PS1-DESIGNING A DIRECT DRIVE TABLE FOR GEAR HOBBING MACHINE

1. A REPORT

ON

DESIGNING A DIRECT DRIVE TABLE
FOR GEAR HOBBING MACHINE


By



Name of student ID Nos.
Ashwin Shirbate 2014B3AB509P
Tushar Sharma 2014ABPS736P
Akshay Golechha 2014B5A4844P
Sukrit Rao 2014A4PS294P
Mohammed Pahadwala 2014A4PS416P
Keerti P. 2014B1AB777P
Pranav Bang 2014B2A4780H
Vivek Prakash 2014ABPS498P





AT

(HMT Machine Tools Division, Bangalore)

A Practice School I Station of



BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI


(July, 2016)

2. 1
A REPORT ON

DESIGNING A DIRECT DRIVE TABLE
FOR GEAR HOBBING MACHINE
by

Name of the student (s) ID No.(s) Discipline(s)

Ashwin Shirbhate 2014B3AB509P MSc (Hons.) Economics &
B.E.(Hons.) Manufacturing
Engineering

Tushar Sharma 2014ABPS736P B.E. (Hons.) Manufacturing
Engineering

Akshay Golechha 2014B5A4844P MSc (Hons.) Physics &
B.E.(Hons.) Mechanical Engineering

Sukrit Rao 2014A4PS294P B.E. (Hons.) Mechanical Engineering

Vivek Prakash 2014ABPS498P B.E. (Hons.) Manufacturing
Engineering

Keerti. P 2014B1AB777P MSc (Hons.) Biology &
B.E. (Hons.) Manufacturing
Engineering

Mohammed Pahadwala 2014A4PS416P B.E. (Hons.) Mechanical
Engineering

Pranav Bang 2014B2A4780H MSc (Hons.) Chemistry &
B.E. (Hons.) Mechanical
Engineering

Prepared in the partial fulfillment of the
Practice School I Course
AT


(HMT Machine Tools Division, Bangalore )

A Practice School I Station of



BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI

(July, 2016)

3. 2
Acknowledgements


For the success of this project, we would like to thank our Manager,
Mr.Venugopal, and our mentor, Mr. Nagraj, for their unparalleled effort and
guidance in every step of this project. This project would have been almost
impossible if not for their help.

We would also like to thank our Practice School Division instructor,
Mr. Chandra Shekhar R K, for his help and support throughout.

4. 3
BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE
PILANI, RAJASTHAN

Practice School Division


ABSTRACT SHEET

Station: HMT Machine Tools division Duration: 8 weeks
Centre: Bangalore Date of Report Submission: June 15, 2016
Date of Start: June 23, 2016

Title of the Project: "Development of a direct drive table for gear hobbing machine"

Name of the Student:
Ashwin Shirbate
Tushar Sharma
Akshay Golechha
Sukrit Rao
Mohammed Pahadwala
Keerti P.
Pranav Bang
Vivek Prakash

Discipline:
MSc (Hons.) Economics &
B.E.(Hons.) Manufacturing
B.E. (Hons.) Manufacturing
MSc (Hons.) Physics & B.E.(Hons.)
Mechanical Engineering
B.E. (Hons.) Mechanical Engineering
B.E. (Hons.) Mechanical Engineering
MSc (Hons.) Biology & B.E.(Hons.)
Manufacturing
MSc (Hons.) Chemistry &
B.E.(Hons.) Mechanical Engineering
B.E. (Hons.) Manufacturing
ID No. 2014B3AB509P
2014ABPS736P
2014B5A4844P
2014A4PS294P
2014A4PS416P
2014B1AB777P
2014B2A4780H
2014ABPS498P
Name of the Mentor: Mr. Nagaraj Designation: Design Department
Name of the Manager: Mr. Venugopal Designation: Dy. Manager (Training)


Name of the PS Faculty: Dr. Chandra Shekhar


Key Words: Gear hobbing, Direct drive, torque motor
Project Area: Solidworks Designing/remodeling
Abstract: The project aims at improving the current design of a gear hobbing machine and replacing the
conventional drive for the work table with a direct drive table. This modification will make the gear
hobbing machine durable and increase working life of the machine. The modification will also make the
machine energy efficient by reducing energy losses, and increase the accuracy and control of the drive
table.

5. 4
Signature of Student Signature of PS Faculty







Date Date

6. 5


BIRLA INSTITUTE OF SCIENCE AND TECHNOLOGY

PILANI (RAJASTHAN)

PRACTICE SCHOOL DIVISION

Response option sheet

Station:HMT Machine Tools Division Centre: Bangalore

ID No. &Name(s): Ashwin Shirbhate (2014B3AB509P) Sukrit Rao (2014A4PS294P)
Vivek Prakash (2014ABPS498P) Keerti P. (2014B1AB777P)
Tushar Sharma (2014ABPS736P) Akshay Golechha (2014B5A5844P)
Mohammed Pahadwala (2014A4PS416P) Pranav Bang (2014B2A4780H)

Title of the Project: Designing a Direct Drive Table for a Gear Hobbing machine

Usefulness of the project to the on-campus of study in various disciplines. Project should be scrutinized
keeping in view the following response options. Write course No. and Course Name against the option under
which the project comes.

