Hoisting Mechanism Gearbox Selection & Speed Ratio Calculation
Key Point Gearbox selection for crane hoisting mechanisms follows a three-step process: ① Calculate the reduction ratio i=n₁/n₂ (n₂=v×m/πD); ② Calculate the output torque T₂=(Q+G)×D/(2×i×η); ③ Select the model from the gearbox catalog and verify the service factor f≥1.25~1.60. This article walks through the complete process using four worked examples — CD1 5t/10t and QD 32t/50t — with a comparison table of ZQ/QJ series.
The gearbox is the core transmission component of a crane's hoisting mechanism, and its selection directly affects lifting speed, load capacity, and service life. Unlike standard off-the-shelf products, calculating the reduction ratio and torque to match different lifting capacities, lifting speeds, and pulley ratios is a fundamental engineering task in crane design. This article systematically derives the complete selection process — reduction ratio calculation, torque verification, and service factor selection — in accordance with ISO 4301 Crane Design Standard and JB/T 9003-2004 "Three-in-One Gear Reducers for Cranes," and demonstrates the process with four worked examples: CD1 electric hoists (5t/10t) and QD double-girder bridge cranes (32t/50t).
Hoisting Mechanism Drive Chain and Reduction Ratio Calculation
A typical hoisting mechanism drive chain consists of: electric motor → coupling → brake → gearbox → drum → wire rope → pulley block → hook. The gearbox's role in this chain is to reduce speed and multiply torque — converting the motor's high-speed, low-torque output into the drum's low-speed, high-torque rotation.
Drum rotational speed calculation: n₂ = v×m/(π×D). Here, v is the lifting speed (m/min), m is the pulley ratio (number of wire rope falls / rope layers on the drum), and D is the drum nominal diameter (m). For example, a CD1 5t electric hoist: v=8m/min, m=2, D=0.3m, giving n₂=8×2/(π×0.3)=17.0r/min.
Reduction ratio calculation: i = n₁/n₂, where n₁ is the motor's rated speed (r/min). Common motor pole selections: 4-pole motors n₁≈1400r/min (1450 in engineering practice), 6-pole motors n₁≈950r/min. Applying the example above: i=1450/17.0=85.3. In practice, a standard reduction ratio of i=80 (closest available) is selected.
Actual lifting speed verification: After selecting the standard reduction ratio, the actual speed must be recalculated: v_actual = π×D×n₁/(i×m). With i=80: v_actual=π×0.3×1450/(80×2)=8.54m/min. The deviation is (8.54-8)/8=6.75%, which is acceptable within the ±10% tolerance.
Gearbox Torque Verification
Wire rope tension at the drum: S = (Q+G)/(m×η_g). Q is the rated lifting capacity (N), G is the lifting spreader dead weight (N), m is the pulley ratio, and η_g is the pulley block efficiency (0.96~0.98 per pair of pulleys; approximately 0.92~0.96 for three pairs).
Drum input torque: T₂ = S×D/2 = (Q+G)×D/(2×m×η_g). This torque represents the load the gearbox output shaft must withstand.
Gearbox selection torque: T_calc = T₂ × f_s. The service factor f_s (also called usage factor or duty factor) is selected based on the work duty classification:
M1~M3 (light duty): f_s = 1.10~1.25
M4 (medium duty): f_s = 1.25~1.40
M5 (heavy duty): f_s = 1.40~1.60
M6~M7 (extra-heavy duty): f_s = 1.60~2.00
Gearbox model confirmation: Based on T_calc, select the model from the gearbox catalog's rated output torque table, ensuring the catalog's rated output torque T_rated ≥ T_calc. Also verify the gearbox's maximum permissible torque (for overload conditions) and instantaneous peak torque.
Motor Power and Gearbox Matching
Initial motor power selection: P = (Q+G)×v/(6120×η_total). η_total is the overall hoisting mechanism efficiency (including gearbox efficiency η_gb × drum efficiency η_d × pulley block efficiency η_g). Gearbox efficiency η_gb: single-stage cylindrical gears ≈0.98, two-stage ≈0.96, three-stage ≈0.94. Overall efficiency is typically taken as 0.85~0.90.
Motor verification: The hoisting motor must lift the rated load at full speed under rated power, and the starting torque ratio must be verified (typically M_st/M_n ≥ 2.0~2.5, corresponding to the motor's overload capacity). For frequent start-stop duty cycles such as M6 and above, high-slip motors should be selected.
Motor-to-gearbox interface matching: The motor output shaft diameter and keyway must match the gearbox input bore. Common motor shaft diameters for Y-series 4-pole motors: 2.2kW — 28mm, 4kW — 38mm, 7.5kW — 42mm, 15kW — 48mm, 30kW — 55mm. The gearbox input bore diameter must be 0~0.5mm larger than the motor shaft diameter.
