Gear tolerances define how precisely a gear's tooth profile, pitch and runout match the drawing. Tolerance classes follow recognised systems such as ISO, DIN and AGMA, and each class balances functional performance against manufacturing cost. Tighter classes improve noise, vibration and load sharing, but they increase machining and inspection effort.
Backlash is the intentional clearance between mating tooth flanks. Some backlash is required in nearly every gear pair so that teeth do not jam, lubrication can form an oil film, and thermal expansion does not lock the mesh. The right amount depends on speed, temperature range and the positioning accuracy the application needs.
Positioning-critical applications such as machine tool feeds and servo drives usually need minimal, controlled backlash, sometimes achieved through preloading or split gears. Heavy reversing drives also demand carefully controlled backlash to avoid impact loading. By contrast, general power transmission can tolerate normal backlash values that keep costs down.
The gear inspection report is where tolerances are proven. Depending on the class, a supplier measures profile deviation, pitch deviation, runout, tooth thickness and backlash against master gears or with a gear measuring center. Asking for the relevant inspection values in the RFQ ensures the delivered parts are verifiable against the agreed class.
When specifying tolerances, buyers should reference a standard and a class explicitly, for example an ISO or DIN quality number, rather than vague phrases such as "precision quality". This removes ambiguity during quotation and acceptance. It also lets the supplier propose a realistic class if the stated one exceeds what the application actually requires.
Practical RFQ advice: state the reference standard and class, the required backlash range if positioning matters, and the inspection documents you need. A manufacturer with in-house gear inspection can confirm feasibility before production and provide the report with each batch.