Failure stress calculation for ball and saddle.
It is assumed that the weakest point or failure point due to the reaction force from the water jet will be the neck of the ball (ø12mm).
For the purposes of calculating the load necessary to cause failure it is assumed that the water jet reaction acts through the centre line of the water tube and that the complete ball is held rigidly in the clamp.
In addition it is assumed that the ball is fitted to an ø12mm rod and does not widen out to the saddle or is attached to the water tube.
Material: Aluminium grade 6082 T6. Maximum stress = 300 x 106 N/M2
Area of rod – A = D2 / 4 = 113 x 10-3 M2
T = 300 x 106 / √3 = 173 x 106
Force needed to break rod, F = T / A
F = 173 x 106 x 113 / 10-3 = 195900 Newtons = 1998 kg
Note: Maximum recommended nozzle reaction force etched on to SoloClamp frame = 50 kg
Safety chain
It is considered that the risk of failure of the ball neck from the nozzle reaction is very small. However, the risk of the neck breaking due to accidental damage, i.e. the device being driven into by a reversing truck, is significant. Failure of the ball neck could allow the nozzle and hose to flail, possibly causing injury. As a precaution the SoloClamp is fitted with a safety chain to restrain the hose and nozzle should the ball neck break. The chain links the water tube to the bracket hinge pin.
Safety chain specification:
7×19 ø2.0mm SS316 stainless steel wire rope looped at each end using crimped SS316 stainless steel ferrules. Rope minimum breaking strain: 232 kg.
Nozzle overload
The reaction force from a smooth bore water jet ø50mm (2”) with a pressure of 10 bar at the tip and a flow of 5250 l/min is 400 kg. However, this combination of pressure and flow is normally not achievable during firefighting operations due to the limitations of the pumps and hoses.
Tests have shown that when an excessive force is applied by the nozzle, and the ball and socket are overloaded; the failure mode is a slow slide to a position where the loading is within the limits of the device and not a catastrophic failure.