By adjusting operating parameters to avoid loads and adhering to more rigorous inspections, conveyor downtime from wood screw failures becomes avoidable

BY ROTATING AND MOVING along the bottom of a pile of wood material that is often more than 80 ft high, screw conveyors transport such products as woodchips, bark, and sawdust through a pulp and paper mill. The component of this conveying system that physically moves the material is the wood screw, a large spiraling mechanism also known as a sawdust, bark, or Wennberg screwAlthough a simple system, screw conveyors have the potential to shut down portions of the mill when screw breaks occur under an inaccessible portion of the pile. However, by monitoring certain operating parameters that influence loads placed on the screw, as well as implementing certain preventive maintenance activities, a mill can reduce the number of failures and minimize unscheduled downtime.

BASIC WOOD SCREW DESIGN. The screw portion of the conveyor has an overall nominal length of 35 ft-45 ft and is made up of five basic parts as shown in Figure 1. Each of these parts is subject to failure and presents a different maintenance challenge.

Barrel. The barrel portion of the wood screw conveyor is typically made of pipe material manufactured according to the ASME specification SAl 06, Grade C Schedule 120 pipe.

If the barrel portion breaks, lead times for obtaining new screws can range from three to five months, because sections of pipe over 35 ft long are difficult to obtain from suppliers.Also, in most mills, welding varying lengths of pipe together to make the barrel is considered unacceptableFlights and digger teeth. Having the appearance of screw threads on a screw, flights are made of bent flat plate material that is welded to the barrel in a helical pattern. Digger teeth, which are small pieces of metal welded to the outer edges of the flights, are sometimes added to the screw to help break up compacted material along its length.

Digger teeth enhance the performance of the screw by digging into the pile, but, since their failures have only a minor impact on the overall operations of the screw, they are considered expendable. However, failure of the flights can reduce system capacity, resulting in damage to the barrel. Damaged flights must be removed from the barrel, then replaced with new ones.This means that the screw is taken out of service for one to three weeks, depending on the extent of damage.

Drive shaft. The drive shaft is a center traversing pipe which drives the screw. It is physically located inside the barrel and extends from one end to the other.

The drive shaft is protected by the barrel, but can be damaged when the barrel fails.Although the pipe material is not difficult to obtain, replacing it means the screw must be taken apart.

End shafts, end heads, and bearings. Located at the ends of the screw, these components allow independent rotational and lateral movement of the screw. The components are not difficult to obtain or fabricate, but replacing them means downtime as the screw is taken apart for repair.

Wear resistant coverings. Wear resistant coverings are placed on the flights and the barrel to extend the screw's life by reducing material loss from erosion. These coverings are hand-applied coatings, wear-resistant plate material, or alloy material (stainless steel, inconel, etc.) welded to the surfaces of the barrel or flights.

Wear-resistant coverings are intended to wear away under normal operating conditions. However, they should be replaced on a regularly scheduled basis since they are easier to replace than the barrel or the flights.

SURFACE CRACKING CAUSES. Most screws fail near the center of the screw or slightly offcenter toward the drive end.

These types of failures are usually load related and are the result of cracking that starts at some type of surface discontinuity such as a dent, ding, irregular weld contour, weld start or stop, or a material microstructure of martensite at the toe of the flight-to-barrel weld.These surface discontinuities act as stress risers, amplifying the effects of loads on the screw.When trying to prevent screw failures, examining the load conditions that initiate surface cracking is just as important as examining the parts affected by the loads.

Imposed Loads. The two major types of applied loads on the screw result in forces that bend or twist it.These forces are perpendicular loads (bending loads) and parallel (twisting loads) applied to the screw axis. The forces that are applied perpendicular to the screw axis are the result of the pile weight acting vertically downward, of the horizontal force on the screw as it is pushed against a compacted or frozen portion in the pile, or of a vertically upward force on the screw as it rides over material between the flights and the floor during startup. These forces tend to bend or bow the screw near the center of its length or at a location under the portion of the pile with the greatest material height.