Yankee creping problems and too many regrinds, a case study

The symptoms

An operator was watching the serpentine path of No. 5's sheet as it wound 350 degrees around the Yankee cylinder on its way to the doctor blades. As the sheet reached its destination, her suspicions were confirmed - the sheet wasn't doctoring off. It'd been a familiar sight on No. 5 for a few days, not to mention a costly problem that had led to reoccurring downtime. What began as an innocent creping problem typically ended with a complete machine shutdown.

Oddly enough, the past year had seen No. 5 receive an alarming number of regrinds. After each regrind, the doctor blades were carefully reset. Upon start-up, the Yankee cylinder would appear to function properly. But once it reached its operating temperature and pressure, the Yankee no longer squared with the doctor and the creping problems would resurface.

What could be causing the frequent regrinds, and why wouldn't the doctors remain squared with the Yankee?

The analysis

Recognizing the unusually high number of regrinds as a symptom of a larger problem, the mill maintenance manager decided it was time to call in a Yankee specialist from the OEM.

Upon visiting the mill, the OEM specialist asked to inspect the Yankee for movement on the front side with the pressure rolls loaded. To do so, he placed a dial indicator on the Yankee. A normal reading would find very little side-to-side movement. The Yankee was found to be out of square by an average of more than .200 inches. This explained the creping doctor's poor performance, but what was causing it to go out of square in the first place?

The cause of the creping problem was buried in a stack of service reports. The OEM specialist discovered that after previous regrinds, mill maintenance realigned the Yankee and adjusted the doctors while the machine was down. Once the Yankee was brought up to operating temperature and air pressure was loaded, the Yankee shifted on the front side.

And what was the cause for the regrinds? To compensate for the misalignment of the Yankee, the mill often increased the air pressure to square the doctors. The additional pressure accelerated wear on the Yankee surface and forced the mill to regrind every six to eight months instead of its normal schedule of three to four years.

The solution

The OEM's final recommendation called for the mill to duplicate the operating position of the Yankee to properly align the doctors. For future regrinds, it was recommended that the ladder housing bearing and rocker support plate be shimmed. The shims should remain in place not only when the Yankee is being aligned and the doctors are set, but also for the regrind. This procedure ensures the Yankee remains square and true during operation.

For more information on proper Yankee and doctor maintenance and setup, contact your Valmet representative.