what chain pitch suits a 25 pound arm joint — Sharp AI

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what chain pitch suits a 25 pound arm joint

Either #25 chain (1/4 inch pitch) or 8mm pitch chain is recommended for a 25-pound arm joint, as both are standard FTC sizes with tensile strengths that far exceed the loads experienced in competition. According to Game Manual 0, #25 chain can withstand up to 930 pounds of force before breaking, which is vastly higher than any force a team will exert on an FTC robot .

When implementing this on an arm joint, Game Manual 0 advises transferring torque through a chain, gear, or belt rather than mounting the arm directly to the motor shaft to better handle axial forces and allow for larger reduction ratios. To ensure reliability under load, Game Manual 0 also recommends using a chain breaker to custom-size the chain run instead of using master links, which are prone to loosening and failing during matches .

Additionally, to prevent the chain from skipping under the heavy load of the arm, the chain must have proper tension and wrap. Game Manual 0 recommends that the chain maintain at least 90 degrees of contact with the sprocket, though 180 degrees or more is best practice to prevent derailment .

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1

Roller Chain > Chain Wrap > Best practices for wrap

Game Manual 0·CC BY-NC 4.0

(Figure: 724 Rednek Robotics Wun, Relic Recovery)

(Figure: 8103 Null Robotics, Rover Ruckus)

Chain can take a beating. No matter what your application is, metal chain is usually up for the challenge. #25 chain can hold up to 930lbs before breaking, and there's nothing you'll do in FTC that will exert that force. (If your chain does break, it's most likely due to a faulty Master Link or sprockets that are not correctly aligned.)
Chain can be however long or short as you wish. If your ratio changes or your shafts move, it's easy to adapt your chain run - just break the chain and put it back together at its new length. You can often do this without even removing the chain from your robot.
Chain can be pretty precise. When properly tensioned, roller chain doesn't have very much slop. However, you really need to get your chain tension right to reduce slop, and you'll probably want an adjustable tensioner for when the chain stretches. This can be done easily if using extrusion systems, as the sprocket can be adjusted for tension.

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2

Roller Chain

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When your shafts aren't right next to each other, roller chain and sprockets will allow you to transmit power securely between your shafts.

Term: Chain — Roller chain is made up of a series of links joined by pins. Each link can rotate around its pins, creating a dynamic loop that can conform to any shape. The pins in the chain engage the gaps between teeth on each sprocket.

Chain number refers to the type and size of chain that is compatible with the sprocket. #25 chain and 8mm chain are both commonly used in FTC®.

If you've ridden a bike, chances are that you've already seen roller chain - the chain on your FTC robot is similar, but it's probably a different pitch (different size). Chains most commonly used in FTC are #25 (1/4 inch pitch) or 8mm pitch.

When using chain, often there is a master link. This is a special type of link that has a removable end capsule in order to shorten the chain. However, as it is removable, it is not a very reliable chain link and can loosen and fall off under prolonged usage. There have been teams who have had master chain links fail during competition, costing them a match in the elimination rounds.

A chain breaker eliminates the need for master links because it can break and join chain at any point.

Attention: It is highly recommended that teams purchase a chain breaker (we recommend the DarkSoul chain breaker for #25 chain, and the goBILDA Chain Breaker for 8mm pitch chain) instead of using master links, which are prone to failure.

(Figure: The removable master link is shown on the right.)

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3

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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4

Roller Chain > Chain Wrap

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Chain should, at the very least, have 90 degrees of contact with the sprocket. The best practice is to have 180 degrees or more of contact, as it is very unlikely to fall off with proper tensioning. Chain skipping, especially on drivetrains or arms, is very possible without proper chain wrap or tensioning.

When tensioning chain, be sure to not undertension or overtension chain. Undertensioning chain can result in the chain falling off the sprocket or chain skipping, where the chain can skip along the sprocket. Overtensioning the chain often results in the motor burning out, or less seriously, a loss of efficiency. Push along the chain run, and if the chain moves slightly without significant resistance, chances are you've done it correctly. If it's too tight, then the chain will barely move under a gentle press.

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5

Arms

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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.

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6

Useful Resources > Mechanical Design and Build > Power Transmission

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SDP-SI Timing Belt and Pulley Handbook (https://www.sdp-si.com/PDFS/Technical-Section-Timing.pdf)* --- A detailed handbook about belts and pulleys. Some of the things mentioned also apply to chain.

SDP-SI Timing Belt Drive Design Guide (https://www.sdp-si.com/Belt-Drive/Designing-a-miniature-belt-drive.pdf)* --- A one-page guide to designing belt and pulley drive systems.

