The E1 vertical edger main drive shaft cracked repeatedly. The problem is not the shaft strength itself but insufficient speed drop compensation at the moment the horizontal rolls bite. Torque testing and simulation only confirm this judgment. When R1 bites, the horizontal rolls suddenly take the rolling force and their speed drops sharply. If the speed difference set between E1 and R1 cannot cover this drop, E1 keeps pushing the rolled material toward the horizontal rolls for a short period and carries a thrust torque far above the normal width-setting load. Cracking of the universal joint fork head is the result of this abnormal load acting repeatedly.
Torque test
Measuring points were set at each universal joint shaft, using strain gauge torque sensors and a wireless dynamic strain acquisition instrument for long-duration, multi-channel real-time recording. The acquisition instrument was powered by a lithium battery module, with an external wireless synchronization module. Data passed through a wireless synchronization communication controller and a switch to the computer, so multi-channel data stayed aligned on the time axis.
Vertical roll torque characteristics
Pass 1: the bite peak was not high and torque was fairly steady. After the horizontal rolls bit, vertical roll torque took a large sudden impact, then fell back close to zero, climbed to a lower torque plateau, and stayed there until tail-out.
Pass 3: the pattern was similar to pass 1, but a large reverse torque impact occurred at the moment of bite, and the vertical roll torque impact caused by horizontal roll biting was also larger.
Symptoms and causes of shaft failure
Vertical roll torque during width setting is normal in itself. The excessive load comes from the torque impact when the horizontal rolls bite. After R1 bites, vertical roll torque falls back while horizontal roll torque gradually rises. This shows the horizontal rolls are overcoming the initial bite speed drop and starting to pull the material. The problem is concentrated in the time window before the speed drop is overcome.
The main cause of shaft failure is that at the instant R1 bites, the speed difference set between E1 and R1 is not enough to compensate for the horizontal roll speed drop, so the vertical roll carries an abnormal thrust load during and shortly after the bite. There are two secondary factors. At low steel temperature, deformation resistance rises and the impact load at the same speed difference is larger. In pass 3 the side reduction is larger, and the braking effect of the vertical roll produces a larger reverse torque at bite, which adds to the forward impact.
Simulation analysis
A simulation model built from the measured torque data shows that maximum stress occurs at the stress concentration area of the section transition, which matches the actual fracture location. Maximum tensile stress in the flange fork and cross shaft both exceed the allowable stress but have not reached the ultimate strength of the material. This means the fracture is not a one-time tensile overload but fatigue cracking under alternating impact. The arc transition on the cross shaft is a typical fatigue weak point in this type of shaft. In engineering practice, design improvements at this area usually aim to reduce stress at the transition fillet, and existing cases have reduced equivalent stress there by about 13% by adjusting joint structure dimensions. Flange forks for heavy-duty universal joint shafts are often forged from 42CrMo and then quenched and tempered. Surface roughness and residual stress at the fork root fillet directly affect fatigue life.
Solutions
Increase the speed difference between R1 and E1 at bite, so E1 has enough buffer margin during the horizontal roll speed drop and does not push the material too hard toward the horizontal rolls in a short time. This is the main measure.
At the control logic level, a dynamic torque protection can be added. While the vertical roll is loaded and the horizontal roll is not yet loaded, switch the main motor from constant torque control to dynamic torque control. When rising vertical roll load torque is detected, reduce vertical roll speed appropriately to slow the rise rate of main drive torque. At the same time, monitor actual vertical roll rolling force. When it exceeds the preset overload rolling force, reduce the speed of both vertical and horizontal rolls together so the speed difference stays stable and the impact speed drop at horizontal roll bite is not amplified further.
Increase tapping temperature and set a minimum entry steel temperature, so impact strength in low-temperature, high-load passes is reduced at the source.
Reduce the side reduction in pass 3 appropriately to lower the reverse torque peak caused by vertical roll braking.
Set warning values for drive shaft torque and main drive motor current, and bring both into daily monitoring.
Implementation results
After the plant adjusted the control system and some process parameters according to the analysis report, the frequent fracture problem on the E1 main drive shaft was solved.
Universal joint fork heads and cross shafts are high-value wear parts in rolling mill drive systems. Hansheng Automation can machine non-standard drive parts such as flange forks, cross shafts, spline shaft sleeves and shaft connection flanges from customer drawings. Material can be alloy steel such as 42CrMo or 34CrNi3Mo, with heat treatment and surface treatment agreed on the drawing. Single units or small batches are supported.
Our precision parts machining capability is not limited to rolling mill drive parts. In addition to tobacco packaging machine parts and filling machine parts, we machine non-standard parts to drawings for printing machines, food machines and other equipment, and we accept small-batch orders for yacht precision parts and RC car precision parts. Hansheng Automation works within precision parts machining. Material, tolerance and surface treatment are agreed on the drawing, and parts are machined and delivered accordingly.
