Design Log DEC 059 erm8erf8 stacking height confirmed rotor stack assembly specification created

DEC-059 - ERM8/ERF8 Stacking Height Confirmed; Rotor Stack Assembly Specification Created

Field	Value
Decision ID	DEC-059
Status	Confirmed
Date	2026-05-28
Author	Copilot (session a38ceaab)
Amends	-

Context

The ERM8/ERF8 rotor-interconnect connector stacking height was previously recorded as approximately 5mm but was disputed (see checkpoint 154). A systematic re-read of both Samtec datasheets was performed to establish the definitive value.

Decision

The -05.0- lead style designation in the selected parts:

• ERM8-005-05.0-S-DV-K-TR (male, rotor Board A)
• ERF8-005-05.0-S-DV-K-TR (female, rotor Board B)

denotes a 5 mm per-connector standing height above each PCB surface. Because the two connectors mate end-to-end, the total PCB-to-PCB inter-rotor gap is:

5 mm (ERM8) + 5 mm (ERF8) = 10 mm

This was independently verified against the Samtec published stack-height range for the ERM8/ERF8 family (7 mm - 18 mm): the minimum of 7 mm = ERM8 -02.0 (2 mm) + ERF8 -05.0 (5 mm), confirming the additive model.

The 1.5 mm contact-wipe figure stated in the ERM8 datasheet is a spring-travel specification and does not reduce the board-separation distance.

The -05.0/-05.0 pairing is the minimum achievable stacking height for the current connector selection. Taller variants are available:

ERM8 lead	ERF8 lead	Total
-02.0 (2 mm)	-05.0 (5 mm)	7 mm
-03.0 (3 mm)	-05.0 (5 mm)	8 mm
-05.0 (5 mm) [current]	-05.0 (5 mm) [current]	10 mm
-05.0 (5 mm)	-07.0 (7 mm)	12 mm
-08.0 (8 mm)	-05.0 (5 mm)	13 mm
-08.0 (8 mm)	-07.0 (7 mm)	15 mm
-09.0 (9 mm)	-07.0 (7 mm)	16 mm
-09.0 (9 mm)	-09.0 (9 mm)	18 mm

Any change to stacking height requires a DEFECT-DEC and must update the affected MPN(s) in all board Design_Spec BOMs and the Consolidated BOM.

New documentation

design/Mechanical/Rotor_Stack_Assembly/Design_Spec.md was created to formally capture:

• The mini-stack concept (5 rotors + boundary boards)
• The 10 mm inter-rotor gap derivation and source
• The full achievable stack-height options table
• Mechanical clearance requirements (TBD - pending prototyping)
• Indicative mini-stack depth calculation (~115 mm PCB-only, 5 rotors)

Files Changed

• design/Mechanical/Rotor_Stack_Assembly/Design_Spec.md: NEW - full specification created.
• design/Mechanical/Rotor/Design_Spec.md: §7 design intent bullets updated; §7 cross-reference block updated; §9 cross-ref table updated.
• design/Mechanical/Rotor_Actuation_Assembly/Design_Spec.md: §8 cross-ref table updated; new Rotor_Stack_Assembly row added.
• design/Mechanical/Complete_System_Assembly/Design_Spec.md: Component Index (§2) updated; Sub-Assembly Reference Table (§3) updated.