Understanding Gear Dimensions: Space Width and Tooth Thickness

In the design and operation of gears, several dimensions are crucial for proper meshing and power transmission. Two important dimensions measured along the pitch circle are the space width and the tooth thickness.

Let's define these terms:

• Pitch Circle: This is a theoretical circle on which the calculations of gear dimensions are based. When two gears mesh correctly, their pitch circles are tangent to each other.
• Tooth Thickness: This is the width of a gear tooth measured along the pitch circle.
• Space Width: This is the width of the gap between two adjacent teeth, measured along the pitch circle.

Ideally, for perfect meshing without any play, the space width would be exactly equal to the tooth thickness on the mating gear's pitch circle. However, in practice, a small clearance is intentionally left between mating teeth. This clearance is known as backlash.

What is Backlash?

Backlash is defined as the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circle. It is the clearance or lost motion when the direction of movement is reversed.

Mathematically, along the pitch circle:

\(\text{Backlash} = \text{Space Width} - \text{Tooth Thickness}\)

This difference is deliberately introduced during manufacturing for several reasons, such as:

• To prevent jamming due to thermal expansion.
• To allow for lubrication between tooth surfaces.
• To compensate for manufacturing errors.
• To prevent excessive wear.

Analyzing the Options

Let's look at the given options in the context of gear terminology:

• Fillet: This refers to the curved portion at the root of a gear tooth, connecting the flank to the bottom land. It is not the difference between space width and tooth thickness.
• Face Width: This is the width of the gear tooth measured parallel to the gear axis. It is not measured along the pitch circle and is not related to the difference between space width and tooth thickness.
• Flank: This is the surface of a gear tooth profile below the pitch circle. It is part of the tooth geometry but not the difference in width between the space and the tooth.
• Backlash: As discussed, this is precisely the difference between the space width and the tooth thickness measured along the pitch circle.

Therefore, the difference between the space width and the tooth thickness along the pitch circle is known as backlash.