A displacement gage for the rock-mechanics laboratory

Abstract

A gage for measuring displacements has been developed for use in the rock-mechanics laboratory and in the field. The gage consists of a support ring that holds a linear-variable-differential transformer (LVDT), a mounting screw, and a leaf spring. The gage is mounted to the test specimen at two points between which displacement is to be measured. At one point, contact with the specimen is by means of an adjusting screw. At the other point, contact is through a dimple in the leaf spring. The leaf spring in turn is rigidly connected to the support ring. An LVDT is mounted in the ring with its axis parallel to the line of measurement and its core rod attached in the dimple in the leaf spring. Any change in the length of the line between support points is directly communicated to the LVDT. Other gages using LVDTs have been constructed; but the technique for attaching the gage to the test specimen relied on the LVDT itself to support the ring. Because of the delicacy of the movement in precision-gage-head LVDTs, only small forces could be tolerated, leading to an unstable, nonrugged gage. For regular LVDTs, the free floating core is not well suited to supporting lateral forces. Using the leaf spring provides a secure mount, capable of bearing reasonable lateral loads with little flexure. The LVDT is left free of all load so that its precision is uncompromised. By its nature, a leaf spring is stiff in its plane which is the direction of the support forces in this case. In the normal direction, the leaf spring can be as compliant as desired. Accuracy is independent of the spring since the LVDT contacts the sample at the same point as the spring does.