Radial vs Axial Load — Sources and Why Both Must Be Calculated
Every planetary gearbox output shaft carries three types of loading simultaneously: torque (the primary drive force), radial load (a force perpendicular to the shaft axis), and axial load (a force along the shaft axis). The torque capacity is what most engineers specify from the catalogue. The radial and axial loads are frequently underestimated or omitted — and their effect on bearing life is far more severe than the equivalent increase in torque.
Radial load sources
A force perpendicular to the output shaft axis — the key planetary gearbox radial load source. Generated by:
- Belt drive: Tight-side + slack-side belt tension resultant. For a flat/V belt with tension ratio T₁/T₂ = 3, the net radial force ≈ 2 × T₁ × cos(wrap angle / 2)
- Chain drive: Chain tension acts tangentially on the sprocket; the resultant of drive-side and slack-side tensions is the radial load on the gearbox shaft
- Rack and pinion: Tangential cutting force on the pinion creates a radial component at the pitch point equal to F_tangential × tan(pressure angle)
- Gear mesh: Spur gear mesh produces radial force = F_tangential × tan(pressure angle)
Axial load sources
A force along the output shaft axis. Generated by:
- Helical gear mesh: The helix angle produces an axial force component = F_tangential × tan(helix angle). At 20° helix angle: F_axial = 0.36 × F_tangential
- Helical coupling: Torque-induced axial force proportional to shaft misalignment angle
- Thrust from conveyor belt: Belt drive with angular misalignment or crowned pulley creates a lateral (axial) force at the shaft end
- Screw conveyor thrust: Material resistance on the screw flighting creates thrust that acts axially on the drive shaft
Why radial load matters more than torque for bearing life
The L10 bearing life relationship is cubic: L10 ∝ (C/P)³. Doubling the radial load P reduces bearing life to (1/2)³ = one-eighth. The same doubling of krútiaci moment typically increases bearing load by much less than doubling (because torque loads the gear teeth, not the output bearing directly). This asymmetry means radial load specification errors have a disproportionately severe impact on bearing life.
The Overhang Multiplier — How Mounting Distance Amplifies Bearing Load
Korea Ever-Power catalogues specify the permissible radial load at a reference point — typically a distance x_ref from the output flange face. When the actual radial load is applied at a different distance (either closer or further from the flange), the effective bearing load changes. The relationship is derived from the bending moment at the output bearing.
OVERHANG LOAD MULTIPLIER DERIVATION
F_bearing = F_r × (x + a) / a
where:
x = distance from gearbox flange face to load application point (mm)
a = distance from gearbox flange face to output bearing centre (mm)
(internal dimension — from Korea Ever-Power datasheet)
Catalogue permissible radial force F_r_perm is given at x = x_ref
→ F_bearing_ref = F_r_perm × (x_ref + a) / a
At actual installation distance x_actual:
F_r_allowable = F_bearing_ref × a / (x_actual + a)
Simplified multiplier k = (x_ref + a) / (x_actual + a)
F_r_allowable = F_r_perm × k
Example: a = 40 mm, x_ref = 20 mm, x_actual = 60 mm
k = (20 + 40) / (60 + 40) = 60/100 = 0.60
→ Permissible radial force reduced by 40% at 60mm overhang
| Actual overhang x_actual | Multiplier k (a=40mm, x_ref=20mm) | % of catalogue F_r_perm | Bearing L10 change |
|---|---|---|---|
| x = 0 mm (flush with flange) | k = 1.5 | 150% allowed | +3.4× longer |
| x = 20 mm (= x_ref) | k = 1.0 | 100% (catalogue) | Baseline |
| x = 40 mm | k = 0.75 | 75% allowed | −58% life |
| x = 60 mm | k = 0.60 | 60% allowed | −78% life |
| x = 100 mm | k = 0.44 | 44% allowed | −91% life |
In Korean gantry machine and automated guided vehicle rack-drive installations, the output shaft pinion is commonly mounted 60–100 mm from the gearbox face to clear the mounting structure. As the table above shows, this seemingly modest overhang reduces the permissible radial force by 40–56% — more than halving the bearing-limited capacity compared to the catalogue value. Engineers who check only the torque rating against catalogue and ignore the overhang multiplier are selecting a gearbox that operates at 2–3× its bearing-rated load, producing bearing failures within months rather than years.
