Hydraulic Control of Machine Tools

579 Pages·1965·26.67 MB·English

by  E. M. Khaimovich (Auth.)

HYDRAULIC CONTROL
OF
MACHINE TOOLS
by
E. M. KHAIMOVICH
Translated by
O. T E D D ER
Translation edited by
K. F O S T ER
LECTURER IN MECHANICAL ENGINEERING
UNIVERSITY OF BIRMINGHAM
P E R G A M ON P R E SS
OXFORD . LONDON · EDINBURGH · NEW YORK!
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Copyright © 1965
PERGAMON PRESS LTD.
First English edition 1965
Library of Congress Catalog Card Number 63-10041
This is an edited translation of the original Russian
Fudponpueodu u audpoaemoMamuKa cmanKoe
(Gidroprivody i gidroavtomatika stankov),
published by Mashgiz, Moscow
Preface
IN THIS country, hydraulic controls have been applied to machine tools
for some time, but an aura of mystery still surrounds many aspects
of their design. This may be due, in part, to the large number of
manufacturers of hydraulic components who are each developing indi­
vidual systems—a situation which makes a comprehensive account
of progress difficult. Professor Khaimovich illustrates what may be
achieved by a few large groups co-ordinating their efforts into a
particular direction.
What is immediately apparent is that the Institutes concerned
with the development of hydraulic systems have tried continually
to correlate theoretical and experimental work in order to provide
systematic progress in the subject. The result is that the book con­
tains a great deal of information which is of use to the hydrauHc
specialist. At the same time, Professor Khaimovich has written the
book specifically about machine tool applications and has included
quite detailed drawings of a great many successful systems which have
been used in Russia in the past few years.
Although the book does not describe devices which are as sophis­
ticated as some modern western electro-hydraulic systems, it is of
immense value to have such a wide coverage of the Soviet progress
in a field which is becoming increasingly more important.
K. FOSTER
IX
Foreword
THE automation of machine tools is a factor of great importance in
the development of the Soviet engineering industry which, in turn,
forms the necessary basis for technical progress in the national econ­
omy. Automation increases the productivity of labour, reduces the
physical strain on operators, improves the quahty of products and
ensures the rehable operation of equipment.
The success of automation depends to a great extent on the quahty
of automatic equipment and, especially, of hydraulic drives and
mechanisms.
Hydrauhc drives are used in many fields of engineering, but their
main application is in machine tools.
A great number of scientific institutes and individual enterprises
are engaged in development on hydrauhc machines and automation
equipment. They include the following institutes especially concerned
with the application of hydraulic equipment to metal-cutting machine
tools: ENIMS-Scientific-Research Institute of Metal-Cutting Machine
Tools (F. I. Zuzanov, S. A. Barsukov, I. Z. Zaichenko, G. I. Kame-
netskii, S. S. Chernikov, and others); STANKIN-Machine-Tool and
Tool Institute (N. S. Acherkan, V. V. Ermakov); MVTU-Moscow
Technical University (P. N. Prokof'ev, B. I. Yanyshin); NIAT-Scien-
tific-Research Institute of Production Engineering (V. A. Leshchen­
ko); Kiev Institute of Technology (the author, A. F. Domrachev,
and others: KhAI-Khar'kov Aviation Institute (M. B. Tumarkin);
Various machine tool manufacturers and design bureaus (L. S. Bron,
B. L. Korobochkin, A. E. Leonov, I. K. Yudin, I. I. Knyazhitskii,
A. Kh. Safronovich, Yu. D. Bragov, and others). Most Soviet machine
tool manufacturers are widely using hydraulic mechanisms.
The use of hydraulics in machine tools rose tremendously after
the Great Patriotic War; a number of special factories have been
built which produce hydraulic pumps, motors, and other equipment.
Many types of machine tools are now fitted with hydrauhc mech­
anisms. Soviet machine tool manufacturers use hydraulics in grinding,
xi
Xii FOREWORD
broaching, planing, slotting, drilling, milling, die-sinking machines,
lathes, transfer machines, and many other special machines designed
for automatic production lines.
The range of application of hydraulic drives and mechanisms is
continuously increasing and their designs improved. It is impossible
to consider in a book of this size all problems concerning this ex­
tremely important branch of engineering especially in view of the fact
that many questions have, as yet, not been sufficiently investigated.
For this reason the author had set himself the task of considering
only a few typical methods, the principles of design of hydraulic
systems and their equipment and also some typical circuits.
