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FIRST® Robotics Competition (FRC®)

Mechanical Resources

FIRST Specific

Gears...Gears...Gears

Gears modify and transmit motor power into torque, speed and direction. Two or more gears working together are called a transmission, gearbox, or gearhead. When two gears have unequal numbers of teeth, then speed and torque change. A large gear driving a smaller gear will give more speed, but less torque. A small gear driving a larger gear will give more torque, but less speed. In essence, torque can be traded for speed or vice versa by adjusting the ratio of the gear teeth between the two interacting gears.
Different Types of Gears

The basic types of gears include:

Spur gears - the simplest and most common type, these are flat with parallel teeth along the edge. These are found in the FIRST robot transmissions.
Worm gear - capable of very high torque thus harder to back drive, but less efficient than spur gears. These are found in window motors.
Rack & Pinion gears - a rack is a long flat bar with teeth, the pinion is a spur gear that forces the rack one way or another. These are found as part of car-type steering systems.
Face gears - changes the direction of force only in a right angle. This is how the spinning drive shaft along the length of a car translates at right angles to drive the rear wheels.
Bevel/Miter Gears - another way to change the direction of force, but at almost any possible angle
Helical Gears - a refinement of the simple spur gear, these are flat gears but with angled teeth. These run quieter and smoother than spur gears.

However, there are many, many more variations of gear types, some very specific to the solution of a particular mechanical problem.

How Gears Work
How Gear Ratios Work
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.

Gear Calculator
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)
Gear Quiz

Steering Differential

This allows opposite wheels to rotate at different speeds when the vehicle is turning. It's how your car goes around corners where the inside wheel has to travel much slower than the outside wheel.

Planetary Gears

These are called "planetary" because they resemble the solar system of Earth and the other planets revolving around the sun in orbits. This design makes for a tight/compact gearbox that fits on the end of a motor making it seem to be part of the motor itself. You'll see these types of gears in the FIRST older 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

Chain/Sprocket

Good for transferring power over a distance as well as for adjusting speed/torque.
Roller chain is called that because it is made with tiny rollers that smoothly engage the sprocket teeth.

Sprockets have to be accurately aligned or the chain will walk off. The distance between the centers of the sprockets should be 1.5 times the diameter of the larger sprocket or greater. Center distance should be adjustable in order to properly tension the chain, the distance of one chain link is enough, or an idler sprocket or Delrin tensioner should be used to adjust chain tension. A little slack is desirable, preferably on the slack side of the drive.
The chain should wrap at least 120¡ around the sprockets, which requires a sprocket-to-sprocket ratio of no more than 3.5 to 1; for greater ratios, an idler sprocket may be required to increase chain wrap around both sprockets. The more fully engaged teeth the better, for example, with only one or two teeth engaged, the chain will be forced right off the teeth.

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

Chain Tensioners

Chain Tools

The most common roller chain sizes used in FRC robotics are #35 and #25. The FRC Kit-of-Parts comes with #35 roller chain. Other chain sized, such as bicycle chain, are uncommon.
#35 chain is more forgiving for beginners if sprockets are misaligned, long runs are involved, or unknown stresses will be put on the chain.
#25 chain is lighter and strong enough for most applications, but must be aligned properly.

Chain breaker tools are designed to remove the pins that hold chain links together. Some tools only remove links and the chain must be refastened with master links. Other types of these tools will both disassemble, as well as reassemble (rivet) chain into one continuous run producing a stronger chain without the weakness/drawbacks of a master link. Available at Go Cart stores and online.

A related tool is a roller chain puller. This is a specialized tool that pulls the two loose ends of a chain together and holds them while you use a breaker tool or a master link to reassemble them.

Master links replace one link, and they also come in a half-link version, as well as a link-and-a-half combo. Example Google search terms include: "#25 Chain Master Link", "#35 Chain Half Link ", "#25 Chain Link And A Half"

Google search for "DarkSoul #25 Chain Breaker Tool"
Google search for "#35 Chain Breaker Tool"

FRC Motor Tables

This was included in the FRC Kit-of-Parts by IFI one year. For the kitbot of that year, it shows how sprocket ratio changes will affect your wheel speed. For a given speed it also shows how much power is required from a pair of CIM motors to attain that full speed and identifies the risky sprocket ratios and extreme ratios that will cause the 40amp breakers to trip.
FRC_DrivetrainRefTables

IFI FRC Drivetrain Reference Tables, 1.4 MB

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.

Mechanical Components

Descriptions of some of the frequent parts found in the FRC Kit-Of-Parts (KOP or Kit).

