Does a Beveled Washer on an Inclined Surface Preserve or Cancel the Incline?

[Amaelle]

TL;DR

Custom 10.23° beveled washer between bolt head and inclined lumbar plate — does it preserve the inclination angle or cancel it? Also: bolt hole becomes elliptical on inclined surface — washer OD sizing implications at 120 kN

*Background**

I am designing a lower torso surrogate adapter for crash test dummy drop tower testing. The assembly works as follows:

- The **lower torso** (EN24T wedge) is bolted **flat** to the drop tower floor
- The top face of the wedge is inclined at **10.23°** — this is intentional, it replicates the anatomical lumbar inclination
- The **upper torso** sits on top of the inclined lumbar plates at 10.23°
- The two are joined by **M16/M20 Gr12.9 through bolts** passing through both lumbar plates and the wedge
- The bolts must carry **120 kN** impact load

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**The Problem**

Because the lumbar plate surface is inclined at 10.23°, the bolt holes are drilled perpendicular to that inclined surface. This means:

- The bolt axis is **NOT vertical** — it is at 10.23° to vertical
- The bolt head bears on the **inclined surface** at an angle
- Without correction, the bolt bends on tightening → stress concentration → bolt fails below rated load

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**Proposed Solution**

Add a **custom machined beveled washer** (10.23° cone angle, EN24T, OD ≈ 40mm) between the bolt head and the inclined lumbar plate surface.

The beveled washer:
- Has an **angled bottom face** matching the 10.23° inclined plate surface
- Has a **flat top face** giving the bolt head a perpendicular bearing surface
- Makes the bolt **perpendicular to its own bearing surface** — full preload, no bending

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**The Question**

My supervisor flagged a concern: does adding the beveled washer change or cancel the 10.23° inclination of the upper torso?

My understanding is **no** — here is my reasoning:

1. The lower torso wedge is already fixed flat to the floor — the 10.23° inclined top face is fixed geometry, it does not move
2. The upper torso sits on that inclined face — it is at 10.23° regardless of what the bolts do
3. The beveled washer only corrects the **bolt-to-bearing-surface relationship** — it does not change the geometry of the interface between upper and lower torso
4. The inclination is defined by the **wedge geometry**, not by the bolt angle

In other words: the beveled washer makes the bolt work correctly **on the inclined surface** — it does not flatten the surface.

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**Can anyone confirm or correct this reasoning?**

And as a secondary question — is a custom machined EN24T conical washer the right approach for 120 kN at 10.23°, or is there a better standard solution (DIN 6319 spherical washer pair, for example)?

Thank you

 

[Baluncore]

TL;DR: Custom 10.23° beveled washer between bolt head and inclined lumbar plate — does it preserve the inclination angle or cancel it? Also: bolt hole becomes elliptical on inclined surface — washer OD sizing implications at 120 kN

Add a **custom machined beveled washer** (10.23° cone angle, EN24T, OD ≈ 40mm) between the bolt head and the inclined lumbar plate surface.

How will you prevent the beveled D washer from rotating during assembly?

The wedge angle of 10.23° will cause the D wedge to move sideways, until the hole in the D wedge contacts the shank of the bolt. What prevents the D wedge from sliding sideway with the bolt head, until the bolt shank is in contact with the lower side of the hole? If that does not happen during assembly, it will happen later during a drop.

As a guide to wedges, Tan(10.23°) = 0.180 ; which is the minimum coefficient of friction needed between the wedge and the surfaces, that will prevent the wedge from being ejected, until being held by the hole contacting the shank.

The relative position and alignment of the two lumbar plates being clamped, should be fixed not by plain bolts alone, but by dowels, or by reamed holes with fitted bolts.

 

[Amaelle]

Thank you for the detailed feedback — the points on washer rotation, sliding, and dowel pins are all valid and I will address them in the design.

However my original question was specifically about whether the beveled washer changes the 10.23° inclination angle — and I don't think that has been answered yet.

To clarify the assembly sequence:
1. The lower torso wedge is bolted flat to the drop tower floor
2. The 10.23° inclination is machined into the wedge top face — it is fixed rigid geometry
3. The upper torso sits directly on that inclined surface — it is at 10.23° by definition
4. The beveled washer sits between the bolt head and that inclined surface — it only corrects the bolt bearing condition

My understanding is that the inclination is defined entirely by the wedge geometry — not by the bolt angle — so the beveled washer cannot change it.

Could anyone confirm or challenge this specific point? Does the beveled washer affect the 10.23° interface angle between upper and lower torso — yes or no?

 

[Lnewqban]

Could anyone confirm or challenge this specific point? Does the beveled washer affect the 10.23° interface angle between upper and lower torso — yes or no?

Why did you call the washer "conical" in two occasions?

I can't understand the concern of your supervisor.
Unless I am missing something that is evident to him, I believe that the beveled washer can't affect the 10.23° interface angle between upper and lower torso.

As correctly pointed above, a slight vertical rotation of the beveled washer during assembly can introduce additional great load to the bolt, as the top surface will not be perpendicular respect to the axis of the bolt.

 

[Amaelle]

Thank you — this is very helpful and confirms my reasoning.

Regarding the terminology inconsistency — you are right to flag it. I used both "beveled washer" and "conical washer" interchangeably, which is confusing. To clarify: I am referring to a single part — a washer with one flat face and one angled face machined at exactly 10.23°, placed between the bolt head and the inclined lumbar plate surface. I will use "beveled washer" consistently going forward.

Regarding my supervisor's concern — I believe the confusion was about where exactly the washer sits in the assembly. The washer is placed under the bolt head only, not between the two torso interfaces. The 10.23° inclination is defined entirely by the machined wedge geometry — the upper torso bears directly on that inclined surface, so the angle is fixed regardless of what the bolt or washer does.

On the rotation point — you and the previous reviewer have both correctly identified this as a real risk. A beveled washer that rotates even slightly during tightening will introduce a bending moment on the bolt shank in addition to the tensile load, which at 120 kN impact could be critical.

I have one proposed fix in mind — machine a shallow counterbore recess in the lumbar plate surface that the washer locates into, preventing both rotation and lateral sliding in one feature.

However I would really value your experience on this: what is the most reliable industry-standard solution to prevent a beveled washer from rotating during bolt tightening on an inclined surface? Is the counterbore recess approach correct, or is there a better established method?

Thank you again for the precise and useful feedback.