How to Design a Chassis
The chassis is the structural component for the robot which contains the drivetrain and allows the robot to be mobile by using wheels, tank treads, or another method. A chassis is sometimes referred to as the robot’s frame. The chassis also provides a structure to attach manipulators such as arms, claws, lifts, plows, conveyor systems, object intakes, and other design features used to manipulate objects.
There are many considerations to be made when designing a robot chassis.
What is the purpose of the robot? Is the robot design for a classroom project or is it for a competition? If the robot is for a classroom project its chassis may be assembled with less concern for repeated interactions with other robots. During a competition, if the chassis bends, twists, or falls apart the robot may no longer be able to compete effectively.
Are there sizing rules for the robot? Many competitions have sizing rules included in the rules of the game. These rules could have a maximum height, width, and length the robot can have at the beginning of a match and the rules may have a maximum expansion horizontally and/or a maximum height limit. The chassis must be sized so all the robot’s components will fit within the sizing rules.
What shape will the chassis be? One of the advantages of the VEX EDR system is it allows for many designs and a nearly endless opportunity for creativity. However, there are some aspects to consider. The structural metal components assemble much easier when 90 o connections are used. The chassis shape should allow space for the robot’s other components such as the control system, motors, wheels, gears, and sprockets. A good design practice is to lay out the chassis with all of the other components before the assembly to assure the spacing will work. Be sure the chassis shape will accommodate the robot’s drivetrain design. If the robot will be used in a competition, are there shapes which will provide an advantage? Perhaps a narrower shape will allow the robot to navigate the field easier and/or fit into a scoring zone easier. Perhaps a wider shape will allow the robot to push more game pieces or provide more area for an intake system. Perhaps a U-shape will allow space for a conveyor and/or a game piece manipulator. Perhaps there is an obstacle the robot needs to go under and it can not be as tall. Perhaps the robot will need to reach high or outside the wheelbase and it will be advantageous to build the chassis shape to fill the maximum size limit and create as large and stable a footprint as possible.
Support of Shafts
It is important that the design of the chassis incorporates two parallel points of support for any shafts which will be inserted into the chassis. If two supports are not provided for each shaft, the shaft will be allowed to slightly pivot up and down on the single point of support and it will make the shaft harder to spin. The heavier the robot assembly the shaft is supporting the more important it is to provide these two points of support.
Examples of Two Points of Support
|1 Point of Support (Poor)||2 Points of Support (Good)||2 Points of Support (Good)|
Structural Metal Pieces
What type of structural metal pieces will be used to assemble the chassis? The VEX EDR system has many available options in steel and aluminum. There are C-Channels available in 5 Hole and 2 Hole width in both steel and aluminum. There are aluminum C-Channels in 3 Hole width available. The wider the C-Channel, the less likely it is to bend or twist, however the chassis will be heavier. There are Angles available in both steel and aluminum with square holes and there are steel Angles with slotted holes. Angles are ideal for attaching and supporting towers. The steel Angle with slotted holes allows for connections which are not 90 o. There are Rails available in both steel and aluminum. Rails have end connectors which provide an additional connection point. Rails are one of the types of structural metal included in the Chassis Kits.
Things to consider when selecting a structural metal material. VEX offers metal structure pieces in two material options: steel and aluminum. There are advantages and disadvantages to using a specific material based on the material properties and the pieces available. Both material options can be cut, drilled, filed, and re-shaped to allow for custom designs.
The steel structural metal was the original material which was available when the VEX EDR system was introduced. When trying to decide whether to use a steel structural piece, here are a few things which should be considered in the decision:
- Steel metal pieces are less expensive than aluminum and this may be a consideration in classroom projects.
- Steel metal pieces do not bend or twist as easily as the same metal pieces made out of aluminum.
- Steel metal pieces are available in the Boaster Kit and the Metal Hardware Kit.
- Steel metal is available in 4 different sized Chassis Kits which can be mixed and matched for a number of different designs.
