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Sprung arrangement

Similarly to the fixed-floating bearing arrangement, the sprung mounting arrangement can be subdivided into a fixed bearing on the tool mount and a floating bearing on the rear end of the spindle. However, the fixed bearing cannot be moved without force, in fact it is generally definitively pre-tensioned by means of a spring arrangement on the outer ring.  This allows changes in length due to the shaft heating up and the kinematic relative displacement to largely take place without any loss in the pre-tensioning force. The moveable outer ring of the floating bearing is mounted using a so-called floating bearing bushing. The axial displacement is frequently assured here through a ball bearing bushing (linear bearing) or a slide bearing.

In the case of hydrodynamic slide bearings, the lubricant is fed, without pressure or with very low pressure, to the contact point between the shaft and the bearing shell. The necessary lubrication film builds up autonomously by receiving the oil during the relative movement of the shaft and the bearing shell. The main application area of hydrodynamic slide bearings is in finishing machines (lathes and grinding machines) and in heavy machine tool construction (presses).

Hydrostatic and aerostatic slide bearings maintain the lubricating film (oil, air) through an external pressure system.
With hydrostatic slide bearings, the shaft and the bearing shell are separated from one another by a permanent film of oil. In contrast to the situation with hydrodynamic slide bearings, the height of the lubricating oil film is independent of the slide speed.      Furthermore, hydrostatic slide bearings always exhibit purely fluid friction and no start-up friction, and for this reason are wear-free. There is no stick-slip, even at low speeds.  Particularly with heavy machine tools and in special machinery construction, main spindles exhibit hydrostatic mounting in many cases.

Aerostatic bearings work on the same functional principle as hydrostatic bearings.    They differ primarily in terms of the characteristics of the lubricant.  Air has a lower viscosity than oil by two to three orders of magnitude, and is hardly affected by the temperature. High load-bearing capacity and stiffness are achieved through a small bearing gap. Aerostatic bearings exhibit very low friction, practically no wear and low noise development.  The disadvantages of this type of mounting are increased manufacturing costs and poor dry-running characteristics.    The application areas of aerostatic bearings are primarily in high-precision machines as well as in machines with high-speed drilling and grinding spindles.

The electromagnetic principle is currently still very rarely encountered in machine tool construction. Electromagnetic bearing systems are well suited to very high rotational speeds thanks to their relatively large air gap and the resultant low friction moment transfer.  They are occasionally used in high-speed machining. Alongside the electromagnetic bearings, additional safety bearings are also provided which are intended to prevent destruction in the event of a failure of the magnetic bearing, a power failure or an overload. Electromagnetic bearings are used in individual situations for special tasks, because their ability to reach higher speeds must be offset against increased costs, regulatory burden and energy expenditure.

 

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