Source: https://guthlabs.ai/docs/instruments/shaft-diameter-sizing-torsion-bending/implementation/

instrument implementation / implementation-contract

# Shaft Diameter Sizing (Torsion + Bending) implementation

A source-bound implementation contract for Shaft Diameter Sizing (Torsion + Bending); deployment and executable qualification remain separate.

Status: **VERIFIED_WITH_ERRATA**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-with-binding-errata

**Evidence ID**: t4-082

**Effect**: Computation or source lookup only

## Input and validation requirements

The formula returns an undefined result when tauallow=0 and returns a nonphysical zero diameter when both bending moment and torque are zero. The specified equation has no actual conditional formula branch; sign crossing does not change the result.

## Required corrections

````
[
  {
    "vector": 1,
    "field": "d_mm",
    "wrong": 46.702,
    "correct": 21.677,
    "cause": "d³ off by 10× (Te 1e5 treated as 1e6); her own v2-v4 use the same formula correctly"
  }
]
````

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "inputs": {
    "Mb": 0,
    "Tt": 100000,
    "tauallow": 50,
    "Kb": 1.5,
    "Kt": 1,
    "units": "Mb,Tt in N·mm; tauallow in MPa"
  },
  "expected_outputs": {
    "d_mm": 46.702
  },
  "working": "d = [16/(π·50)·√((1.5·0)^2 + (1.0·100000)^2)]^(1/3)\n= [16/(50π)·√(0 + 10000000000)]^(1/3)\n= [16/(50π)·100000]^(1/3)\n= [101859.16]^(1/3)\n= 46.702 mm"
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/shaft-diameter-sizing-torsion-bending/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/shaft-diameter-sizing-torsion-bending/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/shaft-diameter-sizing-torsion-bending/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`

- `tranche errata record`