Refer Bulletin for Course No. and Course Name.
Code No Response Options Course No. (s) & Name
1. A new course can be designed out of the project NO
2. The project can help modification of the course
content of some of the existing courses.
NO
3. The project can be used directly in some of the
existing compulsory Discipline Courses
(CDC)/Discipline Courses Other than Compulsory
(DCOC) /Emerging Area (EA) etc. Courses.
NO
4. The project can be used in preparatory courses like
analysis and Application Oriented Courses
(AAOC)/ Engineering Science (ES)/ Technical;
Art (TA) and Core Courses
YES
5. This project cannot come under any of the above
mentioned options as it relates to the professional
work of the host organization
NO




_________________ _________________
Signature of Student Signature of Faculty

7. 6
TABLE OF CONTENTS:

1. Introduction
1.1. Aim: Development of Direct Driven Table for Gear Hobbing Machine.
1.2. About Hindustan Machine Tools Ltd.
1.3. Reasons for Choosing This Topic
2. Gears
2.1. Basic gear Nomenclature
2.2. Types of Gears
3. Gear Hobbing?
4. Difference Between Gear Hobbing and Gear Cutting?
About the H400/H250 Gear Hobbing Machine
5. Conventional Hobbing Machine Design
6. Direct Drive Mechanism
6.1. Direct Drive Motor- Construction and Working
6.2. Advantages
6.3. Disadvantages
6.4. Why Adopt Torque(direct drive) Motors?
7. Choosing the Suitable Motor
8. Rotary Table Alternative Designs
9. Mathematical Model of the Rotary Table
10. Conclusion
11. Solidworks Model of the Design
12. References

8. 7
1. Introduction
1.1 Aim: Development of Direct Drive Table for Gear Hobbing Machine.
1.2 Reasons for Choosing This Topic
After taking a tour of the shop floor with its various sections we chose this topic for the
following:
1. It involved a variety of aspects from studying the design of the machine on solidworks to
the stress-strain analysis on components in the machine.
2. It was a project which would be beneficial to both HMT and us.
2.1 It would help HMT by giving them a model by which they could try reducing losses and
increase productivity.
2.2 It would help us by expanding our existing knowledge on basic manufacturing processes
as well as improving our machine designing skills. It would also help us in understanding
practical limitations being faced in the industry.

1.3 About Hindustan Machine Tools Limited
Hindustan Machine Tools Limited is public limited company which was founded in the year 1953
by then Prime Minister of India Pandit Jawaharlal Nehru. Its aim was to make independent India
self-sufficient in the production of capital goods and thus build a stable base upon which India’s
manufacturing, transport, agriculture and defence industries could be established.
Since its establishment in 1953, HMT has made large strides in a variety of sectors leading to a
diversified set of machine output including tractors, watches, printing machines, plastic process
machinery and CNC systems and bearings. After the closure of their watch division announced in
January 2016, HMT comprises of five subsidiaries viz-a-viz Central Conditioning Division, CNC
Systems Division, Die Casting Division, Precision Machinery Division and Machine Tools
Division, under the ambit of a parent holding company headquartered in Bengaluru, Karnataka.
HMT Machine Tools’ expertise in machine tools has been honed to a point that it can design and
develop any kind of machine. From simple lathes to multi-station transfer lines, from stand-along
CNC machines to flexible manufacturing systems, leading to factory automation, HMT Machine

9. 8
Tools’ Products cover general purpose machines, special purpose machines and CNC machines to
meet the application needs of every engineering industry. To date, over 100,000 machine tools on
par with international standards in quality and performance, manufacture by HMT, are in use all
over India. The Company also manufactures sheet fed offset printing machines in single, two,
four, and five colours, programmable paper guillotines, ball screws, and CNC Control Systems.
HMT’s pioneering spirit and cutting-edge marketing abilities enable it to showcase its products
and services to a worldwide clientele. The establishment of HMT (International) Limited
leveraged the Company’s international trading experience. HMT(I) markets the products through a
global network that extends over 40 countries to service its customers worldwide. HMT(I) has a
diverse clientele with more than 18,000 machines in over 70 countries including the developed
ones.
HMT Machine Tools, jointly with HMT(I) has been instrumental in executing various
international turnkey projects in Algeria, Tanzania, Nigeria, Malaysia, Iraq, Mauritius, Indonesia,
Kenya, Ethiopia, Iran, Maldieves, Senegal, Turkemenistan, Nepal, Zambia - to name just a few.
The Machine Tool Division was once the market leader in production of capital goods and output
levels were sufficient enough to meet the country’s needs. The recent liberalization of the
economy and entry of foreign as well as domestic competitors has lowered its hold over the
market. However it remains an important player in the industry having witnessed record sales for
the year 2014-2015 and recording an annual turnover Rs 73.69 crore.

10. 9
2. Gears
A gear is a simple machine used to transmit motion or torque with the help of cogs or teeth. Gears
can be used to produce changes in torque. Two gears, which are in mesh, always have the same
teeth shape. A system of multiple gears, working in a sequence, is called a gear train.
Gears transmit motion without any slip unlike belt pulley or chain sprocket, making it better
suitable to reduce losses.










2.1 Basic Gear Nomenclature:
Pitch Circle: An imaginary circle passing through the pitch points of the gear.
Addendum: Radial distance between pitch circle and top of gear tooth.
Dedendum: Radial distance between pitch circle and bottom of gear tooth. Dedendum of a gear is
always greater than its addendum.
Whole depth: The total distance between the top and bottom of the gear tooth is called the whole
depth.
Working depth: When two gears are in mesh, the depth of bite of the two gears is called working
depth. It’s equal to the sum of the addendums of the two gears.
Clearance: Mathematically, clearance equals the difference between whole depth and working

11. 10
depth.
Pitch Diameter (d): Diameter of the pitch circle from which the gear is designed. The pitch circle
goes through the pitch points, which are the points of contact when two gears are in mesh.
Number of teeth (T): Number of teeth on the circumference of the gear.
These two variables can be used to find the module of a gear, which is given by m = d/t. Module
of a gear indicates its tooth size. For two gears to mesh, they should have
the same module.
2.2 Types of Gears:
Spur gears: Simplest type of gear with teeth projecting radially
outward. Spur gears are used to transmit motion across parallel axes.

