Resource
Extension spring end types
The hook takes the load and usually fails first. Name the type, the orientation, and the length inside the hooks.
Common ends
Machine hook (machine loop). A simple loop formed on the last coil. The usual, cheapest end. Fine when the attachment is a pin on the spring axis.
Extended hook. The loop stands out on a short straight. Use it when the attachment is a little away from the body.
Crossover hook. The opening sits across the coil so the pull stays on the axis. Side hook. The opening sits to the side, for an offset attachment.
Machine loops and extended hooks are the usual request here. Other eyes and offsets we confirm from the drawing.
Why the hook fails first
The hook is bent to a tighter radius than the body. Stress there is higher. A calculator that only reports body shear stress is a lower bound — see the extension spring calculator. Reduce hook stress with a larger bend radius or a crossover if the drawing allows it.
What to specify
- Hook type at each end (they can differ)
- Orientation: openings in line, at 90°, or opposite
- Length inside hooks
- Initial tension if the hook must not rattle