A mechanical coupling for rotary devices, whereby a wind vane or other rotary tool, can be replaced without the use of a tool. In a preferred embodiment, the invention provides a coupling arrangement for a rotary device and a spindle supported for rotation, the coupling being enabled by a resilient body which acts upon a spheroidal locking member such as a ball bearing. In particular, the present invention provides a coupling arrangement for a wind vane or an anemometer rotating body comprising impeller cups, which impeller cups and rotating body are required for removable fitment, without the use of tools.