The following points highlight the three main types of infinitely variable drives used in industries. The types are: 1. Mechanical Stepless Drives 2. Hydraulic Stepless Drives 3. Electric Drives.
Type # 1. Mechanical Stepless Drives:
Mechanical variable speed drives are of the friction type. In friction type drive a rolling disc and a roller make line or surface contact. (Refer Fig. 11.58). Motion is transmitted from the roller to the disc, and velocity ratio (V.R.) is equal to l1 / l2.
The velocity ratio can be varied by changing value of l2. Fig 11.59 shows a double disc transmission system in which the velocity ratio is equal to l1 / l2 x l 3/ l4.
Fig. 11.60 shows Reeves variable speed transmission system in which a pair of pulleys is connected by a V-shaped belt. Each pulley further consists of D1 and D2; D3 and D4, these discs can slide along the shaft and rotate with the shaft. The diameter of pulleys is adjusted by bringing discs closer (increased) or taking them apart (diameter reduced). In this manner the ratio of driving diameter to driven diameter can be easily changed and therefore any desired speed can be obtained without stopping the machine tool.
Positive Infinitely Variable (PIV) Gear:
In Reeves drive, slip takes place due to friction between pulley and V- belt. In can be made positive drive, if toothed discs are used and V-velt is replaced by a chain (Refer Fig. 11.61). The disc can be displaced axially by rotating the screw which tilts the frame about hinge. Thus the effective diameters can be changed and the transmission ratios between shafts can be changed.
Type # 2. Hydraulic Stepless Drives:
Hydraulic drives are commonly used to obtain infinitely variable rates of rectilinear motion in machine tools.
Hydraulic drives have following advantages and limitations:
Advantages:
(i) Wide range of speed variation.
ADVERTISEMENTS:
(ii) Magnitude of speed and direction of speed can be easily changed.
(iii)It is smooth, quiet and reverses without shock.
(iv) It can be operated by a remote control.
(v) Self-lubricating.
ADVERTISEMENTS:
(vi) Simple design and easy maintenance.
(vii) Automatic protection against over loads.
Limitations:
(i) The operation of the hydraulic drive becomes unstable at low speeds.
ADVERTISEMENTS:
(ii) Oil viscosity varies with temperature and this may cause fluctuations in feed and speed rates. Compensating devices can be used to overcome this problem.
Type # 3. Electric Drives:
Parallel to the growth of machine tools, an equally rapid progress has been made in the field of electric motors. A bewildering variety of electric drives is available today, so that the task of application engineers is steadily on the increase. The best machine is naturally the one in which there is proper co-ordination between the various elements.
A sound knowledge of the problems of the prime mover and controls together with the problems of the driven machine is a necessary prerequisite in the design of any machine.
Machine Tool Drives:
Machine tool drives may be classified as follows:
(i) Main Drives:
Main drives are responsible for cutting, forming and the like. Adjustable speed is a desirable feature.
(ii) Traverse Drives:
These are used for positioning the tool with respect to the work. Though often performed manually, power drive is provided where a rapid tool-traverse is required. Constant speed, intermittently rated high starting torque motors with provision for reserving are used.
(iii) Feed Drives:
These are intended for feeding the tool into the work or vice-versa. Being essentially a constant torque drive, it requires a wide range of speed variation and employs adjustable voltage D.C. Motors.
(iv) Accessories:
Torque motors for inching, positioning etc., and for chucks, motors for circulating the coolant or lubricant come under this category. Torque motors and gear shifting motors are designed as D.squirrel cage motors, (this is the classification according to NEMA standards) or heavily compound D.C. Motors intermittently rated. The rest are ordinarily squirrel cage motors.