Similar to trusses, frames are generally fixed, load
carrying structures.
The main difference between a frame and a truss is that in
a frame at least one member is a “multi force member”
(çoklu kuvvet elemanı).
A multi force member supports three or more forces
or at least two forces and one or more couples.
In contrast with a truss, the force or moment can be
exerted to any point on the frame member; it does
not have to be applied the joint as in the trusses.
Machines are structures which contain moving parts and are designed
to transmit input forces or couples to output forces or couples.
Therefore a machine is an assembly of rigid members that are capable
of generating work by means of some kind of motion.
A machine may contain several mechanisms.
A mechanism is a device designed to transform input
forces and movement into a desired set of output forces
and movement.
Machines are designed to change forces, enhance and
amplify their magnitudes and transmit them.
Whether a machine is as simple as a hand tool or as
complex as an airplane, the main aim is to convert input
forces into output forces.
The main difference between a frame and a
machine is that although frames are rigid
structures, machines are not.
Machines may be fixed to some supporting
surface or body, but they will always consist
moving parts.
The forces acting on each member of a connected system are
found by isolating the member with a FBD and applying the
equations of equilibrium.
The principle of action and reaction must be carefully observed
when we represent the forces of interaction on the separate FBDs.
It would be appropriate to identify the two forces members in the
frame, if there is any, before starting with the solution.
In order to determine the forces in a frame or machine,
it is divided into a sufficient number of members or
groups of members, but initially the support forces to
be used in the analysis must generally be determined
from the equilibrium of the whole frame.
The structure is then, dismembered and the equilibrium of
each member is considered separately.
The equilibrium equations for the several parts will be related
through the terms involving the forces of interaction.
It should be kept in mind that when going from one member to
the other, the direction of the interaction force must be
changed in accordance with Newton’s third law.
In general the FBDs of pins in the structures are not drawn;
pins are considered as a complementary part of one of the
two members it connects. It must be clearly decided which
member the pin will belong to. Whereas, the FBD of a pin
will be considered if:
 It connects three or more members,
 It connects a support and two or more members,
 A load is directly applied to the pin.
work output
h
energy input
Machines are considered as “ideal machines” when the work
output is equal to the energy input. It is impossible to build such
machines. In a real machine friction forces always generate
useless work which causes loss of energy, therefore, work output
is always less than the energy input. In other words, the
mechanical efficiency is always less than one, h<1.
Mechanical advantage is the ratio of the output force of a
machine to the input force necessary to work the machine.
This concept is totally different from the mechanical
efficiency and should not be mixed with it. Mechanical
advantage is generally greater than one.
Output Force
Mechanical Advantage 
Input Force
Sample Problem: Small bolt cutter operated by hand for cutting small bolts and
rods is shown. For a hand grip P=150 N. Determine the force Q developed by
each jaw on the rod to be cut.
The forces acting on the two parts of the bolt cutter behave as mirror images of each other with respect to
x-axis. Thus, we can not have an action on one member in the +x direction and its reaction on the other
member in the –x direction. Consequently the forces at E and B have no x-components and CD is a twoforce member.
Mechanical advantage