lift gear ratio

We recommend using a robust planetary gearbox motor to power your linear lift, and adjusting the system's speed and torque by changing the spool size rather than swapping out the motor's gearbox. Game Manual 0 notes that changing the spool size is often much more convenient than changing the gear ratio to achieve the desired speed and torque balance. Because the translational speed of the slide is proportional to the spool's radius, a smaller spool will increase torque and decrease speed, while a larger spool does the opposite.

If you must change the gear ratio itself, you can gear down (increase the ratio) to gain more torque at the expense of speed, or gear up (decrease the ratio) to gain speed at the expense of torque. Game Manual 0 explains that these two properties are inversely proportional, meaning that any change to one will directly affect the other.

Additionally, Game Manual 0 strongly advises using planetary gearboxes rather than spur gearboxes for high-load mechanisms like lifts. Because spur gearboxes only have a few teeth engaged at any time, they are highly susceptible to stripping and breaking under heavy or sudden loads, whereas planetary gearboxes distribute the load across multiple meshing teeth.

For teams looking for specific lift designs, the REV Robotics DUO Build System documentation provides guides for building both a Three Stage Cascading Lift (https://docs.revrobotics.com/duo-build/linear-motion-kit/three-stage-cascading-lift) and a Three Stage Continuous Lift (https://docs.revrobotics.com/duo-build/linear-motion-kit/three-stage-continuous-lift).

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Three Stage Cascading Lift | REV DUO Build System

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Rigging > Picking the right spool size

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Spools have a special property that isn't often discussed, but is extremely useful when creating linear slide systems. Just as the system's speed and torque can be changed by changing its gear ratio, speed and torque can also be changed by changing the spool size. The motor rotates the spool at a constant angular speed. Thus, the translational speed (the speed of the slide) is proportional to the radius of the spool, and since torque is inversely proportional to speed, changing the spool size changes torque as well.

This is important to recognize, as changing spool size is often more convenient than changing gear ratio to get the desired combination of speed and torque. To illustrate this, say you have a linear extension system with a 3.7:1 gear ratio. You then decide that a 5:1 gear ratio would provide a more desirable combination of speed and torque than your current 3.7:1 ratio.

In many cases, instead of swapping gearboxes, it makes more sense to swap out spools to a smaller one. If your spool is currently 2 inches, your new size should be frac{2*3.7}{5} inches to achieve the same result.

You also need to make sure that when fully wrapped on the spool, your cable or string doesn't overlap. Overlapping can result in a change in spool diameter, which will change the tension in your string.

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Three Stage Continuous Lift | REV DUO Build System

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Gearbox Internals

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Fundamentally, a gearbox is just a collection of gears and an enclosure that connects them. Gearboxes have an output ratio, the final Gear Reduction between the motor input and the final output shaft.

Term: Gear Reduction — Also known as a gear ratio. In any rotational power transmission system (typically involving motors and servos in FTC®), a gear ratio defines both the number of rotations of the system's input and the number of rotations of the output.

For instance, a NeveRest 20 gearmotor consists of an unmodified NeveRest Motor and a planetary gearbox that has a gear ratio of 20:1 (or, when spoken, "20 to 1"). This means that in order for the output shaft of the gearbox to rotate 1 time, the input shaft of the motor must rotate 20 times. Gear ratios are one of the most important design considerations about a power transmission component.

Any motor or servo has two properties: speed and torque (or rotational force). These two properties are inversely proportional, meaning that increasing speed decreases torque, and vice versa. For instance, if one wishes to make a mechanism faster at the expense of torque by doubling the speed of that 20:1 gearbox, they would decrease the gear ratio by a factor of 2. Since 20 divided by 2 is 10, the new desired ratio would be 10:1 (this is referred to as gearing up). However, if one wishes to double torque instead, making the system more powerful and robust at the expense of speed, they would increase the gear ratio by a factor of 2, leaving them with a 40:1 ratio (this is referred to as gearing down).

