Drives in Machine Tools
Introduction
Machine tools are precise and complex machine which are used to produce various type of components by metal cuttings, that is, by removing metal in the form of chips. The work piece is held in a machine tool with the help of various types of work-holding devices such as chucks, collets, face plates, mandrel, etc, or held between centers ( centre lathe, grinders etc) or clamped on a machine table ( planers, shapers, slotters, milling machine, surface grinding machine etc.) . The metal cutting tool is held in various types of tool holders, for example, a tool post of a centre lathe, an arbors of a milling machine, spindle of a drilling machine and so on. Motion in Machine Tolls. or removing the metal from a work piece, a relative motion is necessary between tool and !ob. !ob . The various motion characteristics of machine tools are " #orking #orking motion and auxiliary motions.
$#orking $#orking motion% affect affect the process of o f clip removal. These are transmitted either to the cutting tool, or to the work or to both simultaneously. #orking #orking motion include"
&.'rimary cutting motion .eed motion
'rimary cutting motion provide for cutting the chip from the !ob at the cutting speed which determine the rate at which the chips are formed. 'rimary cutting motion most commonly used are" rotation and straight line reciprocation. The primary cutting motion of a certain machine tools may be uite complex but bit can be described as a combination of rotary and reciprocating motions. *otary motion may either be transmitted to the !ob, or to the tool( milling cutters, drills etc.) or to both simultaneously (cylindrical grinding ). +traight line reciprocating primary motion is employed in plan ers, sharpers, slotters, broaching machine, power hackshaw machine etc. This motion can either be transmitted to the tools or to the work( planers ). The feed motions are the &
movement either of the tool or of o f the work in reference to each other. This This motion enables the cutting operation to be extended to the whole surface of the workpiece to be machined. This motion can be rectilinear or curvilinear. t t may be in a longitudinal direction or in a cross direction. n the example of turning a cylindrical !ob on a lathe, the cutting motion is obtained by the rotation of !ob between centres and feed motion is obtained by the movement of tool parallel tp the axis of !ob and normal to cutting motion.
$uxilliary Motion% prepare the machine, workpiece and tool for carrying out the cutting process and check wether the movement have been properly made. This motion include" loading clamping the !ob, removing the finished wark, clamping and swiveling units on which the work or tolls are mounted, rapid approagh and withdrawal of units carrying the cutting tool, measuring workpiece and other operation. +ince all these motion are non cutting motion, these should be performed as fast as passible to reduce the total time to produce one component and thereby increase the rate of production of machine.
Drives in machine tools
Thr primary cutting motion of machine tools are power driven. +imilarly, feed motion are also power driven except on small machine, where these may be performed manually.
The operating cycle of machine tools, including both working and auxiliary motion, is obtained by means of a drive and definite units and mechanism. The drive of machine tool consist of"
&. sour source ce of of ener energy gy,, and and . evi evices ces for tran transm smit itti ting ng powe powerr from from the sour source ce of ener energy gy to the the oper operat atin ing g elements for producing the cutting motion and feed motion.
Machine tools are driven almost universally by electric motors. /ach machine may be driven individually by its own motor or driven by belt form a line shaft furnishing power to other machine tools as well, this being called 0 1roup rive2
Group Vs Individual Machine Tool Drive . The choice between two depend on
&. 3omparative first cost . total annual operating expenses 4. +uch minor advantages and disadvantages from the production point as can be foreseen from experience in similar installation (a) Group Drive . 1roup drive motor are often mounted overhead and machine tools are driven in groups through line shafts and belts. This limits the si5e of motor to about 67 8# or preferably not over 46.7 kw as they are unwidely to replace in case failure. 1roup drive is usually more economical in fixed charges, power consumption and maintenance (b) Individual Drive . +uch a drive should be used in following situation (&)
in areas reuiring over head crane services
()
on machine which would reuire countershaft if grouped and are likely to be
moved freuently as activity in department varies (4)
on machine that reuire considerable power say about &6.7 kw or more
(9)
on a few machine scattered over a large area
(7)
machine reuiring wide speed variation also are best driven by individual drive
in complex machine, :arious movement are better synchroni5ed electrically than mechanically. ndividual drivers are slightly more expensive but are more flexible, permit better plant layout and changes to facilitates the flow of work through the plant, help maintenance and allow cleanliness and better lighting. #orking ha5ard is thus reduced. /ach drive should be selected on its own merit, but the individual motor drive has largely superseded the group drive. The motion and power is transmitted from the drive motor to the various units by devices called $ transmission element%. n $ electrical drive $ the direct motor drives the machine drive shaft through direct coupling. n 0machine drive 0 the transmission include" belts, chains. The transmission elements between the input and o utput shafts can perform the following function" &. convert rotary motion into translatory motion 4
. convert rotary motion into rotary motion
Mechanical drive .
n a mechanical drive, the transmission element will depend upon the type o f conversion needed between drive shaft and driven shaft
&. Conversion of rotary motion into rotary motion .
