On-machine grinding has proven itself a cost- and time-efficient alternative to dryer cylinder replacement and off-machine grinding. Companies that provide quality on-machine grinding service can refinish and renew cylinders for 40 to 50 percent less than the overall cost of off-machine grinding or even replacement. For many mills, time is an even more crucial factor. On-machine grinding can be achieved in less than 50 percent of the time it takes to grind off-machine or replace a cylinder.
On-machine grinding follows a straightforward three-step process:
In the first step, the entire dryer section and its performance are evaluated to determine whether grinding or another more appropriate solution is required.
During the evaluation, the wall thickness of each dryer cylinder to be reground is checked against its ASME coding removal allowances. Most dryer cylinders or cans have been designed with a shell thickness that permits substantial radial removal. This means dryer cylinders can be refinished several times before replacement is necessary and still maintain the ASME pressure vessel coding.
The drive train must also be inspected because incremental changes in the diameter of a number of dryer cans may alter the draw of the sheet. In some cases, where dryers may have been taken off the drive train because of their poor performance, a regrind may allow a "coasting" can to be reattached to the drive train. This improves the performance of the dryer section and ultimately the entire machine.
Because poorly reset doctors can cause shell damage, they also need to be inspected. Material buildup behind doctor blades and doctor chatter add to the problem. In addition, poor doctoring practices may complicate existing cylinder damage and make it worse.
If on-machine grinding is necessary, preliminary data acquired on cylinders and doctors is used to create a computer simulation with a CAD program. The dryer is recreated digitally, and the grinder is tailored to the specific machine. This simulation ensures the entire process runs smoothly and indicates whether special modifications may be necessary.
The CAD simulation is also used to develop an appropriate timetable and to assist in the scheduling process.
An efficient timetable and accurate scheduling are important because, for many mills, downtime is a greater concern than actual cost since production is lost. Although on-machine grinding takes a little longer than an average downtime, it takes less than half the time of off-machine grinding, logistics are much simpler and fewer people are necessary. In addition, installing new cylinders takes 7 to 10 hours longer per can than on-machine grinding.
The entire regrind process takes 28 to 55 hours, but the actual grinding is much less than that. However, the severity of cylinder damage or special conditions can make the process shorter or longer. Thorough record keeping by the mill saves valuable time at each step of the grinding process.
On-machine grinding can be effectively used to correct dryer cylinder damage or remove organic buildup. Buildup and cylinder damage can cause fluttering, wrinkling, creasing, quality sheet errors and sheet breaks.
Because varying sizes and specifications of dryers exist, the on-machine grinder is custom assembled to each paper machine. A skilled grinder crew can refinish dryer cylinders to OEM tolerances, regardless of which company supplied the cylinders, and provide reliable technical support. They can achieve cylinder tolerancing, or surface accuracy, as low as .005" across a cylinder's linear length and .003" runout anywhere on the shell.
On-machine grinding creates an efficient, smooth finish, which improves heat retention. Ample heat transfer makes the drying process more uniform with less variance and greater heat retention. This allows higher production speeds. On-machine grinding can polish any surface down to 13 RA, which is well below the recommendation for dryers.
On-machine grinding consists of a free-floating grinder installed on the machine between the cans in the dryer section, and it is operated remotely with PLC controls. This capability eliminates the need to remove cylinders or dismantle the dryer section. Up to three cylinders can be ground from a single mounting position.
The grinder is supported by a roller that rests on a dryer cylinder, and it is held in position by rollers in contact with adjacent cylinders. The grind is controlled in three dimensions by calculating pressure load variations against the shell surface using sophisticated sensors and a computer processor. Grinding heads follow the contour of the cylinder to remove surface contaminants and tar buildup without removing any metal that would change the shell dimension. A powerful extractor system, attached to the grinder, minimizes cleanup. While grinding, the system contains and recovers more than 90 percent of the debris and separates metal and contaminants into different containers.
On-machine grinding offers papermakers a great new tool that lowers cost, reduces downtime and allows cylinders to last longer before being replaced.
For more information about on-machine grinding, contact your Valmet representative.