Reference
NAVSEA 0900-LP-060-2010, Technical Manual for Electrical Machinery Repair: Electric Motor Repair Shop Procedures Manual (Series), Chapter 2, "Test Equipment and Measuring Instruments"

Background

The snap gage is an instrument that precisely measures shaft diameters and bearing journals within one ten-thousandth of an inch.

One advantage of the snap gage is that it is a dial indicating instrument that can be read directly. Also, an accurate reading on the dial indicator does not depend on a sense of feel like when using an outside micrometer.

Setting the snap gage requires the use of plug standards before commencing measurements. These standards correspond to the exact inside diameter of most ball bearings that you will find in electric motors. Any deviation (plus or minus) of the surface being measured is readily indicated when the snap gage is properly set and used. The readings obtained are compared to allowable minimum and maximum measurmentsof the size and type of bearing installed.

• An insulated handle that reduces heat transfer from the hands to the instrument, which reduces the possibility of inaccurate readings caused by thermal expansion of the metal
• A dial with digits indicating thousandths (0.001) and incremental lines indicating ten-thousandths (0.0001) of an inch
• A bezel that allows for zeroing the dial indicator
• A bezel clamp that locks the bezel in place
• A guard that protects the dial indicator when the snap gage is laid on its side
• A hex wrench mounted in the handle and used as a lever to turn the size adjustment wheel and adjust the backstop
• Two locking wheels used during plug standard calibration to lock the instrument at the proper setting
• A size adjustment wheel that raises and lowers the upper anvil to make contact with the plug standard
• A backstop that keeps the standard or surface being measured centered between the anvils
• A lower anvil that the plug standard or object being measured rests on