What Service Factor Is and Why It Exists
The service factor (SF in general usage; KA in ISO 6336 notation; the ratio CSF or Ks in AGMA notation) accounts for the fact that a real helical gear drive system transmits peak torques that exceed the nominal rated torque. A nominally 100 kW motor driving a conveyor does not deliver a constant 100 kW — it delivers variable torque that spikes during start-up (motor breakaway torque 200–300% of rated), during product jams (brief full stall torque), and during starting with a fully loaded belt. Each of these transient events accumulates fatigue damage at a rate proportional to the peak torque raised to the 6th power (contact fatigue) or 8th power (bending fatigue).
For a helical gear rated to exactly the nominal transmitted torque (SF = 1.0), any peak exceeding 100% of rated torque accelerates fatigue damage faster than accounted for in the rating calculation. SF > 1.0 provides the margin that covers these transient peaks. The practical consequence: in ISO 6336 terms, KA multiplies the nominal tangential force F_t on the helical gear before all other calculation steps — so a KA = 1.5 gear must carry 1.5× the tangential force, requiring either a larger module, wider face width, harder material, or some combination.
AGMA Load Classification — The Industry-Standard Framework
AGMA 2101 (and its predecessor AGMA 2001) classifies drive systems into combinations of power source character and driven machine character. Each combination gives a recommended service factor:
| Power Source Character | Driven Machine: Uniform | Driven Machine: Light Shock | Driven Machine: Moderate Shock | Driven Machine: Heavy Shock |
|---|---|---|---|---|
| Uniform (electric motor, steam turbine) | 1.00 | 1.25 | 1.50 | 1.75–2.00 |
| Light shock (multi-cylinder engine) | 1.25 | 1.50 | 1.75 | 2.00–2.25 |
| Moderate shock (single-cylinder engine) | 1.50 | 1.75 | 2.00 | 2.25–2.50 |
| Heavy shock (impact press, punch) | 1.75+ | 2.00+ | 2.25+ | 2.50–3.00+ |
Driven Machine Classification — Which Category?
| AGMA Character | Application Examples | Shock Character |
|---|---|---|
| Uniform | Centrifugal pumps and compressors; light fans; conveyors with steady load; generators | Load nearly constant; starting infrequent and unloaded |
| Light shock | Centrifugal fans and blowers; loaded conveyors; crane travel; general machine tools; printing presses | Some variation; infrequent starting with light load; no severe impact |
| Moderate shock | Ball mills (empty); metal cutting machine tools; helical gear drives on papermaking rolls (interrupted cut); mine hoists; paper mills; large fans | Moderate torque variation; periodic loading and unloading cycles; some impact at start-up under moderate load |
| Heavy shock | Ball mills (loaded); crusher drives; large helical gear open drives; rolling mill roughing stands; dredge drives; heavy punching machines; cement kilns | Severe torque variation; frequent heavy starts; impact loading; potential for brief torque reversal |
ISO 6336 KA — The Equivalent in the International Standard
ISO 6336-1 defines application factor KA as the ratio of the maximum external dynamic gear load to the nominal tangential load at the pitch circle. ISO 6336-1 Table 1 provides indicative values:
| Working Condition of Driving Machine | Working Condition of Driven Machine | KA (ISO 6336) |
|---|---|---|
| Uniform | Uniform | 1.00 |
| Uniform | Moderate shocks | 1.25 |
| Uniform | Heavy shocks | 1.50 |
| Light shocks | Uniform | 1.10 |
| Light shocks | Moderate shocks | 1.35 |
| Light shocks | Heavy shocks | 1.60 |
| Moderate shocks | Uniform | 1.25 |
| Moderate shocks | Moderate shocks | 1.50 |
| Moderate shocks | Heavy shocks | 1.75+ |
Special Cases That Require Higher KA
Frequent Motor Starting Under Load
A standard three-phase induction motor produces a starting torque of 150–300% of rated torque at standstill — the full starting torque is applied to the helical gear drive train every time the motor starts. For a drive that starts 10–20 times per day under full load, the accumulated damage from motor starts can be significant. The correction: add a “starting frequency factor” of 1.0–1.3 to the base KA depending on the number of daily starts and the starting torque multiplier. Korea Ever-Power requests starting frequency information for all drives that start more than 5 times per day.
Reversing Drives
When a helical gear drive reverses direction under load — crane hoists, rolling mills on reverse pass, ship propulsion ahead/astern — both the drive flank and the coast flank of each tooth are loaded in alternating tension. ISO 6336 reduces the allowable bending stress σ_F lim by a factor of 0.7 for fully reversing bending loads (YM = 0.7 in the notation), compared with one-directional loading (YM = 1.0). This effectively requires either larger module, wider face width, or harder material for a reversing drive compared with a one-directional drive of the same nominal torque.
Emergency Overload — Peak Torque Assessment
ISO 6336 also requires that a helical gear survive a specified emergency overload (typically 200–300% of rated torque for one or a few events) without tooth fracture. This is checked against the maximum allowable bending stress rather than the fatigue bending stress limit, using a static safety factor S_F_stat ≥ 1.0 at the peak overload torque multiplied by the static overload factor Y_S. For cement mills with frozen charge restart events (torque up to 400% of rated), this check frequently governs the gear module selection rather than the fatigue check at rated torque.
