The precision of the fit between the network of machine components is critical to the success of a papermaking operation. Nowhere is this more clearly illustrated than at the head fit – the interface between the head and shell. Head fits are one of the most misunderstood concepts for a successful roll repair program. Two thousandths of an inch (.050 mm) could be the difference between a good press fit and a loose fit.
Many years ago, most plain press and suction rolls had straight head and shell fits and relied on an interference fit. This meant that the head was larger than the mating shell.
This interference, along with the head bolts and studs, provided the mechanical means that held the head and shell together. Over time, as these rolls are disassembled for inspection and maintenance repairs, the contact between the head and shell can wear and the head and/or shell may need to be rebuilt to restore the proper fit.
For several years now, most suction and many controlled crown (CC) rolls have been equipped with tapered head fits. Many tapered head fits have a .375 inch (9.5 mm) per foot taper on diameter cut into the shell and a duplicate taper cut into the mating head. Because the contact between the head and the shell wears over time, shim packs for the tapered head fits were developed for suction rolls to maintain a proper head fit. As the head and shell wear, shims can be removed to increase the fit.
A proper, tight head fit between the head and shell maintains roll integrity and immobilizes the roll core as the shell rotates around the core at high speed. If the fit wears beyond acceptable tolerance, it has a detrimental effect on paper quality by causing uneven loading and unnecessary roll cover wear. Avoiding the pitfalls associated with poor head fit is accomplished through regular inspections.
Most suction rolls are disassembled every six months or once a year, while CC rolls are disassembled less frequently. Each time a roll with an interference fit is disassembled, the shell and head fits should be thoroughly inspected. These maintenance inspections provide insurance against damage caused by poor head fits.
Unfortunately, because there are few external indications of a poor head fit, the roll must be disassembled to inspect the interface. Proper inspections can detect and correct poor head fits long before external signs appear or damage occurs. A head fit inspection should include the following steps:
Even with proficient maintenance practices, it may be necessary to rebuild the head or shell. Eventually these components wear to the point that maintenance becomes insufficient. This happens when the maximum number of shims that allow a tight fit have been removed.
At this point, material must be added to the head to restore its original dimensions. However, because both the head and shell may wear, it may be necessary to rebuild the head larger than its original specifications. (It is easier to add material to the head than the shell.)
Three repair methods can restore the head and shell: plating, spraying and installing sleeves.
Many mills install a sleeve to restore the necessary fit between the head and shell. However, because a sleeve can weaken the head, roll specialists recommend plating first, metal spraying second and installing sleeves third. Because sleeves are the last recourse, it is recommended that a new head be ordered once a sleeve is installed.
These three repair methods offer quick, affordable solutions to prolong head and roll life. Good roll performance will continue if maintenance inspections and appropriate repairs are made to preserve head fit.
For more information about getting the correct head fit, contact your Valmet representative.