Annex J (informative) — Front panel stiffness and extra mounting holes

Four mounting holes are the usual pattern. EIS places no requirement on hole count. This annex estimates when a wide panel flexes under a plug and where extra holes go when a designer wants them; tools/module.py computes the tables.

J.1 Span

Between screws, the panel is a simply supported beam as tall as the panel and as thick as the sheet. A 10 N push at midspan, about the force of seating a 3.5 mm plug, is allowed to bow it 0.5 mm. The ends are free to rotate. With I = height × thickness³ / 12, that span is

L = (48 E I × 0.5 mm / 10 N) ^ (1/3)

5052 aluminum has a modulus of 70.3 GPa. FR-4, across the weave, has a flexural modulus of 16.5 GPa. At the span these numbers allow, the bending stress stays under 20 MPa. The 0.5 mm bow is reached first.

The recommended panel is 5052 aluminum or FR-4, from 1.6 mm to 2.0 mm, so for each material the 1.6 mm row covers the thicker sheet. At the same span, 1.6 mm FR-4 bows about eight times as far as 2.0 mm 5052.

The span between the outer holes is (N − 3) × 5.08 mm. While that span is inside the limit, four screws are enough. Past it, a panel can take more holes, added two at a time, until every gap is inside the limit. Every gap is a whole number of HP, so splitting the span can leave one gap an HP longer than an equal split, and that step is enough to cross the limit; the count follows the gaps as they are actually placed.

1.6 mm FR-4 is the most flexible of these four sheets. A 104 HP panel of it needs 12 screws on 3U and 16 screws on 1U, and that is as far as the ladder goes. A dash means a 104 HP panel of that sheet still fits the previous screw count. The widths are the secondary pattern.

Sheet	Thickness	3U span	More than 4	More than 6	More than 8	More than 10
5052	1.6 mm	194.9 mm	42 HP	80 HP	—	—
5052	2.0 mm	243.6 mm	51 HP	98 HP	—	—
FR-4	1.6 mm	120.2 mm	27 HP	50 HP	73 HP	96 HP
FR-4	2.0 mm	150.3 mm	33 HP	62 HP	91 HP	—

Sheet	Thickness	1U span	More than 4	More than 6	More than 8	More than 10	More than 12	More than 14
5052	1.6 mm	131.7 mm	29 HP	54 HP	79 HP	104 HP	—	—
5052	2.0 mm	164.6 mm	36 HP	68 HP	100 HP	—	—	—
FR-4	1.6 mm	81.2 mm	19 HP	34 HP	49 HP	64 HP	79 HP	94 HP
FR-4	2.0 mm	101.5 mm	23 HP	42 HP	61 HP	80 HP	99 HP	—

J.2 Even pattern

The even pattern keeps an even number of holes on each rail: 2, then 4, then 6, and so on. Each step adds a symmetric pair, which is four screws on the panel, so it skips six, ten, and fourteen screws. Every gap is a whole number of HP, and the center gap is the wider one. Because the gaps must be whole HP, the count can rise at an odd width and fall back at the next even one. Because it adds holes in pairs, on a 1U panel it can need more screws than the secondary pattern at the same width: 2.0 mm 5052 first needs 12 screws at 98 HP, 2.0 mm FR-4 first needs 16 screws at 95 HP, and 1.6 mm FR-4 first needs 16 screws at 75 HP.

J.3 Secondary pattern

The secondary pattern adds a middle hole. The count on each rail then goes 2, 3, 4, 5, and so on, through 6 holes on a 3U rail and 8 holes on a 1U rail for a 104 HP panel of 1.6 mm FR-4. Each step adds one hole on each rail, which is two screws on the panel. Every gap is a whole number of HP, as equal as that allows. On an odd HP panel the middle hole is on the center. On an even HP panel the center is 2.54 mm off the strip pitch, so the middle hole sits 2.54 mm to one side of it. The tables and the model put it on the left, seen from the front; either side is valid.

On a 3U panel the two patterns first diverge at 42 HP.

Pattern	2.54 mm steps from center	Millimeters from center	Screws
Even	±13 and ±39	±33.02 and ±99.06	8
Secondary	−1 and ±39	−2.54 and ±99.06	6

On a 1U panel they first diverge at 29 HP: the secondary pattern is 6 screws at 0 and ±66.04 mm, and the even pattern is 8 screws at ±25.40 mm and ±66.04 mm.