The prior art discloses many types of multisection telescopic crane booms and actuator mechanisms for extending and retracting the boom sections. In some prior art arrangements, each boom section is telescopically movable relative to another by means of its own individual hydraulic ram. In other prior art arrangements several boom sections are telescopically movable by one hydraulic ram. In the latter arrangements, for example, either releasable pin means are used to temporarily secure one boom section to another so that one ram can move both, or cable and pulley arrangements are used so that movement of one section by a ram effects movement of other sections. U.S. Pat. Nos. 3,396,601 and 3,605,358 disclose arrangements of the latter type wherein numerous pulleys and complex cable systems are required and wherein substantial portions of the cable-pulley systems are located on the exterior of the telescopic crane boom.