The most common ways of gearing up or down are using gearboxes, gears, sprockets and belt-driven pulleys, all of which exist in various sizes.

In FTC, gearboxes may be more common than you think. Nearly every motor and servo has a gearbox attached or built-in to it. These gearboxes are either spur or planetary.

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Gearbox Internals > Spur Gearboxes

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Term: Spur gearbox — A spur gearbox has spur gears which are stacked on top of each other. Gear reduction is achieved through different size gears on the same plane.

Spur gearboxes are an arrangement of gear ratios, often stacked to achieve a large compound ratio (e.g. 40:1). Each individual ratio only has two gears- one may be 8:1, another may be 5:1, but the final ratio will be 40:1. These gearboxes are used in the AndyMark NeveRest Classic series and goBILDA's 5201 series motors, as well as REV HD Hex Motors. Due to the nature of how these gearboxes are built, each reduction only has a few teeth from each gear engaged, and those teeth carry the entire load of the gearbox. It's easy to damage a spur gearbox from shock load, and if one gear breaks, the entire gearbox will stop functioning.

Tip: Using spur gearboxes on high-load applications such as drivetrains or arms is not recommended. Instead, use planetary gearboxes.

(Figure: Example of a spur gearbox inside a servo. Note how all gears mesh with only one other gear.)

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Gearbox Internals > Planetary Gearboxes

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Planetary gearboxes use a more complex system of gears to achieve a robust reduction in a compact space. In automotive engineering, planetary gear sets can achieve a few different ratios without changing gear size, but all planetary gearboxes that you will see in FTC only achieve one gear ratio.

Term: Planetary Gear — Planetary gearing consists of a center gear (sun gear) which has smaller gears (planet gears) revolving around it. The outer radius has a ring gear which holds the other gears in place.

Planetary gearboxes are used in the AndyMark NeveRest Orbital and Hex gearmotors, REV HD Hex UltraPlanetary motors, and goBILDA's wide selection of Yellow Jacket planetary gear motors. As you can see from the graphic below, there are more teeth meshing per stage than in the spur gearbox.

(Figure: Example of a planetary gearbox stage. Note how the sun gear meshes with more than one gear.)

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Choosing a Motor > Choosing The Right Gearbox

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For regular use, any of the planetary gearboxes will fit your needs. Planetary gearboxes are just a tiny bit more expensive, but boast better backlash and efficiency, higher load capacity, and better capacity for shock loads than spur gearboxes. The tradeoffs, cost and mechanical noise, are almost never a factor.

Tip: Because both gearbox types are so similar in price for similar ratios, we generally recommend the use of a planetary over a spur gearbox.

If you already own spur gearboxes, try to use them in lower-load situations, and use planetary motors on your drivetrain and high-load mechanisms.

For small reductions, it may be more economical to choose a motor you already own and build an external reduction using gears, chain, or belts. It should again come down to your desired output shaft, desired gear ratio, and for the UltraPlanetary, whether you want the ability to swap parts out on the fly.

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Design Tradeoffs > Spur Gear Gearboxes Planetary Gearboxes

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Note: Spur gear gearboxes are fine for most applications for a rookie team. We are not advocating necessarily having to upgrade to planetary motors, but there are some advantages which may become useful in more advanced use cases such as high-load systems.

Important: Spur gear gearboxes have inherent disadvantages to planetary gearboxes. Spur gear gearboxes should not be used in high-load situations, primarily because the gears can strip and destroy the gearbox.

An example would be a drivetrain that has to change directions repeatedly and quickly. Planetary gearboxes are much better suited for drivetrain and arms, due to the configuration of the sun and planet gears. In addition, spur gear gearboxes are prone to shock loads; therefore, direct drive is not advisable on drivetrains. Refer to the gearbox anatomy section for more complete information on gearboxes. This refers to using spur gear gearboxes which are attached directly to the pinion gear of the motor. It does not mean external ratios outside of the motor gearbox, which will always be in a spur gear configuration.