The following transmission
elements are used to convert rotary motion to drive shaft to rotary motion driven shaft (a) ;elt drivers. n such driver, a pulley is mounted on drive shaft and another on driven shaft and the motion is transmitted from the drive pulley to driven pulley with the help of belts. Transmission ratio of the drive,
B. 3hain drivers. *oller chain and silent chain are employed in machine tool
drives. The direction of rotation of the driving sprocket and driven sprocket is same
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C. Toothed gearings . toothed gearing serves for transmitting motion between parallel,
intersecting and crossed shafts. +traight tooth and helical tooth spur gears are used to parallel shafts.
2.conversion of rotary motion into rectilinear reciprocating motion.
The following kinematics links are used to convert rotary motion into rectilinear motion (a) Rac and pinion . f the opinion is rotated on the driving shaft the rack will travel in a straight line. or one revolution of pinion, the travel of rack will eual the no. teeth on the pinion, that is, travel of the rack, + will be given as + < p=n mm per min ' < pitch of rack teeth, mm = < no. of teeth on pinion > < speed of pinion ' < m , where m is module of gear + < m =n mm per min
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(b) !orm and Rac . ?pon rotation of the worm, the travel of will be given as + < pnk 8 < no. of threads C. "cre# and $ut .
*otation of a screw, fixed in axial direction, will cause a nut, held against rotation, to travel in a straight line along the screw. The travel of the nut or the screw can be + < pn mm per min > < speed of screw D. "lider and Cran . The no. of strokes per min of silder crank mechanism euals the
speed in rev min of the driving crank, while the silder travel is calculated from the following relation" + < r mm r < radius of crank, mm % Cran and "lotted rocer arm . The travel of silder is determined from the formulaa
+ < @ .r * < crand radius, mm @ < rocker arm length, mm < distance fro,m the rocker arm pivot to the crank centre, mm
A
Characteristics of Mechnacial stepped Drive
The cutting sped is eual in value to the spindle speed. s is clear from diameters are plotted along x axix and cutting speed along y-axix.refer to fig..7,a vertical line at d<4&Bmm is drawn ,and hori5ontal lines for different values of v (eual in value to n) are fond.or example,let us tae the speeds are "
the corresponding cutting speeds will be "97,A4,CD,&7,&BD,7D,477,7DD,and 6&D mEmin respectively.from points corresponding to this cutting speeds on y-axix ,hori5ontal lines are drawn.Their points of meetings the d<4&Bmm lines are plotted .;y !oining all these points to the origin,the ray diagram will be completed.
6
t is clear that due to limited scale of diagram,it is not possible to draw the ray lines for 7DD and 6&D revEmin.n such cases it is convenient to take d<4&.Bmm and vertical lines corresponding to this diameter drawn.it is clear that the cutting speed corresponding to 7DD and 6&D revEmin will be 7D and 6& m.min respectively. +o therays can be drawn as explainrd above. f needed<&7Cmm can also be used. n this case ,it will be eual in value to nE.The most common use of ray diagram is to find the optimum speed of rotation of given values of v and d.for example,if v<&DmEmin and d< &ADmm,the coreesponding point on the diagram,fig will be a .This point will be lies in bEw rays for nA<7D revEmin to increase the life of the tool. >ow productivity of a machine tools can be written in terms of the metal removal rate in mmcubeEmin that is,
B
&."eries of "pindle "p'eeds for Machine Tools . ;efore we discuss the various the
various series,let us study about the loss of productive capacity. @et the max value of the optimum cutting speed be v. rom the speed chart fig let d f-& be the diameter that can be machined at spindle speed >f-& d! the diameter d laying bEw df and f-& .it is clear from fig that for machining diameter ,there is no spindle ray,the speed rays >!-& and >! being the two definite steps for agiven machine tools .+o,d being greater than !, the spindle speed to be selected will be >f-& which is lower than the calculated one from the diameter $d%. +o there is a speed loss of Fv, which is given as
C
>ow productivity of a machine tools can be written in terms of the metal removal rate in mmcubeEmin that is, 'roductive capacity,
G
f feed depth of cut and d are given then GIn. +o if a lower spindle chart, then there will be loss in the productivity capacity . or 5ero loss of productivity capacity,Fv
&D
"eries ofspindle speeds. s discussed above the spindle speeds vary over a discreet no.
