Thirty years ago, Grundfos in Denmark was a pioneer in this field by using alumina ceramic shafts in their central heating circulator pumps. Since that time, Grundfos alone has built around 30 million pumps using ceramic shafts and bearings, most of which are still running today - so we can say quite safely that these materials work well in this application!
The parts in the picture are pump shafts made of 95% pure alumina which Aegis supply for use in small pumps. The applications for these pumps varies from chemical handling to heating water circulation, garden pond and fountain pumping and even for use in aquariums. Shown in the picture are shafts centreless ground to 3.0, 4.0 and 8.0mm diameter, some having anti-rotation features and some without. Sometimes the shafts run in ceramic bearings of the same grade and sometimes they run in small plastic bearings or Glacier-type composite bearings with a non-metallic contact surface. Other grades of alumina can also be supplied, but for most purposes the high strength and low cost of the pink 95% grade make it the first choice.
How do ceramic shafts and bearings work?
The reason that ceramic shafts working against ceramic bearings are so good when working under water or process-fluid lubricated conditions, is not because the ceramics are inherently "good", i.e. low-friction, bearing materials - they are not! If the pumps run dry, the bearings start to squeal and eventually seize up or otherwise damage themselves. For wet running, they work well because the ceramic bearing and shaft are very hard indeed and so are able to grind up and disperse foreign materials such as limescale, rust or sand particles which happen to get into the bearing area. For the best running conditions, quietness of bearing and longevity, the alignment, surface finish, roundness and tolerances of the ceramic components have to be to a very high standard. In such a case, the bearings function hydrodynamically, with a thin film of process liquid separating the rotating elements. However, for applications like small garden fountain pumps, the materials work quite well enough with just unground surfaces on the inside of the bearings. No hydrodynamic action is present but the boundary lubrication of the surfaces is enough to give a relatively long life and a low enough friction level.
What are the properties of alumina ceramic shafts?
As already mentioned, alumina ceramics are well known for being extremely hard. A hardened steel file is about 700 HV. A tungsten carbide drill tip is about 1,400 HV. 95% pure Alumina ceramic is about 1,600 HV and the individual crystals within it, if measured on a microhardness tester, would approach the figure for sapphire, which is 2,100 HV.
Alumina ceramics are also immensely strong - if you use them in compression. "High tensile" steel has a yield strength in compression somewhat over 1,000 MPa. Alumina ceramic has a compressive strength of over 2,000 MPa! Unfortunately for us, it's difficult to design a shaft to work purely in compression, so the more useful parameter to consider is the flexural strength of the material. The flexural strength of 95% alumina is approximately 330 MPa, which compares with brass (300 MPa) or mild steel (385 MPa) - so it is to be considered as a medium strength material.
Ceramics are very stiff. The Young's Modulus for steel is 200 GPa, while for alumina ceramic it is 350 GPa. This is both an advantage and a disadvantage of ceramics. While it means that elastic bending of the shaft will be very small in service, allowing a very precise pump to be built, it also means that if misalignment or bow is forced into the shaft, perhaps by lack of precision in other components, the resulting stress in a shaft strained to a pre-determined degree will be much greater, perhaps leading to failure.
However, the most noticeable difference between ceramics and metals is in what happens just as they fail. Metals fail in a ductile fashion, bending, creasing, stretching or squashing. Ceramics do not. They just break. The impact resistance of ceramics is well known to be quite low, so it is important that shafts are designed to be protected in the case of a pump being dropped onto a hard surface, or else are specified over-size so that they can withstand such an impact.
How are ceramic shafts made?
The ceramic process begins with production of the chemically and physically correct powder. The last production step is diamond grinding (and possibly polishing) to reach the desired tolerance, geometry and surface finish. In between these stages lie the forming and sintering processes which turn a fine white powder into a sapphire-hard near-nett-shape sintered product. In the case of small solid or tubular shafts of 6mm diameter and less, the usual forming process is Extrusion.
For extrusion, the powder is mixed with a small quantity of water and suitable organic binders and is then extruded through a tungsten carbide die using either a de-airing screw extruder or else a batch-type piston extruder. This process is done at room temperature and the extruded ceramic is dough-like and very fragile while damp. After drying, it becomes rigid and has properties similar to school-room chalk; it can be turned, milled, drilled or threaded using tungsten carbide or diamond tipped tools, and can be form-ground using an abrasive wheel.
For larger diameter shafts, or those with steps on the inside, it is more desirable to use Isostatic Pressing as a forming process. This process uses dry powder at room temperature and presses this into a cylindrical shape using hydraulic pressure. The powder (with suitable organic binders) is fed into a cylindrical rubber bag, which may also contain a steel mandrel if the shaft is to be tubular or with a stepped or blind hole. The bag is then sealed and pressurised in a hydraulic fluid so that the powder is compacted inside it, and onto the mandrel if one is present. After de-pressurisation, the rubber bag returns to its normal shape and the compact can be removed. For large scale production, automatic isostatic presses are used which can turn out over 1,000 compacts per hour. The compact is usually relatively misshapen, especially near the ends, so it is then green-machined by turning or form-ginding, sometimes after a pre-firing operation to strengthen the part.
Once shaped to a near-nett shape, it is Sintered at a temperature of around 1,600°C for a furnace cycle time of around 36 hours, and during this sintering the material densifies by around 60%, shrinking by about 20% in each dimension, and taking on its final mechanical and chemical properties. During the sintering process the material becomes quite plastic, and this means that shafts tend to bend during firing unless special techniques are used to keep them straight.
The sintered shafts now have to go through at least one Diamond Grinding process to reach the desired degree of precision. Usually this is done on a centreless through-feed machine with a diamond wheel which is as long as possible. Several passes through the machine are required, since each pass only can take off a relatively small amount of material. Diameter tolerances at this stage may be as coarse as ±0.02mm for light duty products, or may be as tight as 5 microns for the best performance. Superfinishing of the shaft may be called for as a final process to improve the surface quality to a polished finish and to improve the roundness of the shaft. The material is capable of giving surface finishes as low as 0.05 microns Ra, though normally 0.2 microns Ra would be the finest finish necessary.
How much do ceramic shafts cost?
Alumina powder is a relatively inexpensive material, but the long list of production processes necessary to make a finished shaft indicate that the cost of the raw material is not a good indication of the cost of the final product. The price of finished ceramic shafts can be anywhere between £0.30 for a small, ground-only shaft produced in quantities of 50,000, to several £ for a larger high precision shaft made in smaller volumes.
The best value for money will be found by:
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