PTOs & Motor Sharing | Spartan Design

// Section 01

What Is a Power Take-Off?

V5 robots have 8 motor ports. Every motor you give to a mechanism is one less for your drivetrain — and vice versa. A PTO lets you break that tradeoff by sharing motors between systems.

› When Motor Sharing Is Worth It › Pneumatic Shifting › Examples › Tradeoffs › Testing Checklist

A Power Take-Off (PTO) is a mechanism that mechanically connects two subsystems to a shared set of motors. A shifting mechanism — usually pneumatic — connects or disconnects each subsystem from the motor as needed.

The classic VRC application: use your drive motors to power a hang or lift at the end of the match, when you no longer need to move across the field. The motors do not change — only which mechanism they are connected to does.

🎯 Rule of Thumb

A PTO is justified only when you need more power than your remaining motors can provide AND the two shared functions never need to run simultaneously. If drive and the mechanism ever need power at the same time, a PTO will not work. Design your match strategy around the constraint before building the mechanism.

PTO vs Transmission

A PTO shifts which mechanism the motor connects to. A transmission changes the gear ratio between the motor and a single mechanism. These are different things:

PTO: Motor drives Drive OR Lift, not both at once. Motor budget is fixed; you are choosing which system gets power.
Transmission: Motor drives one system but at different speeds/torques. High gear for speed, low gear for force. More complex; rarely used in VRC.

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// Section 02

When Motor Sharing Is Worth It

PTOs add mechanical complexity. They are only worth the build time and failure risk when the motor budget genuinely cannot be solved any other way.

The Motor Budget Problem

A standard V5 robot has 8 motor ports. A typical allocation:

4 drive motors (minimum for competitive drive)
2 mechanism A (intake, lift, shooter)
2 mechanism B (second mechanism)

If you need a third significant mechanism — especially one that requires high torque like a hang — you have a motor budget problem. A PTO solves it by time-sharing motors that are not needed simultaneously.

Good PTO Candidates

Drive motors → hang: At the end of the match (last 30 seconds), you are typically not driving long distances. Use the drive motors to power a high-force hang. This is the most common PTO in VRC.
Drive motors → catapult: If you only shoot while stationary at a fixed field position, drive power is not needed during shooting. Share cautiously — you cannot react to defense during this window.
Intake motor → lift: If your robot alternates between intaking and lifting (never doing both at once), one motor can serve both with a PTO shift.

Bad PTO Candidates

Anything that needs to run while driving. If you ever need to intake, score, or shoot while moving, a PTO on those motors will fail in practice.
Time-critical transitions. Pneumatic PTOs take a fraction of a second to shift. If your strategy requires instant switching mid-match, the delay will cost you.
Teams without pneumatics. Most mechanical PTOs require pneumatics to shift reliably. Without a pneumatic system, you are limited to servo-actuated clutches — which are slower and less reliable.

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// Section 03

Pneumatic Shifting Concepts

Pneumatics provide the force and speed required to engage or disengage a PTO clutch reliably. A hand-built mechanical shifter is rarely fast or reliable enough for competition use.

How a Pneumatic PTO Shift Works

The pneumatic cylinder extends or retracts to move a shifting collar, dog gear, or clutch plate. When engaged, the motor drives the target mechanism. When disengaged, the motor either idles or drives the other mechanism.

Shifting Mechanism Types

Dog gear / shifting collar: A sliding collar with teeth engages one of two gear sets on the shaft. Fast, reliable, mechanically simple. Requires precise alignment — dog teeth must mesh cleanly or they skip.
Friction clutch: A disc or plate pressed by the pneumatic cylinder creates friction between rotating shafts. Slippage possible under high load — less predictable than a dog gear for high-torque applications.
Gear shift (in/out of mesh): The pneumatic cylinder moves a gear in or out of contact with a mating gear. Simple but the gear must re-enter mesh cleanly — a mis-timed shift can strip teeth.

Air Budget for Shifting

Every PTO shift uses a pneumatic activation. Plan your shifts in advance and count them against your air budget. A hang PTO that shifts once is cheap. A mechanism that shifts frequently across a match can exhaust your supply. See the Pneumatics Best Practices guide for air budget calculation.

💡

Test shifts at low air pressure. Your pneumatic system loses pressure across a match. Test the PTO shift mechanism at 60% of your starting pressure to verify it still engages reliably. A PTO that only works at full pressure will fail late in a match or after a long queue.

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// Section 04

Drive-to-Mechanism & Mechanism-to-Drive Examples

Real VRC applications of motor sharing. These examples show the match strategy implications alongside the mechanical design.

🚗 Drive → Hang (Most Common)

When: Last 20–30 seconds of match when the robot is at the hang bar. Drive is no longer needed.
Mechanics: Four drive motors shift to power a high-reduction winch or passive hang mechanism. High torque needed — drive motors can provide it.
Match strategy: Robot must be positioned before shift. After shift, robot cannot drive. Allocate time carefully.

