Hydraulic device for relieving the rotary table bearings on heavy vertical lathes.

Prodan, Dan ; Motomancea, Adrian

1. INTRODUCTION

The work refers to a hydraulic system of relieving the axial loads appearing during the machining process on the rotary tables of heavy vertical lathes (turning and boring lathes).

For the time being, as far as the authors know, such types of relieving processes are not mentioned anywhere and therefore on heavy vertical lathes with rotary table diameter between 1700 mm and 4300 mm, the parts with weight exceeding the maximum capacity imposed by the manufacturer of the axial bearings fitting the bearing assembly of these rotary tables cannot be machined. This fact obviously restricts the capacity of machining some heavy parts, even if their diameters allow the parts to be turned on such machines.

The solution proposed and presented by the authors is an extremely efficient way to solve a highly stringent technical problem in the metal cutting area.

2. DEVICE DESCRIPTION

For the machines currently manufactured, the solution of taking up the weight of the part is of the type shown in Fig.1. (Botez 1977) The part 1, with weight G, is clamped on the rotary table 2. This table is rotated by the toothed rim 3. The weight is completely relieved by the bearing 4 onto the frame 5.

The proposed solution is shown in Fig.2. Besides the elements 1-5, also used for the solution in Fig.1, there are also: the bearing assembly 6 and the rotary and linear hydraulic engine 7. Its piston, with diameter d and working surface S, can rotate simultaneously with the table 2. The piston can also perform a vertical translation movement. The weight G decomposed into the forces [F.sub.1] and [F.sub.2]. The force [F.sub.1] is taken up by the hydraulic engine 7, and the force [F.sub.2] is relieved through the bearing 4 onto the frame 5. The force [F.sub.1] can be controlled by means of the pressure p generated by the hydraulic driving installation that shall provide the possibility of adjusting the pressure according to the machined part, so that the capacity of the bearing should not be exceeded. The hydraulic engine used, as it has already specified, allows the rotation of the piston and also its vertical translation. Losses are drained onto the circuit D.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The construction of such an engine is shown in Fig.3. Part 1 is the one that leans onto the rotary table. It is caught by the piston 2. The piston can rotate, but also can perform a vertical movement, due to the roller bearings 6. These bearings are secured with the caps 3 and 8 and with the flexible rings 5. The eventual losses between the body 7 and the caps 3 and 8 are stopped by the sealing elements 4. The pressure oil feeding is made through the path P. The pressures on the two sides of the lower bearing are balanced through the holes a. The inevitable losses between the piston and the body are drained through the holes b towards the path D. The piston is pushed upwards by the oil pressure and it is driven into rotation by the rotary table. The piston can move vertically due to the fact that there is the possibility of relative movement, on small strokes, between the rings of this type of bearing.

Fig. 4 shows the location of such a system in a vertical lathe currently manufactured. (Prodan & Marinescu 2005). The rotary table 1 leans on the bearing 2 and the hydraulic engine 4. The stresses taken up by the hydraulic engine are also relieved on the frame, by means of the construction 5. This is a schematic diagram meant to explain how the relieving system operates. In practice the rotation transducer is also mounted in the central area.

The bearing load is relieved directly on the frame 3.

[FIGURE 4 OMITTED]

The hydraulic schematic diagram is shown in Fig.5. The pump 1P1 driven by the electric engine 1ME1 sends oil from the basin T, through the check valve 1Ss1, by the relieving system on the path P. The maximum operating pressure is adjusted by means of the pressure valve 1Ssig.1. The pressure is detected and confirmed by the pressure relay 1Rp1.

The accumulator 1Ac1 shall provide a minimum pressure in case of accidental shutdown of the pump during operation. The accumulator is fitted with the security group 1Rb1., 1Rb2 and 1Ssec.1. The gauges and valves 1M, 2M and respectively 1RM and 2RM are used for reading the pressures at the pump and the accumulator. The oil filtration is made in the return circuit, by means of the filter 1F1.

