage for gear

friction

18

Drive Train Design Calculatorhttp://www.chiefdelphi.com/media/papers/1469