of stages in a stepped deive.This steps usually from a series. This series can be .p,g.' or @ogarithm 'rogression
(&) +peed +pectrum in irthmetic 'rogression. s is clear , in these series, there will be a constant common difference bEw any two consecutive spindle speeds.The common diferrence will be given as "
+pindle speeds will be " 7D,&&D,&6D,4D,CD, and 47D revEmin. t is clear that the value of fi is larger speeds and goes on decreasing at higher speeds, snd so it is not constant. +uch an arrangement is uite unsuitable,because thr lower speeds are too widely spaced while the higher speeds are close together.The productivity capacity loss is thus also not constant for each stage.it is more at lower speeds and less at higher speeds. @et us draw the speed spectrum by taking a simple example of small lathe. t is intended to accommodate workpiece upto &DDmm ina diameter and range of five spindle speeds is to be provided.utting speed is 9mEmin.
&&
>ow with the help of above data a speed a speed spectrum can easily be drawn, irstly a hori5ontal line corresponding to cutting velocity of 9mEmin is drawn.then vertical lines from diameter min,d,d4,d9 and dmax are drawn to meet the speed line. The meeting points are !oin to the origin to get the speed rays.Then by drawing the test line, that shows the speed loss as a%saw diagram $for .' series can be drawn
2 "peeds "teps in G.(
#e know that properties of a 1.' sereies. The common ratio bEw consecutive is eual and constant, that is the series can be written as
f *n and fi are given, 5 can becalculated from the above formula.it is then rounded to a whole number and then corresponding value of *n can be found out..
&
Ran and "lotted rocer )rm
n example of this drive is the movement of ram in a shaper. A.
cams plate and cylinder cams are widely used to obtain working and auxiliary
motions in automatics and semiautomatics. 3am mechanism consist of a cam and a follower. To obtain ,motion in the reuired and of the reuired magnitude, intermediate linkage such as levers, are often arranged bEw the cam follower and the final member,
n fig, , are shown the plate can actuated movements. ?pon rotation of the cam ,the the follower or the final member travels up and down (fig), swings about a pivot (fig b),travels vertically (fig c),or hori5ontally fig (d). 'lates cam may be suitabely employed for motions suare with the cam axix . 'lates cams are of one piece design and are used of comparatively short travel or stroke.
&4
3ylinder cams, fig., are drums having helical slots or cam members. #hen the cylinder rotates ,the followers or the final members travels hori5ontally or vertically.These cams are usually employed to obtain motion parallels with the cam drum axix.+uch cams are of short travel.or longers travels cam plates or members are attached by screws to the drum.The magnitude,direction and speed of the travel of the operative element or follower ,may be varied by the imparting a particular form to the cams.3ams discussed above are single edged cams.The actuate the follower in one direction only and depend on some external force produced by a spring or weight to return the follower to its starting point.Two edged cams are positive motion cams and provide for positive follower travel in both direction.+uch cams have a curvelinear groove which accommodates the follower roller.
4.mechanism
for
periodic
*otation.'eriodic
rotation
mechanisms
and 1eneva
mechanism.
"electing the ma*imum )nd Minimum cuttings speed and +eeds .
The cutting speeds depend chiefly on the material to be machined,the material of the cutting tools,the depth of cut and the rate of feed .Maximum minimum si5es that are to be accommodated, are selecting by analy5ing the manufacturing process.The usual range of cutting speeds and feeds for various combination of work material and tool material can be found in standards tables .The higher the cutting speed larger is the rate of production. &9
The applications of cutting speeds ,higher than the recommended values,will decrease the life of cutting tools which will be sub!ect to premature wear of failure. The rate of feed is selected to suit the machining allowance ,provided for the given operation,and the reuired accuracy and the surface finish.naturally, the the maximum feed rate will be for the roughing cut for the hardest tool Jsoftest work material combination and minimum feed rate correspond to the finishing cut .Kther condition being,eual,the rate prLoducion will increase in direct proportion to an increase in the rate of feed. Therefore to reduce the total machining time so as to increase the rate of production,it is desirable to increase both cutting speed and rate of feed within feasible limits.
