- Difficulty
- Intermediate
- Build time
- 70-100 min
- Estimated cost
- $0-$18
- Age range
- 11-17
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The carriage travels at least 8 cm without rotating, advances a repeatable distance per screw turn, and reverses cleanly without leaving its guide.
Learning goals
- Identify how rotation of a threaded shaft produces guided carriage translation.
- Construct and explain a rotary-to-linear system.
- Measure how the thread pitch changes performance.
- Diagnose losses caused by thread sliding friction and guide rubbing.
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.
- Use a long bolt and matching nut captured in a cardboard carriage for a low-speed classroom model.
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.
- Stop before the nut reaches either screw end, and keep fingers away from the moving carriage and fixed stop.
Orient the build
Place the build so rotation of a threaded shaft is on your left and guided carriage translation 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 long guide base
Brace two parallel rails longer than the planned 8 cm travel.
Mark both end-stop zones.
Step 2
Support the screw
Mount the threaded shaft in bearings at both ends while allowing free rotation.
Add a crank outside one support.
Step 3
Capture the follower nut
Lock the matching nut inside a compact carriage so it cannot rotate.
Keep the thread axis centered in the carriage.
Builder checkpoint: After capture the follower nut, the first subassembly should stay aligned when handled gently.
Step 4
Fit carriage guides
Place guide faces above and below or on both sides of the carriage.
Leave enough gap for smooth travel.
Watch for: If this stage binds or drifts, inspect nut backlash before adding more parts.
Step 5
Engage the thread
Rotate the screw by hand to start the nut without cross-threading.
Advance only after several smooth turns.
Step 6
Add physical end stops
Install stops before the nut can run off the usable thread.
Leave space to reverse the crank safely.
Builder checkpoint: After add physical end stops, operate the build slowly and confirm that guided carriage translation begins without binding.
Step 7
Measure pitch travel
Set the pointer to zero and turn the crank exactly five times.
Divide measured travel by five.
Step 8
Test direction and load
Reverse through five turns, then push lightly against the carriage during motion.
Watch for screw bowing or guide twist.
Builder checkpoint: At the final checkpoint, The carriage travels at least 8 cm without rotating, advances a repeatable distance per screw turn, and reverses cleanly without leaving its guide.
See the engineering
Why it works
- Input
- rotation of a threaded shaft
- Output
- guided carriage translation
- Motion
- rotary-to-linear
- Energy losses
- thread sliding friction, guide rubbing, shaft bowing, nut backlash
Why this works
Screw pitch conversion
A mating nut must advance along a helical thread as the screw rotates. Fine pitch produces small linear travel per turn and a large ideal force tradeoff.
Look for: Mark the crank and measure carriage travel after exactly five turns in each direction.
Where the energy goes
Efficiency and losses
The ideal model leaves out thread sliding friction, guide rubbing, shaft bowing, nut 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 thread sliding friction becomes visible or audible.
Math bite
Predict carriage travel
Formula: travel = turns × thread pitch
- Turns = 8
- Pitch = 2 mm/turn
Substitute: travel = 8 × 2 mm = 16 mm
Result: Eight turns move the carriage 1.6 cm ideally.
Finer pitch gives less travel and more ideal force per turn.
Backlash creates lost motion when direction reverses.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Run five unloaded turns from the middle of the guide.
Success looks like: The carriage advances smoothly and returns within 2 mm after five reverse turns.
Measure: Travel per turn and reversal backlash.
Change: the thread pitch
Keep constant: carriage, guide gap, turn count, frame, and start position
- coarse pitch
- fine pitch
- fine pitch after reversal
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The nut spins with the screw | Carriage does not restrain rotation | Hold the carriage lightly and turn | Add wider anti-rotation guides |
| The screw bows | Supports are far apart or load is high | View shaft from the side under light load | Add a center support or reduce load |
| The carriage jams | Guides and screw axes are not parallel | Disconnect nut and slide carriage by hand | Realign rails to the screw centerline |
| Reverse motion has delay | Thread clearance creates backlash | Rock the crank and watch the nut | Add a gentle preload only if it does not bind |
Choose your tradeoff
Fine pitch improves positioning resolution but increases turn count and thread friction. Align the screw and guides before reducing clearance, because a tight misaligned carriage will bind.
Keep experimenting
Try another version
Pointer carriage
Move only a paper pointer with no external load.
Pitch comparison
Measure travel and input turns for two screw pitches.
Position scale
Calibrate a dial that predicts carriage location from crank turns.
Build together
Classroom and access options
Classroom version
Teams can compare the thread pitch while keeping carriage, guide gap, turn count, frame, and start position. 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.
- Fit a large crank and add raised end-stop markers along the guide.
Reflect on the design
- How did the thread pitch change the measured result?
- Where did thread sliding friction affect the build most strongly?
- What evidence shows that screw pitch conversion explains the motion?
- Which change would improve guided carriage translation without creating a new problem?
Glossary
- Screw pitch conversion
- A mating nut must advance along a helical thread as the screw rotates.
- Input
- The action or energy supplied to a system; here it is rotation of a threaded shaft.
- Output
- The useful response produced by a system; here it is guided carriage translation.
- 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.