3. MATHEMATICAL MODELS USED

3.1. Static calculations

For the operating pressure, at an imposed load Fi, the following relationship shall be used (Prodan 2004):

p = 1,1 x [F.sub.1]/[pi] x [d.sup.2]/4 (1)

In the relationship (1) d stands for the diameter of the piston. The output of the feeding pump can be calculated using the expression:

[Q.sub.p] = 2,5 x [pi] x d x p x [j.sup.3]/96 x L x [mu] (2)

In the relationship (2) there are the following notations: j-piston-to-cylinder clearance, L-piston width, [mu]-viscosity of the oil used. The power of the electric engine for pump driving shall be determined with the relationship:

[P.sub.ME] [greater than or equal to] [Q.sub.p] x p/450 (3)

[FIGURE 5 OMITTED]

In the above mentioned relationship, the pressure is expressed in bar., the pump output in L/min. and the resulted power in KW.

The accumulator shall be charged with nitrogen under a pressure [p.sub.0]:

[p.sub.0] = 0.9 x [p.sub.min] (4)

where [p.sub.min] is the minimum operating pressure, determined according to the machine type.

If it is considered that the installation operates with the maximum weight part for which the corresponding pressure is [p.sub.max], the minimum volume recommended for the accumulator [V.sub.0] is (Prodan 2002):

[V.sub.0] = [Q.sub.p] x [p.sub.max] x t/0.9 x ([p.sub.max] - [p.sub.min]) (5)

In the above mentioned relationship, t stands for the period of time when a proper relieving is provided for the load corresponding to pressure [p.sub.min]. This time shall be longer than the time required for stopping the rotary table. If a single accumulator cannot cope with the output requirements, accumulator packs can be used.

3.2. Dynamic calculations

According to the hydraulic diagram shown in Fig.4, the pump output [Q.sub.p], assumed to be constant, is found in the flows passing through the pressure valve [Q.sub.1], through the functional clearance [Q.sub.2] and is dissipated by compression [Q.sub.3], within the hydraulic engine:

[Q.sub.p] = [Q.sub.1] + [Q.sub.2] + [Q.sub.3] (6)

The expressions of these flow rates, as well as the calculation of other hydraulic parameters under dynamic conditions are found in (Prodan&Dobrescu 2000).

4. CONCLUSIONS

The proposed system allows the relieving of the bearing assembly, mainly of its axial component. It is recommended to be used on vertical lathes using bearing assemblies with axial-radial bearings. The load taking up is performed according to the machined part and to the recommendations of the bearing manufacturer. The hydraulic engine, with possibilities of translation and rotation, is actually a more complicated construction that also allows the connection of the rotational transducer for the C-axis at the CNC machines. The functional piston-to-cylinder clearance depends on the construction overall size and the technological possibilities of manufacturing. The actual hydraulic installation allows the continuous adjustment of the operating pressure according to the machined part. The pressure value is permanently monitored by the machine equipment. It is noticed that the time of maintaining the minimum pressure in case of emergency is directly proportional with the accumulator volume. This time should be longer than the time necessary for the complete stop of the rotary table. The system cannot be used in case of the machines with hydraulic lifting force.

5. REFERENCES

Botez, E. (1977) Machine-Tools, Fundamentals Theory of Design, Editura Tehnica, 1977, Bucharest.

Prodan, D.& Dobrescu, T. (2000J, Hydraulic systems of hydraulic relieving, Hydraulics, Hydraulics & Pneumatics Research Institute, Publishing House, December 2000/3-4, Bucharest.

Prodan, D. (2002) Modernization of hydraulic installations on vertical lathes, Hydraulics & Pneumatics Research Institute, Publishing House, June 2002/2, Bucharest

Prodan, D. (2004) Machine-tool Hydraulics, Printech Publishing House, Bucharest.

Prodan, D. & Marinescu, S. (2005) Refabricarea masinilorunelte, sisteme hidraulice, Editura Tehnica, 2005, Bucharest.