Scaravella F.lli
ii
INDICE GENERALE
FOREWORD
QUICK REFERENCE GUIDE FOR CATALOGUE CONSULTATION ............................................................................................... F-1
TECHNICAL DATA
STATIC AND DYNAMIC RIGIDITY ............................................................................................................................................................... 2
Static rigidity of the screw .................................................................................................................................................................. 2
Static rigidity of the screw nut .......................................................................................................................................................... 2
Static rigidity of the balls contact zone ....................................................................................................................................... 2
DEFORMATIONS ................................................................................................................................................................................................ 2
CALCULATION OF APPLIED TORQUE ......................................................................................................................................................3
CALCULATION OF LOADS AND DURATION ...........................................................................................................................................3
OPERATIONAL LIFE ......................................................................................................................................................................................... 3
Mounting ......................................................................................................................................................................................................3
Lubrication ..................................................................................................................................................................................................4
PRELOAD .............................................................................................................................................................................................................. 5
SUPPORT SYSTEMS.........................................................................................................................................................................................6
BUCKLING LOAD ..............................................................................................................................................................................................6
CRITICAL SPEEDS ............................................................................................................................................................................................ 7
Critical speed of screw ......................................................................................................................................................................... 7
Limiting speed ...........................................................................................................................................................................................8
PRECISION ...........................................................................................................................................................................................................8
STANDARD BALLS CREWS TABLES
MODE OF TABLE READING........................................................................................................................................................................ 8b
SINGLE CYLINDRICAL NUT WITH THREADED HEAD .......................................................................................................................9
SINGLE CYLINDRICAL NUT ........................................................................................................................................................................ 11
SINGLE PRELOADED CYLINDRICAL NUT ........................................................................................................................................... 14
DOUBLE CYLINDRICAL NUT PRELOADED ..........................................................................................................................................15
FLANGED SINGLE NUT ................................................................................................................................................................................. 17
SINGLE PRELOADED FLANGED NUT ....................................................................................................................................................34
DOUBLE NUT (anged + cylindrical) PRELOADED ......................................................................................................................... 50
Scaravella F.lli
F-1
FOREWORD
- Quick reference guide for catalogue consultation -
The following pages contain the tables for a rapid individuation of ballscrew nuts into the Catalogue. The ballscrew nuts are organized
by Type of nut (single, preloaded or double), by tipe of ange (DIN or STD - standard), by nominal diameter (Dn) and by pitch of tread.
Single cylindrical nut (MC), Single cylindrical nut with threaded head (MCTF),
Single cylindrical nut
Flanged single nut Central ange single nut
FC
Single cylindrical nut
with threaded head
MCTF
Flanged single nut (MF) and Central ange single nut (FC)
The ballscrew nut MC, MF and FC are realized by a single body with internal helicoidal thread for balls circulation and holes for the
deectors for balls recirculation. The ballscrew nuts are provided WITHOUT PRELOAD (medium axial backlash 0,01 ÷ 0,02mm); on de-
mand can are provided at “0” backlash (zero).
The anged nuts MF and FC may have holed type DIN o STD (standard):
On demand may are realized ange with particular form and holed.
60°60°
OILOIL
SS
90°90°
OILOIL
30°30°
OIL
OIL
S
S
Hole type
standard (STD)
Hole type DIN
(6 hole until 32mm
nominal diameter)
Hole type DIN
(8 hole from 40mm
nominal diameter)
60°60°
SS
OILOIL
L6L6
OILOIL
60°60°
30°30°
SS
L6L6
D2D2
90°90°
SS
SS
Scaravella F.lli
F - 2
Single preloaded cylindrical nut (MCPI) and Single preloaded flanged nut (MFPI)
Single preloaded cylindrical nut Single preloaded flanged nut
MCPI MFPI
The ballscrew nut MCPI and MFPI are realized by a single body with internal helicoidal thread for balls circulation and holes for the
deflectors for balls recirculation. For increase the rigidity of coupling screw/nut the nut helicoidal thread is realized with a “distance
- S” between the two deflector series. The entity of distance define the required preload.
