The Screw and the Jackscrew (MEC037E)

Figure 1 – The screw

Formula:

M1 = F x 6r x tg (α – υ )

Where:

M 1 = turning moment

F = applied force

r = mean radius of thread

σ = angle of friction (tan σ - u, where u is the friction coelhcient)

THE JACKSCREW

The jackscrew is used to raise heavy loads by taking advantage of the theoretical mechanical advantage (TMA) of the screw (see Fig. 2).

Figure 2 – The jackscrew

Formula:

(a) Theoretical mechanical advantage

where TMA = theoretical mechanical advantage

TMA = 1 π r/ρ

r = length of the handle (m)

ρ = pitch, or distance between corresponding points or successive threads (m)

b) Force

F = F_O / 2R

where

F = force applied to the circumference of the screw

Fo = resultant force

R = length of the arm of applied force

h = pitch of the screw

Obs.: Here, we assume the absence of friction