Technical Information

F-3 Hydrostatic Feed Drive and Assembly Technologies for Ultra Large-Sized Machine Tools

F-3 Hydrostatic Feed Drive and Assembly Technologies  for Ultra Large-Sized Machine Tools

Toshiba Machine's ultra large-sized machine tools process machine workpieces weighing as heavy as a few hundred tons with high accuracy. Among the key technologies, the hydrostatic feed mechanism is available, which serves as the feed mechanism of our ultra large-sized machine tools and constitutes one of the essential technical elements. To maintain feed accuracy of this superior feed mechanism, scraping of slideways is a key factor. Now, we introduce the hydrostatic worm and rack mechanism and scraping, which are essential to the ultra large-sized machine tools.

[Platform]
F. Slide and rotation
[Applications]
Ultra large sized machine tools

[Technical points]

  • Fluid lubrication: Low frictional resistance, high efficiency and long service life.
  • High rigidity: The number of teeth that can be engaged at the same time is many.
  • Rack type: No limitation is imposed on the length of the stroke. Best suited for ultra large-sized machine tools.
  • Scraping: Conformity of each slideway is made uniform to reduce frictional resistance.

1. Introduction

Since the hydrostatic worm rack feed mechanism was developed in 1972 for Toshiba Machine's plano milling machine, it has been used in a total of about 100 ultra large-sized machine tools up to now. It constitutes the key factor supporting high rigidity, high accuracy and high cutting performance for axis feed over the ultra-long stroke. We have repeatedly improved this mechanism, moving ahead face-to-face the technological themes to respond to the market requirements for the ultra large-sized machine tools. Additionally, to maintain the high feed accuracy, improvement of accuracy of the hydrostatic slideways supporting the long table, and scraping by a skilled worker are essential.

2. Principle of hydrostatic worm rack

2.1 Basic principle of static pressure

Fig. 1 Principle drawing of hydrostatic pressure

Fig. 1 Principle drawing of hydrostatic pressure

We briefly explain the basic principle of static pressure in Fig. 1. For the relationship between pocket pressure "P", flow rate "Q" fed from the land area, hydrostatic clearance "h" and bearing load "W", the following equations are established approximately.

Q = BPh3/12μL
W = ApP
where,
μ: Coefficient of oil viscosity
L: Land width
Ap: Effective area of load (or pressure)
Ap = (L1 - L) · (L2 - L)
B: Circumference of land B = 2 (L1 + L2 - 2L)
L1, L2: Dimensions of hydrostatic surface outer frame

From the above expression, you can find that flow rate "Q" is proportional to the cube of hydrostatic pressure "h". This signifies that control of hydrostatic clearance is very important. To be more specific, control of hydrostatic clearance of the hydrostatic worm rack and clearance of the hydrostatic slideway of the table is important. Control of clearance is performed by scraping by skilled workers, and the fluid lubricating condition is formed due to hydrostatic oil film to minimize the coefficient of friction.

2.2 Principle of hydrostatic worm rack

Fig. 2 Rack

Fig. 2 Rack

Next, we describe the principle of the hydrostatic worm rack. As shown in Fig. 2, the rack profile is part of the female screw thread, and hydrostatic pockets are provided on its tooth surface. Hydrostatic oil is fed to the both tooth surfaces through the oil supply holes (Fig. 4) provided on the worm as shown in Fig. 3 to produce the opposed hydrostatic pressure.

Fig. 3 Worm

Fig. 3 Worm

Fig. 4 Principle drawing

Fig. 4 Principle drawing

Fig. 5 Characteristic diagram

Fig. 5 Characteristic diagram

As a result, the frictional force between the tooth surfaces of the worm and rack is composed of only shearing resistance of oil in principle, and an extremely effective feed mechanism can be realized.
Fig. 5 shows its characteristic diagram. At neutral point h0 of the hydrostatic clearance, pocket pressure on the both tooth surfaces is equally P0. Then, if thrust load is imposed due to table travel, the hydrostatic clearance changes by δ alone and pressure ΔP is caused between pocket pressure P1 on the load side and pocket pressure P2 on the anti-load side to maintain hydrostatic clearance h0. This mechanism introduces the flow rate control technique (Note 1) for the hydrostatic system.

Note 1: Generally, oil viscosity changes with change in temperature, then oil flow rate also changes accordingly. The flow rate control technique always maintains this flow rate constant in relation to change in temperature.

