And when should they be replaced?

The first question is easy. Tires are essential for softening the ride your employee, equipment and product experiences. There is nothing else to absorb the shocks received as your lumbering machine makes its way across the shop floor or yard. The tire also has factors that involve safety, and for that reason there needs to be more care given to replacement, ignoring the often heard declaration of a tire's soundness from the customer "well they're still round and black".

Lets start with cushion tired lifts [rubber bonded to a steel band]. With a full load your average tire is carrying over 9600 pounds on the drive tires, and about 1300 pounds on the steer tires [lifted to a degree due to the load], the tires do this while your truck turns, stops and accelerates getting the load to and from points within your workplace. To do so safely it should be in the best condition possible while it lives through its wear life [anywhere from a thousand running hours or more dependent upon your floor conditions and use].