L10 Bearing Life Calculation — From Applied Load to Expected Service Hours
Once the actual bearing load is known (accounting for radial force, axial force, and any overhang multiplier), the expected L10 bearing life can be calculated using the ISO 281 standard formula. L10 is the life in millions of revolutions that 90% of a bearing population will reach before fatigue failure.
ISO 281 BEARING LIFE CALCULATION
L10 = (C / P)^(10/3) × 10⁶ rev [for roller bearings, exponent = 10/3]
where:
C = basic dynamic load rating of the bearing (N) — from Korea Ever-Power datasheet
P = equivalent dynamic bearing load (N) — calculated from radial + axial forces
P = X × F_r + Y × F_a
X = radial load factor, Y = axial load factor (from bearing catalogue, depends on F_a/C₀ ratio)
For pure radial load (F_a = 0): P = F_r
Convert to hours: L10h = L10 × 10⁶ / (n × 60)
n = output shaft speed (rpm)
Example: C = 15,000 N, F_r = 5,000 N (pure radial), n = 50 rpm
P = 5,000 N
L10 = (15,000 / 5,000)³ × 10⁶ = 27 × 10⁶ revolutions
L10h = 27×10⁶ / (50 × 60) = 9,000 hours
At F_r = 7,500 N (1.5× overload):
L10 = (15,000 / 7,500)³ × 10⁶ = 8 × 10⁶ rev
L10h = 8×10⁶ / (50 × 60) = 2,667 hours (−70%)
| Load ratio F_r / F_r_perm | P/C ratio | L10 (millions of rev) | Hours at 50 rpm | vs catalogue life |
|---|---|---|---|---|
| 0.5× (half load) | 0.167 | 216 M | 72,000 h | +700% |
| 1.0× (catalogue rated) | 0.333 | 27 M | 9,000 h | Baseline |
| 1.25× (modest overload) | 0.417 | 13.8 M | 4,600 h | −49% |
| 1.5× (significant overload) | 0.500 | 8 M | 2,667 h | −70% |
| 2.0× (severe overload) | 0.667 | 3.4 M | 1,130 h | −87% |
Based on C=15,000N example bearing, n=50rpm output. Your actual C value is on the Korea Ever-Power EP series datasheet. Apply the overhang multiplier from Module 2 to your radial force before entering this calculation.
EP-AF vs EP-AB — The Same Frame, Very Different Radial Load Capacity
Korean engineers specifying planetary gearboxes for belt-drive or rack-drive applications frequently use the EP-AB series because it covers the required torque. What they sometimes overlook is that EP-AB and EP-AF share the same body diameter and mounting flange — but the EP-AF high-rigidity series uses a significantly larger-diameter output shaft and an upgraded output bearing system that doubles or triples the permissible radial load at the same frame size.
The shaft bending stiffness scales with diameter to the fourth power (I ∝ d⁴). An EP-AF090 output shaft that is 1.4× the diameter of the equivalent EP-AB090 shaft has 1.4⁴ = 3.8× the bending stiffness — which directly translates to a proportionally higher permissible radial load before the shaft deflection and bearing moment reach the rated limit.