S
S
Double cylindrical nut preloaded (MC+MC) and Double nut (flanged + cylindrical) preloaded (MF+MC)
Double cylindrical nut preloaded Double nut (flanged + cyl.) preloaded
The ballscrew nut MCPI and MCPI are realized by two nut coupling through a spacer.
The entity of the spacer width “S”, define the required preload.
SS
Scaravella F.lli
F - 3
Fast search Table
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
16 pitch 5
3
Page 11 Page 9
Page 17 STD
Page 18 DIN
4
Page 11
Page 17 STD
Page 18 DIN
16 pitch 10 3 Page 18 DIN
20 pitch 5
3
Page 11 Page 20
Pages 18 - 19 STD
Page 21 DIN
4
Page 11 Page 9
Page 18 STD
Page 21 DIN
3+3
Page 15 Page 14
Page 50 Page 35 STD
Page 34 DIN
20 pitch 10
3
Page 11
Page 19 STD
Page 19 DIN
4Page 9
Page 21 DIN
Scaravella F.lli
F - 4
25 pitch 5
3
Page 11
Pages 18 - 19 STD
Page 22 DIN
4
Page 11
Pages 18, 19
and 20 STD
Page 22 DIN
5 Page 9
3+3
Page 15
Pages 36 - 38 STD
Page 37 DIN
4+4
Page 15
Page 36 STD
Page 37 DIN
25 pitch 6 3+3 Page 38 DIN
25 pitch 10
3
Page 11
Page 19 STD
Page 21 DIN
4
Page 11 Page 9
Page 19 DIN
Page 19 STD
3+3 Page 51 STD
Page 51 DIN
4+4 Page 51 DIN
Page 51 STD
25 pitch 15 3 Page 19 STD
25 pitch 20
2
Page 11
Page 19 STD
Page 19 DIN
2+2 Page 51 STD
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
Scaravella F.lli
F - 5
32 pitch 5
3 Page 11
4
Page 11
Page 21 DIN
Pages 22 - 23 STD
5Page 9
Page 21 DIN
6
Page 11
Page 22 DIN
Page 22 STD
3+3
Page 15 Page 14
Page 51 Page 40 STD
Page 41 DIN
4+4
Page 15 Page 42
Page 51 Pages 40 - 41 STD
Page 52 Pages 37 -39 DIN
32 pitch 6
4 Page 11
5
Page 11
Page 23 DIN
Page 24 STD
3+3 Page 37 DIN
Page 42
4+4 Page 40 DIN
5+5 Page 52
Page 52 DIN
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
Scaravella F.lli
F - 6
32 pitch 10
3
Page 12
Pages 21 - 25 STD
Page 25 DIN
4
Page 12 Page 9
Pages 21 - 25 DIN
Pages 21 - 24 STD
5Page 12 Page 9
Page 25 DIN
3+3
Page 16
Page 53 Page 40 STD
Page 53 Page 39 DIN
4+4
Page 16 Page 52
Pages 52 - 53 DIN
Page 53 STD
5+5 Page 53 DIN
32 pitch 12
4Page 12
Page 26 DIN
4+4 Page 53 DIN
32 pitch 20
2
Page 12
Page 19 STD
Page 21 DIN
3Page 19 STD
Page 21 DIN
2+2 Page 53 STD
32 pitch 25
2Page 12
Page 19 STD
2+2 Page 53 STD
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
Scaravella F.lli
F - 7
40 pitch 5
4
Page 12
Pages 26 - 27 STD
Page 27 DIN
5Page 9
Page 27 DIN
6Page 12
Page 27 STD
4+4
Page 15
Page 54 Page 42 STD
Page 43 DIN
5+5 Page 46 DIN
6+6
Page 15
Page 54 STD
Page 46 DIN
40 pitch 6
4 Page 12
6Page 12
Page 27 DIN
4+4 Page 15 Pages 43 - 44
Page 44 DIN
6+6 Page 54 DIN
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
Scaravella F.lli
F - 8
40 pitch 10
3Page 12
Page 28 STD
4
Page 12 Page 9
Page 28 STD
Page 32 DIN
3+3
Page 15 Page 44
Page 55 Pages 45 - 47 STD
Page 46 DIN
4+4
Page 15 Page 44
Page 55 Pages 45 - 47 STD
Page 57 Page 46 DIN
6+6 Page 15
40 pitch 12 4+4 Page 43 DIN
40 pitch 20
Page 13 Page 33
Pages 29 - 30 Page 31 STD
Page 32 DIN
4 Page 33 DIN
3+3 Page 56 STD
Pages 55 - 57 DIN
40 pitch 40
2Page 13 Page 30
Pages 29 - 30 Page 31 STD
2+2 Page 56 STD
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
Scaravella F.lli
F - 9
50 pitch 5
5 Page 12
6
Page 12
Page 27 STD
Page 32 DIN
4+4
Page 15 Page 48
Page 58 Page 42 STD
Page 46 DIN
5+5 Page 46 DIN
6+6
Page 15
Page 54 Page 48 STD
Page 59 Page 46 DIN
