Mechanical strain indicators and recorders

Abstract

The oldest techniques of strain measurement are those which involve direct mechanical measurement of the change in length of a specimen. The general disadvantages of direct mechanical measurement sometimes overshadow and detract from its general use. Weight, bulkiness, low frequency response and limited range of reading are the major disadvantages when compared with the electrical strain gage. However, many situations arise where dependability of calibration, along with the lack of need for expensive electronic instrumentation, make the use of a mechanical motion-measuring device practical. When temperature compensated to the workpiece, they have the capacity to accurately show changes over long periods of time. Also, one mechanical gage may be used to take readings for any number of positions by using permanent gage-length points as a reference. The basic principle of mechanical-type gages consists of magnifying the relatively small motions to values that are readable by eye. For example, the strain of steel subjected to a uniaxial state of stress of 60,000 psi (413.7 MPa) is .002 in./in. Since the total elongation is the product of the strain and the gage length, it can readily be seen that any mechanical device for indicating strain in terms of motion must be capable of great amplifications without lost motion or play. One of the most fundamental methods of strain amplification is the use o f : (a) simple lever. In this basic category may also be placed : (b) Multiple levers (c) Optical levers (d) Dial indicators (e) Direct measurement over long distances Another large class of motion-amplifying devices also exists. These devices may be classified in the general field