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Choosing a Motor > Gearboxes > Spur Gearboxes

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Danger: Spur gearboxes are NOT recommended due to their shorter lifespan and lower mechanical resilience compared to planetary gearboxes. If you are purchasing new motors, it is highly suggested to purchase planetary gearbox motors instead. Care should be taken to not put load on the output shaft of a spur gearbox. In particular, spur gear motors should NOT be used in high load applications, such as a drivetrain.

Motors with spur gearboxes include AndyMark NeveRest Classic motors (in 40:1 and 60:1 ratios), the REV HD Hex 40:1 Spur motor, and the discontinued goBILDA 5201 Series Yellow Jacket Spur Gear Motors. All of them offer similar performance and reliability, so your choice is primarily dictated by the convenience of mounting and connecting to the rest of your design (e.g., if you use REV kit, you should probably choose REV HD Hex motor, as it uses the same 5mm hex shaft as the rest of REV system).

REV HD Hex Motor with Spur Gearbox - This motor comes in a 20:1 or 40:1 ratio, and uses the same connections (JST VH) as the REV Expansion and Control Hub for power, which means no adapter cables are necessary. The output shaft is a 5mm hex shaft. REV UltraHex has a 5mm hex bore running through the middle of a 1/2" hex shaft, which makes adapting this motor to any length of UltraHex, and by extension, 1/2" hex shaft, very easy.
AndyMark NeveRest Classic motors come in 40:1 and 60:1 ratios. The output shaft is a 6mm D-shaft, and the motors have inbuilt cables with Anderson PowerPole or JST VH connectors to connect to power.

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Encoders > Programming Encoders > Reading Encoders

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Tip: You can calculate a motor's Counts Per Revolution by taking the base motor's Counts Per Revolution and multiplying it by the gearbox ratio. Be careful to use the actual gearbox ratio when doing this! For example, a 5:1 Ultraplanetary motor would have a count per revolution of 28 * (5.23) = 146.44 because the base motor has 28 Counts Per Revolution and the 5:1 gearbox actually has a 5.23:1 gear ratio. Remember, when using two gearboxes on top of each other, you multiply the gear ratios together.

In the following example, we divide the encoder position by its counts per revolution to obtain the number of revolutions the encoder has rotated. You have to replace [Your Counts Per Revolution Here] with the counts per revolution of your motor, found on its product page or calculated using the above tip.

Java

double CPR = [Your Counts Per Revolution Here];

int position = motor.getCurrentPosition();
double revolutions = position/CPR;

There is one more number we can get: the angle of the shaft. Calculating this number is very simple. We can multiply the number of rotations by 360 (since there are 360 degrees in one revolution). You might notice that this number can go above 360 as the shaft rotates multiple times. As such, we introduce angleNormalized, which will always be between 0 and 360.

Java

double CPR = [Your Counts Per Revolution Here];

int position = motor.getCurrentPosition();
double revolutions = position/CPR;

double angle = revolutions * 360;
double angleNormalized = angle % 360;

Putting it all together, we get the following testing program.

Java

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Gears

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Gears, like sprockets and pulleys, are used in power transmission for three common applications: changing the direction of power, changing the amount of torque, and changing RPM. Gears are a less common transmission option than chain, but are still very viable for most use cases. Gears are equally as reliable as chain, but can't be used for transferring power over long distances. Many teams dislike gears when using kit-based channels because the hole pattern limits which combinations of gears can be used. Consequently, it may be difficult to achieve a desired specific gear ratio. However, with extrusion systems, it's very easy to use different ratios, since the gears can be moved around in extrusion.

Term: Gear — A gear is a machine part that has cut teeth, usually written in the form "numberT" (e.g. 32T, 86T). It is a form of power transmission that reverses the direction of rotation when used. The most common material for gears to be made of is aluminum or delrin plastic.

