Mechanical Engineering University of Gaziantep

1. ABSTRACT
Here, design of a power screw driven press is to be discussed. As usual design considerations
include more than one unknowns therefore an engineer’s problems usually do not seems two
unknowns, two equations. But still design should be done, in this aspect I will centre on my
design approach to the screws with regarding safety and then select the nut and motor for this
specific purpose.
3
CONTENTS
Abstract II
1.Introductin 3
2.Design Analysis
2.1 Design Terminology for Power Screws 3
2.2 Design of screw and nut 5
2.1.1 Screw diameter selection 5
2.1.2 Nut height selection 5
2.3 Buckling consideration 6
2.4 Self-locking condition 6
2.5 Torque calculation 6
2.6 Efficiency 7
2.7 Motor selection 7
3.Conclusion and Future Developments 7
References 7
Appendix

2. 1. INTRODUCTION
I will initialize my design by selecting a screw that is available for me (say in the foundation)
and then check it for safety if satisfied or not. And later on consideration of self locking
condition, efficiency and required power input according to the selected screw is going to be
done for the understanding of practicalability to life. Finally a nut for the screw is selected by
regarding bearing and shear stresses.
4
2. DESIGN ANALYSIS
Fig 1.
A schematic example of application of power screws to a power driven press is shown in
figure 1. The some of the desired properties are tabulated as below. By using this table the
screw, nut and motor for our pressing operation is going to be selected from standard tables or
in other words from machineries catalogue.
As seen from the figure 1. , pressing operation is the reverse organization of raising a load so
our job is again as in the raising. This is important in order not to confuse lowering or rising.

3. 5
Screw pitch, p(mm) 6
Screw threads Triple
Factor of safety, n 3
Load per screw , F(tons) 4
Friction coefficient, μ 0,22
Motor speed, n(rpm) 800
Material of screw 080M50CD
Screw Ultimate Strength, Sut (MPa) 650
Screw Yield Strength, Sy(MPa) 510
Screw Modulus of elasticity, E (GPa) 207
Material of nut ASTM Gray Cast Iron
Nut Ultimate Strength, Suc (MPa) 670
Nut Yield Strength, Sy(MPa) 220
Table 1.
2.1 Design Terminology for Power Screws
A power screw of outside diameter, do, is designed to have a thread pitch, p. The application
of the device requires the screw to lift or lower the load. (in our case press the load which is
in the same manner as lifting the load). The coefficient of friction for the thread is μt and
for the collar μc (in our case collar is not used so no collar friction exists).
Fig2.
Unwrapping one thread the following wedge geometry results:
Where >= Lead angle,
L=Lead,
Dm=mean diameter of the contact,
For a single threaded screw L is the same as the pitch but for double or triple threaded screws
L is equal to pitch times number of thread for one rotation of screw. (in our case a triple
threaded screw is used)

4. 6
Fig 3.
2.2 Design of screw and nut,
2.2.1 Screw diameter selection,
From table 1. a screw which is triple threaded and having a 6 mm pitch exists. Since the
diameter of the screw is needed for our calculations, selecting a metric screw having, p=6 and
going on my calculation will not be wrong unless the screw I selected does not ensure the
safety concern.
Our possible choice of diameters for a metric grade 6mm pitch are, 64, 72, 80, 90, 100 for
coarse-pitch series. Let’s start our design with M 64.6 .
Calculations according to M64.6 screw;
mean diameter, dm=d – p/2 = 64 – 6/2 =61 mm,
pitch, p=6 mm,
Lead, L=3*6=18 mm,
F=mg=4000*9.81=40 kN(approximately),
F(load factor of sfety included)=F*n=40*4= 160 kN,
Now check the strength of the screw in order to be sure that we are on the safe side wrt n=4;
650
= = = MPa
10.88
Fn
160000
c 59.7
At
2680
u
= = =
59,7
c
n
which is safer than desired.
Note: If we don’t have to use a 6mm pitch screw with respect to standards a smaller diameter
screw will be enough for strenght consideration.
So we can continue our design with M64.6 screw .
2.2.2 Nut height selection,
Check shearing of threads for h,
Fn
dh
s
2
= s = Ssy = 0.5Sy = 0.5*510 = 255MPa h=6.88 mm
Check shearing of nut for h,
Fn
drh
s
2
= h=7.98 mm,
Check bearing stress of nut for h,

