The Snap Fastener: A Spring That Pinches the Same Every Way

McCormack's socket has two parts seated on opposite sides of the cloth. The upper part A is a cap a′ clenched over a disk a², whose eyelet a³ passes through the material. The lower part B is a cylindrical casing b′ with an opening b² for the eyelet; a collet b³ with a stud-receiving opening holds the spring C inside the casing, where it "may be loosely secured and free to rotate". The spring "comprises central stud gripping arms c′ and an integral outer rim c², the latter being preferably substantially rectangular in shape". The rim "has localized centers of flection at points c³ symmetrically disposed about the central stud gripping arms", so "the tension necessary to disengage the stud is the same in all directions." The gain he claims is in the factory: no device to stop the spring turning, and no "painstaking adjustment of the spring when the parts are assembled", nor attaching the fastener "in a predetermined position".

Set four snap fasteners from the heavy-duty kit into two strips of denim, following the kit's setting tool and a hammer on a hard surface. Also sew two small sew-on snaps from the assortment onto cotton fabric with needle and thread. Close each snap. Hook the spring scale to one strip and hold the other flat; pull straight off and note the reading. Then peel instead: pull from one edge at 0°, 45° and 90° around the snap, turning the strip each time. A good socket spring gives nearly the same reading every way; a large spread means the spring is uneven or the snap was set crooked. Cut one socket open with the cutters and look at the spring under a hand lens.

**George A. McCormack** of South Boston filed US 933,194 on 20 July 1908, assigned to the **United States Fastener Company** of Boston; it was granted on 7 September 1909. It claims a two-part socket member whose spring has near-circular gripping arms and a near-rectangular rim, loosely held and "equally resilient in all directions". Press studs themselves were older. **Honest limits.** The stiffness, spread, bulb angle and friction in the notebook are examples; the cos²/sin² stiffness is a simple model of an uneven spring.