Helical gears: They are gears in which the teeth
of the gear are set at an angle that is not parallel to
the axis of the gear. This helps the gears to mesh
more effectively, causing them to run smoothly and
quietly. They are commonly used when high speeds
and low noise levels are required. A major
disadvantage is the resultant thrust along the axis of
the gear, which can be solved using lubricants or
using a double helical gear.

Bevel gears: Bevel gears are gears whose shapes are conical. When
two bevel gears are in mesh, their axes intersect. Hence, this makes it
possible to change the operating angle.

Spiral gears: These are bevel gears with helical teeth. The helical
design helps reduce vibration and noise than a conventional spur gear.
Their advantages and disadvantages are similar to those of helical gears.

12. 11
Rack and pinion gear system: A rack is a toothed bar
that is in connection with a spur gear. The rotational motion
applied on the pinion causes the rack to move linearly with
respect to the pinion. This system is used in trains to force
them up steep slopes.


3. Gear Hobbing
Gear hobbing is a special form of milling operation used to prepare gears. The teeth or splines are
progressively cut into the workpiece by a series of cuts made by a cutting tool called a hob.
Compared to other gear forming processes it is relatively inexpensive but still quite accurate, thus
it is used for a broad range of parts and quantities.
It is the most widely used gear cutting process for creating spur and helical gears. It is the most
extensively used process for preparing gears not only due to its flexibility and broad range of
output but also since it is quick and relatively inexpensive.
Hobbing machines, also known as hobbers, are fully automated machines that come in many
sizes, because they need to be able to produce anything from tiny instrument gears up to 10 ft (3.0
m) diameter marine gears. Each gear hobbing machine typically consists of a chuck and tailstock,
to hold the workpiece, a spindle on which the hob is mounted, and a drive motor.
Since the gear ratio between hob and blank is fixed, the resulting gear will have the correct pitch
on the pitch circle, but the tooth thickness will not be equal to the space width.
Hobbing machines are characterized by the largest module or pitch diameter it can generate. For
example, a 10 in (250 mm) capacity machine can generate gears with a 10 in pitch diameter and
usually a maximum of a 10 in face width. Most hobbing machines are vertical hobbers, which
means the blank is mounted vertically. Horizontal hobbing machines are usually used for cutting
longer workpieces; i.e. cutting splines on the end of a shaft.
The hob is a cutting tool used to cut the teeth into the workpiece. It is cylindrical in shape with
helical cutting teeth. These teeth have grooves that run the length of the hob, which aid in cutting

13. 12
and chip removal. There are also special hobs designed for special gears such as the spline and
sprocket gears.
The cross-sectional shape of the hob teeth are almost the same shape as teeth of a rack gear that
would be used with the finished product. There are slight changes to the shape for generating
purposes, such as extending the hob's tooth length to create a clearance in the gear's roots. Each
hob tooth is relieved on the back side to reduce friction.
Most hobs are single-thread hobs, but double-, and triple-thread hobs increase production rates.
The downside is that they are not as accurate as single-thread hobs. Depending on type of gear
teeth to be cut, there are custom made hobs and general purpose hobs. Custom made hobs are
different from other hobs as they are suited to make gears with modified tooth profile. The tooth
profile is modified to add strength and reduce size and gear noise.

4. Difference between Gear Hobbing and Gear Cutting?

● Gear hobbing is a generating process.

● The term generating refers to the fact that the shape of the gear tooth that is formed is not the
conjugate form of the cutting tool.

● Rather, the shape of the tooth is generated by the combined motions of workpiece and cutting
tool

● In gear cutting the shape cut into the workpiece is the same as that of the cutting tool.



5. About the H400/H250 Gear Hobbing Machine
The H400/H250 Gear Hobbing machine is ideal for production of spur, helical and
worm gears, taper gears and crown gears required in automobiles, LMVs,defence
trucks, tractors, gear boxes, etc.

14. 13
Salient Features :

● Gear Hobbing Machine H250/H400 for axial and radial feeds and
also for tangential feed.
● Infinitely variable axial and radial feeds.
● Hobbing of taper gears and crown gears through CNC
interpolation.
● Multicut facility ensuring easy hobbing of bigger module gears.
● Auto speed, feed and direction change.
● Ideal for hobbing spur and helical gears, worm gears by radial
hobbing, taper gears and crown gears required in automobile & auto
ancillaries.
● This machine can be offered as :
● Single axis CNC - radial feed (X) axis or
● Two axes CNC - radial and axial feeds (Z and Z axes)




Specifications :

Rapid Traverses
Radial (X) mm/min 3000
Axial (Z) mm/min 1500
Tangential shifting (Y) mm/min 325

15. 14
Positioning Accuracy/ Repeatability
X-Axis mm 0.01 / 0.005
Y-Axis mm 0.02 / 0.01
Z-Axis mm 0.02 / 0.01
Control system Siemens/Fanuc

16. 15

17. 16




6. Conventional Hobbing Machine Design :

Hobbing machines provide gear manufacturers a fast and accurate method for cutting parts. This is
because of the generating nature of this particular cutting process. Gear hobbing is not a form

18. 17
cutting process, such as gashing or milling where the cutter is a conjugate form of the gear tooth.
The hob generates a gear tooth profile by cutting several facets of each gear tooth profile through
a synchronized rotation and feed of the workpiece and cutter. This manner of cutting a gear is
made possible by the hob effectively acting as a mating worm of the gear; however, this “worm”
has gashes in order to provide a way of cutting (generating) the teeth.

