222 | MM Industrial Spectrum | Special Issue | 2015

The electromechanic feed assemblage or the drive by linear servomotors are used for feed implementation at building of machining centres at the current time.

3.3 LINEAR FEEDASSEMBLAGES

The composition of the linear feed assemblage is shown in Fig. 3.3.1. Fig. 3.3.2 shows some essential types of the linear feed assemblage drive. If the drive is performed by means of the motion screw and the nut, there are two essential types available: in the first case, the screw rotates and the nut moves; in the second case, the nut rotates and moves at the same time and the screw stands. It can be sometimes favourable to use the combined type, when the rotary motion is performed by the screws as well as by the nut. The third possibility is to perform the drive by means of the piston and the shaft.

Ball screw and nutThe straight motion at CNC machine tools is either the main feed cutting motion (performed by the tool or by the workpiece) or the auxiliary motion. If the ball screw and the nut (KŠM) are used for the drive, the motor usually performs the rotary motion which is transformed to the straight motion

Feed linear assemblage

drive method guideways positionmeasuring

covering andsupply of media lubrication

• with bearing packs • slideways • hydrostatic • combined • aerostatic

• direct • indirect

• telescopic covers • bellows type • energy carriers

• oil • grease

rotary servo drive linear motor

• ball screw • pinion and rack • worm and rack

• asynchronous • synchronous

Fig. 3.3.2: Type execution example of the linear feed assemblage [Toshulin]Fig. 3.3.2: Type execution example of the linear feed assemblage [Toshulin]

through the ball screw. The rotary AC servo drive is connected to the ball screw (Fig. 3.3.3) directly by means of the clutch through the inserted gear mechanism (belt, toothed wheels), through the inserted

gearbox (P), or by means of the mentioned possibilities. The suitable way is given by the kinematic, dynamic or static relations which must be assessed according to the particular cases.

The rotary servomotors which are most often used for feed coordinates are

synchronous electronically commutated electric motors (AC). They create the type of brushless electric motors with higher quality, based on the simultaneous control of three clamp currents having the harmonic courses [Souček 2004]. This motor type is most often used for the feed drive. The comparison of the torque characteristics of

the brush electric motors and the brushless electric motors is shown in Fig. 3.3.4.

The motor has the subsequent advantages in comparison with the direct current (DC) motor:• it is not necessary to keep the commutator;• the power output is not limited – the maximum power output is also at the maximum speed (Fig. 3.3.4);• the motor is better cooled (losses are in the stator);• good IP 65 covering; • the speed is not limited by the mechanic commutator;• feeding transistors enable to work without the transformer.

The asynchronous electronically com-mutated electric motors (AC) are the motors having the squirrel cage rotor. Feeding of the stator winding is performed by three harmonic currents. The magnetic field is not made by magnets but by induced currents which are induced into the armature due to the slip [Souček 2004]. These motors are used for the spindle drive (Section 3.3.4).

with toothedwheels

with the belt

insertedgearbox

directly

Fig. 3.3.3: Methods of the AC servo driveconnection to the ball screw

Fig. 3.3.1: Morphology of the feed assemblage

Fig. 3.3.4: Comparison of the AC (alternating current) motor and of the DC (direct current) motor

overload

permanent run

Linear feed assemblages

2015 | Special Issue | MM Industrial Spectrum | 223

The servomotors are operated in various modes (Fig. 3.3.5). The best way how to dimension the servomotor is that the servomotor is able to comply with the S1 mode (permanent operation. However, in some cases it is necessary to overload the motor for a short time, e g. modes S2 – 30 min, S6, etc.

Various design elements are situated on the servomotor shaft. These are belt drives or different kinds of gearboxes. The belt drives transfer the required power output at the parallel layout of the belt pulley axes to bigger distances (Fig. 3.3.6). They serve as a damping element in many cases. Acceleration called by the motor causes the different tensile stress in the belt.

The lower part is stretched, the upper part is released. The driven belt pulley will move in a jerking way only after this “springing”.

Fig. 3.3.5: Operation modes of servomotors [Bonfiglioli]Fig. 3.3.5: Operation modes of servomotors [Bonfiglioli]

Fig. 3.3.6: Spatial layout of the belt drive Fig. 3.3.6: Spatial layout of the belt drive

The lower part will be released and the upper part will be stretched, which results in oscillation which disappears only thanks to the damping belt effects. The oscillation amplitude is as follows:JMOT = JRED – the oscillation amplitude is the same one at both belt pulleys;JMOT > JRED – the oscillation amplitude is more obvious at the load, it remains relatively quiet at the motor;JMOT < JRED – the big mass remains quiet, the motor performs bigger rotary motions. These are caught by regulation and adjusted by control. The motor tends to rotary oscillation.

