This article is part of the Prusa XL Technical Series on INVESTEGATE.de
All guides assume that the Prusa XL Fundamentals are correct.
If not, start here:
Prusa XL Basics – Technical Fundamentals
From a mechanical engineering perspective, running a toothed timing belt over a smooth idler on the tooth side is geometrically suboptimal. While the effects may not always be immediately visible in low-load 3D printer applications, the underlying mechanics are well understood.
Effective Pitch Diameter and Belt Geometry
When a toothed belt wraps around a smooth pulley on the tooth side, the belt teeth cannot properly mesh. Instead, each tooth is forced to deform as it passes over the pulley surface. This causes:
- Micro-lifting and re-seating of each tooth
- Variation in the effective pitch diameter
- Slight elongation of the effective belt path
- Increased local vibration
- Accelerated tooth flank wear
This is not a slipping phenomenon. The belt does not slide — it continuously adapts its geometry to a surface that does not match its profile. Over time, this repeated deformation increases mechanical stress within the tensile cords and tooth structure.
Toothed Idlers Maintain Correct Pitch Geometry
A toothed idler (sprocket) that matches the belt profile (e.g., 2GT / GT2 / GT3) preserves the designed pitch engagement. The belt meshes cleanly with the pulley grooves, maintaining:
- Constant pitch diameter
- No forced tooth deformation
- Reduced cyclic stress
- Improved positional consistency
Provided that:
- The correct belt profile is used
- Minimum pulley tooth count and bend radius specifications are respected
the system operates within manufacturer-defined mechanical parameters.
Why the Issue Often Goes Unnoticed in 3D Printers
In typical desktop 3D printers:
- Belt speeds are moderate
- Mechanical loads are relatively low
- Duty cycles are intermittent
Under these conditions, the negative effects may not immediately manifest as failure. However, the absence of visible problems does not imply optimal mechanical design. It simply means the application operates far below the failure threshold.
The manufacturer’s specifications from Gates
Gates has published a detailed PDF that lists everything related to belts:
https://www.gates.com/content/dam/documents-library/catalogs/light-power-and-precision-manual.pdf
Manufacturer Idler Size Recommendations
Page 68 deals with the recommended sizes of the idlers/pulleys.
The image above summarizes manufacturer guidelines for timing belt idler sizing. It differentiates between three configurations:
- Inside Idler (toothed pulley on tooth side)
- Backside Idler (smooth pulley on belt back)
- Inside Flat Idler (smooth pulley on tooth side)
For 2MGT / 2GT belts, the key recommendations are:
- Minimum inside toothed idler: 12 teeth
- Minimum backside smooth idler: 0.50″ outer diameter (12.7 mm)
- Minimum inside flat idler: 1.00″ outer diameter (25.4 mm)
The critical distinction is between toothed and smooth contact on the tooth side of the belt.
A toothed inside idler is allowed at relatively small sizes (minimum 12 grooves), because the belt teeth properly mesh with the pulley profile and maintain constant pitch geometry.
A smooth idler on the tooth side, however, requires a significantly larger diameter. The guideline specifies that its outer diameter must be equivalent to the pitch diameter of a 40-groove sprocket in the same pitch. For 2MGT, this corresponds to approximately 25.4 mm.
This recommendation exists because smooth contact on the tooth side forces the belt teeth to deform while passing over the pulley, potentially increasing wear and noise. Larger diameters reduce bending stress and tooth deformation.
Backside idlers follow a different rule set, as the belt back is flat and does not rely on meshing geometry.
These specifications are based on fatigue behavior, cord stress distribution, and long-term belt durability considerations — not on immediate functional failure.
Practical Implications for the Prusa XL
Using toothed idlers on the tooth side:
- Preserves correct belt geometry
- Reduces long-term wear
- Minimizes micro-vibrational excitation
- Aligns the system with established timing belt design principles
Smooth idlers on the tooth side function in practice — but they represent a compromise rather than a mechanically ideal solution.
For a precision motion system like the Prusa XL, maintaining correct belt meshing is the technically cleaner approach.
| Column | Meaning | 2MGT Minimum | Prusa XL (20T) |
|---|---|---|---|
| Inside Idler (toothed) | Toothed idler on tooth side | 12 teeth | ✔ Above spec (20T) |
| Backside Idler | Smooth idler on belt back | 0.50″ (12.7 mm) O.D. | ✔ Likely compliant |
| Inside Flat Idler | Smooth idler on tooth side | 1.00″ (25.4 mm) O.D. | ❌ Only relevant if smooth used… Thats why you should use toothed one! |
As usual a great article. Something that I wish Prusa would address in perhaps a XL-S model. I am too unskilled to make a change this profound without guidance from Prusa – but I am glad you are pathfinding issues like this.
You should consider crossposting to Redit prusa3d, I think their community engagement person is active there.
Thank you 🙂 Yes, an XL-S upgrade would be nice!
Yep Reddit is good for that. I really need to do that someday. It’s all a matter of available time 😀