Normal wear and tear is the expected and approved consumption of a portion of the reasonably expected life span of a component when the component is used competently and with proper care.
On a Snowcat, an example would be the tracks. A set of tracks can be expected to last 4000 hours or more and around the time of this hour marker, the condition of the tracks is likely to have deteriorated to a point of unreliable service. This deterioration might be worn down grousers or rubber belting that is cracking and separating. Now the service life of a component may be extended through meticulous care or light use, or it may be shorter due to heavy use and mediocre care, and we can’t precisely determine what the service life is of hardware. But the bottom line is that tracks are just like tires on a car; over time and with each mile they wear out. The Lessor is responsible for normal and expected wear and tear of these tracks for the expected and approved use by the Lessee.