A SCREW JACCK IS DESIGNES-64. THER OF TOMMMY BAR FOR AHE DIAMETEPP...

99]

a screw jac

ck is design

ES-64. Th

er of tomm

my bar for a

he diamete

pplied

) Bending m

(a)

to effort ap

moment due

bar due to

ed

the tommy

effort applie

) Torque on

(b)

loads

(c)

(d)

) A percenta

age of axial l

ads

pled with tra

al loads coup

) Some axia

ansverse loa

ES-64Ans. (

IE

(a)

Design of Friction Drives

S K Mondal’s

Chapter 2

(a) Enhance the load carrying capacity of the jack

(b) Reduce the effort needed for lifting the working load

(c) Reduce the value of frictional torque required to be countered for lifting the load

(d) Prevent the rotation of load being lifted.

IES-65Ans. (d)

IES-66. Under service conditions involving jarring, vibration and pulsation of the

working load, the bolt of choice would

[IES 2007]

(a) short bolt with high rigidity

(b) long bolt with increased elasticity

(c) Bolt with a dished washer

(d) bolt with castle nut

IES-66Ans. (d)

IES-67. If P is the pitch of a square thread, then the depth of thread d is given by

(a) 0.5 P

(b) P

(c) 1.5 P

(d) 2.0 P

IES-67Ans. (a)

IES-68. The frictional torque for square thread at mean radius while raising load

is given by

[IES-1993]

(W = load; R

o

= Mean Radius;

ϕ

= Angle of friction; α = Helix angle)

( )

a WR

_o

tan(

φ α

−

) ( )

b WR

_o

tan(

φ α

+

) ( )

c WR

_o

tan

α

( )

d WR

_o

tan

φ

IES-68Ans. (b)

IES-69. What is the efficiency of a self-locking power screw? [IES-2006; 1997]

(a) 70% (b) 60%

(c) 55%

(d) < 50 %

IES-69Ans. (d) We know that the frictional torque for square thread at mean radius while

raising load is given by

WR

_o

tan(

φ α

−

)

Where: (W = load;

R

o

= Mean Radius;

ϕ = Angle of friction; α = Helix angle)

For self locking, angle of friction should be greater than helix angle of screw So that

tan(

φ α

−

)

WR

o

will become positive. i.e. we have to give torque to lowering the load.

IES-70. To ensure self-locking in a screw jack it is essential that helix angle is

(a) Larger than friction angle (b) smaller than friction angle.

[IES-1996]

(c) Equal to friction angle

(d) such as to give maximum efficiency in lifting.

IES-70Ans. (b)

IES-71.

The maximum efficiency of a screw jack having square threads with

friction angle φ is

[IES 2007]

ϕ

−

−

_(b)

1

tan

)

2

/

tan(

(a)

+

−

(d)

sin

sin(

(c)

IES-71Ans. (c)

η

φ

⎛

=

−

⎞

IES-72. Assertion

(A): The maximum efficiency

1 sin

⎜

+

⎟

φ

1 sin

⎝

⎠

of a screw jack is same,

where

ɽ

is the friction angle, for both motion up and motion down the plane.

Reason

(R): The condition for the maximum efficiency for motion up and motion

α

= −

π φ

where α = helix angle. [IES-2003]

down the plane is same, given by

4

2

(a) Both A and R are individually true and R is the correct explanation of A

(b) Both A and R are individually true but R is not the correct explanation of A

(c) A is true but R is false

(d) A is false but R is true

IES-72Ans. (a)

IES-73. A screw jack is said to be self-locking if its efficiency is

[IES-2002]

(a) Less than 50% (b) equal to 50%

(c) more than 50%

(d) 100%

IES-73Ans. (a)

IES-74. Which one of the following is the value of helix angle for maximum

efficiency of a square threaded screw? [

φ

=

tan

⁻

¹

μ

] [IES-2004]

(a) 45

^o

+

φ

(b)

45

^o

-

φ

(c)

45

^o

-

φ

/2

(d) 45

^o

+

φ

/2

IES-74Ans. (c)

IES-75.

Stresses in a screw thread are estimated by considering the thread to be:

(a) Long cantilever beam projecting from the pitch cylinder

[IES-2006]

(b) Long cantilever beam projecting from the root cylinder

(c) Short cantilever beam projecting from the root cylinder

(d) Short cantilever beam projecting from the pitch cylinder

IES-75Ans. (c)

Q.20.

A power screw of 32 mm nominal diameter and 5 mm pitch is acted upon

by an axial load of

12 kN. Permissible thread bearing pressure is 6 MPa;

considering bearing action between the threads in engagement, what is

the number of threads in engagement with the screw?

[IES-2009]

(a) 6

(b) 7

(c) 9

(d) 10