50 pitch 10
4
Page 12
Page 28 STD
Page 32 DIN
6
Page 12
Page 28 STD
Page 32 DIN
3+3 Page 15
Page 58 STD
4+4
Page 15 Page 48
Pages 45 - 47 STD
Page 59 Page 46 DIN
6+6 Page 55 STD
50 pitch 20
3Page 13
Page 32 DIN
4Page 13
Page 32 DIN
3+3 Page 59 DIN
4+4 Page 59 DIN
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
Scaravella F.lli
F - 10
50 pitch 40 2
Page 13
Page 31 STD
Page 32 DIN
63 pitch 10
6
Page 13
Page 28 STD
Page 32 DIN
8 Page 32 DIN
4+4
Page 16
Page 60 Page 49 STD
Page 60 Page 46 DIN
5+5
Page 16
Page 60 STD
Page 60 Page 46 DIN
63 pitch 20
3Page 13
Page 32 DIN
4Page 13
Page 32 DIN
5 Page 32 DIN
3+3 Page 59 DIN
4+4 Page 59 DIN
DIAMETER
PITCH
N° of
CITCUITS
of
BALLS
FLANGE
HOLED
TYPE
Single
cylindrical nut
Double
cylindrical nut
preloaded
Single preloaded
cylindrical nut Flanged
single nut
Double nut (flan-
ged + cylindrical)
preloaded
Single preloaded
flanged nut Central flange
single nut
Single cylindrical
nut with threaded
head
Scaravella F.lli
2
STATIC AND DYNAMIC RIGIDITY
The denition of static rigidity is known, as the ratio between the
applied LOAD and the deformation that is determined:
=F
f(kg/µm)
K stat V
The static rigidity of the screw (K stat V ) is expressed in [kg/µm]
] and corresponds to axial deformation.
For example, for a rigidity of 1,3•103 kg/µm, with an axial LOAD
of 5000 kg, there is a deformation that is equivalent to:
(5000 : 1300) = 3,8 µm.
The overall rigidity of the ball screw is a function of 3 factors:
a. the rigidity of the screw (or threaded shaft);
b. the rigidity of the screw nut;
c. the rigidity in the contact zone of the balls.
Static rigidity of the screw
where: A, shaft cross-section (mm2);
E, modulus of elasticity (21•103 kg/mm2);
I, initial LENGTH of the shaft.
Static rigidity of the screw nut
K stat M, which is normally very high, for the compact shape of
the screw nut itself; it is calculated with the previous formula.
Static rigidity of the balls contact zone
K stat M, , is theoretically determined, as a function of the ball-
groove contact, for loads higher than those of normal operation.
These loads correspond to the deformation of the screw and
screw nut thread and to the ball-groove contact deformation.
Rigidity conditions can be improved by applying a PRELOAD to
the screw nut.
Of course, the rigidity characteristics of the screw must be
correlated with the rigidity of the machine and with the methods
of xing the screw on the machine itself: in particular the screw
supports and their rigidity.
The dynamic characteristics of ball screws are present in the
analisys of designers, who must increasingly take into account
the “responses” of the machines to the stresses that they receive,
and that are not “fully operational”.
The dynamic rigidity is a function of the variations of the LOAD,
applied to the ball screw, and in particular of the relationship
between the frequency of these values and the typical frequency
of the considered part.
F
f
(kg/µm)
K stat VA • E
I • 103
=
It is well known that when equality occurs between the two
frequencies, the Dynamic rigidity is minimal, and resonance
phenomena arise. The parameters on which the dynamic rigidity
of ball screws depends are:
- oscillating weight;
- damping ratio;
- static rigidity;
- own frequency.
