Static Force Analysis

6/19/2015 1Hareesha N G, Dept of Aero Engg, DSCE, Blore

Unit 1: Static Force Analysis
• Static force analysis
– Introduction
– Static equilibrium
– Equilibrium of two and three force members
– Members with two forces and torque
– Free body diagrams
– principle of virtual work
• Static force analysis of
– four bar mechanism
– slider-crank mechanism with and without friction.
6/19/2015 Hareesha N G, Dept of Aero Engg, DSCE, Blore 2

STATIC EQUILIBRIUM
• A body is in static equilibrium if it remains in its state of rest or
motion.
• If the body is at rest, it tends to remain at rest and if in motion, it
tends to keep the motion.
• In static equilibrium,
– the vector sum of all the forces acting on the body is zero and
– the vector sum of all the moments about any arbitrary point is zero.
• Mathematically,
• In a planer system, forces can be described by two-dimensional
vectors and therefore,

EQUILIBRIUM OF TWO AND THREE-FORCE MEMBERS
• A member under the action of two forces will be in
equilibrium if
– the forces are of the same magnitude,
– the forces act along the same line, and
– the forces are in opposite directions.
• Figure shows such a member.
• A member under the action of three forces will be in
equilibrium if
– the resultant of the forces is zero, and
– the lines of action of the forces intersect at a point (known as
point of concurrency).

EQUILIBRIUM OF TWO AND THREE-FORCE MEMBERS (Contd….)
• Figure (a) shows a member acted upon by three forces F1, F2 and F3
and is in equilibrium as the lines of action of forces intersect at one
point O and the resultant is zero.
• This is verified by adding the forces vectorially [Fig.(b)].
• As the head of the last vector F3 meets the tail of the first vector F1,
the resultant is zero.
• Figure (d) shows a case where the magnitudes and directions of the
forces are the same as before, but the lines of action of the forces do
not intersect at one point.
• Thus, the member is not in equilibrium.

• Consider a member in equilibrium in which force F1 is completely
known, F2 known in direction only and F3 completely unknown.
• The point of applications of F1 , F2 and F3 are A, B and C respectively.
• To solve such a problem, first find the point of concurrency O from the
two forces with known directions, i.e. from F1, and F2.
• Joining O with C gives the line of action of the third force F3.
• To know the magnitudes of the forces F2 and F3, take a vector of proper
magnitude and direction to represent the force F1.
• From its two ends, draw lines parallel to lines of action of the forces F2
and F3 forming a force triangle [Fig.].
• Mark arrowheads on F2 and F3 so that F1 , F2 and F3 are in the same
order.

MEMBER WITH TWO FORCES AND A TORQUE
• A member under the action of two forces and an applied torque will
be in equilibrium if
– the forces are equal in magnitude, parallel in direction and opposite in sense
and
– the forces form a couple which is equal and opposite to the applied torque.
• Figure shows a member acted upon by two equal forces F1, and F2
and an applied torque T for equilibrium,
where T, F1 and F2 are the magnitudes of T, F1 and F2 respectively.
• T is clockwise whereas the couple formed by F1, and F2 is counter-
clockwise.

FORCE CONVENTION
• The force exerted by member i on member j is represented
by Fij

FREE BODY DIAGRAMS
• A free body diagram is a sketch or diagram of a part isolated from the
mechanism in order to determine the nature of forces acting on it.
• Figure (a) shows a four-link mechanism.
• The free-body diagrams of its members 2, 3 and 4 are shown in Figs.
(b), (c) and (d) respectively.
• Various forces acting on each member are also shown.
• As the mechanism is in static equilibrium, each of its members must
be in equilibrium individually.

• Member 4 is acted upon by three forces F, F34 and F14.
• Member 3 is acted upon by two forces F23 and F43
• Member 2 is acted upon by two forces F32 and F12 and a torque T.
• Initially, the direction and the sense of some of the forces may not be
known.
• Link 3 is a two-force member and for its equilibrium F23 and F43 must
act along BC.
• Thus, F34 being equal and opposite to F43 also acts along BC.

• Assume that the force F on member 4 is known completely.
• To know the other two forces acting on this member completely, the
direction of one more force must be known.
• For member 4 to be in equilibrium, F14 passes through the intersection
of F and F34 .
• By drawing a force triangle (F is completely known), magnitudes of
F14 and F34 can be known [Fig.(e)].
Now F34 = F43 = F23 = F32
• Member 2 will be in equilibrium if F12 is equal, parallel and opposite
to F32 and

A four-link mechanism with the following dimensions is acted upon by
a force 80N at angle 1500 on link DC [Fig.(a)): AD = 50 mm, AB = 40 mm,
BC = 100 mm, DC = 75 mm, DE = 35 mm. Determine the input torque T
on the link AB for the static equilibrium of the mechanism for the given
configuration.
As the mechanism is in static equilibrium, each of its
members must also be in equilibrium individually.
Member 4 is acted upon by three forces F F34 and F14.
Member 3 is acted upon by two forces F23 and F43
Member 2 is acted upon by two forces F32 and F12 and
a torque T.
Initially, the direction and the sense of some of the
forces are not known.
Force F on member 4 is known completely. To know the other two forces acting
on this member completely, the direction of one more force must be known. To
know that, link 3 will have to be considered first which is a two-force member.

