Selecting a screw conveyor screw involves selecting a lot of components and making sure they work well together. The screw is the most critical part of the screw conveyor. Features like flight thickness, pipe selection, welding, shaft size and various other features can have profound effects on the longevity of this critical component. Weather you know it or not, the center shaft on a screw is a simple piece of pipe. Part of the reason screws can be made so economically is because they use some standard, readily available components. For standard length screws, the pipe size and schedule is standardized based on the shaft size selected. The chart below shows those standard selections for carbon steel only.
|Shaft Size (in)||Pipe Diameter (in)||Schedule|
These combinations are perfectly suited to these standard length, full pitch screws in most conveying applications. Once you veer outside of the standard shaft combinations and applications the pipe size needs to be evaluated for strength, weldability, deflection and material of construction. Below are some details to consider for each category
The strength of the pipe refers to its ability to accept the torque being produced by the drive unit. The torque is expressed in inch-pounds or foot-pounds. There are two specific areas to look at when evaluating the screw strength. The first is the coupling bolt connection and the second is the pipe right after the internal bushing. The strength of the coupling bolt connection is determined by looking at the force the coupling bolt is exerting on the inside of the hole in the pipe. This is called the bearing stress. In the picture below the red arrows are the force of the driveshaft on the coupling bolts and the green arrows are the resistance of the bushing and pipe on the coupling bolts. Because the pipe and bushing are welded together they are considered one piece for calculation purposes.
The next section to evaluate is the pipe directly after the coupling connection. This section has to transmit the power of the drive out to the flights and then into the material being conveyed. The strength of the pipe directly after the bushing is determined by comparing the torque produced by the drive with the available cross sectional area of the pipe. This is called torsional stress.
When welding flighting to the pipe, the heat from the welding will distort the pipe and make the screw wobble when running. This is not unusual and minor distortion can be either thermally of mechanically removed. When using heavy welding >3/16″, continuous welding and/or short pitch flighting, using a heavier or larger pipe is advised. Abrasive and sluggish materials also sometimes dictate a heavier pipe. Standard pipe is schedule 40, but schedule 80, 120 or heavier is not unusual in really demanding applications.
Screw conveyor screws longer than standard length or using heavy flights will deflect the pipe in the center. The picture below can help visualize the spans of the screw as the same as the spans of a bridge. When the span gets long or heavy the middle of the pipe will deflect. Excessive deflection will cause additional stress on the pipe and can even cause the screw to rub on the inside of the trough. It is customary to limit the deflection to 1/4″ at the center of the screw for carbon steel and less for stainless steel. The red triangles represent columns of a bridge and are the bearing supports and the blue line represents the bridge span and deflection of the pipe.
Other factors affecting the screw pipe selection include excessive heat (>220F), inclines or declines, material of construction, galvanizing and many others. Each special condition has its own requirements and should be considered when selecting a screw pipe. Contact us at [email protected] for help with your application. If you have a custom screw requirement, fill out the custom RFQ here for a quote.