Short answer: Overhung load = (output torque ÷ pitch radius) × service factor × load-type factor. The load-type factor is 1 for a chain sprocket, 1¼ for a gear, 1½ for a V-belt sheave and 2½ for a flat-belt pulley. Compare the result with the reducer's catalog overhung-load rating, after correcting that rating for where the sprocket or sheave actually sits on the shaft.
A sprocket, sheave or gear mounted on a gearbox output shaft pulls the shaft sideways. That side pull, the
overhung load, is carried by the output bearings and bends the shaft. A gearbox can have ample torque capacity and still be overloaded here. A small sprocket mounted far out on the shaft is the classic way it happens.
This guide gives the calculation used in gearbox catalogs, the factors for each type of drive, and a complete worked example from Winsmith's engineering data.
What the catalog rating means
Winsmith's catalog lists the maximum allowable overhung load, or chain pull, in pounds for each reducer. The rating assumes the load is applied
one shaft diameter out from the housing or mounting flange. It is limited by the bearings or by the shaft, whichever is weaker. The allowable load drops as the load moves farther from the reducer.
Overhung loads are subject to the same service factor that applies to the reducer, plus a load-type factor for the kind of drive element.
The overhung load calculation
Design overhung load = (T ÷ R) × service factor × load-type factor
- T = torque on the output shaft, in lb-in (convert lb-ft by multiplying by 12)
- R = pitch radius of the sprocket, sheave or gear, in inches. Use the
pitch diameter ÷ 2, not the outside diameter.
- Service factor from the gearbox maker's table for the application and hours per day
- Chain sprocket: 1. Practically no pull on the loose side of the chain.
- Gear: 1¼. Load acts along the line of action and exceeds torque ÷ pitch radius.
- V-belt sheave: 1½. The loose side of the belt also pulls.
- Flat-belt pulley: 2½. The loose side must be kept under tension.
- Variable-speed flat-faced pulley with V-belt: up to 3½. Speed changes by changing belt tension.
Winsmith attributes the gear, V-belt and flat-belt factors to AGMA recommendations.
Quick illustration: 100 lb-ft (about 136 N·m) of output torque through a sprocket with a 4 in (about 102 mm) pitch radius gives 1,200 ÷ 4 =
300 lb (about 1.33 kN) of chain pull. With a service factor of 1.5, the design load is 450 lb (about 2.0 kN). Swap the sprocket for a V-belt sheave of the same radius, and the 1½ factor raises it to 675 lb (about 3.0 kN).
Correcting for where the load sits
The sprocket centerline is often not at the catalog's reference point, one shaft diameter from the housing. When it sits farther out, Winsmith reduces the allowable load:
La
= Lc
× ℓ ÷ (ℓ + Δℓ)
- La = allowable overhung load at the actual position
- Lc = catalog overhung-load rating
- ℓ = a factor from the catalog for each reducer size and model: the distance from the bearing center to the reference point
- Δℓ = how far the actual load sits beyond the reference point
The practical lesson: mount the sprocket or sheave as close to the housing as the layout allows.
Worked example from Winsmith
A size 930DT reducer at 25:1 carries 1,500 lb-in (about 169 N·m) on its output shaft. It drives through a 23-tooth, ¾ in pitch chain sprocket whose centerline is 5.00 in (127 mm) from the center of the reducer. Service is 24 hours a day with uniform loading.
- Service factor (uniform, over 10 hr/day): 1.25
- Chain load-type factor: 1.0
- Sprocket pitch radius (5.508 in ÷ 2): 2.754 in (about 70 mm)
- Catalog overhung-load rating, Lc: 1,350 lb (about 6.0 kN)
- Catalog reference point from housing center: 4.375 in
- Actual load point from housing center: 5.00 in
- ℓ for this size and model (catalog table): 2.600
- Δℓ = 5.00 − 4.375: 0.625 in
Design load: 1,500 ÷ 2.754 × 1.25 × 1.0 =
680 lb (about 3.0 kN)
Allowable at the actual position: 1,350 × 2.60 ÷ (2.60 + 0.625) =
1,088 lb (about 4.8 kN)
The allowable load exceeds the design load, so the unit can carry it.
Direction of pull matters too
Winsmith's listed capacities apply when the chain pull is directed
toward the base, or parallel to the base on the near side of the sprocket. When the pull is directed away from the base, or parallel on the far side, the allowable load may need to be reduced. The catalog rating pages flag this with a footnote by size and model. Note the pull direction before you compare numbers.
Sprocket size: the lever you control
Because the load is torque divided by radius, a larger sprocket lowers the chain pull at the same torque. Doubling the pitch radius halves it. The trade-off is that a larger driving sprocket changes the drive ratio and chain speed unless the driven sprocket changes too, and it needs room. Check the speed with the
gearbox ratio calculator before changing sizes.
What to send us for an overhung-load check or quote
- Gearbox make and full model or part number, plus a nameplate photo
- Motor HP and rpm, and the output torque if known
- Sprocket teeth and chain pitch, or the sheave pitch diameter and belt type
- Distance from the housing face to the sprocket or sheave centerline
- Direction of the chain or belt pull relative to the gearbox base
- Application and hours of operation per day
Frequently asked questions
What is overhung load on a gearbox?
The side load on an input or output shaft from a sprocket, sheave, gear or pulley mounted on it. The shaft bearings carry it.
How do I calculate chain pull from gearbox torque?
Divide the output torque in lb-in by the sprocket pitch radius in inches. For a chain drive, that is the overhung load before the service factor is applied.
Why does a V-belt sheave get a higher factor than a sprocket?
A belt needs tension on both sides to grip, so the slack side adds to the pull. A chain's slack side carries practically none.
Does moving the sprocket closer to the gearbox help?
Yes. The allowable load is highest at the catalog reference point near the housing and drops as the load moves outward.
Summary
Calculate the side pull as torque ÷ pitch radius. Multiply it by the service factor and by the load-type factor for the chain, gear or belt. Correct the catalog rating for the real mounting position and pull direction, and confirm the allowable load exceeds the design load.
Sources & References
Send us your gearbox, sprocket and layout details and we will check the load against our
gearing products.
Related Reading:
