A calender wherein a permanently installed main roll cooperates with at least one exchangeable roll to provide a nip for the passage of a web of paper or the like. The end portions of the exchangeable roll are mounted in bearings which are insertable into and extractible from sockets provided in levers which are pivotably mounted in the frame of the calender. The calender has a transporting unit which is provided with grippers for the bearings and with motors which reciprocate the grippers to extract the bearings from their sockets after the levers are pivoted to positions in which the exchangeable roll is spaced apart from the main roll. The transporting unit thereupon transfers the exchangeable roll onto a mobile rack which can be transported to a station where the exchangeable roll is repaired. The same rack or a discrete second rack can deliver a refurbished exchangeable roll to the transporting unit for introduction of the bearings on the refurbished roll into the sockets on the levers in the frame of the calender before the levers are pivoted to move the refurbished roll into engagement with a running web or with the main roll. Locking devices are provided to releasably hold the bearings in their sockets, and additional locking devices are provided to releasably hold the bearings in sockets which are provided on the rack or racks. The transporting unit can constitute a robot whose grippers then constitute effectors and whose operation is automated to start a roll exchanging operation as soon as one or more sensors in the calender ascertain that the quality of the exchangeable roll is unsatisfactory for further treatment of running webs.