- Steel metal is also available in a number of single type/length metal component packs.
The aluminum structural metal was introduced later in the VEX EDR product line, however its properties make it widely used for designs in robotic competitions. When trying to decide whether to use an aluminum structural piece, here are a few things which should be considered in the decision:
- Aluminum metal pieces are lighter and this provides a competitive advantage because the lighter the structure, the easier it is for motors and pneumatic systems to move it.
- Aluminum pieces are slightly thicker than the steel pieces and in certain orientations, it is more difficult to align the holes between 2 or more pieces.
- Aluminum pieces are softer than the steel pieces which can allow screws and drive shafts to dig into the sides of the square holes when they have a large stress placed on them and this can create a loose connection. However this softness allows aluminum to be cut, drilled, filed, and re-shaped easier than steel.
- Aluminum Metal pieces are available in the Aluminum Structure Kit and the Long Aluminum Structure Kit.
- Aluminum is available in an Aluminum Chassis Kit 25x25.
- Aluminum metal is also available in a number of single type/length metal component packs.
All of these metal pieces can be mixed and matched to assemble a very effective robot chassis. The decision about which type of metal to use does not have to be an all or none. For example, aluminum angle and rails might be used for the drivetrain part of the chassis in order to keep it light and steel C-Channel might be used for the tower part of the chassis in order to provide strength to support a large arm or lift system.
It needs to be noted that metal Plates and metal Bars (which are also available in both steel and aluminum) were left out of this discussion of Structural Metal pieces. This is because plates and bars do not have material which extends in all 3 (X,Y,&Z) spatial axis and therefore do not have the structural strength to be used as a main component for a chassis. However, these metal parts can serve some very important functions in a chassis such as:
- Plates and Bars can be used to support and connect the other structural components to stiffen a chassis.
- Steel Plates or steel Bars can be mounted flush to a piece of aluminum structural metal to reinforce its square holes when a shaft or screw is inserted through the hole and the shaft/screw has a large stress applied to it.
- Plates and Bars can provide a flat surface on a chassis to mount components like the V5 Robot Brain, the V5 Robot Radio, and the V5 Robot Battery.
How are fasteners used to assemble the chassis? Fasteners are parts which connect the metal pieces and other structures together. There are numerous fasteners available to assemble a chassis. Unless the chassis has a structure which is designed to pivot, each junction needs to have two or more connection points. As a general rule, the more stress a junction has the more fasteners should be used, however this will equal more weight for the design. For example, if two 5 hole C-Channels are being connected, placing a screw through all 25 intersecting holes would be excessive. A classroom chassis may not experience as high an amount of stress as a competitive chassis. The classroom chassis may use fasteners which are faster to assemble like the Bearing Attachment Rivets, #8-32 Hex Nuts, Nut Bars, and Thumb Screws. A competition chassis will need to be assembled with screws and nuts. 1-Post Nut Retainers and/or 4-Post Nut Retainers may also be used. Standoffs are also very effective for assembling a chassis. A standoff is used to separate two parts from one another while creating a rigid connection. The #8-32 Standoffs come in various lengths between ¼” and 6”. In addition to these fasteners, the VEX Robotics Competition has a game rule about “Non-VEX screws” which allows any commercially available #4, #6, #8, M3, M3.5, or M4 screw up to 2” (50.8mm) long (nominal), and any commercially available nut, washer, and/or spacer (up to 2” / 50.8mm long) to fit these screws. Chassis junctions may also be reinforced using Gussets, Plates and/or Bars.
The chassis of the robot serves as its skeleton, so it is essential to have a well designed and well-assembled one. The success or failure of the robot can depend on the chassis.
| Safety Hazard:
File or sand smooth any edge od material which has been cut to remove any sharp edges.
| Safety Hazard:
Be cautious around material which has just been cut.
Structural metal and hardware can be purchased at https://www.vexrobotics.com/vexedr/products/structure.