S.T.B.S College of Diploma Engineering,
SURAT
Class: Mech-1051(B)
Prepared.BY:Thumar Sanket C.
EN.NO:106470319020
APP.BY:Mr.CRS
TOPIC : Screw jack
There are two distinct
uses for screw threads
and they usually
demand different
behavior from the
threads :
•a power screw such
as a lathe leadscrew or
the screw in a car lifting
jack which transforms
rotary motion into
substantial linear
motion (or vice versa in
certain applications),
A
power screw is a
device that is common to
tools or machinery that
are used to change
angular motion into
translation. It is also
capable of developing
a large amount of
mechanical advantage.
Familiar applications
include clamps or vises,
presses, lathes lead
screws, and jacks.
Weight
supported
by three
screw jacks.
In each
screw jack,
only the
shaded
member
rotates.
Figure 8-1
Terminology of
screw threads.
Sharp Vee
Threads shown
for clarity; the
crests and roots
are actually
flattened or
rounded during
the forming
operation
(a) Single (STANDARD)-, (b) double-, and (c)
triple threaded screws.
Power screw thread forms. [Note: All threads shown are external (i.e., on the screw,
not on the nut); dm is the mean diameter of the thread contact and is approximately
equal to (d + dr)/2.]
Efficiency
which, is the torque required
to raise the load.
The efficiency is
therefore
T0
Fl
efficiency  e 

TR 2TR