Chassis (AndyMark)
Motors (CIM, Fisher Price, Banebot, Denso)
Drivetrain (wheels, gearboxes, power transfer)
Ways to change your robots speed/torque
- Motor(s) # and type used
- Gearbox ratio (fixed or shifting)
- Sprocket ratio
- Wheel size
Miscellaneous (Igus slides, leadscrew, etc.)

	Chassis The basic frame that the robot sits on, the Kit chassis changes a bit every year, but since 2005 it has given teams an easily assembled basic drive base in a matter of hours. Of course, manipulators and superstructure still seem to take 6 weeks. Extra chassis and individual spare parts can be purchased at AndyMark where youÕll also find step-by-step assembly videos and instructions. Source: AndyMark
	Can Motors These range from small to hand drill power to power mechanisms. Even the smallest of these is too powerful for a 20amp Spike and must use a speed controller. Some people break-in the bronze bushings by attaching the motor armature to a drill or Dremel tool and running it for a short while. Wearing in the bushings can get you 10% more power. Just don't over do it. Specs: RS-550 , RS-775 Source: banebots
	CIM Motor The workhorse choice for drivetrains. Only powered through a speed controller. Designed for 1000 hours mininum life when run at ~17% duty (3 minutes on CW, 2 sec off, 3 minutes on CCW 30 minutes off, repeating) with 64 oz-in load torque and 12V supply. Specs: CIM motor curve CIM testing at max power Source: AndyMark & VexPro
	Mini CIM Motor A third shorter and lighter than a CIM, but otherwise the same dimensions. Powered through a speed controller and designed for long life and stall conditions. Specs: popular motor curves Source: VexPro
	Bag Motor A miniature version of the CIM. Resistant to stall conditions. Specs: popular motor curves Source: VexPro
	Servo These are low power devices that can be ordered to a specific position within a rotational range of ~180 to 270 degrees depending on the model. Specs: HS322HD.pdf Source: super droid robots
	Denso Window motors These come in left/right-handed versions, so they are not interchangeable mounts. The integral gearbox uses a worm gear and can be difficult to backdrive. There are also anti-backdrive pins that can be removed if that fits your design. Most teams make their own electrical connector , and the mechanical coupler comes in the Kit. Can be powered from a Spike (forward/reverse/off) or speed controller (variable speed). The pins are PN: 12129493 (Packard) or 829-12129493 (Mouser). The housing is PN: 12129487 (Packard) or 829-121 (Mouser). Electrical connector: housing: mouser pins: mouser
	Automotive motors Legal for several years these include door lock actuators (as long as they are motors not electric solenoids, seat motors, window motors like those above, windshied wiper, throttle. Source: various and AndyMark
	Kit Wheels These have a sticky solid tread. For a dead axle, like the Kit assumes, each wheel requires two pressed in bearings. Sprockets bolt directly to the wheel. Dead axles are fixed in place and the wheel spins on them. Live axles on the other hand spin and the wheels are fastened solidly in place to the axle. Two bearings, one on either side of the wheel, or one on either end of a live axle are required for balance and prevent the wheel or axle from being twisted out of place by the weigh or stress of the robot. Cantilevered axles have two supporting bearings set some distance apart and the axle projects out from them. Source: AndyMark
	Traction Wheels These sport replaceable tread and different tread choices. For a dead axle each wheel requires two pressed in bearings. Sprockets bolt directly to the wheel. Source: AndyMark & VexPro
	Omni wheels This wheel gives full power in the forward direction, but slides sideways very easily. A pair of these on the front makes for easy turning. For a dead axle each wheel requires two pressed in bearings. Sprockets bolt directly to the wheel. Source: AndyMark & VexPro
	Mechanum Wheels An unusual wheel, not for the rookie. These are expensive, but allow a robot to instantly move in any direction at the expensive of weight and power. Each wheel requires its own motor/transmission. Source: AndyMark & VexPro
	Sprockets & Roller Chain The most common power transfer over any distance used in FIRST robotics. #35 roller chain is the most common and most forgiving when sprocket alignment issues crop up. #25 chain is smaller and lighter, but does require greater attention to proper alignment. Sprocket ratios are very simple to change and play with. Chain is easy to make in specific lengths either using a chain break tool to create a solid length of chain or using master-links or half-links to adjust the length. Chain tensioners for long runs are usually required. They can be cheaply purchased, made from a simple piece of slippery Delrin, or using a floating sprocket. Sprocket source: AndyMark Roller chain tensioners: McMaster-Carr Chain, master/half-link, tensioner source: AndyMark
	Belts & Pulleys An alternative to chains/sprockets, belts/pulleys are quieter, but belts must be purchased in the required length. This is a FIRST primer on the Gates drive belts/pulleys that Gates offers and donates in the FIRST KOP. Belting path/length/tension Designer GatesBeltDrivesForFIRST.pdf GatesBeltsKitInfo-2013.pdf GatesBeltsKitInfo-2012.pdf Source: AndyMark VexPro Gates Stock Drive Products