K din = K stat
A
In our case A takes values approximately between 4 and 5.
To improve dynamic rigidity, the excitation frequency values
must be moved away from those of the natural frequency,
and/or maximum damping must be achieved with appropriate
PRELOAD values.
DEFORMATIONS
It is possible to know the value of axial elastic deformation in a
screw-screw nut assembly subjected to a certain LOAD.
This value is calculated each tima, as it is a function of the number
and diameter of the balls being worked, the conditions of use and
the preload within the screw-screw nut assembly.
With NOT PRELOADED screw nut
With PRELOADED screw nut
0,025
0,020
0,015
0,010
0,005
1000500 1500 2000
Axial load (kg)
Axial elastic deformation (mm)
=
Scaravella F.lli
3
CALCULATION OF APPLIED TORQUE
The value of C torque, necessary for the operation of a ball screw
to which an axial load F is applied is:
Ct = = kgm
F • p
2000 • π • η
where: F, is axial LOAD
p, is screw PITCH
η, is the yield of the helical pair (0,9).
To this should be added the inertia torque of the screw shaft and
the torque due to the PRELOAD of the screw nut.
CALCULATION OF LOADS
AND DURATION
The calculation of the permissible LOAD on the shaft can be set,
especially for long and thin screws, stuck at one end and free
from the other (heavier case), with the processes of the tip loaded
solids (Euler formulas)..
The LOAD to which the ball screw is subjected must be considered
applied in dynamic conditions, sometimes with shocks: therefore,
the sizing must take this condition into account.
It should also be noted that the sizing of the ball screw must
be carried out, taking into account not only the screw, but the
resistance of the screw-nut-ball assembly.
As for the duration of a screw, it is noted that it is correlated with
its resistance to fatigue, and with the number of times the sphere
touches a given point of the groove.
Therefore, the lifetime measurement of a ball screw is expressed
in number of rotations (106 revolutions, or millions of revolutions).
The coefcient Ldin of dynamic LOAD indicates the permissible
LOAD (in kg) for a duration T of 106 revolutions. The coefcient
Lstat of static LOAD corresponds to the maximum permissible
load on the screw in resting conditions, or for very slow rotations.
Beyond this LOAD there is a permanent deformation on the
raceways of 0,0001 compared to the diameter of the ball.
For the choice of the screw it is necessary, however, to know the
medium load Fm: that is the load corresponding to the actual
use of the screw, which is determined by the conditions of use
of the screw itself and can be calculated approximately by the
following formula:
F
v
=
F1
3T1+F2
3T2+...+Fn
3Tn
T
3
where:
F1 is the constant LOAD during T1 rotations;
F2…Fn, are the constant loads during T2…Tn rotations;
T = T1 + T2 + … + Tn, are the number the total number of rotations
during which the loads F1, F2, …, Fn.
The calculation of the life of the screw:
Tv=Cdin
Fm
3
106giri
3
F
m=
Cdin
T
v
106
where:
TvScrew life in number of revolutions
Ldin dynamic LOAD
(see Technical Data Tables, pages 10 ÷ 69)
Fmmedium operating working LOAD
For the calculation of the service life, the average value of the
load is considered for Fm , the medium operating working LOAD,
which affects the service life raised to the third.
Still the report
Cdin
F
m
=Tv
10
6
3
it can be called λ and obtained according to the number of rotations
required by the screw.
OPERATIONAL LIFE
The nominal life of a ball screw is the number of hours of activity
at a constant speed (or the number of revolutions) that the screw
is able to withstand before the rst signs of fatigue (peeling) occur
on the rolling surfaces (screw and screw nut)..
Practical experience has shown that identical screws, working
under the same conditions, have different durability; hence
the concept of nominal life. The nominal life, according to
the ISO denition, is the life reached or exceeded by 90% of a
sufciently large number of identical screws working under the
same conditions (alignment, applied load, speed, acceleration,
temperature, lubrication and cleaning).
The service life is the lifespan of a specic screw before failure.
The failure is not normally caused by fatigue (peeling), but by the
wear of the recirculation system, corrosion, contamination and,
more generally, by the loss of functional characteristics.
