3lbers

Common Bot Types

Drum Spinner

Radii, Misti

A robot whose weapon is a spinning drum with teeth at the front of the robot. Typically, Drum spinners are invertible.

Ring Spinner

Qti, Smitti Werbenjagermanjensen

A robot whose weapon is a spinning ring with teeth that surrounds the entire frame. Generally has bearings which support the ring as it spins.

Pros

• Invertible
• Hits from every side

Cons

• Difficult to make a reliable weapon drive and support system

Shell Spinner

A robot with a spinning "shell" that encompasses the robot. Similar to a ring spinner, the shell has teeth on it which serve as the weapon. They are easier to make than ring spinner however cannot be made invertible.

Vertical Spinner

Dat Boi

A robot with a vertically spinning weapon at the front of the robot. Typically this weapon is complemented by a surrounding wedge to lift opponents into the weapon.

Horizontal Spinner

Schwifti, Ki

A robot armed with a flat spinning bar at the front driven with a pulley.

Key Design Considerations

Drive Train

Speed and Torque

A typical 3lber can move around 50-100 in/s. Usually gear ratios necessary to achieve these speeds can supply more than enough torque to allow the robot to push opponents around the arena.

Ground Clearance

A problem most 3lbers have is being too close to the ground. Debris and uneven arena floor can easily get underneith the robot, propping the wheels off the ground and preventing movement. While considered beneficial for preventing wedges from getting underneath the bot, the loss in control due to low clearance usually nullifies this benefit. It is far better to have a highly mobile bot with high clearance, greater than 0.375" from bottom plate to ground.

Weapon

Weapon Drive

Typical weapon drive employs a belt and pulley drive train. A rubber or urethane belt runs between a motor with a pulley, and a pulley on the weapon. When using a brushless out-runner motor, the outer can of the motor can be used as a pulley directly, but requires guides to prevent the belt from sliding up or down the motor. Direct drive weapons, where the motor shaft is directly connected to the weapon, is highly discouraged, as the shock from weapon hits can shatter the motor magnets, breaking the motor entirely.

Common motor specs at this scale:

• Power: 500-800W
• Voltage: 11.1 (3s) - 14.8 (4s)
• KV: Depends on weapon type. Small spinners such as drums, disks, and bars, aim for weapon speeds greater than 8000RPM. Large spinners such as ring and shell spinners are in the 2000-5000RPM range

Weapon Material

Weapons are usually built from aluminum (6061) and mild steel. Large weapons such as drums and ring spinners employ an aluminum frame with steel teeth. Smaller weapons such as bars and disks can be made entirely from steel. It is critical that steel be used for the part of the weapon that comes in contact with other bots, as aluminum is too soft and deforms on direct impact. Toolsteel teeth may be used however it must be fully heat treated after machining to be effective. It is also quite expensive and usually not worth it.

It is important to run FEA on the weapon design, as it will highlight key points of failure. Typical forces a weapon experiences at this scale ranges between 300-600lbs.

Calculations

Here are some common calculations used to find things such as weapon speed, drive speed, pushing force, and power requirements.

RPM = V*KV

ω = RPM *2π/60

v = ω*r

P = ω * τ

α = τ*J

F = τ*r

T = ω/α *0.63

t = T*3

ω_output = ω_input * e

τ_output = τ_input / e

P: Power (Watts)

RPM: Rotations per Minute

KV: RPM/Volt

V: Voltage

ω = Angular Velocity (rad/s)

α = Angular Acceleration (rad/s²)

τ = Torque (N-m)

J = Mass moment of inertia (N-m²)

F = Force (N)

T = Time Constant (s)

t = Spinup time to reach 98% max RPM (s)

r = Wheel/pulley radius (m)

v = Tangential velocity (m/s)

e = Gear Ratio (Drive gear/Driven gear)

NOTE: It is typically easiest to use si units for calculations then convert to imperial later.

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