Newsham Engine circa 1730
Fig #4: Detail of Bow-tie Gear and Chain in Front of Engine.
Here is a magnification of the bow-tie gear and chain. The chain is attached to the treadle on one end and the top of a "sector" of the gear on the other end. The bow-tie gear itself has a large "groove" on its side that the chain rides in as the gear rotates.
The green uprights support the shaft with a connecting block. They also support the hand-rails extending from the Pump Housing that are used for balance of those working the treadles.