What Is Gear Backlash?
Backlash is the clearance or play between mating gear teeth when measured at the pitch circle. When you hold one gear stationary and rock the other back and forth, the small angular movement before the teeth make contact is the backlash. It exists in virtually every gear system.
Mathematically, backlash is the difference between the tooth space of one gear and the tooth thickness of its mating gear, measured along the pitch circle. Standard backlash for spur gears is typically 0.03 to 0.05 times the module.
Why Backlash Exists
Backlash is intentionally designed into gear systems for several important reasons:
- Thermal expansion: Gears heat up during operation, causing teeth to expand. Without backlash, expanding teeth would bind and potentially seize
- Manufacturing tolerances: No gear can be manufactured to perfect dimensions. Backlash accommodates normal manufacturing variations
- Lubrication: The gap between teeth allows lubricant to enter and exit the mesh zone, preventing metal-to-metal contact
- Deflection: Under load, gear teeth deflect. Backlash prevents contact on the non-driving side of the tooth during loaded operation
When Backlash Causes Problems
While necessary for most applications, backlash becomes problematic in certain situations:
- Positioning systems: CNC machines, robot joints, and antenna positioning systems need precise angular positioning. Backlash creates dead zones where the output does not respond to input changes
- Reversing loads: When loads alternate direction frequently, teeth impact the opposite flank each reversal, causing noise, vibration, and accelerated wear
- Servo systems: Feedback control systems can oscillate or become unstable when backlash creates a nonlinear dead band in the drive train
Methods to Reduce or Eliminate Backlash
Engineers have developed several techniques to control backlash:
- Tighter tolerances: Manufacturing gears to closer tolerances (AGMA Quality 10-12) reduces backlash but increases cost significantly
- Adjustable center distance: Mounting one gear on an eccentric or adjustable mount allows the center distance to be reduced, closing the backlash gap
- Split gears: Two gear halves are spring-loaded against each other, with one half pressed against each flank of the mating gear. This eliminates backlash but adds friction and complexity
- Preloaded duplex gears: Similar to split gears, two thin gears are mounted on the same shaft with a controlled angular offset
- Tapered gears: Conical tooth thickness allows axial adjustment to control backlash
Measuring Backlash
Backlash can be measured in several ways:
- Circumferential backlash: Measured at the pitch circle using a dial indicator on the driven gear while holding the driving gear fixed
- Normal backlash: Measured perpendicular to the tooth surface using a feeler gauge between mating teeth
- Angular backlash: The angular movement of the output gear with the input gear locked, measured in arc-minutes or degrees
Backlash in Different Gear Types
Typical backlash values vary by gear type and quality:
- Spur gears: 0.03-0.05 × module (standard), 0.01-0.02 × module (precision)
- Helical gears: Similar to spur in the normal plane
- Worm gears: Generally higher backlash due to the sliding contact nature
- Harmonic drives: Near-zero backlash (< 1 arc-minute) — ideal for precision applications