The shaft seal such as disclosed, for example, in Patent Literature 1 has been known as a member for hermetically sealing the gap between two members which are coaxially rotatable relative to each other. This shaft seal is comprised of an inner circumference side sealing portion located on the inner circumference side and an outer circumference side sealing portion located on the outer circumference side, these sealing portions being fit to each other such that they can come into slidable contact.
Incidentally, if earth, sand or the like penetrates into the seal slidable contact parts of the inner and outer circumference side sealing portions, wear of the seal is accelerated, extremely shortening the service life of the seal. It is therefore important to provide a means for preventing penetration of earth, sand or the like into the seal slidable contact parts in order to extend the service life of the seal. The shaft seal disclosed in Patent Literature 1 is designed to prevent penetration of earth, sand, etc. into the seal sliding parts by providing a covering member so as to stride over the inner and outer circumference side sealing portions so that early wear of the seal can be avoided.
Crawler track units are widely used as an undercarriage in work vehicles such as hydraulic excavators. Such crawler track units often include roller units in which a floating seal is assembled into an annular space defined between a roller configured to roll in contact with the crawler belt and a roller shaft for rotatably supporting this roller (see e.g., Patent Literature 2).