Internal bucket legs place the belt or chain and buckets inside the supporting casing, while external bucket legs carry the elevator assembly beside an open structural framework.
The choice changes foundation loads, inspection access, environmental exposure, and installation complexity, but does not change the basic capacity equations for bucket style, speed, pulley diameter, head profile, and outlet position [S1][S5].
What Separates an Internal Leg From an External Leg
An internal bucket leg integrates the conveying loop inside the supporting leg or casing, a configuration used when the elevator structure and process envelope are coordinated as one assembly [S3].
An external bucket leg uses a separate frame or tower to support the head, boot, belt or chain, and buckets, leaving more of the conveying path open for inspection. The standard components remain the same in either arrangement: a head pulley or sprocket, boot pulley or sprocket, buckets, casing, drive, and take-up [S3].
Double-leg high-speed centrifugal designs are cataloged for grain and other free-flowing materials, while industrial elevator families also cover centrifugal and continuous discharge with either belt-mounted or chain-mounted buckets [S2].
Comparison by Access, Exposure, Capacity, and Installation
For inspection access, an external leg leaves the belt or chain and buckets more exposed, but an internal leg can still provide service doors at the head, boot, and trunk. Installation work includes bucket and belt attachment, belt splicing, hood installation, drive installation, initial belt tracking, throat-slide adjustment, and in-service belt-tension adjustment [S4].
For environmental exposure, an open external assembly is easier to inspect but more exposed to dust, weather, impact, and accidental contact. An internal leg places the elevator in a defined casing and therefore depends more heavily on casing integrity, access closures, and boot-level containment [S3][S4].
For capacity, both designs use the same governing relationships. Capacity is affected by bucket style, belt speed, pulley diameter, head profile, outlet position, particle size, particle shape, density, angle of repose, coefficient of friction, and terminal velocity [S5].
For installation, the internal arrangement is generally most coherent when the elevator is part of a purpose-designed vertical process structure. The external arrangement is generally more practical when access, retrofit work, or separation between the support frame and the process casing is more important than architectural compactness.
Selecting the Arrangement by Operating Case
Choose an internal bucket leg when the objective is a compact, integrated vertical lift inside a controlled plant structure, especially for free-flowing grain where a double-leg or single-leg centrifugal arrangement is appropriate [S2][S3].
Choose an external bucket leg when maintenance personnel need frequent access to the bucket line, belt tracking, take-up, or drive components without opening a confined structural enclosure [S4].
Do not select either arrangement from vertical lift height alone. The selection must include throughput, available footprint, material characteristics, service conditions, and the required discharge method, because belt speed and head geometry can remove more capacity than the leg architecture itself [S5].
For abrasive or heavy-duty service, mill-duty and super-capacity elevator families are offered for cement, rock, fertilizer, lime, gypsum, coal, fine ore, severe duty, and higher shaft centers. Super-capacity continuous elevators use SC buckets mounted between two strands of chain [S2].
Discharge Design Sets the Operating Limits
Centrifugal discharge normally begins at about the 10 o'clock position and is used for grain and other materials handled at medium to high belt speeds. A minimum and maximum centrifugal belt speed applies to each bucket projection and pulley-diameter combination [S5].
Centri-gravity discharge starts at top dead center, or 12 o'clock, and continues for approximately 50 degrees. It suits powders, sand, cement, and wood chips operated at slower speeds, with CC-S, Starco, AA, and AC bucket styles identified as typical options [S5].
Gravity discharge occurs at about the 2 o'clock position and suits seeds, pellets, and fragile materials. Slow-running continuous buckets, including MF and SC styles, support gentle handling, while the internal or external leg decision must preserve the belt or chain speed required for that discharge method [S5].
The head cover also affects performance. A smooth head cover with no flat-top interruption helps material slide toward the outlet while limiting dust generation and material damage; gaps or vents that cause material to rebound can send it back down the leg casing [S5].
Service, Failure Modes, and Sourcing Requirements
Three failure conditions deserve design attention: a flooded boot overfeeds the elevator and places extra strain on the belt and drive, an incorrect head profile reduces discharge efficiency, and poor belt or chain tension affects tracking and service life [S3][S5].
The outlet must be sized and positioned for the required capacity. For grain and free-flowing particles, the documented positioning basis is at least 45 degrees from the head shaft, while meals, cement, sticky materials, and wood chips require a tailored outlet arrangement [S5].
For procurement, specify the arrangement, capacity duty, material, speed, pulley size, bucket type, head profile, outlet position, drive arrangement, take-up location, and required access dimensions. Catalog selection charts are preliminary tools, and final equipment recommendation should be tied to the application and supplier calculation [S2].
Further evaluation of structural and operating economics can be found in Bucket Elevator Cost per Foot of Leg Height, while bucket material trade-offs are detailed in Polyurethane vs Nylon vs HDPE: Bucket Elevator Material Decision.
The verifiable next step is to issue the same material, throughput, height, footprint, discharge, and maintenance data set for both arrangements, then compare foundation requirements, service access, and supplier-calculated power before approving the leg layout.