jackshaft definition
A jackshaft, also called a countershaft, is a common mechanical design component used to transfer or synchronize rotational force in a machine. A jackshaft is often just a short stub with supporting bearings on the ends and two pulleys, gears, or cranks attached to it.ackshaft [edit]. The oldest uses of the term jackshaft appear to involve shafts that were intermediate between water wheels or stationary steam engines and the line shafts of 19th century mills.