5. compareb with Suc, since we have already included factor of safety n
7
4
pFn
h d 2 dr
2 ( )
b
−
=
by rising force.
4
pFn
2 2 Suc d dr
( )
h
−
=
h=2.49 mm
After these calculations we can decide h for our nut as 8 mm wrt shearing of nut .
2.3 Buckling consideration
Now we should consider buckling for designing the length of the screw.
Since we don’t know the column is either Euler’s or Johnson’s, we need to make an
assumption. Since the loading is axial my suggestion is Euler’s column.
4
*
Inertia, 4
555497.2
64
mm
dr
I = =
C is equal to 0.25 since our situation best fits one end fixed and one end free.
2
C EI
Critical laod, 2
L
Pcr
=
If we equalize Pcr to F*n, then we will get the maximum available length of the screw .
When n=4 and E=207GPa from table 1.; L=1331 mm
Select L= 1m for our case will give Pcr=283.7kN ;
Compare the applied load and the critical load in order to prevent buckling,
Pcr=283.7kN F= 40kN . But now there is one more step which is cheking our
assumption;
44.75
2 2
CE
L
= =
1
Sy
k
62.5
4
= L
=
D
L
k
L
L
Check if
k
k
1
44.75 62.5, Therefore our assumption is correct.
Designing our screw length as 1 meter will be safe for buckling.
2.4 Self-locking condition,
The condition for self locking is μ tan
or
L
μ let’s compare,
dm
0.22 0.09 therfore self-locking condition is established for our situation.
2.5 Torque calculation
After completing screw design, we can calculate the required torque(raising load) for
pressing operation.
μ
*
* *
*
F dm L dm
−
+
=
dm L
Tr
μ
2
when we put the values into eqaution from table 1 and
use M64.6 screw; Tr =1564,3 Nm =1,565 kNm

6. 8
2.6 Efficiency
0.292
FL
To
= = =
2
Tr
T
e
=29.2% Our design has an efficiency of 29.2 percent
2.7 Motor selection
Since there exists a gear ratio of 4 between motor and screw head gear, the velocity ratio will
also be 4. From table 1. the motor’s angular velocity is suggested as 800 rpm so we can
calculate screw velocity w as;
Wscrew=800/4=200 rpm 200rpm=200*2A/60 =20.94 rad/sec.
Power=T*w=1.565 * 21 = 32.865 kw 32.865kw=32.865/0.745=44 hp
So a 45 hp G.E (50 hertz) Electric motor, 400/440 volt, 725 rpm or 50 hp G.G.E Electric
motor, 460 volt, 1170 rpm may be suggested for our design.
We have concluded our power screw driven press design by using a M64.6 grade screw
in length of 1 meter , a 8mm in height nut and a 45 hp electic motor.
3.CONCLUSIONS AND FUTURE IMPROVEMENTS
We have started our design by assumptions and then realized them step by step and ensured
safety by using standardized screws, nuts and selecting a widely used electric motor. As seen
our way of solution design is not a straight forward problem but needs engineering judgement.
After mechanical design analysis a further step should be cost analysis in order to say; our
design is just ready for manufacturing.
REFERENCES
1. J.Hüseyin Filiz. Problems on the design of machine elements.
2. Joseph E.Shigley, Charles R.Mischke, Richard G.Budynas (7th edition). Mechanical
Engineering design.
3. www.FlyPress.com
4. www.Savonaequip.com