For a single-thread hob, the hobbing machine synchronizes each revolution of the cutting tool to
one tooth of the workpiece. For example, such a hob would rotate twenty times per revolution of a
twenty-tooth gear. As the hob feeds across the face of the workpiece at a fixed depth, gear teeth
will gradually be generated by a series of cutting edges, each at a slightly different position. The
number of cuts made to generate the gear tooth profile will correspond to the number of gashes of
the hob. Simply put, more gashes produce a more accurate profile of the gear tooth.
Due to the worm-like nature of the hob with respect to the gear, several cutting edges will be
working simultaneously, which provide significant potential for fast cutting speeds and/or short
cycle times. With this realization, one can see the hobbing process advantage over other cutting
processes.
All gear hobbing machines, whether mechanical or CNC, consist of five common elements, which
are listed below and shown in Figure.

● A work spindle to rotate the workpiece (shown in blue)
● A cutter spindle to rotate the cutting tool, the hob (shown in yellow)
● A means to rotate the work spindle and cutter spindle with an exact ratio, depending on
the number of teeth of the gear and the number of threads of the hob (shown in red)
● A means to traverse the hob across the face of the workpiece (shown in green)
● A means to adjust the center distance between the hob and workpiece for different size
work pieces and hobs

19. 18



In summary, the hobbing process uses a worm-like cutter (the hob) that makes successive
generating.
Hob Feed :
Figures of feed motion of hob show the cutting of teeth on the workpiece with the hob. There are
three principal feed motions with respect to the workpiece [Figs. 5 (a), (b) and (c)]. Feed motion in
the direction OR is the radial-feed motion. It is called radial because it is in the radial direction of
the workpiece. Feed motions in the directions of OA and OT are called axial-feed motion and
tangential-feed motion, respectively. There can be feed motions which are combination of any two
of these primary feed motions.

20. 19



Axial feed is the most commonly used feed, unless there is restriction for axial motion. The hob is
fed to the full depth form and then fed in axial direction to generate the full width of the gear.
Radial feed is used where there is restriction to axial motion, for example, in cutting distributor
gear on camshaft, where there is obstruction on both sides of the gear due to the presence of the
cam lobes. In this case the hob is fed in radial direction. It can be seen from Fig. 5 (a) that the
tooth depth is not constant along the axis. For this reason, the hob has to be set centrally with
respect to the gear being cut. In tangential feed, the hob moves at a direction tangential to the gear
being cut. This method of cutting is used for producing worm wheels.

21. 20

When any two of these feeds are used together, we get a resultant feed which is dependent on the
ratio of the two feeds. The most commonly used combined feed is called diagonal hobbing in
which tangential and axial feed are used simultaneously. This is shown in fig. 6 normally in radial
and axial feed, about 1½ to 3 tooth-pitches do the work, whereas in diagonal hobbing more
number of teeth are in operation. Accuracy of the gear produced by diagonal hobbing depends on
the accuracy of the hob over the number of teeth, and its pressed over number of teeth. As in
radial and axial feed only a limited number of teeth do the actual cutting, the wear of the hob will
be concentrated only on these few teeth. In order to spread this wear over all the teeth of the hob,
many machines are fitted with automatic hob shift.

22. 21
Hobbing Process Diagram :



7. Direct Drive Mechanism


A direct drive mechanism is one that takes the power coming from a motor without any reductions
(such as a gearbox). The concept of Direct Drive technology involves replacing conventional
servo motors coupled to some form of mechanical transmission such as gearbox, timing belts,

23. 22
pulleys or ball screws. Mechanical transmissions introduce backlash, mechanical losses and noise
that can reduce machine performance.


7.1 Direct Drive Motor
Construction and Operation
Direct drive systems couple the system’s load directly to the motor without the use of belts or
gears. In some situations, brushed or brushless servo motors may lack adequate torque or
resolution to satisfy some applications’ needs. Therefore, mechanical means, such as gear
reduction systems to increase torque and resolution, are used to meet system requirements. The
direct drive can provide very high torque in a modest package size and solves many of the
performance issues of the gear reducer. All in a system that is as easy to use as a stepping motor
Fig. 1.45 below shows the construction of a direct drive motor compared to a conventional motor
with a gear reducer. The gear reducer relies on large amounts of frictional contact to reduce the
speed of the load. This gearing effectively increases torque and resolution but sacrifices speed and
accuracy. The direct drive motor is brushless and gearless so it eliminates friction from its power
transmission Since the feedback element is coupled directly to the load, system accuracy and
repeatability are greatly increased and backlash is eliminated.

24. 23
The motor contains precision bearings, magnetic components and integral feedback in a compact
motor package (see Fig. 1.46). The motor is an outer rotor type, providing direct motion of the
outside housing of the motor and thus the load. The cross roller bearings that support the rotor
have high stiffness, to allow the motor to be connected directly to the load. In most cases, it is not
necessary to use additional bearings or connecting shafts.

The torque is proportional to the square of the sum of the magnetic flux (Øm), of the permanent
magnet rotor and the magnetic flux (Øc ), of the stator windings. See Fig. 1.47. High torque is
generated due to the following factors. First, the motor diameter is large. The tangential forces
between rotor and stator act as a large radius, resulting in higher torque. Secondly, a large number
of small rotor and stator teeth create many magnetic cycles per motor revolution. More working
cycles means increased torque.

