design of screw jac k may 2020

Design of Machine Elements
Design of Screw Jack.
Gaurav Mistry
Assistant Professor
Diwaliba Polytechnic, UTU.

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❑ Introduction to Power Screw Design of Machine Elements
The power screws (also known as translation screws) are used to convert rotary motion into
translatory motion. For example, in the case of the lead screw of lathe, the rotary motion is
available but the tool has to be advanced in the direction of the cut against the cutting resistance
of the material. In case of screw jack, a small force applied in the horizontal plane is used to
raise or lower a large load. Power screws are also used in vices, testing machines, presses, etc.
In most of the power screws, the nut has axial motion against the resisting axial force while the
screw rotates in its bearings. In some screws, the screw rotates and moves axially against the
resisting force while the nut is stationary and in others the nut rotates while the screw moves
axially with no rotation.
❖ Types of Screw Threads used for Power Screws

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1. Square thread. A square thread, as shown in Fig. (a), is adapted for the transmission of
power in either direction. This thread results in maximum efficiency and minimum radial or
bursting pressure on the nut. It is difficult to cut with taps and dies. It is usually cut on a lathe
with a single point tool and it can not be easily compensated for wear. The square threads are
employed in screw jacks, presses and clamping devices. The standard dimensions for square
threads according to IS : 4694 – 1968 (Reaffirmed 1996),
2. Acme or trapezoidal thread. An acme or trapezoidal thread, as shown in Fig. (b), is a
modification of square thread. The slight slope given to its sides lowers the efficiency slightly
than square thread and it also introduce some bursting pressure on the nut, but increases its area
in shear. It is used where a split nut is required and where provision is made to take up wear as
in the lead screw of a lathe. Wear may be taken up by means of an adjustable split nut. An acme
thread may be cut by means of dies and hence it is more easily manufactured than square thread.
3. Buttress thread. A buttress thread, as shown in Fig. (c), is used when large forces act along
the screw axis in one direction only. This thread combines the higher efficiency of square thread
and the ease of cutting and the adaptability to a split nut of acme thread. It is stronger than other
threads because of greater thickness at the base of the thread. The buttress thread has limited
use for power transmission. It is employed as the thread for light jack screws and vices.

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❖ Comparison of V threads and Square threads.
‘V’ - threads Square threads
1. Pitch of the thread is relatively less and
does not have adequate strength for power
transmission.
1. Pitch of thread is relatively more compared
to V threads and capable to carry heavy loads.
2. Mostly used for fasteners and pipe fittings. 2. Used for power screws.
3. Easy to manufacture. 3. Relatively difficult to manufacture.
4. Threads can be cut with the help of die and
tap.
4. Cannot be cut with the help of die or tap.
5. Production cost is low. 5. Production cost is high.
6. Used for bolt, nut, studs, screws, etc. 6. Used for screw jack, lead screw, vice, screw
for clamps, screw for press, screw for fly press,
etc.

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❖ Applications of Power Screws
1. For screw jack – to lift the load.
2. In universal testing machine.
3. In machine tools like lathe, shaper, milling, etc. as a lead screw.
4. In vices and clamps for clamping.
5. In indexing mechanism.
6. In sluice valves for opening and closing the valve gates.
7. In turn buckles or couplers to connect two members under tension.
❖ Advantages of Power Screws
1. Gives higher mechanical advantage and velocity ratio.
2. Rotary motion can be converted in to linear motion.
3. No noise during its operation.
4. Can sustain more load.
5. Easy to design and manufacture.
6. Generally it is self locking.
❖ Limitations of Power Screws
1. More power is lost in function.
2. More wear and tear of the elements.
3. Its efficiency is very low.

