Consider a reciprocating engine as shown in figure 8.

Here, the equivalent mass of reciprocating parts is $m_0$ and the total mass of the engine including the reciprocating parts is m.

The crank length and connecting rod length are ‘e’ and ‘I’ respectively. The inertia force due to the reciprocating mass is approximately equal to:

$m_0 \ e \ w^2 \ [sin \ wt \ + \ (e/l) \ sin \ 2 \ wt]$

If ‘e’ is small as compared to ‘l’ , the centrifugal force = $m_0 \ e \ w^2 \ sin \ wt$

Hence if ‘e’ is small, the precision analysis of rotating unbalance is applicable to the case of reciprocating unbalance.