Multi-model battery pack assembly pallets are jigs that repeatedly locate cell stacks of different chemistries on a single line, using a clearance fit tolerance between locating pins and boss holes.
Problem Definition
In August 2026, Hyundai Motor announced that it would invest €55 million to operate an automated battery pack assembly line at its İzmit plant in Türkiye (estimate: based on the official announcement; detailed line specifications are undisclosed). The line will mixed-produce NMC and LFP chemistry packs for the Ioniq 3 on the same line. This creates a mechanical contradiction. When cell thickness differs by chemistry, stack thickness deviation grows larger, and the locating pin-to-boss clearance fit of the shared pallet behaves differently for each chemistry. Can a single clearance fit simultaneously satisfy the repeat-positioning accuracy required for both chemistries?
Kinematic Analysis
Pallet positioning is achieved through a clearance fit between the pin and the boss hole. The pin must fit loosely relative to the boss hole to absorb the stack-thickness deviation between chemistries. Using an interference fit would require replacing the pin at every chemistry changeover, which is unsuitable for mixed production. If the fit clearance is too large, repeat-positioning accuracy collapses and propagates into gripping error at the downstream robot gripper. In the end, the design variable narrows to a single choice: selecting the tolerance grade.
Calculation / Formula Verification
The boss hole is specified as $\phi10\text{H7}$ (10.000–10.015 mm), and the locating pin as $\phi10\text{g6}$ (9.986–9.995 mm).
$$c_{max} = D_{max} – d_{min} = 10.015 – 9.986 = 0.029\text{mm}$$
$$c_{min} = D_{min} – d_{max} = 10.000 – 9.995 = 0.005\text{mm}$$
$c$ is the radial clearance in mm. If the required repeat-positioning accuracy before robot vision correction is set at $\pm0.1\text{mm}$ (estimate: reference to the typical grip-shear positioning tolerance of general EV pack assembly robots; confirm after field measurement), the maximum clearance of 0.029 mm leaves margin of less than one-third of the required value.
Next is verification of the pin shear safety factor against inertial load during an emergency stop of the indexing conveyor. Taking the combined mass of the pallet and stack as $m=85\text{kg}$ (estimate: reference figure for a small module pack plus pallet self-weight; confirm after field measurement) and the emergency-stop deceleration as $a=2\text{m/s}^2$ (estimate: reference to the typical e-stop deceleration range of industrial indexing conveyors), the inertial force is as follows.
$$F = ma = 85 \times 2 = 170\text{N}$$
The pin cross-sectional area (single shear, $\phi10\text{mm}$) is $A=\pi/4 \times 10^2=78.5\text{mm}^2$, and the shear stress is as follows.
$$\tau = F/A = 170/78.5 = 2.17\text{N/mm}^2$$
Primary safety-factor verification (yield basis): applying S45C’s tensile yield strength of 343 N/mm², the shear yield strength is $0.577\times343=198\text{N/mm}^2$, giving $SF_1=198/2.17\approx91$. Secondary safety-factor verification (fatigue basis, estimate: reference to a literature value of 235 N/mm² for S45C fatigue limit, with a 0.58× shear conversion factor applied; confirm after field measurement): based on a shear fatigue limit of $136\text{N/mm}^2$, $SF_2=136/2.17\approx63$. Both safety factors carry large margin, so pin shear strength is not the governing factor in this design. The governing factor shifts to wear from repeated fitting. As the clearance-fit interface wears, the clearance grows and repeat-positioning accuracy gradually collapses.
Shop-notes
An alternative of permanently placing dedicated pin rows for each chemistry on the pallet, with unused rows folded down and fixed (the folding pin structure of Toyota’s assembly pallet patent US9604755B2 corresponds to this approach and is covered in a separate patent-analysis article), was considered, but it was not adopted for this design because the increased part count and drive mechanism add maintenance points. A single clearance-fit pin approach is judged more favorable in cost and maintainability.
Machinability note: the pin surface uses SCM440 material with gas nitriding treatment. To secure a nitride-layer thickness of 0.3–0.5 mm, a post-nitriding grinding allowance must be separately reflected in the drawing, and it is advantageous to machine pre-heat-treatment dimensions tighter than the tolerance mid-value to account for nitriding distortion.
A wear-management criterion cannot currently be finalized from the literature (confirm after field measurement). A gauge-pin insertion inspection every 500 fit cycles is recommended as a first-pass interval (estimate: reference to typical inspection intervals for similar locating jigs).
Design-Reflection Checklist
- Re-calculate boss position after measuring per-chemistry cell stack thickness
- Reflect pin-material nitriding specification in the drawing, and secure post-nitriding grinding allowance
- Confirm emergency-stop deceleration after field measurement
- Establish a gauge-pin inspection procedure at 500-fit-cycle intervals
One-Line Summary: The governing design factor for the multi-model battery pack assembly pallet is not the locating pin’s strength but the tolerance expansion from repeated wear at the clearance-fit interface.