Shear Pin Analysis

Background/Purpose

The shear pin is a small metal/nylon pin used to hold together the steering column that is designed to break when the car crashes. When the pin breaks, the steering column will be allowed to collapse into the dash, thus reducing impact forces on the driver’s chest. The objective is to find a suitable pin diameter for this purpose.

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Force Calculations

The pin will undergo double shear, the formula for double shear is:

To solve for the pin diameter with a safety factor of F/2 (safety factor of two, but designed for the pin to break, therefore the force is divided by 2):

From a research paper found on google books (reference), the force required to cause damage to a human chest was 8800N, therefore, a pin diameter was found where F=8800N with the materials nylon, steel and aluminum. Shear strength reference steel and aluminum, shear strength reference for nylon.

Operational forces on the pin were found for a lower bound on the pin diameters. The operational force was found by using 1200N transferred to the pinion from the steering system (see FEA of rack extension), the torque transferred to the steering column would be 15000 N*mm with a pinion radius of 12.5mm. The inner diameter of the steering column is 19.05mm, therefore the operational force on the pin would be 1575 N.

The minimum diameter for the pin to operate in each material using a safety factor of 2, and F=1575N:

Previous on MSXII, the shear pin used was (believed) to be 4-5mm nylon pin, therefore, it would be suitable for MSXIV to also use a nylon pin, from calculations above, the nylon pin should be between 5.35 to 6mm in diameter. A diameter of 5.5 mm was chosen and the pin will be machined out of 1/4” OTS nylon rod.

Verification

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All three forces are below 1575N with SF of 2, therefore the pin is not expected to fail with normal handling.

References:

OSHA Guideline Pulling Force

Steering Torque study

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