Skewing

Skewing, also known as belt flipping, is a common phenomenon in the tube mill forming machine’s roll process. Due to various reasons, skewing can occur at any time during the operation. It is specifically manifested when the pipe blank comes out from the horizontal or vertical rolls with uneven edges. In severe cases, the pipe blank may flip over, preventing it from smoothly entering the next pass, leading to a forced shutdown for adjustment. This directly impacts the efficiency of production operations.

Causes of Off-Center Skewing in Horizontal Rolls

The primary reasons for off-center skewing in the horizontal rolls are as follows (excluding raw material defects such as “scythe bending”):

1. Misalignment of the Roll Pass Center

When the roll pass center is misaligned, the pipe blank will deviate from the centerline during the rolling process, resulting in skewing. If the deformation angle of the roll pass is greater than 90°, when both the upper and lower roll pass centers are misaligned, the pipe blank will flip towards the center of the roll pass (as shown in Figure 8(a)). When the lower roll pass center is misaligned, the pipe blank will also flip towards the roll pass center (as shown in Figure 8(b)). Conversely, when the upper roll pass center is misaligned, the pipe blank will flip in the opposite direction (as shown in Figure 8(c)). If the deformation angle is less than 90°, except for the overall offset of the upper and lower pass, the skewing direction will be opposite to that of the deformation above 90° (as shown in Figure 9). This skewing is typically caused by axial displacement of the rolls, resulting in misalignment of the roll pass center and leading to the pipe blank skewing. In the adjustment process, the assembly structure of the rolls should be checked for any locking failures or looseness in the positioning device and shafts, and timely adjustments should be made before resuming production.

2. Uneven Pressure on the Upper Roll

When there is uneven pressure on the upper roll, it causes the upper horizontal roll to tilt and skew, resulting in uneven gaps between the upper and lower roll passes. In this case, the pipe blank will skew towards the side with the larger gap. If the deformation angle is less than 90°, the pipe blank will flip towards the side with less pressure (as shown in Figure 10(a)); if the deformation angle is greater than 90°, the pipe blank will flip towards the side with more pressure (as shown in Figure 10(b)). In closed roll passes, the pipe blank will rotate towards the side with less pressure, in order to control the seam direction. To resolve this, adjusting the reduction of the horizontal rolls and maintaining the level position of the upper roll, while assisting with the preceding vertical rolls for directional adjustment, is recommended.

3. Bearing Damage

Both the upper and lower rolls are equipped with bearings, and bearing damage will cause changes in the reduction amount, leading to skewing of the pipe blank. The skewing caused by bearing failure is identical to that caused by uneven pressure. In production, abnormal bearing rotation sounds should be checked, and localized heat should be felt to assess the degree of damage. Timely bearing replacement can prevent unnecessary adjustments.