Torque multipliers for machine-tools.

Prodan, Dan

1. INTRODUCTION

Using two-speed gearboxes with electric switching on some machine-tools such as Heavy Vertical Lathes does not ensure all the speed-torque ranges that can be achieved with classic gearboxes with sliding gears (Prodan, 2008). Most of the Vertical Lathes with classic gearboxes ensure the transmission ratios [i.sub.1] = 1, [i.sub.2] = 1/3 and [i.sub.3] = 1/9. Figure 1 shows torque-speed characteristics as obtained at the pinion on the crown gear level, when machining with constant torque (Botez, 1977)--condition specific to heavy machine tools--and the main motor has the following parameters: rated power [P.sub.N] = 100 KW, [n.sub.nom] = 1500 rpm, [n.sub.max] = 4500 rpm. When a two-speed gearbox is used for a motor with the same parameters as above, then the torque-speed characteristics as shown in Figure 2 are obtained. By comparing the two characteristics one may notice that a lower torque is obtained within the range of 166-373 rpm when using a two-speed gearbox. Also, the maximum possible developed torque is 2.25 times smaller. Many machine-tools manufacturers prefer this type of gearbox due to its compactness and lack of backlash. However, if the characteristics obtained with a classic gearbox are needed, then a more powerful motor should be used or, for lower powers, a couple of two-speed gearboxes with electric switching can be connected. A more powerful motor increase both the size and the cost. The second option increases the cost, too, and it is limited by the value of the maximum input torque of the second gearbox, depending on its technical parameters. The performances of the classic three-speed gearbox can be reached by using a classic gear reducer or a classic gearbox with sliding gear and two speed ranges, but the backlashes make this system improper for the feed kinematic chains (Perovic, 2006).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

2. TORQUE MULTIPLIER WITH TIMING BELTS

Nowadays timing belt transmission is frequently utilized due to high powers and torques that can be transmitted, but also for other advantages such as: simple construction, backlash-free, low noise, lubrication-free and lower costs than those of gear transmission. Usually, one step torque multipliers are used. A problem specific to these transmissions is the need of pulling systems. Figure 3 presents the proposed system. The electric motor ME is coupled with the two-speed gearbox CV. The transmission ratios of the gearbox are 1/1 and 1/4. The motion is brought to the shaft I that drives the clutch C. If the clutch is on position I, then the motion is transferred to the shaft II by means of the transmission 1D1/1D2 = 2/3. The belts 2D1/2D2 = 2/3 transfer the motion from the shaft II to the shaft III. Then, the belt transmission 1D3/2D3 = 1/1 transfer the motion to the shaft IV. Finally, the pinion [Z.sub.1] engages with the crown gear [Z.sub.2] rotating the table. If the clutch C is switched on position II, the motion from the shaft I reaches directly the shaft III and then the table through 1D3/2D3 and [Z.sub.1]/[Z.sub.2]. Therefore the torque multiplier by itself ensures two ratios: 1 /2.25 and 1/1.

The formulas below describe the system working:

C [right arrow] I

[n.sub.5] = [n.sub.1] x 1D1/1D2 x 2D1/2D2 x 1D3/2D3 x [Z.sub.1]/[Z.sub.2] (1)

C [right arrow] II

[n.sub.5] = [n.sub.1] x 1D3/2D3 x [Z.sub.1]/[Z.sub.2] (2)

C [right arrow] 0

[n.sub.5] = 0 (3)

[FIGURE 3 OMITTED]

Figure 4 shows system characteristics. This variant contains four speed ranges that include those three obtained with the classic gearbox. The system is backlash-free and the torque multiplier requires no lubrication. This constructive solution reduces the overall size and uses only one pulling system although there are two belt transmissions. To make such a system suitable for the feed kinematic chains of the CNC machine tools, it is necessary to compensate the backlash occurring at the level of the final transmission pinion [Z.sub.1]/crown gear [Z.sub.2] (Catrina et al., 2005). Depending on the machine-tool type, mechanical or hydraulic systems can be used for backlash compensation (Prodan & Marinescu, 2005). The use of a pneumatic system for actuating the clutch C is recommended by the small forces required and by the fact that no oil should contaminate the belts (Bucuresteanu & Isar, 2007). Figure 5 presents the pneumatic diagram of the actuating system of the clutch C.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The pneumatic linear motors CP1 and CP2 move the clutch C to those three positions I, 0 and II. The motors are telescopic with the active surfaces S1 and S2.

The inductive micro-switches--L1, L0 and L2--confirm those three positions. The clutch C can slide along the shaft I. The directional control valves 1DP1 and 2DP1 ensure the selection of the position. The pneumatically controlled valves 1DP2 and 2DP2 and the check valves 1SS1 and 2SS1 help maintaining the selected position. Noise dampers A1 and A2 reduce the noise.

The cyclogram shown by Table 1 describes the working of the system.

3. CONCLUSION

When using integrated gearboxes with electrical switching of the speed, especially on heavy machine-tools, torque multipliers should be provided for obtaining the torque required to rough machining.

The classic torque multipliers--with gears--are expensive, noisy, require tooth machining and have backlash that might compromise the precision of the operated system. In such situations torque multipliers with belt transmissions are recommended.

The advantages specific to belt transmissions are:

--no backlash

--low noise

--no complicated machining is required

--lubrication-free

--and, nevertheless, they can be procured from specialized producers.

When the gearboxes operate as feed boxes as well--see the Vertical Turning and Boring Mills--the belt transmissions represent a simple, cheapper and more liable solution. If there are special requirements with regard to the precision of the total transmission, then mechanical, hydraulic, electric systems for backlash compensation must be provide.

4. REFERENCES

Botez, E. (1977). Masini-unelte. Bazele teoretice ale proiectarii Machine-Tools. Theoretical Fundamentals in Designing), Technical Publishing House, ISBN C.Z.621.9, Bucharest

Bucuresteanu, A. & Isar, D. (2007). Elemente si sisteme pneumatice. Aplicatii Industriale (Pneumatic Elements and Systems. Industrial Applications), Certex Publishing House, ISBN 978-973-1716-06-0, Bucharest

Catrina, D.; Totu, A.; Croitoru, S.; Carutasu, G.; Carutasu, N. & Dorin, A. (2005). Sisteme flexibile de prelucrare (Flexible Systems for Machining), Matrix Rom Publishing House, ISBN 973-685-981-9, Bucharest

Perovic, B. (2006). Handbuch Werkzeug-Maschinen, Carl Hanser Verlag, ISBN 10:3-446-40602-6, Munchen, Wien

Prodan, D. & Marinescu, S. (2005). Refabricarea masinilorunelte. Sisteme hidraulice (Re-designing of Machine-Tools. Hydraulic Systems), Technical Publishing House, ISBN 973-31-2255-6, Bucharest

Prodan, D. (2008). Masini-unelte grele (Heavy Machine-Tools), Printech Publishing House, ISBN 978-973-718-892-2, Bucharest

Tab 1. The cyclogram of the system.

 SOLEINOIDS

 S1 S2 S3 S4

0 + +
I + +
II + +

 SWITCHS

 L1 L0 L2

0 +
I +
II +