gear ratio for a lift

We recommend using a planetary gearbox motor to power your lift. According to the Game Manual 0 Motor Guide, lifts are powered by 12V DC motors, and because they carry significant weight, they are high-load applications . Game Manual 0 warns that spur gearboxes are prone to stripping and failing under high loads, whereas planetary gearboxes offer better efficiency, higher load capacity, and superior resistance to shock loads .

The required gear ratio depends on your rigging style and stage count. As explained in Game Manual 0's section on rigging, if you choose a cascaded rigging system, the gear ratio must increase as you add stages to maintain the same extension speed; for instance, adding two stages to a single-stage cascaded lift geared at 3:1 requires increasing the ratio to 9:1 . Game Manual 0 also notes that linear extensions are generally preferred over rotational arms because they do not require the extremely high gear ratios and heavy support structures that arms need to handle torque .

For step-by-step examples of multi-stage lifts, the REV Robotics Docs provide guides for 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) .

You can fine-tune your lift's speed and torque by changing the spool size rather than swapping the motor's gearbox. Game Manual 0 explains that because translational speed is proportional to the spool's radius and torque is inversely proportional, a smaller spool increases torque and decreases speed. For example, if you have a 3.7:1 gear ratio and want the performance of a 5:1 ratio, you can swap a 2-inch spool for a smaller spool of (2 × 3.7)/(5) inches .

Was this right?

Sources16

Rigging > Picking the right spool size

Game Manual 0·CC BY-NC 4.0

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.

Open Game Manual 0

Gearbox Internals

Game Manual 0·CC BY-NC 4.0

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.

Open Game Manual 0

Gearbox Internals > Spur Gearboxes

Game Manual 0·CC BY-NC 4.0

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.)

Open Game Manual 0

Power Transmission

Game Manual 0·CC BY-NC 4.0

When building any mechanism, it is important to consider how the mechanism will be powered, as gear ratio and form factor are two factors that affect both spacing and efficiency. There are four main forms of power transmission: direct drive, gears, chain, and timing belt. In addition, there are different ways of powering a mechanism that is mounted on an axle.

Open Game Manual 0

Three Stage Cascading Lift | REV DUO Build System

This source does not publish a licence, so its text is not reproduced here. Open the source to read it.

Open REV Robotics Docs

Three Stage Continuous Lift | REV DUO Build System

This source does not publish a licence, so its text is not reproduced here. Open the source to read it.

Open REV Robotics Docs

Common Mechanisms

Game Manual 0·CC BY-NC 4.0

This chapter covers suggestions for design of common components of FTC® robots: drivetrain, linear motion (slides and lifts), claws, intakes, and more.

Open Game Manual 0

Motor Guide

Game Manual 0·CC BY-NC 4.0

12 volt DC motors are main drivers of FTC® robots. They are used for powering the drivetrain, intakes, lifts, and other mechanisms.

Open Game Manual 0

Gearbox Internals > Planetary Gearboxes

Game Manual 0·CC BY-NC 4.0

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.)

Open Game Manual 0

Design Tradeoffs > Spur Gear Gearboxes Planetary Gearboxes

Game Manual 0·CC BY-NC 4.0

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.

Open Game Manual 0

Choosing a Motor > Choosing The Right Gearbox

Game Manual 0·CC BY-NC 4.0

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.

Open Game Manual 0

Rigging > Retraction Options > Cascade Retraction

Game Manual 0·CC BY-NC 4.0

Cascade retraction entails simply rigging another set of cascade string that can retract the system when engaged (see the image below).

Note: If cascade retraction is being used with continuous extension, the two spools cannot be the same diameter. If the variable N is the number of stages in the system, the diameter of the continuous extension spool must be N times bigger than the cascade retraction spool.

Advantages:

Very space-efficient
All stages retract at the same speed at the same time

Disadvantages:

Requires more string (less strings to tension, less strings that can become loose)
Requires different sized spools if used with cascade extension

(Figure: Cascade retraction)

Additional Considerations:

If the system has only one stage, cascade rigging is identical to continuous rigging
Unlike continuous rigging, each time a new stage is added to a cascaded system, the gear ratio required to maintain the same extension speed increases. For instance, if 2 stages are added to a 1 stage cascaded lift that is geared at a ratio of 3:1, the ratio must increase by a factor of frac{2+1}{1} to maintain the same speed, changing the ratio to 9:1.
One disadvantage of cascade rigging is that each string must be kept tensioned. This is still the case with continuous rigging, but you have many more strings to keep track of, as tension must be maintained on all of them.

You need to make sure, as with any time you work with strings and pulleys, that you are pulling the string straight. Any amount of misalignment can lead to the string coming off of your pulley.

Open Game Manual 0

Choosing a Motor > Gearboxes > Spur Gearboxes

Game Manual 0·CC BY-NC 4.0

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.

Open Game Manual 0

Design Tradeoffs > Single/multi axis arm Linear extension

Game Manual 0·CC BY-NC 4.0

Important: Teams are generally advised to stay away from arms and move in the direction of linear slides, primarily due to the issue of complexity. This is because arms typically are less effective than linear extensions and are harder to implement properly.

Arms require a high gear ratio, so arms must be supported extremely well to bear the torque that the motor provides. A poorly supported and/or constructed arm will cause the driver needless pain as it is exceedingly difficult to line up an arm that constantly shakes. In contrast, linear extensions do not need to worry about gear ratios and gearboxes. They can be optimized to be more efficient than arms, and typically are more precise, as linear motion is easier to control than angular motion. Another positive is that linear slides can have more extension than arms, with some reaching 5+ feet in length.

Open Game Manual 0

Encoders > Programming Encoders > Reading Encoders

Game Manual 0·CC BY-NC 4.0

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

Open Game Manual 0

Chief Delphi > FTC > 3d printed linear lift reliable? FTC

Chief Delphi·Community forum posts — not openly licensed

This source does not publish a licence, so its text is not reproduced here. Open the source to read it.

Open Chief Delphi