(Figure: 56T REV aluminum gear)

Gears are made in different materials, with the most common being 7075 aluminum. Never mesh plastic and metal gears together. It is acceptable to mesh different types of metal gears together, as long as they have the same diametral pitch. It is advised to stay away from TETRIX aluminum gears as they wear down very easily. Some REV gears are made out of Delrin, a self-lubricating plastic. It is a durable material, but keep in mind that it is very possible to strip the bore using a plastic gear. Thus, we advise using the REV Hex Hub Strengthener to avoid stripping the bore on Delrin gears.

Bevel gears are a special type of gear that allows power transmission in two different planes. It is especially useful in tight spaces where a regular motor mounting position would not work.

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Vendor Guide > Where can I get ...?

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Motors and Gearboxes

Spur Gear Motors

AndyMark NeveRest Classic
REV HD Hex & Core Hex
goBILDA 5201 Spur Gear
TETRIX TorqueNado

Planetary Gearbox Motors (recommended)

AndyMark NeveRest Orbital
REV HD Hex Planetary & UltraPlanetary
goBILDA 5202/5203 Yellow Jacket Planetary
(italicized = customizable gearbox ratio)

Servos

HiTec (from ServoCity); 24 tooth spline
REV Smart Robot Servo; 25 tooth spline
Axon Mini & Max; 25 tooth spline
goBILDA Dual Mode Servo; 25 tooth spline
Futaba (Futaba servos are widely used in RC and hobby aircraft); 25 tooth spline
Savox heavy-duty servos; 25 tooth spline
Actuonix (linear actuators)

Sensors

AndyMark
REV

Hardware & Raw Materials

McMaster-Carr
OnlineMetals
Amazon
Local hardware store

Aluminum Components

Actobotics channel, mini channel, X-rail extrusion
REV extrusion, C channel, U channel
goBILDA channel, goRAIL extrusion
TETRIX channel
MiSUMI extrusion

Gears/Sprockets/Pulleys

REV gears, sprockets, pulleys
Actobotics gears, sprockets, pulleys
goBILDA gears, sprockets, pulleys
TETRIX gears and pulleys
AndyMark gears and pulleys

Wheels

AndyMark traction, compliant, mecanum wheel
REV grip, traction, omni, mecanum wheel
ServoCity traction, omni wheel
goBILDA traction, omni, mecanum wheel
TETRIX traction, omni, mecanum wheel
Nexus mecanum wheel

Linear Slide Kits

REV 15 mm extrusion slide kit
Actobotics linear slide kit
goBILDA linear slide kit

Drawer Slides

MiSUMI SAR2 or SAR3 aluminum slide
Long Robotics Slides
Hafele cabinet slide
Steel-rolled drawer slide
Igus

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Drivetrains > Drivetrain selection

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Warning: It is not recommended for teams to use spur gearboxes on their drivetrain. Instead, use planetary gearboxes, as they are less prone to shock loads and breakage.

Traction/Pushing Power: While this feature is often overemphasized, it is still very important. Pushing power describes a drivetrain's ability to endure defense/engage in defense. In addition, traction will be important if the drivetrain must traverse obstacles or some sort of terrain.

Many factors affect the pushing power of a drivetrain, including wheel type, motor gearing, and overall weight of the robot.

Note: If you already have a very agile drivetrain with experienced drivers, a team can avoid defense instead of having to fend it off or engage in it.

Powering the drivetrain: Generally, there are four options for power transmission: direct drive, chain, gear, and belt. More detail about each option can be found in the power transmission section.

Teams should stay away from direct drive, as gearboxes are prone to breaking, especially under shock loads (for example, if the wheel is hit by another robot, or the wheel slams into the wall).

We recommend belted drivetrains, but realize that belt is a difficult option for new teams. Chain and gears are also great options - chain requires a bit more forethought, as 1+ tensioner per side is required to maintain correct tension in the chain.

CAD or a detailed sketch is generally recommended with chain in order to visualize the chain run (where the chain will be placed). Gears are slightly simpler, and can be a fantastic and easy option, especially when using extrusion. We would advise to stay away from TETRIX gears, and use the gears from other kits such as the REV delrin gears (with hex hub strengthener) or REV aluminum gears.

See the /docs/common-mechanisms/power-transmission/index section for more details.

