The continual increase in operating speed, coupled with the demand for quiet operation, shockless motion, infinitely variable sped arrangement, and the simplified control of all types of machines, has produced problems in machine design which are in many cases best solved by incorporation of hydraulic feed or driving motion.
By this arrangement, the introduction of rapid power traverse necessitating long shafts or gearboxes giving a limited number of changes is substituted by a hydraulic pump and cylinder giving an infinitely variable speed range, quiet operation, and shockless reversal.
The machine is safeguarded against breakdown and wear is minimised, because all parts work under ideal conditions in the transmission medium and all the mechanical parts like gears, screws, racks and clutches are eliminated.
The driving motion is achieved by the variable speed gear consisting of two parts, namely the hydraulic transmission pump unit and the hydraulic motor unit, whereas the feeding motion is achieved by a pump and cylinder having a piston driven by oil circulated from the pump and controlled by piping and valve.
Advantages of Hydraulic Transmission:
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The various advantages which the hydraulic transmission has over the mechanical system of transmission are:
(i) Greater flexibility of control, and capability of obtaining an infinite number of speeds between zero and maximum. It is possible to make provision for automatic reduction of the delivery of a feed pump as the resistance of movement of a tool increases. Such an arrangement finds application, e.g. when turning castings or forgings having large fillets, the feed being gradually reduced as the cut becomes heavier.
When a hydraulic transmission comprising a variable delivery pump and motor is employed e.g., for a lathe spindle drive, the speed of rotation may be automatically varied to maintain a constant surface speed when taking a wide facing cut.
(ii) Quick reversal without shock;
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(iii) Simple and efficient control both manually and automatically;
(iv) Silent operation, with automatic lubrication by the transmission oil;
(v) Lower power consumption, which varies automatically to meet resistance;
(vi) Safety ensured by relief valves;
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(vii) It has been found that life of tool on hydraulically operated machine tools is increased by as much as 50%.
The inclusion of a pressure gauge in either a feeding or driving circuit affords a reliable indication of the force applied and consequently of the condition of the tools. Thus the dulling of the cutting edges is at once made apparent before actual breakdown occurs and this feature is of special value where expensive milling cutters or broaches are used.
(viii) It is possible to traverse the hydraulically transmission operated slide or table against a positive stop with the assurance that the pressure exerted thereon will never exceed a predetermined value. This is achieved simply by providing a relief valve in the circuit set to operate at the setting corresponding to the maximum force which the piston is to exert.
Limitations of Hydraulic Transmission :
(i) Hydraulic drive can’t be employed in cases where an accurate relation must be maintained between two coordinated motions, such as is required when cutting screw threads.
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(ii) The occurrence of slip in hydraulic transmission drives though has the advantage in increasing the life of cutting tools, but it has disadvantages where it is desired to maintain a uniform rate of traverse. By special precautions the variation in feed rate with rising pressure may normally be reduced to one per cent or even less, but only at the expense of simplicity.
(iii) Apart from the fact that it is not possible to obtain an absolutely uniform feed when a table or slide is operated hydraulically, the speed of movement and consequently the rate of output tends to vary somewhat due to variations in the viscosity of the oil with change in temperature.
In some cases this drawback may be largely overcome by providing a reservoir of ample capacity so that the oil is suitably cooled before recirculation.
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