Common Gearbox Series and Parameters
Three gearbox series are commonly used in crane hoisting mechanisms:
ZQ/JZQ series (medium-hardened tooth flank): The classic two-stage cylindrical gear reducer, with gear tooth surface hardness of 300~350HBW. Center distance ranges from 250~1000mm, rated output torque from 0.5~50kN·m, and reduction ratio range of 8~50. Suitable for small-to-medium tonnage cranes in M5~M6 work duty classifications. Advantages: low cost and simple maintenance; disadvantages: large size and relatively low tooth surface hardness.
QJ series (hardened tooth flank): A three-stage cylindrical gear reducer per the JB/T 9003-2004 standard, with gears carburized and quenched to a hardness of 58~62HRC. Rated output torque ranges from 10~200kN·m, with reduction ratios from 16~200. Suitable for medium-to-large cranes in M7~M8 work duty classifications. Advantages: high load capacity, compact size, and long service life; disadvantages: approximately 30~50% higher cost than the ZQ series.
Planetary gearbox: A sun gear + planet gear + internal ring gear configuration, with three planetary stages achieving reduction ratios of i=100~300. Efficiency η≥0.95, and the unit is 40~60% smaller than a parallel-shaft gearbox at the same reduction ratio. Ideal for high-reduction, space-constrained applications. However, planetary gearboxes demand high manufacturing precision and are more difficult to service.
Kelude Heavy Industry uses QJ series or custom gearboxes in its electric hoist product line, and QJ series hardened gear reducers in its QD double-girder bridge cranes.
Worked Calculation Examples
Example 1: CD1 5t electric hoist (M4 work duty classification)
Q=5t=49,050N, v=8m/min, m=2 (two rope falls), D=0.3m, spreader G≈500N. 4-pole motor n₁=1,450r/min. n₂=8×2/(π×0.3)=17.0r/min. i=1,450/17.0=85.3. Select QJ series standard speed ratio i=80. Actual v=π×0.3×1,450/(80×2)=8.54m/min. Power P=(49,050+500)×8/(6,120×0.85)=49,550×8/5,202=76.2kW. Correction: This power value is clearly too high — the actual CD1 5t motor power is 7.5kW (due to the electric hoist's spring braking and different overall efficiency). This shows that in practice, the CD1 gearbox is already integrated inside the hoist, with total efficiency taken as η_total≈0.80 (including gearbox + drum + pulleys + wire rope friction), giving P=(49,050+500)×8/(6,120×0.80)=49,550×8/4,896=80.9kW. However, the actual motor power of 7.5kW is far below this calculated value — because the CD1 hoist's pulley ratio m=2 halves the wire rope tension, and the hoist uses multi-stage planetary gearing internally (with higher overall efficiency than a parallel shaft design). The accurate formula is P=(Q+G)×v/(1,000×η×60)=50kN×8/(1,000×0.90×60)=7.4kW. Select a 7.5kW motor. Gearbox output torque T₂=S×D/2=(Q+G)/(m×η_g)×D/2=49,550/(2×0.96)×0.3/2=25,807×0.15=3,871N·m. With f_s=1.25 (A5), T_calc=3,871×1.25=4,839N·m. The QJ12.5 gearbox (center distance 125mm) supplied with the CD1 hoist has a rated output torque of ≈5,000N·m ≥ 4,839N·m. The standard CD1 5t hoist uses a 125mm center distance gearbox matched with a 7.5kW 4-pole motor.
Example 2: CD1 10t Electric Hoist (A5 Work Duty)
Q=10t=98,100N, v=7m/min (10t lifting speed is typically reduced to 7m/min), m=3 (three rope falls), D=0.35m, spreader G≈800N. n₂=7×3/(π×0.35)=19.1r/min. i=1,450/19.1=75.9. Select standard speed ratio i=71. Actual v=π×0.35×1,450/(71×3)=7.48m/min. Power P=(98,100+800)×7/(1,000×0.90×60)=98,900×7/54,000=12.8kW, select a 13kW motor. The actual CD1 10t motor power is 13kW. Gearbox output: S=98,900/(3×0.95)=34,702N, T₂=34,702×0.35/2=6,073N·m. With f_s=1.25, T_calc=7,591N·m. The QJ16 gearbox (center distance 160mm) has a rated torque of ≈8,000N·m ≥ 7,591N·m.
Example 3: QD 32t×22.5m Bridge Crane (A6 Work Duty)
Q=32t=313,920N, v=5m/min (low speed for heavy loads), m=6 (six rope falls on drum), D=0.6m, hook block G≈2,000N. n₂=5×6/(π×0.6)=15.9r/min. Select a 6-pole motor with n₁=950r/min (A6 multi-speed motor for frequent start/stop duty). i=950/15.9=59.7. Select QJ series standard speed ratio i=56. Actual v=π×0.6×950/(56×6)=5.32m/min. Power P=(313,920+2,000)×5/(1,000×0.88×60)=315,920×5/52,800=29.9kW, select a 30kW motor. Gearbox selection: S=315,920/(6×0.94)=56,017N, T₂=56,017×0.6/2=16,805N·m. With f_s=1.50 (A6), T_calc=16,805×1.50=25,208N·m. The QJ25 gearbox (center distance 250mm) has a rated torque of ≈30,000N·m ≥ 25,208N·m. The actual QD 32t bridge crane is equipped with a 30kW motor and QJ25 gearbox.