25. 24


7.2 Advantages
● Increased efficiency: The power is not wasted in friction (from the belt, chain, etc., and
especially, gearboxes.)
● Reduced noise: Being a simpler device, a direct-drive mechanism has fewer parts which
could vibrate, and the overall noise emission of the system is usually lower.
● Longer lifetime: Having fewer moving parts also means having fewer parts prone to failure.
Failures in other systems are usually produced by aging of the component (such as a
stretched belt), or stress.
● High torque at low rpm.

26. 25
● Faster and precise positioning. High torque and low inertia allows faster positioning times
on permanent magnet synchronous servo drives. Feedback sensor directly on rotary part
allows precise angular position sensing.
● Drive stiffness. Mechanical backlash, hysteresis and elasticity is removed avoiding use of
gearbox or ball screw mechanisms.


7.3 Disadvantages
The main disadvantage of the system is that it needs a special motor. Usually motors are built to
achieve maximum torque at high rotational speeds, usually 1500 or 3000 rpm. While this is useful
for many applications (such as an electric fan), other mechanisms need a relatively high torque at
very low speeds, such as a phonograph turntable, which needs a constant (and very precise) 331⁄3
rpm or 45 rpm.
The slow motor also needs to be physically larger than its faster counterpart. For example, in a
belt-coupled turntable, the motor diameter is about 1 inch (2.5 cm). On a direct-drive turntable, the
motor is about 4" (10 cm).

Also, direct-drive mechanisms need a more precise control mechanism. High speed motors with
speed reduction have relatively high inertia, which helps smooth the output motion. Most motors
exhibit positional torque ripple known as cogging torque. In high speed motors, this effect is
usually negligible, as the frequency at which it occurs is too high to significantly affect system
performance; direct drive units will suffer more from this phenomenon, unless additional inertia is
added (i.e. by a flywheel) or the system uses feedback to actively counter the effect.

27. 26
7.4 Why Adopt Torque Motors?

Reduced cost of ownership
Direct coupling of the payload to the rotor eliminates the need for mechanical transmission
elements such as gearboxes, timing belts, speed reducers and worm gear drives. Unlike brushed
rotary motors, there is no contact between rotor and stator; therefore there is no mechanical wear
resulting in excellent reliability and long lifetimes. Fewer mechanical parts also minimizes
maintenance and reduces the system cost. The direct drive technology intrinsic to a torque motor
system results in an efficient and effective gearless assembly.




Easy integration

Conventional rotary table
Torque motors are available in a wide range of sizes and can be easily adapted to most
applications. The use of magnets and limited air gap results in a large hollow shaft or bore for easy
integration of cables, cooling tubes, or other application related equipment. The ring-like
configuration of a torque motor minimizes the volume required for mounting. This gives the
machine designer great flexibility in locating the motor to work with bearings, feedback devices,
and payload.

28. 27
Dynamic performance
Dynamic performance is drastically improved by using direct drive due to the very high control
loop bandwidth that can be achieved on the overall system. The direct coupling of the load and
position feedback to the motor has the advantage of eliminating all phenomena that limit the
dynamic performance on non direct driven machines. Eliminating long-time drift, elasticity, and
backlash is a huge advantage for machine performance and lifetime.
Torque motor applications have a wide range of dynamic performance requirements. Depending
on the specifics of a system’s duty cycle, the peak torque, continuous torque, or both will drive the
selection of a motor.





Wide torque-speed range
Direct drive torque motors deliver high torque over a wide range of speed, from a stalled or low
speed condition to high angular velocities. While torque motors can achieve high velocities (up to
5'000 rpm), there is a trade-off in torque as the motor becomes limited by speed dependent losses
increase. The performance of a torque motor over its velocity range can be seen in the
torque/speed curve present in the corresponding data sheet.
The torque motor is part of a complete direct drive solution which includes a position controller.
High-end digital controllers like the ETEL position controllers, which have been designed
specifically for direct drive applications provide excellent control loop quality ensuring optimum
stiffness, smooth motion, and excellent velocity control with low torque ripple.

29. 28
8. Choosing the Suitable Motor
First condition for choosing of the suitable motor should be an analysis of the estimated load:

Case 1: Continuous operation
A motor running in continuous operation must transmit its lost heat to the ambience so that the
max. coil temperature is not exceeded. In the technical data the current values and corresponding
torques are shown for a max. coil temperature of 100°C, water cooling with a cooling temperature
of 20°C implied. The coil temperature is monitored by temperature sensors. For interpretation in
the control system a KTY sensor and a PTC for each motor phase are available. In continuous
operation a regular load of all three phases and a regular temperature distribution can be assumed.
Because of comparatively slow temperature changing the monitoring of the motor can be achieved
with the KTY as well as with the three PTC‘s. To increase operational safety we recommend the
interpretation of both sensor types.


Case 2: Interval operation
In interval operation the value of the surface below the load graph of the motor is important. The
effective torque is calculated with the well known formula as follows:

For the effective current applies accordingly:

Momentary current and torque may reach twice the values of those during continuous operation.
The effective values calculated with the formulas shown above must not exceed the values of the
continuous current shown in the tables.

