trackedbot

Tracked Bot - Mechanical Assembly Guide

Mechanical construction manual, chassis assembly procedure, track tensioning, and hardware mounting guide for the tracked_bot autonomous mobile platform.

For electrical wiring, power connections, and electronics configuration, refer to the Hardware Documentation (../hw/).

Reference Documentation & Media:

📋 Table of Contents

  1. Mechanical Overview & T101 Specifications
  2. Bill of Materials (BOM) & Fasteners
  3. Tools Required
  4. Step 1: Aluminum Frame & Crossbeam Construction
  5. Step 2: DC Gearmotor Mechanical Installation
  6. Step 3: Drive Sprockets, Bearings & Idler Wheels
  7. Step 4: Caterpillar Track Installation & Tensioning
  8. Step 5: Upper Deck Standoff Mechanical Placement
  9. Step 6: Mechanical Inspection & Maintenance

🚜 Mechanical Overview & T101 Specifications

The tracked_bot utilizes an all-metal T101 aluminum tracked chassis designed for high ground clearance, structural rigidity, and all-terrain skid-steering locomotion:

Tracked Chassis Platform

Platform Visual Breakdown (images/body_platform.png):

Mechanical Specifications:

| Parameter | Specification | Notes | | :— | :— | :— | | Chassis Model | T101 Mini Tracked Tank Chassis | Modular aluminum construction | | Frame Material | Aluminum Alloy (6061) | 2 mm thickness, sandblasted & anodized | | Track Material | Engineering Plastic / Rubber | Interlocking modular track links | | Overall Dimensions ($L \times W \times H$) | ~190 mm $\times$ 165 mm $\times$ 60 mm | Compact desktop/lab scale | | Chassis Weight | ~530 g (unladen) | Frame, wheels, motors, and tracks | | Max Payload Capacity | ~2.0 kg – 3.0 kg | Dependent on motor torque and surface friction | | Drive Architecture | Dual Independent Track Drive | Differential skid-steering kinematics | | Ground Clearance | ~18 mm | Center underbody clearance |

🔩 Bill of Materials (BOM) & Fasteners

Component Quantity Material / Description Purpose
Side Structural Panels 2 2 mm Anodized Aluminum Left and right track mounting chassis walls
Central Load Crossbeams 2 Aluminum U-channel / Stiffeners Structural rigidity and deck support
Upper Deck Plate 1 Pre-drilled Aluminum Plate Mounting surface for controller and battery
DC Gearmotors 2 DC Motors with Integrated Gearbox Independent left and right track power
Driving Sprockets 2 Machined Aluminum Alloy Transmit torque from motor D-shafts to tracks
Bearing Idler Wheels 2 – 4 Aluminum with Dual Ball Bearings Guide tracks and support vehicle weight
Track Assemblies 2 Interlocking Track Belts Ground contact and traction
Track Hinge Pins 2 Stainless Steel Pins Track belt closure and sizing
M3 Countersunk Screws 12 Steel M3 $\times$ 8 mm Frame crossbeam fastening
M3 Machine Screws 8 Steel M3 $\times$ 6 mm Motor faceplate attachment
M3 Set-Screws (Grub) 4 M3 $\times$ 4 mm Socket Head Locking sprockets to motor D-shafts
M3 Standoffs & Nuts 4 sets Brass M3 $\times$ 12 mm Standoffs Electronics deck elevation and isolation

🔧 Tools Required

🏗️ Step 1: Chassis Orientation & Frame Structure

Referencing the assembled platform photograph (images/body_platform.png):

  1. Orientation Check:
    • Identify the front vs. rear of the chassis: The rear features the transverse mounting points for the two cylindrical DC gearmotors directly driving the rear toothed sprockets, while the front features the bearing idler wheels.
    • Orient the top aluminum deck so that the central circular cable pass-through hole ($\varnothing \approx 18\text{ mm}$) is positioned centrally, and the four longitudinal adjustment slots extend forward and backward.
  2. Frame & Structural Crossbeam Assembly:
    • Position the left and right structural aluminum side panels against the center deck/crossbeams.
    • Hand-thread the M3 countersunk screws through the side panel clearance holes into the threaded frame members.
    • Check chassis squaring using a small square or calipers (diagonal measurements between opposite corners should match within $\le 1.0\text{ mm}$).
    • Torque all frame screws evenly in an alternating diagonal pattern to eliminate any twist or torsion in the chassis.

⚙️ Step 2: DC Gearmotor Mechanical Installation

As visible in the platform photo (images/body_platform.png), the cylindrical metal gearmotors sit protected directly beneath the main aluminum deck:

  1. Motor Orientation:
    • Insert each 25 mm DC gearmotor into the lower equipment bay so its output D-shaft protrudes through the side panel clearance bore.
    • Orient the motor solder terminals inward toward the chassis center cavity, ensuring the wiring harness cannot contact rotating tracks or sprockets.
  2. Faceplate Fastening:
    • Align the threaded faceplate holes with the chassis mounting pattern.
    • Fasten each motor with four M3 $\times$ 6 mm machine screws.
    • Apply a small drop of medium-strength threadlocker (e.g., Loctite 242) to each screw to withstand vibration during skid-steering turns.
    • Verify that the motor shafts extend outward perpendicular to the side panels.

