Swivel Assembly (EPLT) The Swivel Assembly (EPLT) allows different portions of the tool string to rotate independently. It allows unrestricted 360 degree rotation by means of an internal slip-ring assembly.

A swivel isolates an instrument from the normal torque induced as the spiral-wound wireline is lowered into and pulled out of the well. This torque causes the tool string to rotate typically one or two rotations per 100ft. (30m) of depth.

Excessive rotation may degrade some well log data, such as the data from Diplog-type tools. In addition, some instruments (notably the Density and Z-Density tools) feature a powerful motorized caliper arm that causes the tool to “seek” and hold orientation across the widest axis of the borehole. Other tools have positioned to force this orientation or to hold another specific orientation. This forced orientation prevents wireline torque from rotating the string.

The built-up torque can cause the tool to break loose suddenly and spin, resulting in an interval of reduced log quality. In severe case, the built-up torque can actually damage the wireline by causing the armor wire to unlay, resulting in high strands or “birdcage”. The use of EPLT in these situations allows the top portion of the string to rotate, relieving wireline torque, while the bottom portion (below the swivel) is able to maintain the preferred orientation.