The belts used at building the CNC machine are V belts (based on the principle of increased friction of the drawing layer in the V groove) and synchronous belts (the drawing layer is equipped with transversal teeth – Fig. 3.3.7). The drawing layer is stiffened by steel, glass or aramid fibres.

The original V belts were made of rubber. At the current time, the essential material of V belts is represented by polychloroprene having the outstanding friction properties, which also determines

Fig. 3.3.7: V belt and synchronous belt[Gates – Ulmer] Fig. 3.3.7: V belt and synchronous belt

the possible transferred circumferential force. The geometric V groove shape enables to transfer the circumferential force by pressure on the belt sides and developing the friction force. The ideal belt vertex angle is 36°, after mounting on the belt pulley it is in the interval of <34°–38°> [Brož 2006]. At the multiple V belts Micro V, the vertex angle is of 40°. The belt stiffness is influenced by the stiffness of the drawing layer. The power output which can be transferred by the V belt is

drivingbeltpulley

drivenbeltpulley

3.3 LINEAR FEEDASSEMBLAGES

224 | MM Industrial Spectrum | Special Issue | 2015

equal to the product of the circumferential driving force and of the speed, while the circumferential driving force is given by the difference of the drawing branch traction and of the free branch traction.

The friction force between the belt and the belt pulley is dependent on the radial force and this radial forces changes in dependence on the traction in the belt. The active belt length in the belt pulley is given by the angle of contact. The small belt pulley is always decisive for the belt length in the belt pulley. E. g. Gates company supplies the following V belt types:• Hi Power, low classical profile Hi Power MN (Z, A, B, C);• Super HC;• Quad Power II (XPZ, XPA, XPB, XPC, SPZ, SPA, SPB, SPC) for great power outputs;• Powerband (SPB, SPC, 9 J, 15 J, SV/25 J, JVX);• Micro V (PJ, PL, PM);• Polyflex JB.The V belts Micro V and Powerband are connected on sides (3–4 profiles), the other ones are supplied independently and their delivery does not require any special selection.

Fig. 3.3.8: Herringbone arrangementof the synchronous belt [Goodyear]Fig. 3.3.8: Herringbone arrangement

Fig. 3.3.9: Connection of the shaft with the hub by means of the gripping sleeve [Tollok] Fig. 3.3.9: Connection of the shaft with the hub by means of the gripping sleeve [Tollok]

The synchronous belts are manufactured for the purpose to keep reliably the angle position of the connected shafts. The first synchronous belts had the transversal tooth having the trapezoidal shape, but as the time was running, the shape was rounded. The teeth are made of polychloroprene with the protective nylon layer. The drawing layer is created from cord fibres which are manufactured of glass, aramid or steel according to the stress kind. The belts with the transversal tooth (synchronous belts) have a smaller built in area compared to e. g. V belts. The belt pulleys of synchronous belts are equipped with side parts of pressed steel metal sheet, they have the bent entering end

(approx. 15°). Gates company manufactures the synchronous belts, type Power Gripp GT2 and Power Gripp GT3. Some synchronous belts have the herringbone arrangement of teeth (Fig. 3.3.8), which increases their loading capacity and reduces the noise level.

The V belt pulleys as well as the synchronous belt pulleys are more often attached by means of the cone gripping sleeves. The gripping sleeves represent the safe and economically acceptable connection of the shaft with the hub of the mechanic gear mechanism (toothed wheel, belt pulley, etc.) – Fig. 3.3.9. The advantage can be seen in the rather simple assembly, resistance against the external load, the shaft cross

Fig. 3.3.10: Epicyclic gearbox with gear shifting mounted on the servomotor flange [ZF]Fig. 3.3.10: Epicyclic gearbox with gear shifting

Fig. 3.3.11: Epicyclic gearboxwith the fixed gear step (ZF)

section is not reduced, high torques can be transferred, the radial and axial hub fixation and the backlash free connections are able. The gripping shaft connection works on the principle of creating the specific pressure between the hub and the shaft. The similar formula is used for the calculation of the minimum hub diameter, like for the thick walled hollow cylinder.

If the toothed wheels (or another mech-anism) are mounted with the shafts into the gear casing, we will get gearboxes. The