Designing a drivetrain
Lay out a chassis that is square, serviceable, and actually assembles in the order you think it does.
By the end you can
- Lay out a drivetrain from a master sketch so width and wheelbase stay adjustable
- Check that the assembly sequence is physically possible
- Design for bearing support and motor access rather than only for geometry
On this page · 4 sections
The drivetrain is the part of the robot most punished by a modelling mistake, because everything else bolts to it.
Start from a master sketch
Put the numbers that define the drivetrain into one sketch on a mid-plane:
- Track width — wheel centre to wheel centre, across.
- Wheelbase — front axle to rear axle.
- Plate spacing and plate thickness.
Build both side plates from that sketch. Changing track width then becomes one edit rather than a careful update of two mirrored parts.
Use variables for the numbers you will argue about
Declare #trackWidth and #wheelbase as Onshape variables. When someone asks "what if it were 20 mm narrower", you answer in ten seconds instead of rebuilding.
Check the assembly sequence
This is the check that separates a drivetrain that works from one that gets modified with a hacksaw at 11pm.
The classic failure
A perfectly modelled chassis where the motor bolts are only reachable before the second side plate goes on — and the second side plate has to go on before the motor can be aligned. The CAD is geometrically correct and the robot cannot be built.
Walk the build order explicitly:
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Say the order out loud while dragging parts in the assembly. Side plate, bearings, axles, wheels, motors, second side plate.
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At each step, ask what tool reaches the fastener. A hex key needs straight-line access and room to swing.
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Ask what comes off for maintenance. A motor will fail. If replacing it means disassembling the drivetrain, redesign now.
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Check belt and chain access. Tensioning needs to be reachable with the robot assembled.
Bearings, not holes
A shaft running in a hole drilled through aluminium will wear the hole oval within a few matches. Support every rotating shaft in a bearing, and support it in two places where you can — a single bearing lets the shaft cantilever and cock under load.
Model the bearing, not a 8 mm hole
Insert the actual bearing part. Its outer diameter, width, and shoulder requirements all affect the plate around it, and a plain hole in the model hides every one of those constraints.
Check yourself
Your drivetrain model shows no interference anywhere and every part is properly mated. What has the model still not told you?
Next
The same discipline, applied to the mechanism that actually scores.
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