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Transferring Motion

The Antidote - FTC 14320

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Motion

Most FTC mechanical systems require powered rotational motion of an object.
There are four main ways to connect an object to a motor or servo for transmission of powered rotation:

Direct Drive
Geared Drive
Chain Drive
Belt Drive

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Our Intake mechanism for grabbing stones in the 19-20 season

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Vocabulary

Speed: Rotational velocity of a rotating object (Shaft, Gear, etc.).
Torque: Rotational force of a rotating object. Higher torque allows movement of heavier objects.
Driving: The gear, sprocket, or pulley which is turning another gear, sprocket or pulley, usually the one connected to the motor/servo.
Driven: The gear, sprocket or pulley which is being turned or driven.

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Driving Gear

Driven Gear

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Direct Drive

Direct Drive is when the motor/servo is directly connected to an object via the drive shaft.

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Directly driven winch system for linear slides

Motor

Rotating Winch

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Direct Drive Pros & Cons

Pros:

Direct Drive is the most compact way of transferring rotation, allowing for compact designs
Directly driven systems are incredibly easy to create, perfect for prototyping or simple mechanisms

Cons:

Direct drive does not offer torque or speed reductions, so the powered object will spin at the same speed and with the same torque as the driving motor.
Any shock delivered to the object will impact the motor, leading to possible damage to the motor shaft or gearbox.

Especially problematic for use in high-stress situations such as drivetrains

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Gear Drive

Gear Drive is when a motor/servo rotates an object through side-by-side gear transmissions.

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Motor

Driving Gear

Driven Gear

Output Gear

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Gear Ratios

Gears, along with pulleys and sprockets, allow for reductions, which is the increasing or decreasing of speed and torque.

A larger driving gear increases the speed of the driven gear but decreases the torque.
A smaller driving gear decreases the speed of the driven gear but increases the torque.

Calculating Gear Ratios:

Number of teeth (Driving) : Number of teeth (Driven)

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48 teeth : 48 teeth

1:1 Gear Ratio

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Gear Positioning

Gear driven transmissions requires gears to be set at precise distances from each other for optimal performance
For gears to work best together, the pitch diameters need to be tangent.

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Pitch Diameter

Gear pitch diameters can usually be found on the vendor’s website

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Gears at Angles

Gears can be used to transfer motion at angles as well
Specialized gears are required for angular transfer of motion, known as bevel gears.
There are two main types of bevel gears:

Standard bevel gears
Miter gears

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Bevel Gears

Standard bevel gears are used for non-90-degree angles or gear ratios other than 1:1.
Most common and reliable bevel gear is the 2:1 GoBILDA bevel gear set

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Miter Gears

Miter gears are specially designed for 1:1 90-degree angles
GoBILDA’s miter gears are extremely reliable COTS (Commercial Off The Shelf) parts for use in these situations

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Gear Drive Pros & Cons

Pros:

Easy use of reductions to increase or decrease speed and torque for the final output
Specialized gears such as bevel gears allow transferring rotation at angles
More compact for transferring rotation over small distances

Cons:

Difficult to transfer rotation over long distances.
Small room for error when placing gears

Need to be a set distance apart for optimal performance

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Chain Drive

Chain Drive is when a motor/servo rotates an object through chain-connected sprockets.

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REV Robotics Chain Drivetrain

Sprocket

Chain

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Gear Ratios With Chain

Gear ratios work the same way with sprockets as with gears.
A larger driving sprocket gives the driven sprocket more speed
A smaller driving sprocket gives the driven sprocket more torque

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High-torque reduction from 18-19 season robot

16 tooth sprocket

60 tooth sprocket

REV Robotics Chain Guide

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Sprocket Positioning

Positioning sprockets is much easier than positioning gears because of the leeway chain allows while still transferring motion
Even though chain allows more leeway, it is important to remove as much slack from the chain was possible.

The less tension in the chain, the more the sprockets will wiggle independently

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REV Robotics Chain Guide

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Sprocket Positioning Continued

In order to find the length of chain required for two sprockets a set distance apart, a chain length calculator can be used.
By searching for chain length calculators on the internet, various websites can be found that use the distance between and size of sprockets to determine the number of chain links required.
Most calculators can also use the chain length and sprocket size to calculate the optimal sprocket distance.
Some useful sites are:

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Chain Breaking

In order the adjust the length of the chain, it must be broken at the appropriate links and put back together.
Breaking chain and reconnecting it is extremely simple, and it can be reconnected in two different ways:

Using a master link
Resetting the chain

A great guide created by REV Robotics for chain breaking can be found here:

https://www.revrobotics.com/content/docs/REV-41-1442-UM.pdf
The above link uses the REV chain breaker, but it can be done with any chain breaker rated for the chain being broken (#25 chain breaker for #25 chain)

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Chain Drive Pros & Cons

Pros:

Easy to transfer motion over large distances
More forgiving of spacing errors
Simple to put together
Durable

Cons:

Cannot transfer motion at angles
Slightly less efficient than gears and belts

A small amount of motion is lost to the sprockets wiggling independently.

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Belt Drive

Belt Drive is when a motor/servo rotates an object through belt-connected pulleys.

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Pulley

Belt

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Timing Belt

The timing belt is the most used belt for FTC
Timing belts are belts with teeth on the inside to move the pulleys with more friction
Timing belt pulleys are similar to sprockets, however the teeth are usually shorter and smaller

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Pulley Positioning

Pulleys need to be accurately positioned for belt drive to work
Too low tension prevents the belt from pulling on the teeth of the pulley
Too high tension snaps the belt
The best way to position pulleys is to use a belt length calculator

Very similar to a chain length calculator
The belt length calculator can calculate belt size given pulley size and spacing
It can also calculate pulley spacing given belt length and pulley size