30. 29
Case 3: Peak operation
Round table operation is a typical example for peak operation. Here acceleration and deceleration
may increase up to triple continuous torque because between the peaks hardly any power is
necessary. Also in this case the effective values must not exceed the values of the continuous
operation. Depending on motor temperature the permanent magnets come into danger of
demagnetization. Therefore similar applications and the choice of the suitable motor should be
discussed with our engineers.
Torque Motor Selection
Torque motors as well as high speed electro spindles can be treated as the direct drives.
Comparing the parameters of the existing electro spindles it can be stated that maximal torque of
the asynchronous motor is approximately 20 – 40 per cent smaller than the maximal torque of the
synchronous motor of the same size. It is the result of the lower efficiency caused by the emitting
heat. Application of the synchronous motor with the permanent magnets fixed on the rotor causes
neither need of supply nor cooling its secondary part. Such simple design enables ignoring various
design problems and improves the motor life. That is the reason, the contemporary high torque
direct drives are produced with application of synchronous motors.
The first step in drive selection is determination of the torque and rotational speed that are
necessary in particular case. The loading torque changes its value in time. The accurate course of
the torque change is very difficult to determine. It is impossible to calculate the substitutional
torque as it is possible for machines that operate with the predicted periodical way. This torque
can be estimated, anyway. Basing on the existing solutions and taking into consideration the
criterion of maximal acceleration, the value of the maximal torque in amount of 500 Nm seems to
be sufficient.

31. 30

Fig. 2 Auxiliary draft to define the motor torque
The other essential selection criterion of the motor is criterion of size. It is rather important
considering the fact that torque motors are built in a shape of a big diameter ring. More compact
design of rotary table less place will be taken from the machining space. It has been assumed that
dimensions of the rotary table should not exceed the dimensions of the conventional rotary tables. It
has been estimated that the outer motor diameter (DA) should be approximately equal to 300 mm
and the motor length (L) should be equal to 100 mm.

Fig. 3 Alternative designs of placing the bearing in rotary table with the direct drive: a) parallel structure (bearing
inside the motor), b) series structure, c) parallel structure bearing outside the motor)

32. 31
Maximal unit feed force (τ) in the machine tool electrical drive is in the range of 40 – 50 kN/m
2
.
This force can be defined as:

where dF is the elementary force acting on the area segment dA according to Fig. 2.
Increasing the τ value is contemporarily strongly limited because of the risk of demagnetizing the
permanent magnets of the secondary motor part and because of appearing the excessive amount of
heat. The torque motors are supplied with special ducts in rotor and they enable the water cooling.
Considering this fact, maximal value of τ has been assumed as τ = 50 kN/m
2
. The maximal torque
(Tmax), for the motor geometry presented in Fig.2 can be determined by the equation:
T , (2)
where D is the outer diameter of the rotor. Outer diameter of rotor has been connected with the
outer motor diameter by the dependence:
DA =1,2⋅ D , (3)
This dependence is adequate for the motors actually available on the market. Considering the
dependencies (2) and (3) it is possible to obtain:
T V , (4)

33. 32

Fig. 4 Alternative designs concerning the clamp position in the rotary table with direct drive a),c) and d) clamp is on
cylindrical surface b) clamp is on the face surface
According to equation (4) maximal torque limited by allowable assumed motor volume has been
determined as 491 Nm. Considering the further analysis it has been decided to select torque motor
with following parameters: nmax = 350 rpm, Mmax = 460 Nm, PN = 4,1 kW.
Due to the fact that rotational speed of the torque motors is much lower than the rotational speed
of conventional motors, the power of the torque motors is in the range of maximal power of the
conventional servo drives. This causes the possibility of application the standard drive units in
direct high torque motors.

34. 33


Fig. 5 Versions of location the measuring encoders in the rotary table with the bearing fixed inside the motor: a –
encoder in the axis of the table with the port b­­ – encoder in the axis of the table with the blind hole, c – encoder
connected with the disc of the table – reading by means of measuring head
The proposed positions of rotary table clamp have been presented in Fig. 4. The clamp is used in
case of machining when the high rigidity of the work piece fixing is demanded. Such conditions
occur while machining with high cutting speeds and with the great cross section of the chip. Such
conditions usually occur while rough and forming machining. Assuming that motor will have
enough power to overcome machining resistance, and that occurred errors will be in allowable
range, it is possible to consider not to apply the rotary table clamping.
In considerations the rotary table is going to be applied for shaping the face toothings. In this case
the machining forces are mainly of the radial character so they are not transferred directly on the
motor. These facts support the idea of omitting the clamping system in rotary table. It simplifies the
general design and decreases total costs. Nevertheless, the additional space has been predicted
inside of the motor, in case application of clamping would appear to be necessary.
Additionally various possibilities of fixing the measuring encoder have been analyzed. The greater
diameter of converter disc, the higher accuracy of measured angle is obtained. In case of

35. 34
application encoder in the shape of the ring it is possible to perform the port in the table that can be
useful for fixing various elements in the middle of the table.

9. Rotary Table Alternative Designs
Several alternative designs of the rotary table have been analyzed in the paper. The designs have
been distinguished according to:
• bearing position,
• table clamping position,
• measuring system position.
Considering the dimensional dependencies for motor and bearing it is possible to determine three
basic alternative designs that have been shown in Fig. 3.