"pindle speeds for Rotatory Motion .
fter determining the extremes values of the cutting speed :max and the :min and the extreme value of the diameters of !obs max and min which can be accommodated on a machine tools,the extreme value of spindle speeds >max and >min can be determined by the formula"
The ratio of >max and >min is called the range ratio of the spindle speed variation,that is, the range ratio is "
The value of r will depend upon the work-material combination and the tool life. or a fixed tool life , the various combination of work tool can given a velocity range of 9 to &7
&DD , but on the average it varies from & to &7. The diameter range ratio will depend upon the characerstics of a machine tool.t is normally ranges from 9 to A.
,pper and -o#er speeds limits of a -athe .
s is well known,the maximum diameter of a!ob that can be accommodated on a centre lathe is the swing of layhe. /xperience has shown that it is logical to relate the diameters range ratio to the height of the centers,h,above the lathe bed.Therefore,
>icolson and smith have also suggested a formula for >min .ccording to them ,:min should be selected such that a finishing cut at a cutting speed of &7 f.p.m. can be taken on a work piece diameter of slightly less than the swing of the lathe,with a .+.+ tool. They suggested max <(h-rooth),h in inches
. &A
"tepped and "tepless drives .
#ith a constant speed motor , there is need of some method of varying the speed over this range. The infinitely variables speed rotation is possible with suitable mechanical hydraulic and electrical drives. owever, the torue characteristics of available stepless drives do not meet the reuirements of spindle dives which demand
and increased
driving torue to the spindle at lower output speeds in order to maintain a constant rate of metal removal. The stepless drive which posses the reuired torue characteristics ,are limited by the speed range over which these character stics can be mad. n order to provide for a wide range of operating speeds together with adeuate torue at lower spindle speeds. t is necessary that spindle range be convered in a number of discreet steps.This is achieved by a constant speed motor used in con!unction with cone pulleys or a gearbox which provide for a series of spindle speeds in a mechanical stepped drive.The number of speed steps provided in a machine tool is likely to represent a compromise between what is desirable for efficient operation and the cost of drive (it will increase with the number of steps) which can be !ustified in given conditions.
Characterstistics of mechanical stepped drive .
f the work piece diameter d and the optimum cutting speed v are given or known,then the corresponding spindle speed can be determined with the help of eucuttings speed charts of the rectilinear type are often used to uickly determined the spindle speeds for a give cutting speed, ,/ach slanting line or $ray% repersen a definite step,in rpm, of the spindle speed of the given machine tool logarithmic chart of the type.fig., is often used instead of the rectilinear chart to determine spindle speeds. 3onstructions of ray diagram.t is clear from eu
:< dnE&DDD mEmin
Then v N d with n being constant. This relation is represented graphically by a straight line passing through the origin,because v
&6
of straight lines or rays emerging from the origin for a given cuttings speed range will be eual to the number of steps in this range. This is achieved as follows"
: < nE&DDD.d< nE4&B.d
d<4&Bmm, v < n
&B
"tepless mechanical drives
+teeples or infinitely variable main and feed drives have found considerable application in modern machine tools. Their main advantages are"
the possibility of
setting up the optimum cutting conditions (speed and feed ) with higher accuracy than with a stepped drive and the possibility of changing speed of the main drives or feeds without stopping the machine.the steeples, drives can be" mechanical, electrical ,hydraulic or combined drive. they have their own advantages and disadvantages . the selection will depend upon " purpose of the machine ( general or special )O for roughing ,finishing or micro finishing " range ratio, the power reuired and the cost. Mechanical steeples drives are of" friction type or positive type . the operation of friction type drives involves friction losses .other drawbacks of mechanical drives are-rigid kinematic characteristics and the variation in the maximum transmitted power when the speed of changed.
