Polytetrafluoroethylene (PTFE) shaft seal bellows with integrally sealed seams have solved a significant problem in mills that blend paint ingredients at Rust-Oleum Corp.s plant in Pleasant Prairie, Wis. The bellows, which now have been in service for more than three years, stand up to aromatic solvents and conditions that caused previous seals to fail within days.
At the plant, two sizes of mills are used to blend various ingredients for solvent-based paint products. In the mills, either one or two shafts extend down through the housing into a sealed chamber where they drive a mixing impeller. Depending upon the viscosity of the material, the shafts are moved up or down to change the location of the vortex in the ingredients and achieve homogeneity. Temperatures typically average 140° to 160°F and the bellows area is permeated by a steamy, solvent-laden vapor, says Robert Johnson, the plants engineering and maintenance manager. To contain the solvent fumes, the mixing chambers are kept at a slight positive pressure of about 2 in. of water and a sealed system with an inert nitrogen atmosphere is maintained. The mills also incorporate a control system that shuts them down if the oxygen level rises above 3%; sealing the shafts is critical to maintaining the inert atmosphere.
The plant generally operates 24 hours per day, five days a week, but six days a week during the busy season. Were running them [the mixers] an average of 16 to 18 hours a day, explains Johnson, who now has retired.
A tough task
Each shaft is sealed where it enters the housing with a bellows that can accommodate the travel of the shaft as its moved during the process. The corrugated bellows originally supplied with the mixers only lasted a short time because aromatic solvents such as xylene softened the elastomer-coated material, causing the bellows to collapse. Replacement bellows made by stitching together rings of elastomer-coated nylon fabric fared no better the solvents dissolved the stitches and were absorbed into the material. We would see weeping right away and within a couple of shifts we were babysitting them on every batch, says Johnson.
As a bellows failed, it would get soft and collapse, so we had to pull it back up and avoid raising or lowering the mill. Then it took more time to make the paint because we couldnt set the right height for the viscosity of the batch. Even more time was lost because replacing the bellows required disassembling the mill. The impact on our production and production scheduling was significant, Johnson notes.
To solve the problem, the company worked with A&A Manufacturing Co., New Berlin, Wis., a specialist in the design and manufacture of protective bellows. Samples it supplied were tested by Johnson to determine their resistance to typical solvents used. This led to selection of bellows made of PTFE material using a specialized version of the Gortite Vulca-Seal process. Developed by A&A, its designed specifically for demanding applications where the seams inherent to a sewn bellow are unacceptable. In the process, the sections are cut separately and then sealed using heat and pressure, resulting in a seamless cover.
The mills use two sizes of bellows. The larger size, for twin-shaft machines, has a 6.75-in. inner diameter (ID) and 12-in. outer diameter (OD), with an extended length of 44 in. and a retracted length of 2 in. The smaller size, for single-shaft machines, measures 12 in. ID × 18 in. OD, with an extended length of 22 in. and a retracted length of 1 in.
While the price of the PTFE bellows is four to five times that of the previously used seal, eliminating the expense of replacement bellows and avoiding maintenance time and lost production make the change very cost-effective.
Steve Piacsek is R&D Manager for A&A Manufacturing Co., New Berlin, Wis. E-mail him at email@example.com.