Team358.org - Robotic Eagles - FIRST Robotics Competition

Upcoming Events:

Sep 14, 2010 1:00 PM:
SBPLI Golf Fundraiser
Oct 16, 2010 7:30 AM:
Deer Park Invitational
Nov 9, 2010 7:30 AM:
Brunswick Eruption
More...

Mechanical Resources

IFI FRC Drivetrain Reference Tables, 1.4 MB

Gearing a Motor

The motors FIRST gives us to work with are all rating in watts. The most powerful like the CIM at 300+watts are best for your drivetrain. Intermediate strong motors like the Fisher-Price at 140 watts and the van door are great for tough jobs. Globe motors at 50 watts with 1/6th the power of a CIM are good for actuators and other low power tasks. Servos and window motors at 20 or so watts are used as triggers but require enough mechanical leverage.
In choosing a motor Power = Force x Velocity, so as an example to hoist a 150 lb robot 1 foot into the air in 5 seconds:
1 Watt = 1 Newton-meter per second
1 lb = 4.45 Newton (N)
1 inch = 0.0254 meter (m)
So Power = (150lbs * 4.45N) * (12in *.0254m) / 5sec = 39 watts
In this example a globe motor but not a window motor could be used.

There are other factors to consider.

Proper gearing is critical to accomplishing every task and trades off speed for torque or vice versa
Some motors are designed to operate as fast spinning applications rather than stalled and hardly moving. The Fisher-Price motor for instance must spin to cool itself, so if that motor has to strain and slow to do a job, then the internal fans stop cooling it and the motor will eventually burn up.
Some motors come with internal thermal protection, so they shut themselves off when they get too hot. They turn back on after a couple of seconds of cooling down.

Here is a handy calculator that lets you specify different drive train characteristics such as motor, robot weight, motors per side, wheel diameter, gear ratio, etc. It's intended to help pick batteries, but you can use it to determine the gearing you need to reach a target top speed.
Motor Torque / Gearing/Amp-Hour Calculator
Same calculator converted to a local program with some FIRST specific motors added
Advanced Drivetrain seminar
JVN's Mechanical Design Calculator (2008)

Plexiglas vs. Lexan

When you want see-through protection…
Plexiglas is less expensive than Lexan, but Lexan protects against impacts. Plexiglas is much more brittle than Lexan and can easily break if flexed wrong and is prone to chipping and cracking. Lexan is more prone to scratching.
How-To

Gates Belts

This is a FIRST primer on the Gates drive belt/pulley system that came in the FIRST 2009 KOP. These timing belts are an alternative to chain drive.
GatesBeltDrivesForFIRST.pdf

Center Distance for a Given Chain Length

The center distance sprocket-to-sprocket for a tight chain may be determined by:
c = center-to-center distance in inches
L = chain length in pitches
P = pitch of chain [in inches] (.250" for #25 chain and .375" for #35 chain)
N = number of teeth in large sprocket
n = number of teeth in small sprocket

c=P/8*(2*L-N-n+sqrt((2*L-N-n)^2-0.810*(N-n)^2))
Begin with a chain length approximately the distance you are looking for and solve for it, then pick the nearest whole number for "L" to get the final distance "c".

Sprockets must have sufficient chain wrapped around both the driving and driven sprockets to avoid skipping teeth. Shoot for an optimal 180° around each sprocket, but don't depend on less than a minimum of 135° unless there is virtually no-load on the system. Be especially wary of small sprockets where the number of teeth engaging the chain is very few.
Here are several detailed references:
Martin Sprocket and Tsubaki Library and Tsubaki's - The Complete Guide to Chain

Planetary Gears

You'll see these types of gears in the FIRST Globe motors (if you take apart the black gear head on one end of the motor). The Globe gear casing itself is the Sun gear while the planet gears rotate against the inside.

Here are some explainations of how it all works- planet gears with the sun gear.
Epicyclic gearing or planetary gearing
University of Denver Epicyclic Gear Trains

G-Forces & Bumpers

Calculations of G-Force with respect to speed and bumper size. This is a graph that came in the 2000 FIRST Robotics Competition manual.

Framing

Alternatives to putting together a chassis, superstructure, arm, what have you include wood, aluminum, fiberglass, graphite composites, etc. Steel tubing can be a bit heavy, but can be used in limited amounts.
The FRC KOP comes with a very good bolt together chassis, but the rest of the robot has to be designed and assembled through your own resources.

Aluminum Tubing

Standard 1" square aluminum tubing from a local metal supply or even Home Depot or Lowe's is light, sturdy and can be easily cut to any size desired. The tubing can be welded if you have that capability, bolted together, gusseted, rivited, and/or you can use connectors, such as, Brunner or McMaster

Aluminum Extrusion Framing Systems

Complete framing systems are available from a number of manufacturers, such as, 80/20. Bosch, and Item. These are great for prototyping unfinished designs as they are infinitely adjustable and the parts can be reused. The framing members are very strong. The connectors tend to be the expensive parts.

How To...

General Tool Operating Procedures ( Adobe PDF

, 64 KB)

Guide to Using a Milling Machine ( Adobe PDF

, 470 KB)

Guide to Using a Lathe ( Adobe PDF

, 1.31 MB)

Sharpening Lathe Tools ( Adobe PDF

, 80 KB)

How to Use a Drill Press ( Adobe PDF

, 263 KB)

Bandsaw Tune Up ( Adobe PDF

, 101 KB)

Broaching Keyways ( Adobe PDF

, 108 KB)

Instruction on a wide-range of shop tools and short basic written tests complete with answer keys.
Texas Technology Education Guide ( Adobe PDF

, 1.6 MB)