Apparatus for controlling the tension of a web during transport, such as a label-carrying web in a heat transfer labelling machine. The web travels from a supply reel, around a dancer roll and metering roll, through various other web handling devices to another dancer roll, and is collected at a rewind reel. Each dancer roll is linked to a clutch for the corresponding supply or takeup reel, so that each clutch exerts a braking torque determined by the web tension at its dancer roll. Each dancer roll is further linked to an air cylinder to provide a user-determined tension level. The carrier web includes pin holes which are engaged by a pin wheel rotatably mounted to the metering roll, so that the pin wheel swivels when subjected to web tension differentials. Swivelling of the pin wheel opens and closes a valve within the metering roll, thereby providing a variable vent for high pressure air supplied to the supply air cylinder. The system provides automatic adjustment of web tension in response to a tension differential at the metering roll.