For design a) the bearing is placed inside the motor. In this case it is possible to obtain maximal
possible torque for given overall dimensions. In this case the motor is of the largest diameter.
Placing the bearing outside – design c) would demand to increase the table diameter or to decrease
the motor dimensions. The effect of such solution is the increasing the table load capacity but at the
same time supplying the cooling water is much more complicated. Anyway, in this case the cooling
is not necessary, but lack of cooling decreases the maximal torque. Such tables (without water
cooling) can be successfully applied for precise positioning when the large torque is not demanded.
In case of continuous machining with great loading, alternative design a) will be preferred.
There is also intermediate alternative design with the bearing positioned above the motor. The
disadvantage of such solution is the increased height of the table which is not so important when the
table is integrated with machining center. In the case it has been predicted that the designed rotary
table is the autonomous equipment that is fixed on the machine tool table as the auxiliary element.
The proposed positions of rotary table clamp have been presented in Fig. 4. The clamp is used in
case of machining when the high rigidity of the workpiece fixing is demanded. Such conditions
occur while machining with high cutting speeds and with the great cross section of the chip. Such

36. 35
conditions usually occur while rough and forming machining. Assuming that motor will have
enough power to overcome machining resistance, and that occurred errors will be in allowable
range, it is possible to consider not to apply the rotary table clamping. In considerations the rotary
table is going to be applied for shaping the face toothings. In this case the machining forces are
mainly of the radial character so they are not transferred directly on the motor. These facts support
the idea of omitting the clamping system in rotary table. It simplifies the general design and
decreases total costs. Nevertheless, the additional space has been predicted inside of the motor, in
case application of clamping would appear to be necessary. Additionally various possibilities of
fixing the measuring
encoder have been analyzed. The greater diameter of converter disc, the higher accuracy of
measured angle is obtained. In case of application encoder in the shape of the ring it is possible to
perform the port in the table that can be
useful for fixing various elements in the middle of the table.
10. Mathematical Model of the Rotary Table
In order to make easier the initial set up of the control unit regulators for different work conditions
and loadings, a mathematical model of the direct driven rotary table together with drive unit, has
been performed. The simulation of the table work has been performed using Matlab Simulink
program.
An asynchronous motor applied in the table may be successfully modeled in the same way as DC
motor with permanent magnets. The working principle of both motors is very similar. The
difference is based on changing the function of rotor and stator. In synchronous motor the stator is
in position of the armature and the rotor is in position of field magnet with permanent magnets. In
this way there is no need to deliver current to rotating part and therefore the mechanical
commutator is not necessary. In this case commutation is obtained electronically by means of the
inverter.
Considering both electrical and mechanical part of the rotary table the following equations have
been presented (after Laplace transformation):

37. 36

where:
Ώ – angular velocity,
I – current intensity,
S – Laplace operator,
R – wiring resistance of motor,
ke– voltage constant of motor,
km – torque constant of motor,
L – wiring inductance,
Mob – loading torque,
J – moment of inertia

Fig. 6 Block scheme of torque motor without considering the friction

38. 37

Fig. 7 Example diagrams presenting friction torque vs. rotary speed. Denotations: Mf – total moment of friction,
Mv, Mc, Ms accordingly moments of: viscous friction, Coulomb friction, static friction considering
the Stribeck’s effect

Fig. 8 Block diagram of the friction model
Some parameters that are presented in equations (5) and (6) should be determined very carefully
as their improper selection may cause the incompatibility of the model with the real object. Some
data are given by the motor producers in their product catalogue. Values taken from catalogue are:
R = 6,3 Ώ, L = 35,3 mH, ke = 1,16 V/min
-1
, km = 14,9 Nm/A. Loading torque and the moment of
inertia should be assumed as they are influenced by the motor work conditions. Both loading torque
and distribution of the inertia mass may change. The equations (5) and (6) can be presented in the
form of the block diagram shown in Fig. 6.
Moment of inertia in the model has been determined only for moving elements of the rotary table
and assumed as 0,46 kgm
2
. This moment increases in case additional work piece will be fixed on
the table. Loading torque is composed of friction moment and technological resistance moment.
The Matlab Simulink program enables to input resistances of any value and course. This program
also accepts friction of any value and course. This program also accepts friction simulation

39. 38
The modeled object has only some parts that involve friction. Bearing and the V – ring type seal
fixed on the table circumference are the only movable elements contacted unmovable body.
In order to obtain friction phenomena representation, so called extended static model considered
friction, has been used. The extended model is composed of following friction combinations [3, 6,
14]: • Coulomb friction (MC),
• viscous friction (MV),
• static friction (MS),
• Stribeck’s effect

Fig. 9 Position of friction block at the table block diagram

Fig. 10 Block diagram of rotary table model with the drive unit

40. 39


Where
Mel – electromagnetic model produced by the motor,
N– rotational speed,
nS – Stribeck’s speed,
Mf – total friction moment
In equation (7) some problems connected with numerical solutions have also been considered.
These problems can be presented at the block diagram (Fig. 8). Friction parameters have been
determined approximately. Their values have been overestimated in order to better exposure of
friction effect. For further investigations these values will be determined in experience way.
The accuracy of the obtained friction model has been considered as sufficient in spite of the fact
that it does not take into account all the phenomena occurring during the motion. For example, it
does not enable to model the effect of friction force hysteresis while increasing and decreasing the
rotational speed. It does not allow also the machine idle time that has influence on the static friction
value. For convenience and in order to improve the readability of the block diagram, the part
describing friction has been placed in separate subsystem named – “Friction” (Fig. 9). The motor
that has been applied in rotary table integrally cooperates with the equivalent control unit. In spite
of the applied simplifications it has to be remembered that the motor is not the DC motor. In the
case the ECODRIVE 03 is in charge of the control unit. The control unit producer Rexroth
Indramat has developed and has made available a comprehensive documentation [7] with the proper
description of: parameters, connections, diagnostic codes and so on. It has been also presented

