The force exerted by gas pushing on a cup or set of cups at the front of the tool pulls the tool through the line. Differential pressure acting between the front and back of the drive cups provides a force along the pipe axis. This force propels the drive cups, which in turn pull the rest of the tool.

The driving force on a tool must overcome the friction between the tool and the pipe and the magnetic drag that the tool exerts. The pressure differential required to move most tools is small; one measurement program showed the pressure differential for a 24-inch tool to be between 6.9 psi and 10.4 psi ^[Eiber91]. Higher pressure differentials are needed to start a non-moving tool, since both static friction and the magnetization force of attraction must be overcome. These pressure differentials are relatively small compared to the typical pipeline pressure during in-line inspection of 300 to 600 psi. This reduction from typical operating pressures of 800 to 1,000 psi is required to reduce the tool speed to the preferred range of 4 to 6 mph.

The amount of force (differential gas pressure) needed to move a tool through a line depends on the age and condition of the drive cups. The driving force and the cup resistance affect how constant the tool velocity is. Old cups require less force than new cups. So, the driving force changes with time. Large velocity fluctuations can occur at low absolute pressures and when wall thickness changes are encountered ^[Shannon88]. Other pipeline conditions that cause velocity fluctuations include bends, valves, obstructions such as dents, and the internal condition of the pipe. The effects of velocity fluctuations are discussed in more detail elsewhere in this report.

The drive cups are typically at the front of the tool. Additional cups can be used to center each tool segment in the pipeline. To prevent gas pressure from pushing a trailing segment of the tool into the leading segments, the gas is vented through holes in trailing cups or through a bypass on the tool body. Wheeled assemblies rather than auxiliary cups are sometimes used to provide centerline support for trailing segments. In this case, no gas bypass is needed. In addition, wheeled assemblies provide less drag than cup assemblies, and so, they require less driving pressure.

MFL tools often have two or more drive cups. Multiple drive cups help the tool traverse pipeline connections. At tee connections and at valves, the differential pressure across a cup can drop if gas bypasses around the cup. A second set of drive cups helps overcome this problem if the cups are spaced far enough apart so that one cup is always away from the connection or valve.