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❖ Torque Required to Raise Load by Square Threaded Screws
Torque required to overcome friction between the screw and nut,
And the torque required to overcome friction at the collar,

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❖ Torque Required to Raise Load by Square Threaded Screws
❖ Torque Required to Lower Load by Square Threaded Screws

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❖ Efficiency of Square Threaded Screws

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❖ Maximum Efficiency of a Square Threaded Screw

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❖ Over Hauling and Self Locking Screws
The effort required at the circumference of the screw to lower the load is
In the above expression, if  < , then torque required to lower the load will be negative. In
other words, the load will start moving downward without the application of any torque. Such a
condition is known as over hauling of screws.
If however,  > , the torque required to lower the load will be positive, indicating that an
effort is applied to lower the load. Such a screw is known as self locking screw.
In other words, a screw will be self locking if the friction angle is greater than helix angle or
coefficient of friction is greater than tangent of helix angle i.e. μ or tan φ > tan α.

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❖ Efficiency of Self Locking Screws

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❑ Design of Screw Jack Design of Machine Elements

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❑ Design of Screw Jack Numerical Design of Machine Elements
A screw jack is to lift a load of 80 kN through a height of 400 mm. The elastic strength of
screw material in tension and compression is 200 MPa and in shear 120 MPa. The material
for nut is phosphor-bronze for which the elastic limit may be taken as 100 MPa in tension, 90
MPa in compression and 80 MPa in shear. The bearing pressure between the nut and the
screw is not to exceed 18 N/mm2. Design and draw the screw jack. The design should include
the design of 1.screw, 2. nut, 3. handle and cup, and 4. body.

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Exercise: Design a screw Jack taking above dimensions into consideration.

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A screw jack has a capacity of 100 kN. The screw has triple start threads having root diameter
of 60 mm and pitch of 10 mm. Coefficient of friction for threads is 0.15. Determine the
following.
(i) Compressive stress in screw.
(ii) Shear stress in threads ( assuming 14 threads in a nut).
(iii) Efficiency of the screw.
(iv) Whether the screw is self locking or not?
Solution: Given Data
Pitch p = 10, Load W = 100 kN = 100000 N, Threads on nut nt = 14, root dia dc = 60 mm,
coefficient of friction μ = 0.15
1. Compressive stress in screw:
=
100000
𝜋
4
𝑋 602
= 35.39
𝑁
𝑚𝑚2.
2. Shear stress in threads of nut:
𝜏 𝑛𝑢𝑡 =
𝑤
𝜋. 𝑑. 𝑡. 𝑛 𝑡
t (thickness of thread)= p/2 = 10 /2 = 5 mm (explain)
d (dia of nut) = (dc + p) = 60 + 10 = 70 mm . (explain)

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𝜏 𝑛𝑢𝑡 =
100000
𝜋 𝑥 70 𝑥 5 𝑥14
= 6.5
𝑁
𝑚𝑚2.
3. Efficiency of Screw:
𝜂 =
tan 𝛼
tan(𝛼+ 𝜙)
(𝛼 = helix angle, φ = friction angle)
We know tan φ = µ = 0.15.
Therefore, φ = 8.53 degree.
Now tan 𝛼 =
𝑆
𝜋.𝑑 𝑚
S (lead) = Z (number of start) x p (pitch) = 3 x 10 = 30 mm
𝑑 𝑚 (mean diameter of screw)=
𝑑+ 𝑑 𝑐
2
=
70 + 60
2
= 65 mm
Therefore, tan 𝛼 =
30
𝜋 𝑥 65
= 0.1469
𝛼 = 8.35 degree.

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𝜂 =
tan 𝛼
tan(𝛼+ 𝜙)
= 𝜂 =
tan 8.35
tan(8.35 +8.53)
= 0.484 = 48.4 %.
4. For the screw, here
φ = 8.53 degree.
𝛼 = 8.35 degree.
Therefore φ > 𝛼 , the screw is self locking (also as 𝜂= 48.4 % < 50%, the screw is self locking.)
REFERENCES:
1. A textbook of Machine design, R. S. Khurmi, S. Chand.
2. Design of Machine Elements, S. B. Soni, Atul prakashan.
3. www.google.com

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design of screw jac k may 2020

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