11 into the value of the hydraulic pressure fed to the one of the units 9 corresponding to that probe in operation the web 7 is continuously pressure rolled for calendering with the control system 13 initially applying uniform hydraulic pressure to each of the units 9 this continuing as long as the probes measure the same temperature at each of the zones as calendering continues the polyurethane layer 5 inevitably begins to heat to non uniform temperatures lengthwise of the roll 1 at any zone where there is a temperature increase the control system 13 lowers the pressure applied to the corresponding one of the pressure applying units 9 thus lowering the nip pressure at that zone at any zone where there is a temperature decrease the pressure of the unit 9 at that zone is increased in the above way the temperature of the polyurethane layer 5 is kept uniform from end to end and because this temperature is dependent on the nip line pressure the latter is therefore automatically kept uniform throughout its extent localized heating of the polyurethane layer 5 to degrading and possibly destructive temperatures is prevented academically manual operation is possible with each of the lines 12 provided with its own temperature measuring readout and each of the pressure lines 10 manually controlled an operator can adjust the pressures of the lines 10 as required to obtain uniform temperature readings from all of the probes 11 in.