M.S. Bundur, V.A. Prokopenko, P.P. Petkov, N.A. Pelevin

The St.-Petersburg State Polytechnical University

THE RESERCH OF CONTROL SUSTEMS FOR HYDROSTATIC BEARINGS OF THE MODERN MACHINE-TOOLS

Using of hydrostatic bearings (HSB) allows almost completely eliminates the wear of pads of a spindle. Loading ability, a damping, accuracy, vibration resistance essentially are increased, a friction losses are decreased, thermostability of machine-tools are essentially increased.

Among the major variety of control systems for hydrostatical pads (HSP) it is possible to pick out following basic groups:

· with the total pump and various control throttles before everyone HSP;

· with separated pumps before everyone HSP.

Different possible alternatives of the circuit design of control HSP in the HSB of spindle unit (SU) are observed in discussion:

Alternative 1: in a front (FP) and a back (BP) pads - two throttles at each co-ordinate axis (it is used in SU of machine-tool LR520PMF-4);

Alternative 2: in FP and BP SU are used two separate pumps;

alternative 2, a - pumps of the equal flow rate; alternative 2, b - the pump flow rate Q₂ is twice more than charge of the pump flow rate Q₁; alternative 2, c - the pump flow rate Q₂ is twice less than pump flow rate Q₁;

Alternative 3: in FP and BP SU are used one throttle and the pump.

According to constructive sizes of machine-tool LR520PMF-4 the technological overhang of spindle is l_w=0…1000 mm. That is why all further calculations was executed for two cases: l_w=(1/3)·1000=330 mm.

At a reversion of external load the operating point of static characteristic of HSP differ for loading and unloading its branches. Therefore for definition of admitted bearing ability of SU it is necessary to make separate calculation for loading and unloading branches of the static characteristic.

For realization of the greatest productivity of treatment it is necessary to compute the maximum value of external loading R_Zmax at edge HSP displacement δ_∑ not more than 50 % of a nominal gap h. It is determined that for the static characteristic this R_Zmax = 9500 kN.

The comparative estimation of alternatives is made using of static characteristics h=f(p) and loading characteristic R=f(p) which are resulted at fig. 1, a, b (where p – pressure at HSP).

a) b)

Figure 1: Static characteristics: a – h=f(p) for various alternatives of circuit designs, b – R=f(p) for the same alternatives

Under the greatest loading for alternative 1 the static displacement is 4 µm. The static displacement at same case of loading for alternative 2, а is also 4 µm, but pressure in HSB is decreased more than 2 times. At increase of productivity of one for pumps 2 times (alternative 2, b) the static displacement relative to alternative 1 is decreased more than 30 % (is 2,7 µm). At decrease of productivity 2 times (alternative 2, c) the static displacement is increased 30 % (is 5,2 µm). For alternative 3 the static displacement under above mentioned loading is 5 µm. Thus in spite of the fact that alternative 3 provides the least effect of raise of stiffness and loading capacity, it is the most preferable to replacement of base alternative, because pressure and consequently the flow rate of working fluid, significantly are decreased. That is simplifies maintenance of SU. The throttle permits to realize adjustment of initial parameters of HSB that is unattainable for all alternatives of modifications of the pump-pocket.

The comparative analysis of behaviour hydrostatical pads in dynamics is made on the basis of the analysis logarithmic amplitude-frequency and phase-frequency characteristics of model. Key parameter of an estimation of dynamic quality is the phase stock (Δφ) at frequency of a shearing (ω_c). It`s estimated that all alternatives have Δφ = 14,8º – 18,16º and it`s not enough for high quality of dynamic systems. Research of possibilities of increase of dynamic quality at the expense of conversion to asymmetrical scheme automatic control system (ACS) by radial displacement of an axis of a spindle without using of correcting RC-chains. The characteristics of dynamic quality for concentric spindle positions (δ=0) and for various displacements of an axis of a spindle (δ≠0) were calculated for all considered alternatives.

For all alternatives Δφ are more low of minimum recommended value Δφ=40º. The increase of displacement provides rising of Δφ. The nearest to minimum admissible phase stock are alternative 2, c (Δφ = 36,2º) and alternative 3 (Δφ = 34,1º) for displacement δ=15 µm (2,5 times exceeding a matching parameter of serial machine-tool).

Transient process for alternative 2, c was obtained (Fig. 2). Curve 1 for initial alternative has oscillations and consequently dynamic quality is low. In case of radial displacement of the spindle (curve 2) oscillation process is particularly eliminated. The best results gives introduction of mentioned above RC-chains simultaneously with the displacement, δ (curve 3). This case provides absence of oscillations.

Figure 2: Transient process for alternative 2, c

In case of necessity of essential raise of dynamic quality of ACS of HSB may be recommended introduction of either radial displacement of spindle or RC-chain correction of HSB or else both these means.