41. 40
general block diagram of the drive unit (Fig. 10). Here, there have been presented three areas that
delimitate groups of blocks according to their function and their position in equivalent control loop.
There are three following loops with feedback:
a) position regulation,
b) speed regulation,
c) current value regulation.
Separate regulator is assigned to each loop. The setting values are introduced to the equivalent
controller memory cells that are marked with the special codes e.g. S – 0 – 0104 is the cell address
for amplification value Kv, Kp is introduced using S – 0 – 0100 code. By means of codes many
other parameters can be set up. Particular description of codes is provided by the drive unit
producer in special technical documentation. If necessary, it is possible to control: current value
(equivalent to moment), speed value or position value. ECODRIVE03 drive unit enables also to
obtain the demanded value either from computer or directly from the machine tool.
The designed by model, after checking its compatibility with the real drive, will be used to make
easier the setting up of regulators. In order to find optimal setting up parameters it is possible to use
tool data provided by NCD (Nonlinear Control Design Blockset) library located in
Matlab/Simulink software package. NCD block several times makes the simulation, evaluates each
time the output signal, and then changes are introduced to regulator settings. Applying the iterative
optimization methods the NCD block tries to match the model response diagram with the user
demanding. If necessary, the described model can be changed into discrete one.
In Fig.10 there have been shown not only the three loops of the feedback but there is also the
special loop called feed forward (FF). Its using is optional. This loop enables for so called initial
control by means of increasing the sensitivity upon change of the control signal and in this way it
speeds up the total system reaction.
In Fig.11 there has been presented the comparison of the angular position change in rotary table
movement according to the demanded course in the form of the forcing “S” for the system with and
without the regulator. The influence of the friction moment upon the motor that drives the rotary
table has been shown in Fig. 12 and in Fig. 13. The control has been performed without regulators.
The input signal has been forced in the form of voltage that increased linearly in arbitrary way up to
the value equivalent to 10 rotations per second. The visible certain delay in the movement
beginning has been caused by the static friction. The diagram of the friction moment has been

42. 41
shown in Fig. 13. Before the beginning of movement this moment has been equal to
electromagnetic moment.

43. 42
11. Conclusion
Rotary tables with direct drive have also some disadvantages. The greatest disadvantage is the
questionable drive dynamic rigidity. The value of the obtained rigidity depends on the automatic
control system and on the resolution of measuring converter.
The great rigidity together with the small masses of the driven work units, cause that dynamic
characteristics of the drive in the range of sampling frequency ( in the circuit of the current and
work speed regulation) up to about 1kHz, are comparable to the dynamic characteristics of the
inertialess object. In practice this means, the dynamical properties of the drive depend on the
applied control system and not on the mass elastic properties of the system. The development of
new structures and design of the movable parts (eg. openwork structures, wafer structures or profile
structures) is connected with the minimization of the movable parts in such drives. On top of that,
they are applied new lighter materials with better strength and damping properties.
The positioning accuracy of the table with the direct drive depends only on the accuracy of the
angular measure encoder and on the quality of the control system.
The contactless work character of the direct drive excludes its mechanical wear. The life of
bearing describes in this case the life of the whole system. Actually, the cost of the direct drive is
higher than the simple servo drive with mechanical gears. On top of that, the structure of the rotary
table with direct drive is in the form of regular solid without extending elements which decrease the
working space of the machine tool.
The dynamic development of the rotary direct drives as well as their advantages enable to state
that in the near future they will be more and more applied in modern NC machine tools.

44. 43
12. Solidworks Model of the Design
SOLIDWORKS MODEL OF THE DESIGN

45. 44
13. REFERENCES:
● November, 1999 edition of “20-minute Tune-Up”
● ‘Direct Drive Technology and its impact on gearmotor business’ by Masazumi Suzuki.
● Andresen E., Anders M., A three axes torque motor of very high steady state dynamic
accuracy, IEEE Power Tech, Stockholm, 1995.
● Andresen E., Binder A., Anders M., A novel spherical linear PM motor for direct driving
infrared optical telescope, IEMDC Seattle, 1999.
● Aström K.J., Canudas de Wit C., Gäfvert M., Lischinsky P., Olsson H., Friction Models
and Friction Compensation
● Banon L., Feusi H., Servos with high torque motors for direct drive, Automation, Motion
Drives and Control (AMD&C) International Magazine, May 1997, s.234-239.
● Benefits of using direct drive technology, Brochure of Kollmorgen, s.48-65,
http://www.danahermotion.com/, 2002.
● Björklund Stefan, Sören Andersson, Söderberg Anders, Friction models for sliding dry,
boundary and mixed lubricated contacts www.sciencedirect.com/science
● ECODRIVE03 Drive for Machine Tool Applications, www.boschrexroth.com
● Encoders for feed axes with direct drives, Technical information, Heidenhain,
http://www.heidenhain.com/, 10.2005.
● Erat D., Torque-Motoren in Werkzeugmaschinen, Präziser Antrieb, Motion Control, 2001,
nr 3, s.27.
● Espanet C., Miraoui A., Kauffmann J.-M., Torque and electromotive force of a brushless
motor destinated to be integrated in a wheel, Third International Workshop on Electric and
Magnetic Field - From Numerical Models to Industrial Applications, Liège, Belgique,
1996, s.159-164.
● Holzknecht A., Direct drive torque motors for machine tool applications, Technical
publication, http://www.etelusa.com/. [12] Holzknecht A., Torque motors do the trick,
http://www.machinedesign.com/.
● Karmous M., Espanet C., Vaucher D., Kauffmann J.M., Control of a brushless motor
integrated in the wheel for an electrical bike, European Conference on Power Electronics
and Applications’97, Vol. 4, Trondheim, Norvège, 1997, s.643-647.

46. 45
● Karnopp D., Computer simulation of slip-stick friction in mechanical dynamic systems.
Journal of Dynamic Systems, Measurement and Control, 107H1I:100–103, 1985