The HPE skip arrestor is a long-stroke ‘shock absorber’ cylinder and valve system with the ability to accurately position the rock skip and absorb the impulse of rock loading. The skip arrestor is sized to support say 50% of the empty skip weight as the skip is lowered into the loading position to avoid a ‘slack rope’ condition. This is achieved by a fixed head of water acting on a piston area inside the cylinder. The water supply source is typically from the service water column or from a dam above requiring a minimum head of 100m.When the load of rock from the flask falls into the skip, the arrestor cylinder resists the impulse force slowing the downward movement of the skip and thus reducing spillage and rope stretch.
Most of the energy of the falling rock is absorbed in the arrestor cylinder and blown off by a relief valve. The discharge can be returned to the header tank. The relief valve is adjustable and provides a constant resisting force. This constant resistance characteristic is preferred when absorbing energy as it results in the lowest force for a given stroke and energy absorption requirement. (This is in contrast to an orifice based device where the resistance is speed dependent – low at slow speeds and high at high speeds).
The HPE skip arrestor has a long stroke (typically 3 m) to allow over-wind trip tests and is clamped to the shaft guides with friction grips that allow the skip to slide downwards in the event of an over-wind.