Isolator for engine with progressive lock-up for isolation spring

Abstract

In an non-limiting embodiment, an isolator is provided for isolating a device driven by an engine via an endless drive member. The isolator includes a shaft adapter that is connectable with a shaft of the device, a rotary drive member that is engageable with the endless drive member, a torsion spring that is positioned to transfer torque between the shaft adapter and the rotary drive member, and a torque limiting surface. The torsion spring has a first helical end, a second helical end and a plurality of coils between the first and second helical ends. The torque limiting surface has a selected non-zero spacing from the coils. The spacing between the torque limiting surface and each coil is selected such that, during operation, the torsion spring has a spring rate that is below a first spring rate during relative movement between the shaft adapter and the rotary drive member within a selected angular range from a neutral position, and has a spring rate that increases progressively above the first spring rate during relative movement between the shaft adapter and the rotary drive member beyond the selected angular range from the neutral position.