There are only two types of variables active in any dynamic system. These are given the general
names of through variable and across variable. These names are descriptive of their actions
within the system. A through variable passes through the system. An across variable exists across
the system. The power in the system is the product of the through and across variables. Table 10-1
lists the through and across variables for various types of dynamic systems.
We commonly speak of the voltage across a circuit and the current flowing through it. We also can
speak of the velocity across a mechanical "circuit" or system and the force which flows through it.
Just as we can connect electrical elements such as resistors, capacitors, and inductors together
in series or parallel or a combination of both to make an electrical circuit, we can connect their
mechanical analogs, dampers, springs, and masses together in series, parallel, or a
combination thereof to make a mechanical system. Table 10-2 shows the analogs between three types
of physical systems. The fundamental relationships between through and across variables in
electrical, mechanical, and fluid systems are shown in Table 10-3.
Recognizing a series or parallel connection between elements in an electrical circuit is fairly
straightforward, as their interconnections are easily seen. Determining how mechanical elements
in a system are interconnected is more difficult as their interconnections are sometimes hard to
see. The test for series or parallel connection is best done by examining the forces and
velocities (or the integral of velocity, displacement) that exist in the particular elements.
If twoelements have the same force passing through them, they are in series. If twoelements have
the same velocity or displacement, they are in parallel.
Combining Dampers
DAMPERS IN SERIES Figure 10-6a shows three dampers in series. The force passing through each
damper is the same, and their individual displacements and velocities are different.
or : combining :
F = c1( x 1 - xz )= c2 ( x 2 - x 3 )= c3x3
F . . F . . F .
-= x 1 - x 2 ; -= x 2 - x 3 ; -= x 3
0
. ( . . ) ( · . ) . F F F
Xrotal X1 - xz X z - x3 + x3
C1 Cz C3
Cb) Parallel
FIGURE l0-6
then :
Xiotal = F -1 = F( !_ +_! +_! )
Ceff CJ Cz C3
1 1 1 I
-= -+-+
Cejj C1 Cz C3
1
Cejj = 1 1 I
-+-+
CJ Cz C3
Dampers in series and in parallel
The reciprocal of the effective damping of the dampers in series is the sum of the recipru cals of
their individual damping coefficients.