cranequin

from The Century Dictionary.

• noun An implement for bending the stiff bow of the medieval arbalist, consisting of a ratchet working on a small wheel turned by a windlass. Also called a rolling purchase.
• noun Hence The arbalist itself: as, a hundred men armed with cranequins.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device used to arm a crossbow by cranking a gear.