Introduction: A DIN 6-5 marking describes tooth flank tolerance, not a noise rating, so accuracy, backlash, and alignment have to be read together to predict how quietly a geared motor runs.
Two geared motors can carry the same torque figure and the same ratio and still sound completely different on the machine bed. Buyers who compare spec sheets often assume the one with the tighter accuracy grade will be the quieter unit, then feel confused when the installed result does not match the label. The label is real and useful — it just measures one link in a chain. this guide explains what a DIN 6-5 flank actually controls, where backlash and shaft alignment take over, and why grinding a gear to a tight tolerance is a starting condition for smooth running rather than the finish line.
On a finished gear, the DIN 6-5 marking is a statement about how closely the tooth flank follows its theoretical shape. The DIN system sorts cylindrical gear accuracy into grades by grouping individual deviations: total profile deviation, total lead deviation, single pitch deviation, and total cumulative pitch deviation. Grade 6-5 means the gear sits inside the band between grade 6 and grade 5, and smaller numbers mean tighter control. For a parallel shaft helical gearbox running at ordinary pitch line speeds, that band is a sensible production target — tight enough to keep contact even across the face width, and achievable on modern CNC form grinding equipment without drifting into the cost territory of a zero-backlash servo reducer. Grinding is what makes the grade meaningful in practice. Cutting a soft blank accurately is one thing; holding that geometry after the part has been hardened is another. Profile grinding works on the finished flank, removing distortion and cutting the tooth to its final shape, so the stated class describes the gear as it ships rather than the gear as it was roughed out. In the FC series, gears are stated at DIN 6-5 with flanks finished by CNC form grinding, and that accuracy is quoted for a range that runs from 180 to 18,000 Nm of standard torque with ratios from 4 to 205. The payoff shows up as transmission error, which is the small gap between the rotation the output shaft should deliver and the rotation it actually delivers as each tooth pair takes up load. Every mesh cycle with nonzero transmission error produces a tiny velocity ripple, and that ripple pushes on the housing at mesh frequency and its harmonics. A flank held to a tight grade keeps the ripple small, so a ground gear runs smoother and cleaner-sounding than a soft-cut gear of the same size and ratio.
A gear pair never runs with zero clearance, and the amount of clearance built in changes the character of the sound a gearbox makes, not just its smoothness. Two units with identical flank accuracy can produce noticeably different noise signatures once backlash and installed alignment differ, which is why field reports and bench results often diverge.
Backlash is the circumferential clearance between meshing teeth, and its job is to leave room for lubrication film, thermal growth, and manufacturing tolerance. When it is generous and the load fluctuates or reverses — hoist travel, indexing conveyors, reversing drives — teeth separate and re-engage instead of staying in continuous contact. That impact pattern reads as rattle or a knock on reversal rather than a steady whine, and it can dominate the noise a listener notices even when the flanks themselves are excellent. Too little clearance creates the opposite problem: oil gets trapped between teeth, mesh temperature climbs, and the steady mesh tone gets louder and harsher. Backlash is therefore set as a working compromise, and it belongs to the assembled unit rather than to the gear grade alone.
Alignment errors sit upstream of the mesh and can easily outvote a good flank grade. A coupling that is angularly or parallel misaligned, a shaft with excessive runout, a soft foot, or a foundation that settles after commissioning all push vibration into the drivetrain at once-per-revolution frequency and its multiples. Those forces pass through bearings and into the housing, where they excite panels that radiate sound far more efficiently than a well-formed tooth contact does. Service experience across industrial plants is consistent on this point: noise that appears weeks after a clean startup usually traces back to mounting, coupling condition, bearing wear, or base stiffness rather than to a change in gear geometry. A DIN 6-5 flank keeps mesh excitation low; it cannot cancel an excitation created 300 mm away at the coupling hub.
Noise is a system property. Sound leaves a gearbox through three paths at once: the mesh contact itself, the bearings and shafts that carry vibration outward, and the housing walls that act as a speaker. Grinding improves the first path and leaves the other two untouched. A rigid HT250 cast housing with well-supported bearing seats damps and distributes vibration differently from a thin-walled box, a resonance near mesh frequency will amplify a tone that would otherwise disappear into the background, and a heavily loaded unit at low output speed sounds different from the same unit running light. That is why the accuracy grade is best treated as a contributor to smooth running, one that works alongside backlash control and correct installation. There is a practical way to use the number when comparing offerings. A parallel shaft geared motor manufacturer that publishes a flank grade, states how the flanks are finished, and quotes the clearance range it assembles to is giving a reader something checkable. A grade with no finishing method behind it, or with no backlash information at all, tells you much less. Buyers comparing a helical geared motor supplier on noise should also ask how the unit will be mounted, what the coupling arrangement is, and whether the base is stiff enough to keep alignment stable — the answers shape the result as much as the class does. Any industrial gearbox manufacturer's grade claim is an input to the noise outcome, not a prediction of it, and a machine that sounds quiet on a bench can still develop a tone once it is bolted into a resonant frame. The realistic expectation is this: precision grinding removes the gear as a major noise source. It reduces mesh excitation, keeps contact patterns even, and gives the rest of the drivetrain a calm input to work with. What remains — bearing tones, coupling whine, structural resonance, oil level effects — is engineering outside the tooth flank, and it is exactly where most real-world noise complaints are eventually traced.
DIN 6-5 on a ground gear flank means the profile, lead, and pitch deviations sit inside a tight band, and that tightness translates into lower transmission error and a smoother mesh. It is a genuine quality signal, and it is well matched to a parallel shaft helical geared motor that has to run steadily for long shifts. What the grade cannot do is absorb backlash that is set wrong for the duty, alignment that drifted after commissioning, or a housing that rings at mesh frequency. Read the accuracy class as one of three levers — grinding, clearance control, and installation quality — and the noise expectation you carry into a purchase will line up much better with what the machine actually delivers. For the stated DIN 6-5 flanks and the torque and ratio span behind them, the FC series specification page is a reasonable next stop.
A: It places the finished tooth flank inside the accuracy band between grade 6 and grade 5, measured across profile deviation, lead deviation, and pitch deviation, with smaller numbers being tighter. In practice it describes a gear whose flank has been ground to final geometry and holds a small transmission error, which supports even contact across the face width and a smoother mesh at normal pitch line speeds.
A: Backlash is the clearance between meshing teeth, and it changes the type of sound more than the volume alone. Generous clearance lets teeth separate and re-engage under fluctuating or reversing load, which reads as rattle or a knock on reversal. Very tight clearance traps lubricant between teeth, raises mesh temperature, and makes the steady whine harsher, so the assembled clearance is set as a working balance rather than as tight as possible.
A: Because the gear is one part of a sound path that also runs through bearings, shafts, and the housing walls. Coupling misalignment, shaft runout, a soft foot, foundation settling, bearing wear, or a housing resonance near mesh frequency can all generate or amplify noise that no flank accuracy can cancel. A ground gear lowers mesh excitation; it does not control what the rest of the installation adds.
Gear Backlash | KHK Gear Manufacturer