Gears Educational Systems Guide to Chain Drive Systems (https://web.archive.org/web/20191020193018/http://gearseds.com/documentation/deb%20holmes/2.5_Chain_drive_systems.pdf) --- A guide to roller chain, featuring some useful equations.

Gear Efficiency Comparisons (https://www.meadinfo.org/2008/11/gear-efficiency-spur-helical-bevel-worm.html) --- A comparison between different types of gears (spur, bevel, worm, etc).

How Gears Work (https://ciechanow.ski/gears/) --- An interactive visual guide to how gears work.

KHK Introduction to Gears (https://www.khkgears.co.jp/kr/gear_technology/pdf/gear_guide_060817.pdf) --- A handbook covering fundamentals about the mechanicis of gears.

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7

Roller Chain > Center-to-Center calculations

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The equation to calculate center-to-center for chain is quite complicated. Many online calculators (https://reca.lc/chains) can calculate C-C distances without going through the tedious calculations. However, the complete formula is below.

C=(P)/(8)*(2L-(N+n)+sqrt{(2L-(N+n))^2-(8)/(π²)*(N-n)^2})

L=(2C)/(P)+(N+n)/(2)+frac{P((N-n)/(2π))^2}{C}

C = center-to-center distance, inches

L = chain length in pitches

P = pitch of chain

N = number of teeth in large sprocket

n = number of teeth in small sprocket

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8

Roller Chain > Disadvantages

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Chain stretches over time. As it's used, the connections between the links and rollers can stretch a bit. While it doesn't look like much, this stretching can introduce lots of slop into your chain run and even derail it in some cases. You'll most likely need an adjustable tensioner to keep your tension over time - some teams have used spring-loaded dynamic tensioners to automatically compensate for any changes.
The smaller the sprocket, the faster the chain stretch. This is because when the chain is run on a smaller sprocket, more force is applied due to a smaller radius.
Chain wrap, especially in one chain run, can be problematic. Typically, teams use either one or two chain runs (pieces of chain) per drivetrain side. However, one chain run can require more than one idler sprocket and get very complex in order to maintain proper chain wrap.
Sprockets are really big. If you want a really high reduction using chain, you'll pay for it in the space that it takes up. Sprocket teeth are much larger than gear or pulley teeth, so your reductions are going to be much larger.

(Figure: 7244 OUT of the BOX Robotics, Relic Recovery)

(Figure: 9794 Wizards.exe, Rover Ruckus, plastic chain on intake)

(Figure: Properly done chain wrap with the REV system)

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9

Power Transmission

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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.

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10

Power Transmission Glossary

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Pitch Diameter (PD) — An imaginary circle around a component used for various calculations. For gears, it is the imaginary circle that mates with any other gear's pitch diameter when the gears are properly spaced. For chain and belt, it is the circle which is traced by the middle of the belt or chain as the pulley or sprocket rotates. For gears, the pitch diameter will be smaller then the Outside Diameter (OD), but with chain and belts, the pitch diameter will be larger. PD = (tooth * pitch)/pi

Pitch — Pitch refers to the distance between the center of one tooth of a gear, sprocket, or pulley to another. In chains it refers to the distance from one pin to another, and in belts it refers to the distance betewen one groove to another.

Outside Diameter (OD) — The diameter of the imaginary circle traced by the outermost face of the teeth on a gear, sprocket, or pulley. This will be larger then the Pitch Diameter (PD) for a gear, but smaller then the Pitch Diameter (PD) for a pulley or sprocket.

Clearance Diameter — The clearance diameter is the diameter of the imaginary circle that contains the entire pulley, sprocket, or gear. For pulleys and sprockets, this includes the belt or chain on the component. Clearance diameter is used to check for interferences with other mechanisms. It is usually larger then Pitch Diameter (PD) and Outside Diameter (OD)

C2C — Center to center (C2C) refers to the distance between the centers of a pair of sprockets, pulleys or gears. This will affect chain/belt tension and gear meshing, so calculating this correctly is essential.

Idler — An idler gear, sprocket, or pulley is one that is purposely not used for driving anything else on the shaft. The purpose of this idler is, in the case of gears, to transfer power to another direction.

For chain and belt, idlers are more common, and are usually adjustable to maintain tension.

