Clearance compensation hydraulic system forvertical lathes with milling unit.

Prodan, Dan ; Motomancea, Adrian ; Bucuresteanu, Anca 等

1. INTRODUCTION

If in case of turning the rotation motion of the plate is the main cutting motion (Botez, 1977), in case of the milling processing, on vertical lathes, the same motion become a circular advancement motion. In this latest case, the existence of the back turn clearances, in particular in the processes needing interpolations and direction changes, may lead to high errors and to compromising the processed part. The back turn clearances occur as a summation of the clearance in the gear box and the clearance in the last mechanism, the pinion-crown mechanism, this one being also the highest. These are running clearances, clearances of which existence provides for the mechanisms' running. Their effects may be increased both by the structure elasticity and by the dynamic effects caused by large masses and inertia moments, specific to these machines (Botez et al., 1978). These influences are not taken into account hereinafter.

In the case of the most existing machines, the plate may be rotated by two distinct cinematic chains:

1. The main cinematic chain, where the existence of clearances does not create problems

2. The circular advance cinematic chain, independently driven, where the back turn clearance is setoff.

This advance box is a massive construction, quite difficult to be executed and having a high price. There must not be neglected also the complications introduced by the presence of the second electrical motor, besides the one meant for the main cinematic chain to be driven.

Due to development of the toothed belt transmissions, which are clearance free transmissions, it was possible for vertical lathes to be executed, with significantly simplified cinematic chains (Prodan, 2008).

Such a lathe exclusively meant for turning operations is introduced in figure 1, where the following notations were made: 1--electrical motor; 2--gear box; 3--special toothed belt transmission, for high power; 4--belt stretching system; 5--secondary shaft; 6--pinion; 7--crown; 8--plate and 9--framework. The secondary shaft does not have a conical group any more, which eliminated a large portion of clearances. However, due to the clearance between the pinion and the crown, for possible milling operations, a cinematic chain must exist, different from the first one (Botez, 1977) and (Bozina, 2006).

The pinion for these machines is of the type shown in figure 2. The pinion is installed on the framework 2 by ball bearings 3. The pinion drives the crown on the flank of the toothed gear corresponding to the rotation direction. Upon stopping or direction reversal, due to running clearances, the contact flanks change, which leads to positioning errors.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

2. DOUBLE PINION CLEARANCE COMPENSATION SYSTEM

Starting from the construction of the pinion shown in figure 2, a variant may be made, which to allow for the back turn clearance to be compensated when it is necessary only, therefore in milling operations and in accurate positioning. This variant, which makes the subject matter of a patent application, and its mode of operation, is schematically shown in figure 3.

The main pinion 1 and the second pinion 2 are coupled by bolts 3. These are introduced by pressing in pinion 2 and they work by sliding in pinion 3. The piston 4 is fixed on the pinion 2 by the screw 5 and the safety washer 6. The element 7 provides for tightness between pinion 2 and piston 4. Piston 4 and pinion 2 are sealed by element 9. The two pinions are kept in contact by the pre-tensioned springs 10. These springs are guided by bolts 8. The pressure due to pre-tightening is borne by washers 11. All the bolts are ensured with the elastic rings 12.

[FIGURE 3 OMITTED]

In the turning phases the pinions 1 and 2 work on the same flank of the crown. When milling operations are performed, oil under pressure p is brought through the way P. The pressured oil drives the piston 4, with diameter D, pushing it upside. The two pinions have angular teeth. Due to this reason the pinions will work on different flanks of the driven crown, the clearance being as such compensated. Possible leakages are recovered by way a, b.

If the teeth angle to the vertical is p, the following relations may be considered:

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

[F.sub.N] = [F.sub.A]/sin [beta] (2)

[F.sub.T] = [F.sub.A] x ctg [beta] (3)

Where: [F.sub.A]--axial force, [F.sub.N]--force normal to the teeth, [F.sub.T]--tangential force.

Force [F.sub.A] is so dimensioned as to provide for the permanent clearance compensation, whatever the instantaneous value of the moment reduced to the pinion level.

3. CLEARANCE COMPENSATION HYDRAULIC DEVICE

As mentioned above, the clearance compensation device becomes active in the milling or positioning phases only. The hydraulic scheme shown in figure 5 is proposed (Prodan, 2008).

Pump 3, driven by electrical motor 2, sucks up oil from the tank T through oil strainer 1. The pressure valve 6 and the distributor 7 are fixed on the plate 10. When the electromagnet S1 is activated the pressured oil is sent through filter 8 to the rotary distributor 10. This is mounted on the double pinion 11, which gears with plate 12. The plate is mounted on the framework through the ball bearings 13 and 14. The existence of the necessary pressure is confirmed by the pressure relay 9. When the decompression of the clearance compensation system is desired, the electromagnet S1 is activated, which leads to the oil being spilled directly into the tank.

[FIGURE 4 OMITTED]

Possible variations of the resistant moment led also to variations of axial force, [DELTA][F.sub.A]. These, in their turn, may cause variations of the axial position of the piston, [DELTA]y:

[DELTA]y = 16 x [V.sub.0]/E x [DELTA][F.sub.A]/[[pi].sup.2] x [D.sup.4] (4)

Where: [V.sub.0]--pressured oil volume, E--oil elasticity module, D--piston diameter. For the values: [V.sub.0]=1l, E=1.5 [10.sup.4] daN/[cm.sup.2], [DELTA][F.sub.A] = 1000daN, D=100mm., [DELTA]y=0.1mm is obtained.

4. CONCLUSION

The compensation of the back turn clearance is compulsory in the vertical lathes equipped with the possibility of milling processing. For machines with classical gear boxes, with sliding pinions, the presence of special advance boxes is compulsory. In small machines with integrated gearboxes, with electrical switching, if only toothed belt transmissions are used, the circular advance motion may be accomplished with the main cinematic chain. For large machines, with integrated gearboxes but with final pinion-crown mechanism, pinions with hydraulic clearance compensation may be used, instead of advance boxes.

5. REFERENCES

Botez, E. (1977). Machine-tools, Theory, Editura Tehnica, C.Z. 621.9, Bucharest

Botez, E.; Moraru, V.; Minciu, C.; & Ispas, C. (1978) Machine-Tools, Organology and Precision Editura Tehnica, 1, C.Z.621.8, Bucharest

Bozina, P. (2006). Handbuch Werkzeug-Maschinen, Hanser, ISBN 10:3-446-40602-6, Berlin

Prodan, D. (2004), Machine-Tools Hydraulics, Editura Printech, ISBN 973-718-109-3, Bucharest

Prodan, D. (2008). Heavy Machine-Tools, Fabrics and Refabrics, Editura Printech, ISBN 978-973-718-892-2