This book represents a new and completely revised edition of the
book with the same title published in 1953.
CHAPTER I
General Description of
Hydraulic Drives and Other Equipment
THERE are in existence a great variety of machine drives; they can be
divided into four groups: mechanical, electrical, hydraulic and pneu­
matic.
In addition, various types of combined drives are also widely used;
examples include electromechanical, hydromechanical, pneumo-
mechanical as well as hydro-electrical and hydropneumatic types.
Needless to say these designs are being continuously improved.
Drives with stepless control of speed and feed, also devices for the
automatic control of operations are of particular importance for mo­
dern machine tools.
At present mechanical drives are most widely used. However, high
unit pressures and frictional forces acting in mechanical drives with
infinitely variable speed cause rapid wear of the contacting surfaces
with a corresponding loss of efficiency. In addition, mechanical trans­
mission with stepless speed control and mechanical automation equip­
ment are complex in design, bulky and expensive.
Electric drives in the form of simple a.c. motors are more popular
and are also often used as component parts of drives for other types.
Variable-speed electric motors are widely used although they are less
compact than hydraulic motors. Their motion is less smooth and their
response less rapid. Electric automatic equipment has the advantage
over mechanical t3rpes in being able to operate over remoter distances.
Their disadvantages are that they are often more bulky, more com­
plicated and less dependable in operation.
The compressed air used as the working medium in pneumatic
equipment requires a compressor plant to produce it. Considerable
changes in the volume of air with pressure cause variations in the
speed of the mechanisms. Pneumatic drives are therefore used
mainly where the uniformity of speed is less iraportant or where their
2 HYDRAULIC CONTROL OF MACHINE TOOLS
use is desirable from the safety viewpoint, for example in power drills
and grinders, pneumatic hammers and chisels, chucks, pneumatic
stockfeed devices, turret lathes, etc. Owing to the low viscosity of air
high speeds can be obtained with pneumatic equipment without sub­
stantial frictional losses, an example being the spindles of grinding
machines which reach speeds of 100,000 rev/min and instantaneous air
velocity in pipes up to 300 m/sec. In order to avoid an excessive drop
in temperature during the sudden expansion of air and the freezing of
water contained in it, and also in order to avoid excessive leakages,
only pressures of up to 6-7 atm. are normally used.
Pneumatic equipment is often successfully used at considerably
higher pressures reaching to a maximum of 200 atm. in stand-by or
emergency drives. These are intended only for occasional, rather than
for continuous use, in which case excessive cooling is not likely to
aflfect normal operation of the mechanism.
The drop of air temperature during expansion is sometimes used in
pneumo-hydraulic drives for cooling the hydraulic system. This is of
great importance in high-precision machine tools where temperature
cannot be permitted to rise too high.
Low-pressure pneumatic automation equipment is less compact than
hydraulic systems and its movements are less smooth than those per­
formed hydraulically.
Quite recently hydro-pneumatic devices have become extensively
used which combine the advantages of pneumatic drives with those of
the hydraulic equipment.
Owing to its many advantages hydraulic equipment with a liquid
working medium (usually a mineral lubricating oil) is now widely used
in many metal-cutting machine tools.
The assets and advantages of hydraulic transmission include:
l.The ease with which high forces and power are obtained by
means of compact and light mechanisms. Relatively low inertial forces
and moments acting in hydraulic mechanisms.
2. The ease with which a stepless regulation of speeds and feeds is
obtained, which is vital for efficient operation.
3. Controlling cutting conditions by varying the speed and feed dur­
ing operation and automatic variation of these conditions in accord­
ance with a definite programme.
4. Smooth motions which increase the life of cutting tools and some­
times make possible the use of higher speeds and feed rates (for instance
in broaching machines).
HYDRAULIC DRIVES AND OTHER EQUIPMENT 3
6. The simplicity of obtaining linear motion.
6. Frequent and rapid changes of linear and rotary motions.
7. The absence of bulky and complex mechanical transmissions which
require continuous lubrication, maintenance and are subject to consid­
erable wear.
8. Convenient positioning for the hydraulic devices in machines
regardless of the location of mechanical transmissions.
9. Self-lubrication of mechanisms by the hydraulic fluid.
10. Ease of automation.
11. Simplicity and ease of control.
12. Possibility of continuous supervision of pressures and forces act­
ing on the working mechanisms by means of simple and cheap instru­
ments such as pressure gauges.
13. Easy prevention of damage and overloading of machine-tool
mechanisms and tools. The possibility of operations with rigid dogs.