The practical consequence: for any application where the output shaft carries a belt, chain, or gear that imposes a radial force, always check the radial load specification — not just the torque specification — and compare EP-AB vs EP-AF at the same frame size before finalising the order.
| Frame / Model | Output shaft Ø (mm) | Rated torque (N·m) | F_r_perm at x_ref (N) | F_r ratio AF/AB |
|---|---|---|---|---|
| EP-AB 060 | 22 | 37–190 | 730–1,200 N | — |
| EP-AF 060 | 28 | 37–190 | 1,500–2,400 N | ~2× |
| EP-AB 090 | 32 | 120–550 | 1,600–3,000 N | — |
| EP-AF 090 | 45 | 120–550 | 4,000–7,500 N | ~2.5× |
| EP-AB 140 | 48 | 450–1,750 | 4,000–6,000 N | — |
| EP-AF 140 | 65 | 450–1,750 | 9,000–14,000 N | ~2.3× |
Values are indicative. Confirm exact F_r_perm and reference overhang distance x_ref from the Korea Ever-Power EP series datasheet for your specific model and ratio. F_r_perm varies with ratio as bearing pre-load changes across the ratio range.
Any time the application involves a belt, chain, gear, or rack load on the output shaft — and the calculated radial force at the actual overhang distance exceeds 60% of the EP-AB permissible value — switch to EP-AF at the same frame size. The cost increment is typically 20–30% for the shaft upgrade, vs the cost of an early bearing failure and unplanned production stop. The upgrade requires no machine redesign: EP-AF uses the same mounting flange and body diameter as EP-AB at the same frame size.
Right-Angle Gearboxes — How Bevel Gear Separation Force Adds to Shaft Load
Right-angle planetary gearboxes integrate a bevel gear stage to redirect the output shaft by 90°. The bevel gear mesh generates gear separation forces — radial and axial components — that act internally on the bevel shaft bearings. These internal forces are already accounted for in the permissible radial load specification for the EP-ABR, EP-ADR, and EP-AFR right-angle series. However, when the right-angle output shaft also carries an external radial load (from a mounted sprocket or pinion), that external load adds to the already-loaded bevel shaft bearing system.
The practical rule for right-angle gearboxes with additional external loads:
- Check the permissible radial load specification on the right-angle output shaft specifically — this value is lower than the inline series at the same frame size, because the bevel stage pre-loads the shaft bearings
- Apply the overhang multiplier from Module 2 to the external load at the actual mounting distance
- Confirm the combined bearing load (internal bevel separation + external radial) does not exceed the right-angle shaft permissible value
- If the external radial load is substantial, use the EP-AFR (high-rigidity right-angle) rather than EP-ABR at the same frame — the enlarged right-angle shaft diameter provides proportionally higher capacity
A Korean 5-axis machining centre used an EP-ABR090 P0 right-angle gearbox for the B-axis (tilting) with a 60mm overhang pinion driving the rotary table ring gear. The overhang multiplier at 60mm reduced the permissible radial force by 36% from the catalogue value. Combined with the table ring gear tangential force creating a bevel shaft axial component, the actual bearing load exceeded the EP-ABR permissible. Switching to EP-AFR090 (same frame, high-rigidity right-angle) with 1.7× higher shaft load capacity resolved the bearing failure without any machine design change.
EP-AFR: High-rigidity shaft · Same flange/body as ABR · ~1.7–2× higher external radial load capacity · First choice for any right-angle drive with significant external radial load
Worked Design Example — Korean Conveyor Belt Drive Shaft Selection
A Korean food processing belt conveyor drive has the following specification: conveyor belt tension (tight side) 1,800 N, belt wrap 180°, pulley pitch diameter 200 mm (radius 100 mm), gearbox output speed 45 rpm, pulley mounted 50 mm from gearbox flange face, reference distance from Korea Ever-Power datasheet x_ref = 20 mm, a = 40 mm. Required service life ≥ 20,000 hours.
