The Challis Extractor is placed over the bushing and rotated until it engages the slot. It is then firmly secured to the bushing using a standard grip screw. This keeps the tool in constant axial contact and alignment with the bushing, something that is very difficult to achieve when using a screwdriver, even one made for the purpose.
When the force required to turn the bushing with a screwdriver exceeds the strength of the bushing's narrow rim, the rim will begin to shear off by peeling away. If there is not something to prevent this, the rim will easily peel away because the driving forces are progressively acting on a small cross-sectional area. The Challis Bushing Extractor supports the very top of the bushing by clamping it tightly against the tool. It also precisely fits the shape of the bushing at the rim's circumference, and the grip screw prevents material from peeling inward. Because of this, the rim has nowhere to go. This means the driving forces are acting on the entire cross-section of the rim, from slot to slot. Because of this, the force required to shear the rim from the body increases dramatically, possibly by an order of magnitude.
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