DEWBOT VII Arm Drive

What to do, post-FLR?

Situation status quo Arm is driven using a 15mm wide HTD5 synchronous

belt – 500mm long – this is a “nothing special” belt We shredded two of these belts in playoffs – teeth

sheared off Drive motor is 2011 FisherPrice with a 256:1 reduction

Banebots 4-stage planetary gearbox delivering: 101 ft lbf torque at stall 1.35 rev/s unloaded speed

There is 56/20 (2.8:1) pulley reduction from gearbox to arm

The motor is too fast – slowed in software Deux uses an earlier FisherPrice motor. They are not

the same. (could account for “jitters” in prime’s arm)

Objectives

Must - Adopt a more durable arm drive mechanism that the current one

Desired – Increase the amount of mechanical reduction between gearbox and arm – relax software speed limits

Constraints The arm pivot is 3.063” (CAD model) from

the back of the robot frame – this constrains the diameter of any sprocket, pulley or gear used to drive the arm

We should continue to use the FisherPrice motor and Banebots gearbox

Gearbox shaft is ½” (keyed) Arm shaft is 3/8” (keyed) There is a need to be able to manually

lower the arm with the robot powered-down

Arm drive options

Pulleys – status quo Type 35 steel chain & sprockets Spur gears Helical gears Worm gears

Pulleys

A significantly stronger belt would be needed

There is room for a larger driven pulley than currently used

Low probability of delivering needed durability

Do not pursue

Type 35 steel chain & sprockets Easy to execute

Existing motor mount & tensioner should work Parts either on-hand or easily & inexpensively

obtainable A 42:10 (4.2:1) reduction is feasible Well-known ground for the team Probably strong & reliable enough Chain drive will have some slop &

nonlinearity – tensioner should limit slop & backlash.

Probably fits in budget without compressor change

Spur Gear Stronger More linear than chain – some

backlash Requires precision fixed mount plate

for arm pivot pillow blocks & motor 84/15 (5.6:1) reduction possible More costly – compressor change will

be needed Parts are obtainable

Helical Gears Stronger & smoother than spur gears for

same contact area Most linear Requires precision fixed mount plate for

arm pivot pillow blocks & motor (same as spur)

Expensive (but probably in-scope after compressor change)

Limited parts availability ~4:1 reduction seems to be limit, based on

findings so far.

Worm Gear

Mounting would be challenging Generally cannot be back-driven – a

problem for us High reduction possible Basically, doesn’t meet the problem’s

needs Do not pursue