2025-06-16 14:03:53 +03:00

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Two-Disc Cycloidal Gearbox Design

1. Project Description

The two-disc cycloidal gearbox is designed to convert high-speed, low-torque input from a stepper motor into low-speed, high-torque output. It utilizes two synchronized cycloidal discs that rotate in opposite phases, interacting with drive pins to transmit motion to an output shaft.

This configuration minimizes backlash and distributes load across multiple contact points, offering smooth and powerful transmission. The system is fully enclosed and features multiple mounting options for external attachments.

2. Application & Use Cases

Applications

Robotics joints and arms
CNC rotary tables
Precision camera mounts
Automated actuators
Small-scale industrial machines

How to Use

Mount the gearbox base to your motor using the provided holes.
Connect the NEMA-17 motor shaft to the input via the motor shaft connector.
Attach your load or mechanism to the output shaft area.
Power the motor through a driver board and microcontroller (e.g., Arduino).
Control the motion using programmed step sequences.
- Ensure lubrication (grease) between the discs and drive pins for smoother performance and longer lifespan.

3. Engineering Drawings

Description

The design is a compact, scalable two-disc cycloidal gearbox intended for high-torque, low-speed applications. Its parametric structure makes it easy to adapt in size and material. The current prototype has a 90 mm diameter and 54 mm height.

To clearly illustrate the internal mechanics, structural design, and assembly steps, the following visuals are provided:

Included Media
- Assembly Video – starts with an exploded view and step-by-step buildup
- Disassembly Video – shows teardown process in reverse order
- Transparent Body Image – reveals internal components without hiding the casing
Visual Assets & Drawings
- Engineering Drawing Sheet
- Cross-Sectional Analysis (x2)
- Isometric View

4. List of Materials & Components

Standard Components

Part	Quantity	Approx. Size
Bearing	2	Ø65 mm (outer) × Ø50 mm (inner) × 7 mm (thickness)
Bearing	4	Ø32 mm (outer) × Ø20 mm (inner) × 7 mm (thickness)
Stepper Motor	1	NEMA-17

Bolts & Nuts (with available size approximations)

Note: If a bolt is slightly longer than needed, it can be trimmed using a saw. Nut sizes should fit snugly but not too tight.

(A) Connecting Base to Base Cover
- 7 × Flat Head Machine Screws (Type B)
  - Size: M4 × 50 mm
  - Head Diameter: ~7.5 mm
- 7 × M4 Hex Nuts
  - Edge-to-Edge: ~6.9 mm
  - Thickness: ~3.2 mm
(B) Drive Pins Area (Replaces 5 mm Pins)
- 4 × M3 × 50 mm Bolts (hex or socket head)
  - Head Diameter: ~5.5 mm
- 4 × M3 Hex Nuts
  - Edge-to-Edge: ~5.5 mm
  - Thickness: ~2.4 mm
(C) Mounting Points on Top of the Gearbox
- 7 × M4 Hex Nuts
  - Edge-to-Edge: ~6.9 mm
  - Thickness: ~3.2 mm
(D) Shaft Connector
- 2 × M3 × 40 mm Bolts
  - Head Diameter: ~5.5 mm
- 2 × M3 Hex Nuts
  - Edge-to-Edge: ~5.5 mm
  - Thickness: ~2.4 mm
(E) NEMA-17 Shaft Connector
- 1 × M3 × 6 mm Bolt (may require sanding)
  - Head Diameter: ~5.5 mm
- 1 × M3 Hex Nut
  - Edge-to-Edge: ~5.5 mm
  - Thickness: ~2.4 mm
(F) NEMA-17 Connector
- 4 × M3 × 15 mm Bolts
  - Head Diameter: ~5 mm

3D-Printed Components
- Material: PLA (tested), PETG (recommended for durability)
- Parts: Housing, base cover, discs, shaft connectors, output caps, circlips
- Infill: Suggested 40% or more for torque applications

5. Cycloidal Drive Strength Analysis

1. Motor Output Torque

Motor Stall Torque: 0.45 Nm
Gear Ratio: 21:1
Estimated Efficiency: 85%


\text{Output Torque} = 0.45 \times 21 \times 0.85 = \boxed{8.03 \, \text{Nm}}

2. Load Handling Capability (Based on PLA Disc)

PLA Parameters:

Yield Strength of PLA: 50 MPa
Safety Factor: 3 → Allowable stress = 16.7 MPa
Contact Area (per lobe): 5 mm²
Lobes in contact: 4
Radius from center to lobe force point: 33.5 mm = 0.0335 m
Infill Density: 25%

Calculations:

Force per lobe:


F_{\text{lobe}} = 16.7 \times 10^6 \times 5 \times 10^{-6} = 83.5 \, \text{N}

Total force from 4 lobes:


F_{\text{total}} = 4 \times 83.5 = 334 \, \text{N}

Torque handling at 33.5 mm radius:


\tau = 334 \times 0.0335 = 11.2 \, \text{Nm}

Account for 25% infill:


\tau_{\text{safe}} = 11.2 \times 0.25 = \boxed{2.8 \, \text{Nm}}

3. Drive Pin Strength (Bolts with PLA Sleeves)

Assumption:

Bolts used instead of precision 5 mm mild steel pins
Outer diameter maintained at 5 mm using PLA sleeves
Inner bolt shaft assumed 4 mm (typical M4 bolt shank)
Steel Yield Strength: 260 MPa

Cross-sectional Area (4 mm bolt):


A = \frac{\pi D^2}{4} = \frac{\pi \cdot 4^2}{4} = 12.57 \, \text{mm}^2

Force per bolt:


F_{\text{bolt}} = 260 \times 12.57 = 3,268.2 \, \text{N}

Total force (4 bolts):


F_{\text{total}} = 4 \times 3,268.2 = 13,072.8 \, \text{N}

Torque capacity:


\tau = 13,072.8 \times 0.0335 = \boxed{437.9 \, \text{Nm}} \quad \text{(Safe✅)}

Note: PLA sleeves do not contribute significantly to strength — they act as spacers or guides. Only steel bolts are load-bearing.

4. Max Load at 50 mm Arm


F = \frac{\tau_{\text{safe}}}{r} = \frac{2.8}{0.05} = 56 \, \text{N}


\text{Mass} = \frac{56}{9.81} = \boxed{5.7 \, \text{kg}}

Final Summary Table

Category	Value	Safe?
Motor Stall Torque	0.45 Nm	✅
Output Torque	8.03 Nm	❌ Exceeds PLA safe limit
PLA Torque Limit	2.8 Nm	✅
Bolt + PLA Pin Torque Limit	437.9 Nm	✅ (way overbuilt)
Max Load (at 50 mm arm)	5.7 kg	✅

Final Conclusion

The cycloidal drive system is likely to fail at the PLA disc lobes under full output torque, especially if printed with only 25% infill. Although the bolts (with PLA sleeves) acting as drive pins are structurally sufficient, the PLA disc is the mechanical weak point.

If tested:

The drive will function under light loads (below ~5.7 kg at 50 mm arm).
Under full load or high-torque demand, plastic deformation or cracking is expected at the lobes.
The bolts will hold without issue, but localized PLA wear or failure around the lobes and sleeves is likely.

Recommendations:

Increase infill to at least 75% or use a stronger material like PETG.
Consider fillet reinforcement around lobes to distribute stress.
Reduce gear ratio or torque output if high load isn't needed.

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