Sprocket — A sprocket is a mechanical part that transfers power through its cogs, which fit into chain. It is similar to a gear, except that instead of meshing with another gear, the sprocket meshes with chain. See Chain

(Figure: Delrin 20 Tooth #25 sprocket)

HTD Belt — HTD belt is a type of synchronous timing belt commonly used on drivetrains. It is available in different widths to accommodate different sized pulleys. The most common is HTD3 (3mm Pitch) and HTD5 (5mm Pitch) belts, as these are compatible with goBILDA COTS pulleys. See Timing Belt

GT2 Belt — GT2 belt is a type of synchronous timing belt commonly used on drivetrains and other mechanisms. Its available in different widths and pitches, although the most common is GT2 3mm (3mm Pitch), compatible with REV Robotics COTS Pulleys. See Timing Belt

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11

Rigging > Belt or chain driven elevators

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Almost ubiquitous in FRC®, belt and chain elevators have existed seemingly forever. The basic premise is to have sprockets or pulleys mounted at the top and bottom of the elevator to the robot superstructure. Then, the belt or chain is hard bolted to the elevator near the bottom sprocket or pulley. When the chain or belt is driven, the elevator will move up and down. It is possible for elevators to have multi stage designs, but powering them will be more complex.

(Figure: 7236 Recharged Green, Rover Ruckus)

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12

Arms > Arms for End Effectors

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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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13

Arms > Arms for Extension > Multi-Axis Arms

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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)

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14

CAD Tutorials > CAD Tutorial Part 1 - Drivetrain in an Hour > Additional Considerations

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Powering wheels can be done is a couple different ways through either belts and pulleys, chains and sprockets, gears, or even powered directly from the motor. Direct drive and chains are the simpler of the options, with direct drive not needing a calculated distance at all just have to set the motor exactly where the center of the wheel is. Chains allow for a little bit of slack not needing an exact center to center distance in the wrap like belts and pulleys do. Finally gears which need to be a certain distance apart from each other to mesh properly and not skip or bind.

Mounting motors is done in a plate style by face mounting the motor into the innermost drivetrain plate. It can also be done by mounting the motors to a 3rd plate, located in between the outside and inside. This allows for the motor to take up less space in the middle of the robot, but adds complexity. Motors should always be as low as possible and depending where you want the center of mass, either the middle or towards the back of the robot. It is also worth keeping in mind the type of power transmission and the expediency of doing so in light of the motor placement.

Ground clearance is all dependent on if there are any obstacles on the field, as well as what your team wants to do in that game in regards to said obstacles.

For example, in Rover Ruckus some teams with tank drivetrains decided to enter the crater. Therefore, they left enough space to not beach themselves on top of the crater, a common mistake that inexperienced teams often make.

Other teams decided to ignore driving over the crater and decided to reach over with an arm or slide system, which meant they didn't need a lot of ground clearance for their drivebase.

Typically, anywhere from .25 inches of clearance to .5 inches (if you want to be safe) on a completely flat field will allow for the weight of the robot to push into the foam tiles. Nothing else from the robot should touch the ground.

Something you can do is set the robot in CAD onto a field. Set up obstacles such as the crater and simulate driving over the crater by moving it across like you think it would in the real world.

If either of the plates intersect with the obstacle, add some more clearance so you don't get beached like a sad whale.

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Motion Mounting > Zombie Axle (Coaxial) > Advantages

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Only power transmission method that can power two mechanisms on the same revolution point

Has the disadvantages of both live and dead axle
More mechanically complex

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CAD Tutorials > CAD Tutorial Part 3 - Custom Pulley Template

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When designing methods of power transmission, it's useful to have an adjustable pulley generator to rapidly rearrange C-C (center to center) distance for design changes. Typically, FTC teams use the HTD5 belt profile due to its deep tooth profile, which adds resistance to slipping and increases load capacity. This tutorial will focus on the HTD5 profile, but it is relatively easy to adapt for different profiles.

To make the pulley fully parametric (adjustable without redoing the base sketch), we will use Equations (in Solidworks and Creo), Parameters (Fusion 360 and Inventor) or Variables (Onshape). Equations allow a user to quickly adjust values and change multiple dimensions in a sketch or feature.

First, define a new variable "n" and set a default value of 24. This is crucial since "n" will affect the number of teeth, which will define the angle between teeth and the circular pattern.

Copy the sketch below.

The 15° equation is done by typing =360/"n" into the text box.
Note that 5mm dimension at the top describes arc length, which is done in Solidworks by first selecting the two points and the connecting arc.
The two big circles are tangent to the two smaller circles, but the two smaller circles are not tangent to each other.

Leave this sketch as a reference and use "Convert Entities" to create sketches for additional features.

Next, extrude the outer bold circle. Cut-extrude the profile in the reference sketch. Do these features separately.

Now, just create a Circular Pattern. Define Direction 1 as the top face and create "n" instances of the cut-extrude feature.

Now just sketch on the side plane and sketch the flanges. This is up to you, but I prefer to keep the outer point vertical to a point pierced through the outer circle. That way, the flange changes with respect to "n".

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