What gear ratio makes an arm stronger?

To make an arm stronger by increasing its torque and making it more powerful, you must increase the gear ratio, a process known as gearing down. Game Manual 0 explains that speed and torque are inversely proportional, meaning that increasing the gear ratio increases the system's rotational force at the expense of its speed. For example, doubling a 20:1 gear ratio to a 40:1 ratio doubles the torque, making the mechanism more robust.

Because arms require substantial torque to operate, they rely on these high gear ratios. Game Manual 0 states that long arms need large reduction ratios to drive, highlighting a single arm example that uses a 254.5:1 reduction. These high gear ratios allow the arm to bear the torque provided by the motor, though the arm must be well-supported to handle these forces.

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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

Gears

Game Manual 0·CC BY-NC 4.0

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.

Open Game Manual 0

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.

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Arm Gear Assemblies | REV DUO Build System

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Open REV Robotics Docs

Arm Gear Assemblies | 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

Arm Assemblies | 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

Arm Assemblies | 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

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

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.

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Arms

Game Manual 0·CC BY-NC 4.0

Arms encompass a large variety of mechanisms. In FTC, they are used to add extension and flexibility to end effectors. Arms are especially powerful when paired with other mechanisms such as linear extension and mini-turrets.

Arms can be a useful tool for extension. Long arms need large reduction ratios to drive. The example in the Single Arm section uses a 254.5:1 reduction. It is recommended to transfer torque to the arm through a gear, chain, or belt rather than directly mounting to a motor shaft due to axial forces and the ability to get a larger reduction. Due to the large amount of momentum these arms can carry, if suddenly stopped they may be able to break a motor's gearbox or even tip your robot over. Software control can help to reduce the load on motor gearboxes (see /docs/software/concepts/control-loops).

Depending on the application and implementation, arms can be either faster or slower than other extension options.

Open Game Manual 0

Kinematics > Manipulators

Game Manual 0·CC BY-NC 4.0

Often in FTC, teams have arms that swing out around an axis. Controlling these arms requires a bit of thought as depending on the angle they're at, the effect of gravity drastically changes.

Our first step is defining a reference frame. Our recommendation is to define zero as straight to the side, as this is when gravity has the greatest effect on your arm. Other reference frames will still work, but the exact trigonometry will change.

(Figure: 11329 I.C.E. Robotics)

In a reference frame like this, the relative force of gravity will be equal to the cos of the angle. This makes sense as when the cos function is evaluated at zero degrees, it returns 1, while at 90 degrees where there is no effect of gravity, it returns 0.

Fg = gcosθ

Assuming you have an encoder on your arm, you can find theta (the angle of the arm relative to the vertical) using something similar to the following pseudocode.

Note: DEGREE_PER_TICK can be found by taking 360 divided by your encoder resolution all multiplied by your gear ratio. For example, for a 19.2:1 goBILDA Yellow Jacket (https://www.gobilda.com/5203-series-yellow-jacket-planetary-gear-motor-19-2-1-ratio-24mm-length-8mm-rex-shaft-312-rpm-3-3-5v-encoder/), there are 537.7 ticks per revolution, and so frac{360}{537.7} degrees per tick at the gearbox output shaft. If there was a 2:1 gear ratio after that, then the ticks per revolution would instead be frac{360}{2 times 537.7}

current_angle = (TICKS_AT_ZERO - current_tick) * DEGREE_PER_TICK

The typical way to utilize the effect of gravity you just found, is to pass it in as a feedforward parameter to a PID controller. We cover this in gravity-compensated-feedforward.

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

Arms > Arms for Extension > Multi-Axis Arms

Game Manual 0·CC BY-NC 4.0

A multi-axis arm consists of multiple rotational joints. A multi-axis arm provides the extension single arms lack in addition to providing much more flexibility in positioning the end effector. While multi-axis arms seem like the obvious solution to the problems single axis arms face, they are much more difficult to design, manufacture, and control, having issues with accuracy, backlash, and packaging, while providing little benefit over linear slides and other extension methods.

Warning: This is highly discouraged for inexperienced FTC teams.

(Figure: 11146 Barker Redbacks, Into The Deep, multi-axis arm)

(Figure: 20700 Snap, Freight Frenzy, multi-axis arm)

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

Arms > Arms for End Effectors

Game Manual 0·CC BY-NC 4.0

Arms are the most common way to add flexibility and control to end effectors. These arms typically have one or more rotational joints and/or linear extension. Due to the relatively small power requirement of these arms, they are often driven by servos instead of motors.

Attention: End effectors are likely to see collisions and with arms acting as a lever, it is imperative to take extra caution to mount and support these servos properly (see /docs/power-and-electronics/servo-guide/usage-tips).

Single and multi-axis arms can add control to the vertical and horizontal distance of an end effector. In Into The Deep, this type of arm was common in intakes to allow the intake on a horizontal extension to go over the submersible barrier and then "reach down" for samples. They can be used to line up intakes and outtakes to facilitate transfers or just generally add flexibility to where end effectors can act.

Linear extension is usually seen on outtakes to increase the range of the robot's deposit.

(Figure: 15972 TehnoZ, Into The Deep, Finalist Alliance Captain (Edison), arms used on both intake and outtake, linear extension is used on outtake.)

(Figure: 12993 RoboKings Aurum, Centerstage, Finalist Alliance 1st Pick, high degree of freedom outtake arm with horizontal extension.)

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Gears > Advantages

Game Manual 0·CC BY-NC 4.0

Gears are a solid and proven power transmission method. Early examples of gears date back to the 4th century BC, so you're using technology with millennia of development behind it. When it comes to gears, there's not much that we haven't figured out.
Gears are simple to use with both channel and extrusion. On channel, your gears are already spaced correctly - you just need to choose the right pair of gears. Extrusion gives you even more flexibility - just slide your gears into mesh, and you can have whatever ratio you want.
Gears can give you big reductions in small areas. Depending on the gear combination, one can achieve big ratios in reduction in very small spaces (for example, a 10 tooth gear and a 100 tooth gear will take much less space than a 10 tooth sprocket and a 100 tooth sprocket).
Gears require no tensioning: once the spacing is correct, the gears will operate quickly. Unlike chain or belt, there is nothing further transferring the power, which cuts out the need to properly tension chain or belt. This of course has the drawback of not being able to transfer power far distances.

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