In reciprocating engine mechanism, hammer blow is the maximum magnitude of unbalanced force perpendicular to the line of stroke. This force B.ω2.b. sin θ occurs when we place a balancing mass B at radius b to balance primary unbalance force. By analyzing this force, it will be maximum when θ is 90° or 270°, so at that time the force is called hammer blow.
With this hammer blow, the effect of variation in pressure between wheel and rail occurs. To know hammer blow analytically, let us consider P be the downward pressure on rails.
Net pressure between wheel and rail = P B.ω2.b
If the above result came negative then wheel will move lift from rails and if this result came positive then wheel will not lift from rails.
P = B.ω2.b