Parece ser que "face" se refiere a superficie. Estos sellados de contacto de superfice se utilizan para evitar la corrosión de piezas qeu están expuestas normalmente a fricciones (típicamente los rodamientos, pero en general cualquier pieza de maquinaria):

Descricpión en ingles de "mechanical face seal":
http://www.federal-mogul.com/cda/content/front/0,2194,2601_6...

"Static and dynamic sealing technology is concerned with providing sealing solutions for machinery applications in which conventional seals lack the required durability.

The drive mechanisms for tracked vehicles and construction machinery, or the power transmission equipment for conveyor systems, sand conditioning plant, concrete and cement mixers, mining machinery and similar heavy plant are subjected to extreme abrasive stresses in the environments in which they work.

Failures can severely disrupt operations, and repairs are very costly. Our metallic **mechanical face seals** have served well in such arduous conditions for years. Robust and efficient, they guarantee customers high serviceability, durability and maximum productivity of their equipment."

En español:
http://www.dodge-pt.com/pdf/brochures/bearings/A-423-5mx.pdf
"Las chumaceras SC/SCM/VSC de DODGE usan el sello de contacto de superficie PROGUARD™ con protección de blindaje de acero."

Un saludo.