• As link 3 is a two-force member [Fig.(b)], for its equilibrium, F23 and
F43 must act along BC (at this stage, the sense of direction of forces F23
and F43 is not known). Thus, the line of action of F34 is also along BC.
• As force F34 acts through point C on link 4, draw a line parallel to BC
through C by taking a free body of link 4 to represent the same. Now,
as link 4 is three force member, the third force F14 passes through the
intersection of F and F34 [Fig (c)].
• By drawing a force triangle (F is completely known), magnitudes of
F14 and F34 are known [Fig(d)].
From force triangle, F34 = 47.8 N

• Now, F34 = -F43 =F23 = -F32
• Member 2 will be in equilibrium [Fig. (e)] if F12 is equal, parallel and
opposite to F32 and
T=-F32 x h = 47.8 x 39.3 = -1878.54 N.mm The input torque has to be
equal and opposite to this couple i.e. T= 1878.5 N/mm (clockwise)
h=Perpendicular distance between two equal and opposite forces.

Figure shows a slider crank mechanism in which the resultant gas pressure 8x104 N/m2
acts on the piston of cross sectional area 0.1m2 The system is kept in equilibrium as a
result of the couple applied to the crank 2, through the shaft at O2. Determine forces
acting on all the links (including the pins) and the couple on 2. OA=100mm, AB=450mm.
6/19/2015
Hareesha N G, Dept of Aero Engg, DSCE,
Blore
15

6/19/2015
Blore
16
Force triangle for the forces acting on(4)is drawn to some suitable scale.
Magnitude and direction of P known and lines of action of F34 & F14
known.
Measure the lengths of vectors and multiply by the scale factor to get the magnitudes
of F14 & F34.
Directions are also fixed.
Since link 3 is acted upon by only two forces, F43 and F23
are collinear, equal in magnitude and opposite in direction
i.e., F43 = -F23 =8.8xl03 N
Also, F23 = - F32 (equal in magnitude and opposite in
direction).

6/19/2015
Blore
17
Link 2 is acted upon by 2 forces and a torque, for equilibrium
the two forces must be equal, parallel and opposite and their
sense must be opposite to T2.
There fore,
F32 = -F12 =8.8xl03 N
F32 & F12 form a counter clock wise couple of magnitude,
F23 * h = F12 *h
=(8.8xl03)x0.125 = 1100 Nm.
To keep 2 in equilibrium, T: should act clockwise and
magnitude is 1100 Nm.
Note: h is measured perpendicular to F32 &F12

For the static equilibrium of the quick return mechanism shown in Fig (a), determine the input
torque T2: to be applied on link AB Tor a force of 300N on the slider D. The dimensions of the
various links arc OA=400mm, AB=200mm, OC=800mm. CD=300mm

• For the mechanism shown in Fig., determine the torque on the link
AB for the static equilibrium of the mechanism.

• Answers from Composite Graphical Solution

Graphical Solution by Superposition method

In a four-link mechanism shown in Fig., torque T3 and T4 have
magnitudes of 30N.m and2 ON. m respectively. The link lengths are AD
= 800 mm, AB = 300 mm, BC = 700 mm and CD = 400 mm. For the static
equilibrium of the mechanism, determine the required input torque T2

Neglecting torque T3

Neglecting torque T4

PRINCIPLE OF VIRTUAL WORK
• The principle of virtual (imaginary) work can be stated
as the work done during a virtual displacement from the
equilibrium is equal to zero.
• Virtual displacement may be defined as an imaginary
infinitesimal displacement of the system.
• By applying this principle, an entire mechanism is
examined as a whole and there is no need of dividing it
into free bodies.

• Consider a slider-crank mechanism shown in Fig.
• It is acted upon by the external piston force F, the external
crankshaft torque T and the force at the bearings.
• As the crank rotates through a small angular displacement
δθ, the corresponding displacement of the piston is δx, the
various forces acting on the system are
– Bearing reaction at O (performs no work)
– Force of cylinder on piston, perpendicular to piston
displacement (produces no work)
– Bearing forces at A and B, being equal and opposite (AB is a
two-force member), no work is done
– Work done by torque T= T δθ
– Work done by force F = F δx
• Work done is positive if a force acts in the direction of the
displacement and negative if it acts in the opposite
direction.

The negative sign indicates that for equilibrium, T must be applied in
the opposite direction to the angular displacement.

Static Force Analysis

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