Linear Guides and Bearings

The main elements of a processing or measuring machine are linear guidance of the movements of the feed and work units and bearings of the main spindles and rotary axes.

The following requirements are of special interest:

high accuracy combined with low wear over the total life of the machine
low procurement and maintenance costs

To achieve this, certain properties are required:

good tribological properties and without stick-slip for accurate positioning and compact feed drives
low wear and good dry running properties in order to maintain a long-term accuracy
high static and dynamic stiffness and pre-stressed elements to minimize changes in the position of the guided components under load
good damping properties in feed and load direction in order to avoid vibrations resulting from the feed drives and the processing

To meet these requirements, different principles and kinds of guidances are used:

electromagnetic,
hydrodynamic,
hydrostatic,
aerodynamic,
aerostatic,
Guideways and bearings.

Electromagnetic Guideways and Bearings (Magnetic Bearings)

To bear components contact free, stable, and with high precision, fascinated and occupied us for many years. In the first years especially applications on technical special areas or in research have been implemented. These types of bearings for high speed gas centrifuge have been used quite early in uranium enrichment. A practical, well-known example are also floating vehicles like the Transrapid developed by Siemens and ThyssenKrupp.

Because of their useful properties active magnetic bearings are used for rotors:

without grease,
reduction of the mechanical effort,
without wearout,
unbalance compensation,
improved operating characteristics of the machines, reduced costs for maintenance, and longer life,
low energy consumption (reduction of lost power in the bearing),
high scale speeds.

The implementation of these technological advantages in a wide range of applications and system components is not yet complete.

Hydrodynamic Slideway and Journal Bearings

In earlier times the hydrodynamic sliding guide was the most widely used type of guidances in the machine tool industry. An important reason were the low manufacturing expenses in comparison to other guidance principles. Inititally metal bearings were used; the development of sliding coatings made of plastics such as Turcite®, moglice®, or PTFE / epoxy compound sliding coatings by Kuhn-Reaktorharztechnik has contributed to run-flat properties and tribological properties. In the meantime main spindles with hydrodynamic journal bearings are found only rarely. In horizontal machines for finishing one may occasionally find some hydrodynamic spindle bearing systems.

Hydrostatic Slideways and Roller Bearings

The hydrostatic bearing is a further development of the hydrodynamic bearing. The hydrodynamic effect on the production of the supporting pressure in the lubrication gap has been replaced here by the hydrostatic principle, though. Hydrostatic bearings have significant advantages over the well known rolling bearings and hydrodynamic bearings, because the pressure for separating the sliding planes is generated outside the bearing / the guidance with a pump and because the flow way can be controlled and regulated in a way that the oil pressure and height keep constant in all operating conditions.

The advantages of the hydrostatic lubrication principle have long been known, but only the current state-of-the-art technology has brought further development of this lubrication technique:

The continued improvement of the accuracy and efficiency of machinery and equipment and not at least the now reached operational safety and cleanliness of hydraulic equipment were determining for that.

The operations in the lubrication gap of the hydrostatic bearings and guides under various conditions have been studied quite thoroughly in the last few years. Based on these results, the computational optimization of certain properties is quite possible.

In detail the following advantageous features were found:

Great static stiffness, which is in full effect from the slightest load on and independent of speed and feed rate, so one has complete gear backlash.
Great dynamic stiffness as a result of the great damping.
Starting and stopping friction are missing, so that operation is possible at the lowest speeds and oscillating movement.
Little friction, which in the pocket recess takes almost negligible friction values.
Great permissible sliding speeds, which allow great bearing loads even at high speeds, allowing sufficiently large diameter bearings.
No wearout of sliding surfaces, the accuracy remains permanently.
Low heat, good ways of cooling, and with that no high temperature-deformation of machines.
Great movement accuracy, reduced impact of manufacturing errors, contour accuracy and surface quality of the sliding surfaces.
No running noises.

For precise small and medium-sized, but especially large machines, the hydrostatic principle (just because no scraped sliding surfaces are necessary) led to significant cost savings and quality improvements.

That on the other hand the hydrodynamic bearings work without pump unit, this is because it itself contains, in principle, a pump. The bearing lubrication hydrodynamic pressure is built in the wedge-shaped gap between the sliding surfaces, when the lubricant is carried away by the sliding movement due to its adhesion and viscosity and pushed into the narrowing gap. This is resulted by a residual lubricating gap in which there is a balance between load and pressure lubrication. Such a pump should be referred to as a viscosity pump, because its mode of action is based on the viscosity of the oil. In similar energy considerations between hydrostatic and hydrodynamic bearings the assumption is made that all energy losses are resulting from the sum of the bearing pump bearing friction losses plus internal losses:

It is easy to see that hydrodynamic bearings, which so-called viscosity pumps work at a maximum of 33 % efficiency, have considerably higher losses of enery with increasing sliding speed. The hydrodynamic bearing is bound to a very unfavorable „pump System” (bearing cones) in order to produce pressure, while for the hydrostatic bearings one can choose the best pump systems.

In hydrodynamic bearings, the pressure generation also depends on factors that cannot always be controlled; starting and stopping are always critical phases in the operation of such bearings.

Aerodynamic Bearings

An example for aerodynamic bearings are conical spiral groove air bearings. The built of pressure is achieved by wedge-bearing segments. Today, more frequently applied aerodynamic bearings are designed as spring bearing. The damping behavior of such bearings, however, is critical.

Another principle is the spiral bearing. They can often be found in in hard drives and CD players. In machine tools aerodynamic spiral bearings are used in very high speed spindle systems for special applications.

Aerostatic Linear Guides and Bearings

Bearings and linear guides that work with gas lubrication work on the same functional principle as those with liquid lubrication. They differ mainly by the properties of the used lubricant. As an active medium for gas lubricated bearings usually air is used. They are therefore often referred to as air bearings. Compared with oil air has got a lower by 2 to 3 orders of magnitude viscosity. The viscosity of air is almost independent of temperature. The low viscosity of air requires a very narrow bearing gap to obtain high stiffness and load carrying capacity. Due to the small shear the friction is very little. Although at very high relative velocities a warming of the bearing system can not be excluded.

Typical pressure is 4, at maximum 10 * 10⁵ Pa. This air bearings' with normal pressing technical limits are set regarding load capacity and stiffness. Aerostatic bearing systems are used in high precision machines. They are also used in spindles for high speed grinding (HSG) and boring machines.