14. The use of standard sub-assemblies and mechanisms.
The shortcomings of hydraulic drives include:
1. Frictional losses in the hydraulic fluid in the piping and at points
where the speed and direction of the flow changes. These losses rapidly
increase with increasing speed, thus limiting the velocity of liquid in
the pipes (which normally does not exceed 9-10 m/sec) and the speed
of hydraulic pumps and motors used in machine tools (up to 3500
and more rev/min).
2. Internal and external leakages which adversely afifect the effi­
ciency and operational properties of systems. Considerable leakage
of the liquid under load results in non-uniform motion of the me­
chanisms thus imposing a limit on the speed range and making the
operation of the mechanism at low fiow rates difficult.
3. The variations in the supply of liquid and the speed of mechanisms
due to the fluctuations of temperature and viscosity of the hydraulic
fluid and to its compressibility. The effect of the compressibility of the
liquid upon the operation of the hydraulic system is small when the
volumes involved are small but it can become appreciable when large
volumes are involved. Occasionally the compressibility of the liquid
is a positive factor which ensures the smooth operations of mecha­
nisms.
4. The leakage and compressibility of the fluid make it impossible to
form an accurate co-ordination of motions, for example in the mecha­
nisms of screw-thread and gear cutting machines.
5. The leakage of air into the hydraulic fluid and the formation of
4 HYDRAULIC CONTROL OF MACHINE TOOLS
air pockets in hydraulic systems which, because of the compressibility
of air, result in non-uniform or jerky operation of the mechanisms.
6. The compression and expansion of piping in hydraulic systems
which may lead to the loosening of couplings and the failure of pack­
ings.
7. The possibility of a considerable heating of the hydraulic fluid
especially at low efficiencies and low heat transfer rates. The resulting
changes of the temperature can affect the precision of machine tools.
In order to prevent excessive heating of the machine there is a tendency
to reduce the leakage and heat transfer by using fiuid coolers.
8. The inflammability of mineral lubricating oils used as hydraulic
fluids.
9. The necessity to work with very small clearances between parts
(e.g. valves) which involves the use of high-quality heat-treated ma­
terials. Ageing (in order to prevent distortion), grinding, grinding-in,
or lapping are used to increase the accuracy of machining of essential
parts. This makes their manufacture more complicated and expensive.
The above tabulated shortcomings of hydraulic mechanisms can be
reduced or compensated by a discriminating selection of designs and
hydraulic circuits.
The hydraulic equipment of a machine incorporates the following
main parts; the drive, transmissions and control, and auxiliary mecha­
nisms.
The drive consists of energy converters. Its primary part is the pump
which takes energy from an electric motor, or a mechanical drive, and
transforms it into potential and kinetic energies of the hydraulic fluid.
The pump delivers the pressure liquid into the secondary part of the
drive which is the hydraulic motor. This transforms the potential and
kinetic energy of the fluid into the mechanical energy which drives
the working mechanisms of the machine tool.
Hydraulic drives and transmissions can be divided into hydrodyna-
mic and hydrostatic. Hydrodynamic drives utilize mainly the kinetic
energy of the flow.
Drives and transmissions of this t3φe (hydraulic couplings and tor­
que convertes) can be effectively used when the transmission of large
amounts of energy is involved and when the peripheral speeds are high
(about 25-50 m/sec). However, the substantial drop of their speed
with increasing load, and the difflculties of their regulation and rever­
sal, restrict the use of this type in machine tools.
Hydrostatic drives utilize the potential energy (pressure energy)
HYDRAULIC DRIVES AND OTHER EQUIPMENT 5
of the fluid delivered into them by the pump. The hydrostatic drives
of displacement type can be designed to have speed and feed character­
istics required in machine tools. They are compact and their regula­
tion and reversal presents no difficulties. These are the reasons why
the hydrostatic displacement drives in the form of cylinders with
pistons and rods, cylinders with plungers, vane, gear, screw, and piston
pumps and motors enjoy almost exclusive use.
Hydraulic drives can transform the linear motion of the pump into
a linear or rotary motion of the motor, or the rotary motion of the
pump into the rotary or linear motion of the motor.
The hydraulic system of a metal-cutting machine involves complex
equipment: apart from the pump and motor it requires devices for (a)
transporting of the hydraulic liquid (piping), (b) the control of the
system (valves, accumulators, etc.), and (c) auxiliary equipment such
as tanks, filters, etc.