Impact breakers are introduced in terms of their history in the United Kingdom, mode of operation and operating problems. The practical need for an index of breakability is discussed with reference to fracture mechanics and indentation problems. Cyclic loading is reviewed and discussed as an analogy for repeated impact. The importance of work hardening and softening is discussed and the hardening diagram introduced. The case for constant strain energy cyclic testing is argued. The Nottingham University impact test rig is described together with some of its results and intended uses. A mechanistic approach to the problems of wear of impact breaker picks is given in the light of experimental results
Interest in hydraulic breakers in the United Kingdom stems from the need for mechanisation of road-heading in rocks unsuitable for the rotary boom-cutter type of machine. The longwall advance systems popular in U.K. collieries require 'ripping', i.e. the gate roadways must be kept up with the coalface in such a manner that the stone debris can be used for gateside packs and the ripping/packing machinery does not, as far as possible, interfere with access to and from the face. In the late 1960's 'impact rippers' were developed using hydraulic breakers as the cutting tool. The popularity of these machines has remained localised but the market for impact breakers has diversified with quarry, demolition, trenching and gold mining applications. In practice many of the problems with impact breakers come from the machine which manoeuvres them (especially where 'misuse' occurs), however, this paper deals with a series of experiments which it is hoped shed light on the action of the breaker.
Hydraulic Breakers
The mechanism of an hydraulic breaker can be described thus: The hydraulic system pushes a piston back into the breaker, compressing as it does so a quantity of nitrogen. This gas acts as a spring and also activates a valve which allows the hydraulic fluid to escape. Thus the piston is shot forward under the pressure of the gas and impacts upon the pick.
This is in turn in contact with the rock. Changes in the pressure at which the valve operates affects the blow frequency (which may be up to 12 c.p.s., determined either by the capacity of the hydraulic supply or the frictional heating effects at the rock/ pick interface). Blow energies have been measured in the National Engineering Laboratory as being in excess of 4000 joules. The pick is normally pushed into the rock with a 'Crowd Force'. Thus the energy available for penetration in a single impact is the impact energy plus the crowd force times the distance penetrated, i.e. the more plasticity occurring during impact the more there is to be gained from a high crowd force.
There is a dilemma with impact breakers as to the extent to which a 'dynamic' solution is appropriate (strain wave phenomena) and to what extent a pseudostatic treatment is justified.