An important step is determining what you want out of your drivetrain.

Do you want speed?
Pushing power?
Ability to get over terrain?
Do you need to strafe?

All of these questions should be answered before choosing a drivetrain.

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Gears > Disadvantages

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Sometimes, the ratio you want might not be easy to build. Channel spacing limits gear ratios, but this can be circumvented with compound ratios and a bit of creativity.
Long distance power transfer is impractical with gears. If you need to transfer power long distances, gear combinations can become complicated very quickly, so belt/chain is preferable.
Meshing gears can be tricky. It's only made worse by the sensitivity of a gear mesh. However, channels do solve this problem, providing pre-spaced holes to easily mesh your gears. Do keep in mind that gear mesh may not be perfect, even with channel.
Gears usually wear faster than sprockets if there is too much friction between the gears. Teams can use white lithium grease or similar lubricant to help remedy this problem.

(Figure: 11115 Gluten Free gear-based drivetrain)

(Figure: 13075 Coram Deo Robotics, Rover Ruckus gear-based drivetrain)

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Choosing a Motor > Legal Motors

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Bare motor. In all cases above, this is a 12V motor of RS-555 type, with free speed around 6,000 RPM and stall current around 10A. The motor specs posted by different vendors might be slightly different, but the difference is mainly due to different testing methods. In real life, the bare motors used by AndyMark NeveRest motors, REV Robotics HD Hex motors, and goBILDA motors are virtually identical. The most reliable specs can be found in the motor-data section.
Gearbox. The gearbox is attached to the front of the motor and reduces the speed while increasing the torque. For example, a 20:1 gearbox reduces the speed by a factor of 20, resulting in a no-load speed of around 300RPM. See Gear Reduction. A gearbox also contains the output shaft (typically a 8mm REX shaft, 5mm hex shaft, or 6mm D shaft) and mounting holes. The gearbox can also be replaced; this is FTC legal but requires some skill.
Encoder. Attached to the back of the motor and protected by a plastic cap, the encoder contains a sensor registering motor shaft rotation. It can be connected to REV hubs and used for precise control of motor speed or rotating to a specific position.

Since the bare motor is similar for all motors discussed above, the choice of the right motor is dictated by the gearbox: the gear ratio, output shaft type, and ease of mounting.

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Motor Glossary

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Core Hex Motor — The Core Hex Motor, sold by REV, is different from the standard RS-555 series motors that are generally used by FTC® teams. It features a 90 degree orientation and does not contain an output shaft. Thus, teams will have to cut 5 mm hex shaft to length as needed. The Core Hex motor has a slow gear ratio (72:1), and is much less powerful than the RS-555 series motors.

Warning: We advise teams to go against the Basic Bot Guide provided by FIRST®, as Core Hex Motors should NOT be used to power drivetrains.

HD Hex Motor — The HD Hex motor, sold by REV Robotics, is a RS-555 series motor with spur gear and planetary gearbox options. The motor has a 5mm hex output shaft compatible with REV's motion system.

NeveRest Motor — The NeveRest Motor, sold by AndyMark, is a RS-555 series motor that is available in spur gear and planetary options. It has a 6 mm D-shaft output.

Yellow Jacket Motor — Yellow Jacket motors are the RS-555 series motor and planetary gearbox sold by goBILDA. They come with a 6 mm D-shaft (5202 series) or 8 mm REX shaft (5203/5204 series) and are available in many different gear ratios from 3.7:1 up to 188:1.

UltraHex — UltraHex is 1/2" aluminum hex shafting sold by REV Robotics. There is an inner 5 mm hex bore in the middle, which allows compatibility with REV's 5 mm hex shaft motion system. The 5mm hex bore also allows for a 1/4-20 or M6 screw to be tapped into it. 1/2" hex is also compatible with many FRC® vendors.

RS-550 Series Motor — The RS-555 series motor is the standard motor in FTC. It forms the base for the AndyMark NeveRest, REV HD Hex, and goBILDA Yellow Jacket motors.

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