FTC:Swerve Drivetrains
There are many different types of drivetrains used with the FIRST Tech Challenge community. Many teams use mecanum drivetrains since they are very versatile, but one of the most innovative drivetrains a team can create is a swerve drivetrain. Here is the webinar host by FTC 14779 Spontaneous Construction that this page is based off of, it contains all of the same information and more!
What is a Swerve Drive?
A swerve drive is a form of drivetrain that utilizes “pods” that are able to rotate independently of each other. Each pod has a wheel that drives the robot similarly to how a stealth drive works. However, since each pod can rotate, the drivetrain can move in any direction. This allows maneuverability similar to a mecanum drive while having the strength and power of a stealth drive.
Why Use a Swerve Drive?
The main reason to use a swerve drive in FTC is because of innovation. Since only a few teams use a swerve there is a lot of room for innovation in the design. Since mecanum is one of the most popular drivetrains it is fair to compare swerve to it.
| Swerve | Mecanum |
|---|---|
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Why Not Use a Swerve?
Although it has advantages in its strength and innovation, swerve has several downsides. It can take teams a long time to complete and refine a swerve drive to be competitive. Many teams have to use the entirety of the off-season to create theirs. In addition, it can be costly to prototype and produce due to the large number of custom parts required to make a swerve drive. There are also many unique challenges that each team can encounter when developing their swerve. Designing a competitive swerve requires advanced CAD skills since it will have to be fully custom designed. Due to this and the design challenges associated with swerve, inexperienced teams should focus on building simpler drivetrains like mecanum or stealth.
Types of Swerve
There are two main types of swerve drives, coaxial and differential. The main difference between the two is how it rotates the pod and moves the wheel. In a coaxial swerve one motor rotates the pod and one motor rotates the wheel. However, in a differential swerve, the rotation of the pod and the movement of the wheel is the result of both motors working together.
Coaxial
As mentioned previously, coaxial swerve works by having one motor rotate the pod and one motor rotate the wheel. However, the motor that rotates the pod doesn’t have to be a DC motor, it can be a servo. Many teams who create coaxial swerves do this to allow them to use DC motors in other parts of the robot, especially since a team is only allowed 8 DC motors in total.
Differential
The way a differential swerve works is a lot more complex than a coaxial drive. It has two sets of gears, an upper set and a lower set. In between these gears is another gear connected directly to the wheel with an axle. When the upper and lower gears move in the same direction, the central gear stays put and the whole pod rotates. When the upper and lower gears move in opposite directions, the central gear also rotates, which in turn rotates the wheel. If the top and lower gear are moved at different speeds, the two actions can be combined.
Which Should You Use
Both drivetrains have their own benefits and considerations. Each team should consider these and other factors to decide which is best for them. Here is a short list of the major pros and cons of each drivetrain
Coaxial
| Benefits | Considerations |
|---|---|
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Differential
| Benefits | Considerations |
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Considerations When Building a Swerve Drive
Attaching a Pod to the Drivetrain
While figuring out how to attach a pod to the drivetrain may at first seem easy, it is actually rather difficult as one has to consider how to prevent it from wobbling and to keep the gears from skipping. One way to do this is by using large diameter thrust bearings in conjunction with a central bolt. The large diameter bearing provides a larger surface area for the parts to contact, meaning higher stability, and using the central bolt with nuts to apply pressure to the bearing, which is known as preloading, allows it to spin smoother and decreases the wobble. Another way to stabilize the pod is to use herringbone gears which are a form of self aligning gears. Teams can also use rails mounted to the drivetrain to help support the pod and gears.
Here is an example of how FTC 14779 attached their swerve pod to the drivetrain using needle bearings.
Thrust Ball Bearings vs Needle Bearings
There are two main types of bearings that work well in attaching a pod to the drivetrain: thrust ball bearings and needle bearings. Thrust Ball Bearings are self-aligning bearings with a ring of balls sandwiched between two grooved rings. They work very well due to their self aligning nature, however, they are really expensive in the sizes required for a swerve. Needle bearings are an effective and cheaper option. They are composed of small rollers sandwiched between two flat washers. Their larger surface area allows them to take more weight than other types of bearings. However, they require guides in order to stay aligned.
Gears
There are two types of gears used in a swerve drive, spur gears and bevel gears.
Spur Gears
Spur Gears are the most common type of gear to come across. When using them remember that the ratio between turning the pod and the wheel is going to be different. Also, use odd gear ratios (ie. 21:10) to ensure even teeth wear. Using herringbone gears is recommended since they are able to self align and make the pod more stable.
Bevel Gears
Bevel gears are used to connect gears at a 90 degree angle. Teams who want to make a swerve drive will need to make their own bevel gears as there aren’t any easily available in the right size. CADing them may be hard so it is recommended to follow online tutorials when creating them.
Design Considerations
When starting to design the pod, work out the space that is available for use and figure out how large of a wheel should be used. Teams should create several prototypes to ensure that the design works well and is stable enough for a full drivetrain. All of these prototypes need to be tested extraneously. If a team runs into issues they aren’t able to solve they can contact local engineers for advice.
Space
It is important to keep in mind the amount of space each pod will take up. Gears, bearings, and screws will all take up space and teams need to remember them when CADing their design. It may seem appealing to create the smallest pod possible, however, this may cause it to be difficult to clean and repair.
Materials
There are many types of materials that can be used to make a swerve drive. It is possible to entirely 3D print a swerve drive. Here is a list of filaments and their recommended uses
| PLA | PETG | Nylon-11 | |
|---|---|---|---|
| Use | General Parts | Structural Parts | Gears |
| Cost | Cheapest | Somewhat Expensive | Most Expensive |
| Strengths | Hard but brittle | Stronger and can handle more stress | Low friction and high durability |
Complexity
Creating a swerve drive is very hard and complex. They require many moving parts that have to work together seamlessly. A team creating one has to ensure they have enough time to make it. Because of this, most teams create their swerve drive during the off-season to have enough time to create and refine their swerve drive to a competitive level. Talking to engineers can help teams figure out complex problems with their swerve drive. It is not recommended for beginner teams to create swerve drives because of the complexity and difficulty in creating one.
Additional Resources
- FTC 14779 Spontaneous Construction's Swerve Drive
- FTC 14779 Spontaneous COnstruction's Swerve Pod
- FTC 14779 Spontaneous Construction Swerve Webinar
- Gluten Free's Swerve Drive
- Video on How to Create Custom Bevel Gears in Fusion 360