The Most Common Service Factor Error — Ignoring Motor Start Torque
The most common error in helical gear service factor selection is treating an electric motor drive as “Uniform” without accounting for the motor’s starting torque characteristics. A direct-on-line (DOL) started cage induction motor applies 250–350% of rated torque at the instant of start-up. If the drive starts 3–5 times per day, this effectively means the gear experiences approximately 3–5 peak-torque events daily at 2.5–3.5× rated — equivalent to an AGMA “Moderate shock” source classification, not “Uniform”. Selecting KA = 1.0 for such a drive and then experiencing premature tooth bending fatigue at 30–50% of the calculated service life is the direct consequence.
The solution: either apply a start-up torque correction to the service factor, or specify a soft-starter or VFD (variable frequency drive) that limits motor starting torque to 100–120% of rated — converting the actual drive to a genuine “Uniform” source that justifies KA = 1.0–1.25.
Application-Specific Service Factor Reference
| Application | Typical KA | Primary Driver |
|---|---|---|
| Centrifugal pump (steady flow, VFD start) | 1.00–1.10 | Uniform helical gear drive + uniform load; VFD eliminates start shock |
| Industrial fan, centrifugal blower (DOL start) | 1.25–1.50 | DOL motor start at 250% torque; fan inertia prolongs start period |
| General conveyor, belt driven (loaded start) | 1.25–1.50 | Loaded starting; product jams; belt slip events |
| CNC machine tool, printing press | 1.25 | Servo-controlled with torque limiting; low shock |
| Mine hoist (cage) | 1.50–2.00 | Frequent starts under full load; rope snatch events |
| Ball mill, SAG mill | 2.00–2.50 | Ball charge cataracting; frozen charge restart; periodic overload |
| Jaw crusher, impact crusher | 2.00–3.00 | Direct impact loading at rock entry; tramp iron events |
| Rolling mill roughing stand | 1.75–2.25 | Bite shock at billet entry; speed matching errors |
| Marine main propulsion (twin engine) | 1.50–2.00 | Engine torque irregularity; emergency stops; manoeuvring loads |
| Wind turbine gearbox (per AGMA 6006) | Spectrum-based (not single KA) | AGMA 6006 requires full load spectrum analysis — single KA inadequate |
Korea Ever-Power — Service Factor Documentation with Every Order
Correct service factor selection for this heavy industrial helical gear drive requires knowing the power source character (electric motor with DOL or soft start), the driven machine character (shock level), and any special events (frequent starting, reversal, emergency overload). All four must be declared before the gear specification is confirmed.
Korea Ever-Power requests the following information from customers for all helical cut gear orders where the application involves starting under load, shock, or reversal: power source type (motor type, starting method), driven machine type and shock classification, daily start frequency, reversal load presence, and any known peak overload events. This information is used to determine the KA applied to the gear rating calculation, which directly sets the minimum required module and material grade for the specified service life. As a direct helical gear manufacturer, Korea Ever-Power includes the applied KA value in the gear order documentation, so the complete technical basis for the gear specification is transparent and auditable. Browse the helical gear product range for standard and custom industrial gears.
Frequently Asked Questions
Yes — provided the VFD is programmed to limit acceleration torque to 100–120% of rated during starting and stopping, and the driven machine has a uniform load characteristic. A VFD eliminates the DOL motor start torque spike, making the drive genuinely “Uniform” from the AGMA power source perspective. However, if the driven machine itself introduces shock (crusher, mill), the driven machine KA contribution applies regardless of the power source. A VFD driving a ball mill is Uniform power source + Heavy shock machine = AGMA combination still requiring KA ≈ 1.75.
KA directly affects helical gear size and material grade in the fatigue rating calculation, not DIN accuracy class. DIN accuracy class is set by the speed and noise requirements. However, larger module gears (required by higher KA) are sometimes easier to achieve at a given DIN class because the absolute tolerance values scale with module. A helical gear at M10 DIN Class 6 has larger absolute profile and lead tolerances than an M5 DIN Class 6 gear — making the M10 gear somewhat easier to produce to the specified class. This is a secondary effect and does not justify changing the DIN class specification.
KA accounts for external load variations from the helical gear drive system — loads generated by the power source or driven machine characteristics (shock, starting, reversal). KV accounts for internal dynamic loads — the additional tooth forces generated by the gear pair’s own transmission error and the mass-spring dynamics of the gear system at operating speed. Both multiply the nominal tangential force in the rating calculation. At low pitch-line velocity (<3 m/s), KV ≈ 1.05–1.15 (small contribution); at high speed (>20 m/s) for a DIN Class 8 gear, KV can reach 1.4–1.8. A complete helical gear rating uses KA × KV × KH × KF as combined factors — not just KA alone.
Yes — if the helical gear failure mode was pitting or tooth root bending fatigue at less than 50% of the calculated design life, the design KA was likely too low for the actual operating loads. Korea Ever-Power’s failure analysis service (see the failure analysis article) identifies the failure mode and compares the required KA at the observed failure life against the originally specified KA. If the actual peak torque events are known, the replacement helical gear is specified at the correct KA from the first order. If the actual loads are unknown, instrumented torque measurement during two weeks of normal operation is recommended before specifying the replacement — otherwise the new gear may fail at the same interval.
Confirm the Correct Service Factor for Your Helical Gear Application
Provide your power source type, driven machine character, daily start frequency, and any peak overload events. Korea Ever-Power determines the correct KA, shows the resulting module and material grade requirement, and documents the basis in the order specification — giving full auditability for the fatigue life calculation.
AGMA 2101 classification · ISO 6336 KA · Start frequency correction · Reversing load factor · KA documented in every order
Editor: Cxm
