J — Dictionary of Quarrying Terms

jackhammer: A light percussive drill used in the hand or with some light support.

jar-bar feeder: The jar-bar grizzly feeder is a heavy-duty feeder which may form the bottom of the feed hopper of a primary crusher. It consists of a series of bars of elliptical cross-section with the major axes of adjacent bars at right-angles. Material is moved forward by the rotating bars, with a tumbling, rocking motion which makes this type of feeder suitable for applications where fines and clay have to be removed from the feed.

jaw crusher: A compression crushing machine consisting of a fixed crushing face and a mobile face which moves so as to increase and decrease alternately the gap between the two faces.

jawstock: The swing jaw of a jaw crusher.

jet-piercing: The use of high-velocity jet flames to cut blastholes in hard rock.

jet pump: A pump in which a jet of water at high pressure rises up a pipe, taking the sand, gravel and water, in which it is immersed, with it.

jib: see ‘boom’.

jig: A machine in which the feed is stratified in water by means of a pulsating motion and from which the stratified products are removed separately, the pulsating motion being usually obtained by alternate upward and downward currents of water.

jigging: A mineral-processing operation in which particles are separated on a density basis by water pulsation.

jim crow: A rail bender operated by hand.

job evaluation: The techniques of helping to determine a fair day’s pay for a particular job. It is the assessment of the value of the job not the individual and must reflect fairly the skill, experience, responsibility and condition of the job.

joint: 1 A plane or parting in rock, normally involving no visible displacement. Joints often occur in parallel sets, cutting the rock into blocks. 2 A discontinuity in any of the layers forming a road occurring either between the units or incidentally to the process of construction.

jointing sleeves: Insulating thimbles placed over the connected ends of detonator leads coupled in large rounds of shots, and over the connections between the detonator leads and the shotfiring cable.

joist: A beam which supports a floor or ceiling.

joule: The work performed when the point of application of a force of one newton is displaced through a distance of one metre in the direction of the force. Symbol: J (=lNm).

journal: The part of a shaft which is supported and held by a bearing.

journal bearings: Bearings which support rotating shafts loaded in such a way that the load is applied in a direction at right-angles to the axis of the shaft.

jumbo: A drill carriage usually holding several drifters for tunnelling work.

Jurassic: A period of the Mesozoic era between 213 and 144 million years ago, and the corresponding system of rocks.