3. Structure of hydrostatic worm rack

Fig. 6 Structural drawing

Fig. 6 Structural drawing

Fig. 6 shows the structural drawing of the hydrostatic worm rack feed mechanism. Main components are the rack, worm, worm shaft and distributor.

Fig. 7 Relations between oil supply phases

Fig. 7 Relations between oil supply phases

Fig. 7 shows the relationship between the oil supply phases. Hydrostatic pockets are arranged on all right and left tooth surfaces of the rack, and oil supply holes evenly distributed into eight on respective right and left tooth surfaces of the worm are provided by the number of crests. Hydrostatic oil is fed to the rotating worm shaft through the distributor serving as a rotary joint, which is then fed to the worm. As shown in Fig. 6, the hydrostatic oil is fed only to the places where the worm meshes with the rack through the hydrostatic pocket of the distributor. The hydrostatic oil of a predetermined flow rate is fed to both tooth surfaces by means of independent flow rate control valves.
Additionally, auxiliary oil is fed to this mechanism to prevent splash of oil out of the oil supply hole on the worm which is not engaged with the rack and entry of air to such oil supply hole while the worm is rotating. Pilot oil serves to fill the hydrostatic pocket of the rack just before engagement of the worm and rack. Both the auxiliary oil and pilot oil are separately controlled by means of each flow rate control valve so that the optimal flow rate can be supplied.

Fig. 8 Hydraulic circuit diagram

Fig. 8 Hydraulic circuit diagram

Both the hydrostatic oil and auxiliary oil are fed to the worm tooth surface through the same oil supply holes, and the pilot oil is fed to the rising area on both ends of the worm through separate oil supply holes. The hydrostatic oil, auxiliary oil and pilot oil are controlled by means of respective flow rate control valves.
The hydraulic system of the hydrostatic oil, auxiliary oil and pilot oil is shown in Fig. 8.

4. Features of hydrostatic worm rack

We have explained the principle and structure of the hydrostatic worm rack feed mechanism as shown above. The features of this mechanism are shown below.

  1. High efficiency:
    Due to fluid lubrication, the frictional resistance is small and efficiency is extremely high.
  2. High accuracy:
    As the number of teeth that can be engaged at the same time is many, pitch error of the worm and rack can be averaged due to hydrostatic oil film.
  3. High rigidity:
    As the number of teeth that can be engaged simultaneously is large, rigidity is high.
  4. High cutting performance:
    Due to hydrostatic oil film, attenuation is high to enhance the cutting ability.
  5. Realization of axis feed over ultra-long stroke:
    Due to the linked rack type, no limitation is imposed on the length of the stroke.
  6. Without backlash:
    Thanks to the opposed hydrostatic pressure structure, backlash is extremely small.
  7. Semi-permanent service life:
    Due to fluid lubrication through the hydrostatic oil film, metallic contact between the tooth surfaces is absent and the parts will not worn out.

As known from the above this mechanism has the outstanding features to serve as the feed mechanism of the ultra large-sized machine tool.

5. Scraping for Realizing High Accuracy

Fig. 9 Scraping technique is implemented on a ultra large-sized plano milling machine

Fig. 9 Scraping technique is implemented on a ultra large-sized plano milling machine (Table length 26m, table width 6m, bed length 55m, maximum loading capacity 500 ton)

The table, cross rail and spindle head of ultra large-sized machine tools such as ultra large-sized plano milling machine, and the workpiece weigh from several tens of ton to several hundreds of ton. To move and locate each of them smoothly and accurately, a linear guide mechanism is used, which incorporates a hydrostatic guide and hydrostatic worm rack described above.
In terms of accuracies (surface roughness, straightness and flatness) required of the slideways of the linear guide mechanism including the bed (divided into eight sections: 6m-wide and 55m-long) and table (divided into four sections: 6m-wide and 26m-long), skilled scraping technique is essential for the finishing. How such technique is to be handed down from generation to generation is, therefore, one of the big themes yet to be settled. Fig. 9 shows how the scraping is going on over the ultra large-sized plano milling machine. Red lead or blue paste is applied to each slideway of the bed and table, which is then piled up and moved by several tens of mm to make uniform the conformity of the slideways. Scraping is done repeatedly until even conformity and straightness can be assured over the full length of the slideways.

6. Conclusion

Requirements for high-efficiency machining of ultra large-sized machine tools is ever-increasing in the years ahead. To continually promote the high speed and high accuracy, there are many technological topics left for the hydrostatic worm rack feed mechanism, and we have to make utmost efforts to further promote the development of relevant technologies.