- Difficulty
- Advanced
- Build time
- 120-180 min
- Estimated cost
- $0-$25
- Age range
- 13-18
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The selector engages either ratio without contacting both at once; both modes run ten unloaded turns without skipping and produce clearly different output speeds.
Learning goals
- Identify how rotation of a common input shaft produces one of two selected output ratios.
- Construct and explain a selectable rotary-to-rotary system.
- Measure how the selected gear path changes performance.
- Diagnose losses caused by four bearing sets and unused gear drag.
Before you build
Materials, tools, and safety
Reuse-material cost: Usually under $5 with an existing kit. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Removable tape for motion marks
Low-cost swaps
- Use equivalent brick-compatible parts from any kit.
- Use cardboard beams and straw bearings for a larger demonstration model.
- Build two fixed gear paths and swap a removable output pin by hand when no sliding collar is available.
Project-specific safety
- Keep fingers, hair, and loose sleeves clear of moving parts.
- Turn the mechanism by hand; do not attach a high-speed motor.
- Shift only while every shaft is stopped; forcing engagement into moving teeth can eject parts.
Orient the build
Place the build so rotation of a common input shaft is on your left and one of two selected output ratios is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Build the parallel shaft frame
Brace input and output bearings at both ends with equal spacing.
Leave side access for the selector.
Step 2
Install the input gears
Lock a small and large gear to the input shaft with enough separation.
Confirm both rotate as one shaft.
Step 3
Fit free output gears
Mesh the matching large and small gears on the output shaft.
Let each gear spin independently around the shaft.
Builder checkpoint: After fit free output gears, the first subassembly should stay aligned when handled gently.
Step 4
Create the engagement collar
Build a sliding keyed hub that turns with the output shaft.
Add dogs that can meet either output gear but not both.
Watch for: If this stage binds or drifts, inspect gear backlash before adding more parts.
Step 5
Add selector travel stops
Build a fork that moves the collar between torque, neutral, and fast positions.
Limit travel before parts rub the frame.
Step 6
Brace and hand-check
Rotate input in neutral and inspect both gear pairs.
Verify the output shaft remains still or nearly still.
Builder checkpoint: After brace and hand-check, operate the build slowly and confirm that one of two selected output ratios begins without binding.
Step 7
Test each stopped shift
Stop, select torque mode, count turns, then repeat in fast mode.
Return to neutral before moving the selector again.
Step 8
Add light output resistance
Use a paper friction flag to compare turning effort in both modes.
Stop if a collar begins to disengage.
Builder checkpoint: At the final checkpoint, The selector engages either ratio without contacting both at once; both modes run ten unloaded turns without skipping and produce clearly different output speeds.
See the engineering
Why it works
- Input
- rotation of a common input shaft
- Output
- one of two selected output ratios
- Motion
- selectable rotary-to-rotary
- Energy losses
- four bearing sets, unused gear drag, selector rubbing, gear backlash
Why this works
Selectable transmission ratio
Two gear pairs provide separate paths between input and output. Free-spinning output gears remain disengaged until a sliding collar locks one chosen gear to the output shaft.
Look for: Mark both free gears and the output shaft, then watch only the selected gear share the shaft's motion.
Where the energy goes
Efficiency and losses
The ideal model leaves out four bearing sets, unused gear drag, selector rubbing, gear backlash. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where four bearing sets becomes visible or audible.
Math bite
Compare the two modes
Formula: output/input = driver teeth / driven teeth
- Torque mode: 12/36 = 1/3
- Fast mode: 36/12 = 3
Substitute: Fast-mode speed divided by torque-mode speed = 3 ÷ (1/3) = 9
Result: The ideal fast output is nine times the torque-mode speed.
Torque mode offers the opposite ideal torque advantage.
Bearing drag and unused gears reduce both real outputs.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: With no load, select torque mode while stopped and turn the input six times.
Success looks like: Each mode stays engaged, neutral disconnects output, and measured rotation matches direction and approximate ratio.
Measure: Input and output rotations plus selector position.
Change: the selected gear path
Keep constant: frame, gear pairs, crank rate, output flag, and shift-at-rest procedure
- torque mode
- neutral
- fast mode
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| Both ratios engage together | Selector travel or dog spacing is wrong | Move slowly through the center while stopped | Add a neutral gap and firm end stops |
| A mode slips out | Dog engagement is shallow or fork flexes | Apply light output resistance and watch collar position | Deepen engagement and brace the fork |
| Neutral still drives output | A free gear rubs a collar or spacer | Spin gears separately in neutral | Add side clearance between rotating parts |
| Shifting is difficult at rest | Dogs are tooth-to-tooth | Rotate input a few degrees and retry | Chamfer contact geometry or add more engagement positions |
Choose your tradeoff
Firm selector stops matter more than a tight sliding fit. Minimize free-gear rubbing, keep a definite neutral position, and never trade smooth shifting for simultaneous partial engagement.
Keep experimenting
Try another version
Pin-selected gearbox
Use a removable locking pin instead of a sliding collar.
Ratio timing test
Measure ten input turns in both modes under the same pointer load.
Three-position indicator
Add a mechanical display tied to the selector.
Build together
Classroom and access options
Classroom version
Teams can compare the selected gear path while keeping frame, gear pairs, crank rate, output flag, and shift-at-rest procedure. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Use high-contrast tape to distinguish input and output parts.
- Replace a small crank with a wider handle for an easier grip.
- Use a large selector with raised FAST and TORQUE end labels plus a center neutral notch.
Reflect on the design
- How did the selected gear path change the measured result?
- Where did four bearing sets affect the build most strongly?
- What evidence shows that selectable transmission ratio explains the motion?
- Which change would improve one of two selected output ratios without creating a new problem?
Glossary
- Selectable transmission ratio
- Two gear pairs provide separate paths between input and output.
- Input
- The action or energy supplied to a system; here it is rotation of a common input shaft.
- Output
- The useful response produced by a system; here it is one of two selected output ratios.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
One build can start the next.
Share what you learned, change one variable, and help another builder understand what worked.
Explore more guidesSources and build notes
An original BrickLabClips interpretation of a standard mechanical mechanism.
- Mechanism verification: Standard kinematics were checked for motion direction, constraint, clearance, and likely friction points.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.
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