To obtain a useful life equivalent to the nominal life, the screw
must be subjected to an real medium load not exceeding 80% of
the dynamic load along a stroke of not less than 4 times the pitch.
The determination of the “size” of the screw to obtain the required
durability is provided by the experience gained with similar
applications; it is also necessary to consider the specic structural
needs such as the robustness of the terminals (shanks) and the
attachments of the screw nut due to the efforts applied to these
elements.
Mounting
In order to ensure the expected life of the screw it is important to
ensure a correct alignment of the screw with the sliding guides.
Radial loads and eccentric thrusts are absolutely to be avoided
because they reduce in signicantly the service life of the screw.
Scaravella F.lli
4
Lubrication
The lubrication of ball screws must be appropriate in quantity and
quality, even more so for those screws that work at high speed.
The quantity, distribution and frequency of lubrication must be
determined appropriately and constantly monitored. At high
speeds the lubricant on the screw surface can be expelled by
centrifugal force.
It is important to keep this phenomenon under control during
the rst strokes at high speeds and then adapt the lubrication
frequency, ow and quality of the lubricant.
If necessary, use a higher viscosity lubricant. The optimization of
the lubrication frequency and the amount of lubricant must be
determined taking into account the temperature reached by the
screw nut and its trend. Good lubrication is essential to ensure
the proper functioning and durability of the screw.
The screws can be lubricated with oil or grease.
In general, the amount of oil needed to ensure proper lubrication
is between 3 e 5 cm3/h, for each turn of the balls.
Grease lubrication is recommended ONLY for low rotational
speeds. The recommended amount of fat is about half the free
volume inside the screw nut.
Proper lubrication allows you to obtain:
a) a long operational life in accordance
with the calculation values
b) a suitable heat dissipation
c) the reduction of wear and corrosion
Oil lubrication
Diagram for determining the kinematic viscosity of lubricating oil
Average speed nm (rpm) Viscosity ISO VG oil (mm2/sec. a 40°)
Screw diameter Dn (mm) Operating temperature (°C)
- The most suitable lubrication system is at oil centralized lubrication system.
- Lubricant quantity: 3÷6 cm3/h for each ball circuit.
- - For operating temperatures between 10°C and 70°C the viscosity should be placed between ISO VG68 and ISO VG220.
- It is used for low-speed operating conditions.
- Use grease according to Class 2 DIN 51825 every three months max.
- In the presence of high loads, use grease in accordance with DIN 51818
- Introduce grease in quantity corresponding to at least half of the free volume inside the screw nut.
- Carry out complete grease replacement every 12 months.
- Signicantly reduce lubrication intervals in particularly adverse working conditions: sudden changes in temperature, humidity
dusty or saline environments, etc.
Grease lubrication
Scaravella F.lli
5
PRELOAD
PRELOAD is used when maximum rigidity and absence of backlash
is required. The PRELOAD reduces elastic deformation in the
screw/screw nut coupling, increases accuracy and improves
responsiveness to command impulses.
Lower preload values reduce rigidity, higher values increase friction;
In both cases, the accuracy of the positioning or the durability of
the screw is respectively affected.
Table of viscosity classes
(*) Recommended viscosity class
Deformation under load
operating
deformation of the nuts
I and II underload
Axial load F in N
Fv
F
Fm
Force direction Force direction
Spacer
Nut A Nut B
Screw
Ball screws with single nuts without preload denote axial
clearance and have a modest rigidity, as soon as loaded, following
an unfavorable contact.
Therefore the relative movement screw - nut can reach high
values. If a ball screw without backlash is required, with a high
positioning precision and therefore a high rigidity, preloaded screw
nut systems must be used.
Higher values mean higher torques, lower yield and durability.
Too high an increase in the preload of the screw nut produces a
Limiting increase in rigidity, but a considerable increase in
preload torque and therefore in operating temperature. The
preload normally applied at the factory turns out to be about
6% of the dynamic load and is considered optimal and does not
need to be increased.
The PRELOAD is obtained by inserting a spacer of appropriate
size between the two screw nut and acting with a tensile force on
them.
The value of the PRELOAD is determined by acting on the thickness
of the spacer.