STEP-BY-STEP SHAFT LOAD CALCULATION
F_r = 2 × T₁ × sin(wrap/2) = 2 × 1,800 × sin(90°) = 3,600 N
(180° wrap → tight side + slack side resultant = 2×T₁ for 180°)
Step 2 — Drive torque:
T = T₁ × r_pulley = 1,800 × 0.10 = 180 N·m
Step 3 — Overhang multiplier (x=50mm, x_ref=20mm, a=40mm):
k = (20 + 40) / (50 + 40) = 60 / 90 = 0.667
F_r_effective = 3,600 N (actual applied force)
Required catalogue F_r_perm ≥ 3,600 / 0.667 = 5,398 N
Step 4 — Series selection:
T = 180 N·m → EP-AB090 (rated 120–550 N·m) ✓ for torque
EP-AB090 F_r_perm ≈ 3,000 N → 3,000 × 0.667 = 2,001 N effective
Actual load 3,600 N > 2,001 N allowed: EP-AB090 FAILS radial load ✗
EP-AF090 F_r_perm ≈ 7,500 N → 7,500 × 0.667 = 5,002 N effective
Actual load 3,600 N < 5,002 N allowed: EP-AF090 PASSES radial load ✓
Step 5 — L10h verification (EP-AF090, C ≈ 22,000 N):
P = F_bearing = 3,600 × (50+40)/40 = 3,600 × 2.25 = 8,100 N (at bearing)
L10 = (22,000/8,100)³ × 10⁶ = 7.14³ × 10⁶ = 364 M rev
L10h = 364×10⁶ / (45×60) = 134,800 hours ≫ 20,000 h target ✓
The EP-AB090 was adequate for the torque requirement (180 N·m within 120–550 N·m range) but completely inadequate for the radial load — the 50 mm overhang with 3,600 N belt tension exceeded the EP-AB090’s bearing capacity by 80%. Without the overhang calculation, a Korean engineer specifying only on torque would select EP-AB090, which would fail its output bearing within 2,000–4,000 hours. The EP-AF090 at the same frame size provides over 100,000 hours of bearing life for the same application — a fundamentally different outcome from a 20–30% cost increment.
Axial Load Capacity — Limits, Calculation and Common Exceedance Cases
Axial load (thrust force along the shaft axis) is typically the less critical of the two shaft loads for most Korean applications — but several common drive configurations generate significant axial forces that must be explicitly checked against the gearbox specification.
Korea Ever-Power EP series permissible axial load F_a_perm is typically specified as a fraction of the radial load capacity — often 30–50% of F_r_perm for standard EP-AB and EP-AF. The output shaft bearing design is optimised for radial load; axial load is a secondary design parameter. When axial load approaches or exceeds F_a_perm, consider the EP-AFH ultra-precision series whose cross-roller bearing output provides higher axial load capacity in the same frame size.
F_a = F_tangential × tan(β), where β is the helix angle. At β = 20° and 500 N tangential force: F_a = 500 × tan(20°) = 182 N. For high-torque helical drives this becomes significant — at 5,000 N tangential force: F_a = 1,820 N. Verify against F_a_perm.
Material resistance on screw flighting creates axial thrust proportional to the pitch force. At high throughput, this can reach 30–50% of the maximum rated output torque in axial force terms. Always calculate screw conveyor axial thrust separately and confirm against F_a_perm.
Angular or parallel misalignment in flexible jaw couplings generates a small but continuous axial force that acts on the output bearing. For precision drives, ensure shaft-to-shaft alignment is within 0.05 mm TIR to minimise coupling-induced axial force.
Axial load capacity guide
≈ 30–50% of F_r_perm
EP-AFH (cross roller):
Equal radial and axial
capacity in both directions
→ For high axial duty
EP-AH New Line:
High axial + radial via
angular contact bearings
Frequently Asked Questions — Radial and Axial Load Capacity
Confirm Your Radial Load Specification with Korea Ever-Power
Korea Ever-Power’s application team calculates the actual bearing load from your drive geometry — belt tension, overhang distance, chain configuration, or rack-pinion force — and confirms whether EP-AB or EP-AF is the correct series for your installation. Same working day in Korean.
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