There are five different types of chair pads:
- Full floating
- Welded (fixed)
- Rim – mounted on shell
- Splined, or tangential suspension
In the full floating type design the chair pad is not attached to the shell tube. Why this is important? – because the thermal expansion rate is different than shell. In this case the different designs (welded) creates more operation problems and failures.
The full floating design has two metal rings (called retaining rings) located both sides of tire, providing much better operation contact then designs using only stop-blocks. The effect of that solution – mechanical wear is reduced. The retaining rings are locked to the shell so the contact surfaces is much bigger than the solution with only keeper installed.
The advantage of this kind of design is renovation works as well. Chair pads can be removed easily because they are not welded and attached at all. Exchanged of shimming take place without cutting. The floating type, due to more parts and longer installation time, is more expensive. But overall if we consider its service life and failure probability is the most cost-saving solution.
We distinguish 3 types of full floating type designs:
- Simple full floating design – chair pad, welded shim keeper, tire pad keeper, side keepers.
- Full floating type with retaining rings – additional retaining rings.
- Full floating with extra high stop-block – used when the tire has been severely undercut.
The semi-trapped design consists of fewer parts then mention above full floating solution, so it is a little less expensive. Each chair pad is welded at one side. This construction still allow to expand the component, but the potential probability of weld crack is higher.
The welded design was used very often in old installation, but still we can find in many rotary kilns. The main problem with these solution – the welds cracks very often. These design is not used on new installations and generally replaced over time to floating type.
During each inspection of char pads of any type we have to check any crack, loose welds or broken edges. Very often even the smallest problem with the chair pads can lead to big failure of kiln operation especially kiln shell.
The most advanced chair pad component is splined design (patented by Polysius in 1969). This solution is composed with: x-blocks, axial blocks, flexible bridging strip, tensioning device, self-adjusting key and internal tire spline.
Simply in these case the load of rotary kiln is transmitted to retaining blocks (welded and x-shaped), floating tire shoes, wedges and tire splines. The force is distributed on foundation, tires, rollers and bearings.
Due to construction the wedges are pushed between bridging strips by springs to reduce the clearance and stress. The axial securing means absorb the force, adjusting of the rollers and the axial movement of the kiln. The securing means bear on the face of the tire splines. The other ends are welded to the kiln shell. The tire shoes are secured (axially) by stoppers to the end faces.
The advantages of splined design are:
- Ovality of kiln shell is reduced
- Very low thermal stress
- Equal support of each fixing unit, even when the shell temperatures are different.
- Division of axial and bearing load.
- Good torque transition from the tire to the shell.