This document outlines the design of a screw jack. It describes the components of a screw jack including the screw spindle, nut, and handle. It provides equations to calculate the compressive and shear stresses on the screw spindle, bearing and tensile stresses on the nut, shearing stresses on the nut threads, and torques applied to the handle. The design aims to ensure the components can withstand the stresses and forces exerted during lifting of heavy weights.

1. Screw Jack Design
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2. What is a Screw jack?
 A screw jack is a mechanism that converts rotational
motion to linear motion, and a torque (rotational force) to a
linear force.
 They are mainly used to lift heavy weights.

3. Schematic diagram of screw jack:

4. 1. Design of screw spindle:
• Considering screw spindle under compression
 Resisting Area(A)=
𝜋
4
*dc
2
 Compressive strength of screw spindle(W)= 𝜎C ∗
𝜋
4
dc
2

5. • Considering torsional shear stress for shrew spindle
T1=
𝜋
16
* (dc)3
* 𝜏
𝜏 =
16𝑇1
𝜋 𝑑𝑐 3
T1 = Torque required to rotate the screw
T1 =W*tan(𝛼+∅)*d/2
Maximum shear stress theory
𝜏max =1/2 𝜎𝑐 2 + 4𝜏2
Maximum normal stress theory
𝜎cmax =1/2*{𝜎c + 𝜎𝑐 2 + 4𝜏2

6. 2. Design of NUT
• Considering bearing failure of nut
 Resisting area (A)=
𝜋
4
(do
2 –dc
2 )
 Bearing strength of nut(W)=A*Pb
W=
𝜋
4
(do
2 –dc
2 )*n*Pb
 Height of nut(h)=n*p
here p=pitch of screw
Pb =bearing pressure

7. • Considering tensile failure of nut collar
 Resisting area(A)=
𝜋
4
*(D1
2 -do
2 )
 Tensile strength of nut(W)=A*𝜎t
W=
𝜋
4
(D1
2 -do
2)*𝜎t
• Considering shearing failure of nut collar
 Shearing area(A)=𝜋𝐷1 *t1
 Shearing strength of nut(W)=A*𝜏
W=𝜋D1*t1*𝜏

8. • Shearing failure for thread in nut
 Resisting area(A)=𝜋*do *t
 Shearing strength of nut(W)=A*𝜏
W=𝜋*do *t*𝜏
t=thickness of each thread=p/2

9. 3. Design of handle
 Total torque applied at the head
T=T1 +T2
T1 = Torque required to rotate the screw
T2 = Torque required to overcome friction between cup and head
T1= W*tan(𝛼 + ∅)*d/2
T2= 𝜇1 *W*Rm
Where 𝜇1 =Co-efficient of friction between cup and head
Rm =Mean radius

10. Total torque(T)=p*L
Where p= effort applied at the handle