20 XERIUM EUROPE


Pulp Paper & Logistics


The safety factor “SStd” for a standard seam felt is calculated as:


Figure 3: Double felted shoe press 3


To estimate the safety performance and security reserve of a seam felt in a double felted shoe press, it is necessary to calculate the risk. The safety factor is defined as:


Load on one seam loop FL:


A safety factor of 1.0 means that there is no security reserve and local tension differences during the run can lead to the seam opening. In this case the felt must be removed. This happens in the first one or two days when the felt is not yet compacted. The lower specific load of a shoe press additionally delays the compaction of the press felt.


Load per yarn of each seam loop FY:


and gives information on safety reserve. In this respect, values more than 1.5 can be judged as safe and anything under 1.1 as critical.


Safety factor calculation for a standard seam felt To calculate the force on each seam loop, the total felt tension has to be broken down to the load onto a single loop. The maximum force which can be applied to the yarn of a seam loop is equal to the yarn anchorage FYA measured in the laboratory:


The felt tension at the tension roll is considered to be 5.0 kg/cm.


Within the press position the bottom felt has to carry additional rolls and drag by suction boxes. This situation results in a significantly higher felt tension at the entrance of the press nip. The felt tension can be up to between 150 and 200 N/cm which is three to four times as much as the display in the control stand shows. Load increase on the yarn:


Safety factor calculation of a ProSeam SP felt The maximum force that can be applied on the yarn of the ProSeam SP seam loop is equal to the yarn anchorage FYA measured in the laboratory:


For the ProSeam SP calculation, the same felt tension and loop density as before are valid. Felt tension: 5.0 kg/cm = 49.1 N/cm Loop density: 88 loops/10cm = 8.8 loops/cm Load on one seam loop FL:


Load per yarn of each seam loop FY: The loop density of the seam felt is:


In the above-mentioned double-felted shoe press the bottom felt has to drag along the top felt and the shoe press belt. The force on the bottom felt is thus increased and the seam of the seam felt is further stressed. It is estimated that this increases the load on the felt by another 1.5 to 2.0 times.


Load increase on the yarn due to the situation in the press circuit:


Load increase due to situation in the double felted shoe press (dragging along the top felt and the shoe press belt):


The safety factor SSP for the ProSeam SP seam felt is calculated as:


Figure 4: Balance of forces on a seam loop


November 2013


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