+riction type stepless mechanical drives. "-
These drives are based on the principle " the driven link contact the driving link either directly or through some intermediate element ( roller ,disc, ring or belt ). the driven and driving elements are held tightly toghter and the friction force developed will cause one element to rotate when the other is rotated . if the diameter of contact on both the elements (driving and driven ) or on at least one of them is varied, then the transmission ratio of the drive will very accordingly . there are many designs of friction type dev ices .some of these will be discussed below " &. Roller and disc drive / this drive ,fig. is a very elementary friction drive . single control lever permit smooth variation in speed ratios over a wide range from 5ero when the roller is over the recessed portion of the disc to a maximum when the roller contacts the disc at its outer end. the transmission ratio being given as O
i< rE* (* being variable) direction of rotation can be completely reversed by bringing the friction roller into contact with the disc either side of the recessed center portion . the roller must be mounted on a splinted shaft to move it across the face of the disc , while being positively &C
driven by the shaft .drawbacks of this drive are " uneven wear of disc and the rapid wear of the roller . maintaining proper pressure bEw the contact surface is another cause for trouble .
. +riction cone #ith flat 0elt drive "- The principle of this drive is the same as that of the stopped cone pulley and belt system ,except that there are no fixed steps but rather it proviedes the possibility of very slight changes in speed ratio over a wide range of ad!ustment of speed is obtaind by means of a belt frome slipping , the cone angle is usually &Ddia. . the drive is not suitable for large powers due to the flexibility of the belt.
D
4. "preading conical pulleys drive/ fig. shows the distance bLw the pairs of driving and driven cone pulleys can be changed by axially moving one member in each pair. #ith this the diameter of contact of the belt with driving or driven pairs of cone pulleys can be hardened metal ring as the frictional member can be used.
1. 3onical discs and friction roller drive/ fig. shows friction roller arranged bLw
spherically-shaped cones or discs on the driving and driven shafts may be inclined in different position.this will change radii of contact bLw the roller and the d riving and driven discs and thereby the transmission ratio. in the position shown in the figure, (n) driven (n) driver (r being r ). when the axes of the top and bottom rollers coincide vertically , the speeds of driving and driven pulleys will be same. the speed of the driven shaft is transmitted to the spindle through a :-belt drive. by changing the position of the rollers along the spherical surface of cones , spindle speeds can be infinity varied from the minimum to the maximum values. the range can be 9 to B. by combining this infinity variable speeds can be obtained
&
(ositive drive. the positive infinity variable drive is a variant of Pspreading conical
pulleys friction drive P in which a chain is used in places of a belt and the conical faces of the alloy-steel wheels are grooved . the self tooth forming chain engages with these grooves. a control hand wheel moves the control lever about central pivolts to change the ratio of the effective wheel diameter. the chain links are slidable transverse slots that form the power transmitting teeth. a shoe mechanis, with spriong tension, applies pressure to the slack side of the chain to keep it in ad!ustment. all moving parts run in oil with autometic splash lubrication.
ydrolic drives
ydrolic drives are videly used to obtain infinitly variable rates of rectilinear motion in machine tools.mostly, it refer to feed motions but in some machine tools main drive speeds are varied in the way, the advantage of hydrolic drive are " &. rapid and infinitly variable ad!ustment obtainable during operation , for the length, speed and direction of travel of a machine tool unit. . faster , revers and acceleration rates are possible because of less inertia and cushioning effect of the fluid. 4. the drive is smooth and reverses without shock. 9. automatic protection against overloads. 7.ability to stall against an obstruction without damage to the tool or to the machine. A. convenience of remote control and its automation. 6. self lubrication of the system. Dra#0ac &. nsufficiently flat characteristic curve mresulting from leakage.
. effect of the temperature on the oil viscosity. 4. at slow speed (& to &7), the operation of the drive becomes unstable. hydraulic drive for straight line motion consist of" (a) pump, which delivers oil under pressure (b) n operating cylinder with a piston and piston rod (c) control devices.
/ither the piston rod or the cylinder is attached to the operative unit of the machine tool and the other is stationary. The speed of travel of the operative unit will depend on the volume of oil delivered by the pump in a unit of time. The speed can be ad!usted in two ways" (i) by using a variable delivery pump which can deliver different amounts of oil per unit of time. (ii) by using a constant delivery pump and ad!usting the speed with the help of a throttle valve.
The variable delivery pumps are more expensive and complex than constant delivery pumps. This system is more efficient than a throttle type system. however, in many cases, the difference is not large and the saving in operating costs alone does not !ustify the more expensive euipment. There are two ways of controlling the speed in the throttle type system"
(a) Metering in control fig. / the throttle valve is mounted in the pipeline through which oil is delivered to the cylinder.
