Drag Conveyor Basic Design Fundamentals – Part II
Q. How do you spec drag conveyors? — R.L.
A. In our last newsletter we presented an overview of drag conveyors and their basic design fundamentals.
Whatever style of drag conveyor you have (round bottom, flat bottom or tubular) it is made up of these basic components:
- Chain and Paddles Unit
- Head Section
- Tail Section
- Drive and Motor
- Accessories/Ancillary Devices
Housing options have a 10-gauge minimum, which depend on its capacity and length. The standard housing section length is typically 10 foot, with a housing flange on each end of the 10 foot length to bolt conveyor sections together. They can be galvanized, painted or stainless steel. The covers may be flat or hip.
Housing construction types are typically formed metal or welded side wall assemblies with bolted bottoms and covers. Chain returns are made up of sprockets, rollers, a feed pan, and center and side rails.
Inlet types may be direct feed, bypass or pan feeder. Direct feed inlets are used when material going into the conveyor is controlled or metered. Bypass and pan feeder inlets are used to control the flow going into the conveyors (choke loaded).
Access doors are inspection doors on the intermediate housing covers. They allow for chain inspection and the observation of material flow. The intermediate discharge assembly allows for discharge throughout the process.
Chain and Paddles Unit
The chain conveyor pull must be calculated to properly size the chain. There are many different styles available depending on the application, but the most common are cast, roller and all-steel welded.
Paddles or flights can be made from UHMW, steel, non-ferrous metal or other polymers. Tall flights and double flight spacing should be used for excessive chain weight on inclines greater than 15°.
The head section assembly includes the head sprocket, head shaft and bearings.
The sprocket diameter and head shaft RPM selection is determined by the desired capacity in TPH, CFH or BPH and correlates with housing size. More sprocket teeth will reduce the wear. Sleeve construction is two-piece split construction (hardened).
Bearings and shafts must be sized appropriately for drag conveyor capacity, length, horsepower and chain pull. The bearings should be self-aligning pillow block roller style.
Head section covers are typically a minimum 3/16” construction with 12 or 14-gauge cover. It should be easily removable and have inspection doors for ease of maintenance. The service platform that provides access to head section components should allow a minimum 24” clearance to the handrails.
This section will have a minimum 3/16” construction, depending on the capacity and housing size. It is made up of two-piece split (hardened) construction sprockets, pillow block bearings and an inspection/access cover. The sprocket size for the tail section is often the same as the head section, although it is offset in some cases. The length of the screw take-up terminal will depend on the conveyor’s length. The inlet must be located far enough away from the tail sprocket so that materials do not contact the sprocket and cause unnecessary wear.
Drive and Motor
There are a few types of gearboxes:
- Indirect drives, such as shaft mount helical gear reducers with sheaves and belts, are preferred for heavy duty industries.
- In-line gear reducers with chain and sprockets are another type of indirect drive that is mounted to motors or used with shaft coupling.
- Direct coupled gear reducers are not belt driven and are instead attached to the motor shaft.
Motor selection is based on the classification of the area on which the conveyor is installed per NEC:
- Explosion proof
- Various voltages
- Cast iron frame/fin cooled/1.15 SF/NEMA Design B
To calculate horsepower:
|Total HP||=||HP empty + HP live + HP lift
There are a number of factors in this formula, such as light friction, material friction, chain flight/weight, conveyed product weight, conveyor length, chain speed and lift.
Service factor classes are:
- Class I: Steady loading up to 10 hours/day
- Class II: Steady loading over 10 hours/day. Moderate shock load allowed for 10 hours/day
- Class III: Moderate shock loads for over 10 hours/day. Heavy shock loads allowed for 10 hours/day
OSHA requires guards for belts and shafts to protect workers from pinch-points.
The V-Belt should allow for a minimum of 2.0 service factor and maximum operating speed of 5,000 RPM. It must be flame retardant and static conductive.
The following ancillary devices are available:
- Speed switches: Alarm sensors indicate if the conveyor is operating at an undesirable speed. It detects excessive speed or no motion. A tachometer can be set to display the actual speed at which the system is operating.
- Bearing temperature sensors monitor the motor using resistance temperature detectors (RTDs) or discrete sensors.
- Protective coatings to prevent corrosion.
- AR steel liners for housing the sidewalls and bottom to eliminate abrasion.
- Slack chain detection switches.
- Plug switches can be located far down on the conveyor to provide reaction time for problems. These monitors come in capacitance, diaphragm or lift-cover styles.
- Return cups can be placed along the chain for optimal performance.
- Intermediate gates can be installed to provide discharge points along the conveyor. They can be opened and closed for various process lines.
Specifying the proper drag conveyor style and associated components and options will result in an efficient, long lasting piece of material handling equipment.
Answered by Curtis R. Cook, Vice President of WL Port-Land Systems, Inc. He has over 36 years of experience in design, operations, and maintenance of bulk solids storage and handling facilities.