🔍 Key Concepts

• Backlash as the free movement between gear teeth before the driven gear begins to move • Which tools measure linear movement (travel) versus clearance by thickness • How backlash in an enclosed gear case (like the illustration) is typically checked without inserting anything between the teeth

💭 Think About

• Look at the cutaway: would you practically be able to slip something between the meshing gear teeth while assembled, or would you measure how far one gear can move while the other is held fixed? • Which option names a device that reads very small movements on a graduated scale, and which options are mainly used for checking clearances or defects by thickness or color? • Think about maintenance practice: when adjusting gear mesh shims, what kind of instrument do mechanics mount on the housing to watch the pointer as they rock the gear back and forth?

✅ Before You Answer

• Identify which choice is specifically designed to show thousandths of an inch of movement rather than thickness or visual indication • Consider how each tool would physically be applied to the gears in the closed housing shown • Eliminate any tools that are primarily for bearing crush, surface crack detection, or flat surface gaps, not for gear backlash measurement