After explaining some specific terms of damages and repair methods and to be able to describe a occurred damage as exact as possible we want to continue to show and explain how to measure and protocol the damage .
This should be done whenever a
· damage occurs , whenever
· you require and need a specialist from outside and
· to update your belt maintenance – and history book for your own planning and safety.
3.1 Waviness : The waviness describes periodical appearing of single flatness deviations of a belt by wave height and wave length. The wave height is the distance between the lowest point of the bottom and the highest point of the peak of a wave. The wave length is the distance between two adjacent peaks or bottom points. Waves are zones with bigger length than the surrounding area and can be snapped to the opposite side by hand pressure ( from outside to inner side and vica versa ). Waves can be located on the belt edges as well as in the center of the belt. The size of waves is mainly depending on the tension of a belt, therefore the tension of a belt must be considered in any case. Waves are caused by thermal or mechanic overload of the belt material. This can happen e.g. when the belt collides with the framework of the press or the belt is exposed to local overheat. Waves can be repaired only in very few cases
A waviness can be expressed by using the flatness index I.
I = H² / L² x 250.000 ( H = wave height , L = wave length , in mm )
The maximum allowed waviness of a new Berndorf belt (according to Berndorf - Standard Specification) can be found in following table:
for the welding area is valid:
3.2 Cross Curvature: The cross curvature describes the size and the direction of the flatness of a belt in the cross direction. The direction can be a concave ( troughing shape ) , a convex ( roof shape ) , or a mixed one. It is influenced by the internal stress condition of the belt and by the geometry of the supporting .
The reason for the increase of the cross curvature of a belt in comparison to the original condition when delivered , are mainly changes of the internal stress. This is caused mainly by roll and friction stress in the customers machine or by scratches in the belt surface. By this , compression stresses are induced into the belt surface which change the cross curvature condition.
At the measuring position the belt must show a free sag and should be between two adjacent rollers
It must be checked prior to the measurement if the cross curvature is uniform all over the length of the belt. If not the worst deformation area must be found , measured and protocolized.
The measuring ruler is placed parallel to the belt over the whole width of it and the weight of the ruler should not influence the measuring result by deforming the belt shape.
Every 100 mm , over the whole width , starting at one belt edge , the distance between the ruler and the belt surface will be taken ( measured ) and these values are noted in the appropriate column of the measuring protocol.
If the measurement of the cross curvature is to be done within the scope of a repair ( shot peening or flattening the belt ) it is necessary to measure the deformed belt shape before and after the repair .
Please find below a correct filled out cross – curvature protocol.
3.3. Straight Running Deviation: The straight running deviation describes the amount of the sideward movement of a belt edge of a well – tracked belt during one revolution in relation to a fix point of the press construction ( or drum edge )
Mark the measuring points on the belt in an interval of 10 meters over the whole length of the belt , starting with the cross welding seam.
Take a fix point ( preferable e.g. the edge of the drum where you are positioned or a point at the frame of the press ) and measure the distance of the belt edges to that fix point.
The difference between the minimum and the maximum value is the :
Maximum straight running deviation : = ( 2 / 10.000 ) x L ( = belt length )
And should not exceed 20 mm ( Berndorf Band Standard Specification )