	Kit Gearbox A single speed transmission. Speed & torque can be changed by varying the output sprocket ratio or wheel diameter. Made to be powered by 1 or 2 CIM motors. Source & spare parts: Banebots
	Multi-speed or Shifting gearboxes When your strategy calls for both speed and torque, two or gear ratios can get those for you. These can be shifted by servo or pneumatics, but pneumatics is faster/surer. A good source is AndyMark: AndyMark
	Single-speed gearboxes Good for mechanisms as well as drivetrains. Source: AndyMark
	Planetary Gearboxes AndyMark has a selection of these good for mechanisms or drivetrains: AndyMark Banebots also has a selection of potential alternative gearboxes: Banebots
	Versaplanetary Gearboxes VexPro has developed a modular gearbox system with ratios of 3:1 up to 100:1, a variety of easy mounting options for motors, and a variety of output shafts great for mechanisms. Source: VexPro
	Igus Energy Chain When wires for motors or sensors or pneumatic tubing must be attached to a moving mechanism they risk becoming snagged or tangled. This flexible plastic snake is used to guide and protecting moving wires, for example, on an elevator mechanism. Source: Igus Plastic corrugated split flex tubing is a cheaper alternative for this purpose, but doesnÕt guide the wires.
	Igus DryLin linear guides A low friction slide system. Plastic pieces slide along frictionless coated aluminum rods. Source: Igus
	Leadscrew w/nut Translates rotary motion into linear motion. One use is to power slides with a spinning motor. Source: haydonkerk
	Framing Alternatives to putting together a superstructure, arm, manipulator or what have you include wood, aluminum, fiberglass, graphite composites, etc. For FRC, steel tubing can be a bit heavy, especially for a top-heavy superstructure, 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, riveted, 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 and Bosch. These are great for prototyping unfinished designs as they are infinitely adjustable and the parts can be reused. They have slots to attach or slide other parts anywhere along their length. Connector styles include standard bolts, slot nuts, right angles, gussets, joining plates, slides and angle adjustment, or they can be drilled through and bolted as you would aluminum tubing. The framing members are very strong, but heavier that standard aluminum square tubing. The connectors tend to be the expensive parts. Other Construction Techniques With guidance there are other construction techniques, such as Unibody/Monocoque which is made from thin aluminum sheets bent and shaped to provide its own support without needing framing members.
	Plexiglas vs. Lexan (or acrylic vs. polycarbonate How-To Work Acrylic & Polycarbonate When you want see-through protection. As a general rule Lexan is much better for our robots than acrylic simply because it resists shattering. Plexiglas brittle - it can easily break if flexed wrong and is prone to chipping and cracking scratch resistant (better optics in the long run) less expensive than Lexan Lexan Lexan protects against impacts up to 18x what acrylic can withstand (thick enough its bulletproof) more prone to scratching
Bearings With moving parts some type of bearing surface is required to smooth motion, avoid destructive wear and tear on members, and in general make it a dependable and long lasting joint. Wheels or live axles in particular need high speed bearings. Slower moving joints can use oil-impregnated brass bushings or Delrin thrust washers to allow two pieces to pivot against each other. How Bearings Work
	Pop Rivets Useful for quickly fastening two pieces of metal together, pop rivets only require a pre-drilled hole and can also be quickly removed by drilling them out. Rivets come in different sizes and in both aluminum and steel versions and are very cheap. A good rivet tool is worth the money if you're doing a lot of riveting. Some teams assemble their entire robot frame with rivets, rather than bolts.
	Springs-Springs-Springs Constant force springs, Constant torque springs, variable force springs, coiled springs, torsion springs Car springs strong enough to throw a ball over a wall, constant force springs to maintain tension on a stretched cable, even surgical tubing (giant rubber band) to counter-balance a heavy manipulator to make it easier for motors to move. Source: Vulcan Springs (a KOP donor) spring brochure
	Tungsten Inert Gas (TIG) Welding Some robot teams have aluminum welding capabilities. Aluminum can be difficult to weld because at welding temperatures the metal will vaporize if oxygen is present. To avoid destroying the aluminum, a steady flow of inert gas must surround the hot spot pushing the oxygen away and acting as a gas shield. Furthermore, with aluminum the two pieces are not just melted together, but a filler rod is also used to add a strengthening metal to the join.

How To...

Machine shop tips - Nice series of videos by a retired Illinois high school shop teacher

Here's a set of instructional videos produced by MIT. Intended for students who use their model shop, they emphasize safety and the right way to use grinders, sanders, bandsaws, drill presses, mills, and lathes.

Some Others

How to coil and uncoil a bandsaw blade safety
From other sources we have:

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)

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