4
(b) Metering/out control fig. / here,the throttle valve governs the discharge oil from the opposite end of the cylinder.The amount of oil passing through the ad!ustable throttle valve per unit time,will determine the piston speed.if the amount of the oil delivered to the working end of the cylinder is too large.surplus oil is draid to the tank through a relief valve.
(umps., the pumps used in hydraulic drive can be " &.constant delivery pumps (non-ad!ustable) 2.variable delivery pumps (ad!ustable)
ccording to their principle of operation and design, pumps are classified as, (i)
gear type pump (ii)vane type pump (iii) piston type pump
(a)
Gear pump/ it is the simplest and the most commonly used type of pump. it is a
constant displacement pump , and is designed for operating pressure up to 7 atm. it consists of two spur gears, of eual diameter. The discharge pressure can be increased by increasing the speed of the pump and the number of teeth of the gears.
9
(b) Vane pump/ in this pump rotor revolves in a housing . the rotor caries vanes located in slots of the rotor. these pump ,also have constant displacement and designed for working pressues upto A7 atm .
(c) rotary piston pump/ these pump may be designed with either radically or axially arranged pistons. they may operate at pressure up to &DD atm. in the radial arrangement fig. (c), the cylinder rotate centrically in the core of a circular housing.theoil inlet and outlet ports are located near the centre of the rotating cylinder block. when the rotating piston moved outward, oil will be sucked in the rotating cylinder and it will be discharged out when the piston moves inwards. these pumps have delivery ranges of D. to 9lLmin. for small si5e models and &B to ADDLmin. for large si5e models. ydraulic drives with a power rating of Q D.97 kw are economically sound. a hydraulic drive is advisible for dev elopment high torues and pulling force. its cost is less as compared to electric drive of same rating . the application of hydraulic drive in copy system and flow forming lathes is also of great importance.
ydraulic drive for rotary motion. variable displacement pumps are inversible and
can, therefore, be employed either as either as pumps or rotary motion. fig. shows the layout of a hydraulic drive for rotary motions. the variable capacity fluid pump with one direction of flow is run by an electric motor. the rotary motion of the hydraulic motor is utillised to have a rotary motion of the rotary hydraulic motor. check valve is operated when the rate of flow of oil circulating in the system is changed as a result of a change in the pump or hydraulic motor continues for a certain time to run by interia at the same speed but with a changed rate of flow. when this valve opens, the reuired amount of oil 7
is admitted into the system, so as to compensate for the insufficiency of the circulating volume of oil . shut off valve can be used to stop the shaft of the hydraulic motor rapiply without stopping the pump. back pressure valve and cushioning or damping valve protect the system against shock loads in the periods of bracing and reversal. the motors for rotary hydraulic drives are particularly complex, expesive and of low effciency after wear and are not common.
A
(neumatic drives
compressed air is already available in most plants and can be put to work with rather inexpensive euipment. air flow is fast. but the use of compressed air has several disadvantages that limit to light service. 'ressure available are usually not high. a compressed air system by itself is hard to control because of the compressibility of air. feeds and speeds are inclined to vary too much as the load changes and the euipment may not stop and levers within desired limits. an air driven but hydraulically controlled circuit, fig., mitigates some of the shortcomings of air, compressed air is admitted into the oil tank at a pressure of D.9 to D.7 nLmm. thus, the speed of the piston depends on the si5e of the sat in the throttle valve through which oil flows.
6
%lectrical drives
The trend in the development of machine tools drives has been towards more complete motori5ation. the electric drives of machines tools comprise one or several drive motors and devices for their control. up to date electric motors can reversed or braked. the motori5ation of machine tools and an ever wider use of electric controls will continue to be one of the principal factors in their improvement, in increasing the rate of production and reliability of machine tools and in reducing operator fatigue. three phase suirrel cage motors for a power supply of DL4BD for 7DDv , 7D cEs are most freuently employed in machine tools. such motors have aflat characteristics. the speed flats only slightly with an increase of load. three phase electric motor have speeds in rpm near to the followings series " 4DDD , &7DD, &DDD, 67D,ADD and 7DD. shunt wound direct current motors also have a flat characteristics. the speeds of these motors are usually infinitely variable in a speed range ratio of 4 or 7. the main advantage of these motors is that their speeds are infinitly variable. electric motors permit momentry overloads upto &.7 to times the rated vaues.this feature is used in starting a motor since a considerable overload is experienced at this time.
B
