HYDRAULIC INSTALLATIONS FOR HEAVY MACHINE-TOOLS

This paperwork presents the most frequent applications of the hydrostatic drives on heavy machine-tools and tries to modularize them. The focus is on CNC heavy machinetools that have been manufactured after year 2000. Hydraulic drives are used for heavy duty machine-tools when a high power density is required, especially for the auxiliary kinematic chains: locking, toolholder or workpiece clamping/unclamping, balancing of vertically moving loads, compensation of geometric errors etc.


INTRODUCTION
The machine-tools approached within this article are: heavy horizontal lathes (NL) and vertical lathes (VL), gantry milling machines (G), horizontal boring mills with or without quill (AF or AFP) and some special heavy machines (SMU).
At present, the hydraulic installations designed for these machines belong to some systems that are not involved directly in the driving of the generating kinematic chains [1], such as the main and the feed kinematic chains.The hydraulic drive which is characterized by a high density of power and by the capability to develop high forces seems to be at present irreplaceable in some of the specific applications.Some of these applications are going to be presented hereinafter.

CLASSIFICATION OF THE HYDRAULIC DRIVING SYSTEMS USED ON HEAVY MACHINE-TOOLS
For the machine-tools mentioned above, the hydraulic drives are often met under the working conditions shown within the Table 1.
No matter the function they meet, the hydraulic systems that are used can be presented as shown in Figure 1.
The source of hydraulic power is the pump HS, that can have a constant (PCF) or variable (PVF) flow [2].It provides a flow Q and it is subjected to a pressure p. Usually, the pressure and the flow have the values shown in the Table 1.In order to accomplish the function, at the level of the final element which usually is a linear hydraulic motor (HM or C), the values of the used flow Q u and working pressure p u , are adjusted within the hydraulic system HI by the control and distribution equipment [2].The adjustment can be made initially or during the operation, usually by means of the Control Equipment.In some cases, there are provided position or speed transducers T1 which close the loop for the automatic systems.The adjustment can be made also through the pressure control loop, if variable flow pump with pressure regulator and specific transducers T2 are used.Excepting the VL, it is about hydrostatic system on feed kinematic chains.
For large VL it is on main for turning and on feed for milling

HYDRAULIC EQUIPMENT USED FOR HEAVY MACHINE-TOOLS
In most cases, the used pumps are driven by 3-phase electric motors with 1470 rpm speed at 50 Hz.The types of pumps frequently used on heavy machine-tools are presented in Table 2.The gear pumps are the most often used -in 85 -90% of the cases, one of the reasons being their low cost.If the pressures and flows required are high, then the lost power is around 4-5 kW, and thus, the pumps with regulator -usually those vane and simple action -are preferred [3].The main types of hydraulic equipment used for machine-tools are shown in Table 3.The selection of the optimum linear hydraulic motor is done by the mechanical and hydraulic designers.The selection of the hydraulic equipment is to be done by the hydraulic designer [4,5].The selection is based on technical criteria, but also on economic grounds.The cost of the hydraulic installations depends on the size and complexity of the machine tool and varies within 1000 and 100,000 Euro.

HYDRAULIC SYSTEMS FOR HEAVY MACHINE-TOOLS
Roughly, there are two ways to approach the design of the hydraulic installations of the heavy machine-tools: -V1 -a reduced number of pumps, even just one HS, to ensure the required functions, with adjustments specific for each one, HI1-HIn and the driven working elements FW1-FWn; -V2 -distribution of the functions on several pumps HS1-HSn, usually making the pressure adjustments required for each of the systems HI1-HIn, which operate the working elements FW1-FWn.
The diagrams in Figure 2 show these two variants.Table 4 presents the advantages and disadvantages of the two variants.
a .b .Fig. 2. The ways to approach the design of the hydraulic installations of the heavy machine-tools.A larger number of pumps and electric motors.More tanks.The power supply (motors) system is more developed.
Of course, there are cases where intermediate variants are adopted, where the functions are grouped on the optimum source.The design of the installation results by grouping the functions, but also taking into consideration the location on the machine.An optimal cooperation between the mechanical designer and the hydraulic one could lead to a reduced number of working pressure values on the machine and therefore to a simplification of the installation and a decrease of the costs [4].The paperwork presents a summary of the authors' theoretical and practical research in the field of these machine-tools.An upcoming paperwork will present some examples of modularization of the hydraulic installations intended for the heavy duty machinetools: compensation of geometric errors, balancing systems, toolholder clamping/unclamping etc.

CONCLUSIONS
The hydraulic drives are successfully used to accomplish various functions on heavy machine-tools.These functions are: toolholder clamping/unclamping, working axes locking/unlocking, switching of the speed range in the gearbox [5], backlash compensation, indexers actuation, vertical balancing of the weights, geometrical errors compensation and hydrostatic bearing.These functions can be brought together on the same schematic, as previously shown.When putting into practice, the constructive particularities of the machine should be taken into consideration to avoid too long hoses and cable carriers.By increasing the working pressure, the size of the hydraulic equipment is reduced, and consequently the price.When dimensioning the hydraulic equipment components it is recommended to use specialized software of simulation under dynamic condition.

Table 1 .
The hydraulic systems for heavy machine-tools.

Table 2 .
The types of pumps frequently used on heavy machine-tools.

Table 3 .
The main types of hydraulic equipment.

Table 4 .
The advantages and disadvantages of the two variants.