🔄 Step 3: Drive Sprockets, Bearings & Idler Wheels

Notice in images/body_platform.png the visual distinction between the driving sprockets and the idler wheels:

  1. Rear Drive Sprockets:
    • Slide the toothed aluminum drive sprockets onto the motor output D-shafts.
    • Align the threaded hub grub screw hole squarely over the flat facet of the D-shaft.
    • Apply threadlocker to the M3 grub set-screws and tighten securely with a 1.5 mm hex key.
    • Maintain a 1.0 mm to 1.5 mm clearance gap between the sprocket hub and the aluminum chassis wall to prevent frictional rubbing.
  2. Front Bearing Idler Wheels:
    • Press the dual miniature ball bearings into each idler wheel hub.
    • Mount the idler assemblies onto the front chassis support axles using M3 axle bolts and nylon locknuts.
    • Tighten the locknuts until all axial play is eliminated, while confirming the idler wheel spins freely without friction.
    • Verify co-planar alignment: Sight down the side of the chassis to confirm that the rear drive sprocket and front idler wheel are in the same vertical plane.

⛓️ Step 4: Caterpillar Track Installation & Tensioning

As shown in images/body_platform.png, the robot uses modular black engineering plastic (POM) continuous track belts with external anti-slip traction ribs:

  1. Track Orientation:
    • Position the track belts so the outer chevron tread ridges face forward in the direction of travel for optimal traction.
    • Ensure the interior center guide lugs align between the teeth of the drive sprocket and the dual flange of the idler wheel.
  2. Track Sizing & Hinge Pin Insertion:
    • Wrap the track around the drive sprocket, along the bottom contact surface, and over the front idler.
    • Adjust the link count (adding or removing links using needle-nose pliers or a pin punch) so the ends meet with moderate tension.
    • Insert the stainless steel track hinge pin through the joined link knuckles. Ensure the pin is fully recessed and does not extend past the outer edge of the track.
  3. Track Sag & Tension Verification:
    • Support the chassis off the work surface and measure the track sag along the upper horizontal run:
      • Optimal Sag: 5 mm to 8 mm midway between the drive sprocket and front idler.
      • Tension Too High ($< 5\text{ mm}$ sag): Strains motor bearings, increases no-load current draw, and overheats the L293D H-bridges.
      • Tension Too Low ($> 10\text{ mm}$ sag): Risk of track throwing or tooth skipping during high-torque zero-radius skid spins.

📐 Step 5: Upper Deck Standoff Mechanical Placement

Referencing the top deck features in images/body_platform.png:

  1. Standoff Installation:
    • Thread four M3 $\times$ 12 mm brass hexagonal standoffs into the pre-drilled holes on the upper deck matching the standard Arduino Uno R3 mounting hole pattern ($53.3\text{ mm} \times 27.9\text{ mm}$).
    • Fasten each standoff from the underside with an M3 nut and split lock washer.
  2. Equipment Harness Routing:
    • Route the motor power leads and battery cables vertically through the central cable pass-through hole ($\varnothing \approx 18\text{ mm}$).
    • Install a rubber grommet or protective edge trim around the hole circumference to prevent wire insulation chafing against aluminum edges.
  3. Lower Bay Battery Securing:
    • Secure the 2S 18650 Li-ion battery holder (or 7.4V LiPo pack) inside the lower chassis cavity using hook-and-loop straps through the bottom chassis slots.
    • Placing battery mass low and centered between the tracks ensures a low center of gravity (CG), maximizing climbing stability and braking control.

🔍 Step 6: Mechanical Inspection & Maintenance

Before moving to electrical wiring and firmware deployment, perform this mechanical pre-check:

Inspection Item Verification Method Acceptance Criteria
Sprocket Set-Screws Apply rotational torque by hand to drive sprockets Zero slipping or backlash on motor D-shafts.
Idler Wheel Freedom Spin idler wheels with tracks detached Spins smoothly without grinding or axial wobble.
Track Alignment Roll the chassis manually along a flat surface Tracks stay centered on sprockets without walking off.
Fastener Rigidity Inspect all M3 frame screws Fully seated and torqued; no frame flex under hand pressure.
Chassis Squaring Measure diagonal distances across opposite corners Difference between diagonals $\le 1.0\text{ mm}$.

Post-Operation Maintenance:

[!NOTE] Once mechanical assembly is verified, proceed to the Hardware & Electrical Wiring Guide (../hw/) to complete motor soldering, power distribution, and controller installation.

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