Example 4: QD 50t×28.5m Bridge Crane (A6 Work Duty)
Q=50t=490,500N, v=3.5m/min, m=8 (eight rope falls), D=0.7m, hook block G≈3,500N. n₂=3.5×8/(π×0.7)=12.7r/min. 6-pole motor n₁=950r/min, i=950/12.7=74.8. Select standard speed ratio i=71. Actual v=π×0.7×950/(71×8)=3.68m/min. Power P=(490,500+3,500)×3.5/(1,000×0.88×60)=494,000×3.5/52,800=32.7kW, select a 37kW motor. S=494,000/(8×0.93)=66,478N, T₂=66,478×0.7/2=23,267N·m. With f_s=1.50, T_calc=34,901N·m. The QJ31 gearbox (center distance 315mm) has a rated torque of ≈45,000N·m ≥ 34,901N·m. The QD 50t crane typically uses a QJ31 gearbox with a 37kW motor. For complete selection parameters of QD type bridge cranes, refer to QD Type Double-Girder Bridge Crane — Tonnage, Models, Prices, and Selection Guide.
Engineering Parameter Reference Table
The following table provides recommended gearbox selections for common hoisting mechanisms:
| crane | Lifting Capacity(t) | v(m/min) | Pulley Ratiom | Drum D(mm) | Motork W | Speed Ratioi | Gearbox / Reducer Model | Service Coefficientf |
|---|---|---|---|---|---|---|---|---|
| CD1Hoist | 5 | 8 | 2 | 300 | 7.5 | 80 | QJ12.5 | 1.25 |
| CD1Hoist | 10 | 7 | 3 | 350 | 13 | 71 | QJ16 | 1.25 |
| QDBridge | 16 | 7 | 4 | 400 | 18.5 | 63 | QJ20 | 1.40 |
| QDBridge | 32 | 5 | 6 | 600 | 30 | 56 | QJ25 | 1.50 |
| QDBridge | 50 | 3.5 | 8 | 700 | 37 | 71 | QJ31 | 1.50 |
Frequently Asked Questions
Q: Is a higher service factor f_s always better for a gearbox?
A: Not necessarily. A higher service factor means a larger gearbox, which increases cost, weight, and installation space. For A5 duty, f_s = 1.25 is economically sound; oversizing the service factor wastes resources. Conversely, if the actual duty is A6 but you select f_s = 1.25 intended for A5, the gear contact fatigue life will drop significantly. The correct approach is to match f_s to the actual work duty—neither overestimated nor underestimated.
Q: Can the reduction ratio be adjusted if the lifting speed is too fast or too slow?
A: Yes. Lifting speed is inversely proportional to the reduction ratio—increasing the ratio lowers the speed (while raising output torque), and decreasing it raises the speed. However, ratio changes must stay within the standard reduction ratio range (QJ series standard ratios: 20/22.4/25/28/31.5/35.5/40/45/50/56/63/71/80/90/100/112/125/140/160/180/200), with adjacent ratios differing by roughly 12% in speed. If none of the standard ratios match your target speed, consider changing the motor pole count (e.g., from 4-pole to 6-pole, reducing speed by about 34%) or adjusting the pulley ratio.
Q: Is the gearbox used in an electric hoist the same as the one in an overhead crane?
A: The working principle is the same, but the configurations differ. CD1/MD1 electric hoists use a three-stage planetary gearbox (gears integrated inside the drum) with a high reduction ratio (i = 60–120) for a compact footprint. QD double-girder cranes use the QJ series parallel-shaft gearbox (externally mounted) with a medium ratio (i = 16–100) for easier maintenance. Hoist gearboxes are typically custom non-standard designs, while QD types use the JB/T 9003 standard models. Kelude's electric hoists feature a three-stage planetary reduction system, and the QD series comes standard with a QJ-type hardened gear reducer.
Q: What causes gearbox oil leaks and how can they be prevented?
A: Common causes of gearbox oil leaks include: ① aged or improperly fitted oil seals (70% of leaks originate from the input/output shaft seals); ② failed sealant on the housing mating surfaces; ③ overfilled oil level (above the dipstick maximum); ④ a blocked breather cap causing internal pressure buildup; ⑤ housing deformation from prolonged overload. Preventive actions: use a combination of radial shaft seals and labyrinth seals, maintain oil at the dipstick midpoint, replace seals periodically (every 2,000 hours or annually), and keep operating temperature below 85°C. The QJ series comes standard with dual-lip radial shaft seals and an oil return groove leak-proof design.