⬆️ Drive → Lift (Stacking Endgame)

When: Game requires a high-force lift at endgame (tower balance, stack height) that needs more torque than 2–3 dedicated motors provide.
Mechanics: Two or four drive motors shift to assist or power the lift at full extension.
Match strategy: Robot must be in scoring position before shift. No repositioning after.

⚙️ Intake Motor → Scorer (Alternating Cycle)

When: Robot intakes in one field zone and scores in another. Never does both simultaneously.
Mechanics: One or two motors alternate between powering intake rollers and a scoring conveyor via a PTO shift.
Match strategy: Requires precise driver coordination. Any simultaneous demand from both subsystems stalls the shared motor.

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// Section 05

Tradeoffs, Failure Points & Maintenance

PTOs add mechanical complexity that most teams underestimate. Know the failure modes before committing to the design.

Tradeoffs

Benefit	Cost
More effective motors for high-demand tasks	Added mechanism weight and part count
Enables capabilities not otherwise possible in 8-motor budget	Dedicated pneumatics required for shifting
Can improve endgame performance significantly	Strategy constraint: shared systems cannot run simultaneously
Well-designed PTO is very reliable once tuned	Takes significantly more design and build time than a dedicated motor solution

Failure Points

Shifting collar misalignment. If dog teeth are slightly off when the cylinder actuates, they impact tooth edges instead of engaging. Over time, this rounds the teeth. Ensure the shifting collar can only move axially — no rotation allowed during the shift.
Pneumatic leak causing partial shift. A slow leak can cause the shifter to partially engage — a state where neither system is fully powered. Check all pneumatic fittings on the PTO circuit specifically.
Driver shifting at wrong moment. Shifting while motors are at full speed under load can cause shock loading on the dog gear. Train the driver to throttle down before shifting.
Air exhausted before endgame shift. If your PTO shift is planned for the last 20 seconds and you run out of air at 30 seconds, the hang never happens. Budget air specifically for the PTO shifts.

⚠️ Stop Building If…

Shifting is unreliable at reduced pressure
Test at 60% starting pressure. If it fails, the PTO will fail late in matches.

Dog teeth chipping after 20 shifts
Gear mesh misaligned. Fix axial alignment before adding air cycles.

Driver forgets which mode the robot is in
Add a LED or Brain screen indicator. Operator mode confusion causes matches to be lost.

Maintenance Schedule

Inspect shifting teeth visually after every competition day
Check pneumatic fittings on PTO circuit — any hissing means a leak
Confirm shift engagement from a known motor-off state before each match
Verify Brain screen / LED mode indicator is working

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// Section 06

Testing Checklist & Notebook Evidence

A PTO must be tested as a system — not just the shift mechanism in isolation. Test the full match cycle including timing and driver procedure.

🔬 PTO Testing Checklist

20 consecutive shift cycles — no missed engagements

Shift from mode A to mode B and back. Count any partial engagements.

Shift tested at 60% starting air pressure

Must engage reliably at reduced pressure. Simulate end-of-match condition.

Full match simulation with planned shift timing

Run 1m30s match simulation. Execute shift at planned moment. Verify both modes work.

Mode indicator (LED or screen) visible and correct

Driver must know which mode is active without looking at the mechanism

Dog teeth inspected after 20 shifts — no chipping

If teeth are chipping, fix alignment before competing

Air consumption counted for full match with PTO shifts

Verify air budget allows full match + all planned shifts + any pneumatic mechanisms

Driver shift procedure practiced 10 times minimum

Throttle-down protocol, timing, mode verification — must be automatic

Notebook Evidence

Motor budget table: all 8 motors listed with their functions before and after PTO shift — shows the design decision clearly
Air budget calculation including PTO shifts — demonstrates you verified the system against competition constraints
CAD screenshot of shift mechanism in both engaged and disengaged positions
Reliability data: shift success rate over 20 cycles at nominal and reduced pressure
Match strategy diagram showing when in the match the PTO activates and what the robot can/cannot do in each mode

⚙ STEMEngineering: Systems Integration and Constraint Analysis

A PTO is a systems engineering problem — you are managing shared resources under constraints. This is identical to how software engineers schedule CPU time on a multi-process system, or how automotive engineers share a transmission between drive modes. The constraint is time-sharing: two users, one resource, no simultaneous demand. Documenting this constraint, your design solution, and the validation data is exactly what separates a Design Award notebook from one that just describes what you built.

🎤 Interview line: “We had a motor budget problem — we needed 4 drive motors and 4 mechanism motors, but the hang required 2 additional high-torque motors we didn’t have. We designed a pneumatic PTO so the drive motors double as hang motors in the last 20 seconds. The match strategy constraint is that we must be positioned before shifting. We validated this in 10 full match simulations before competition.”

💨

Pneumatics Best Practices

Air budget, shifting, solenoids

⬆️

Lift Systems Guide

The mechanism PTOs usually power

🤖

Full Competition Code

Mode logic for PTO states

✂️

Custom Parts & Fabrication

Cut shifting collars and dog gears

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