Design Step 6 - Check torsional ratings and screw deflection

Torsional Rating

Screw conveyors are limited in overall length and size by the amount of torque that can be safely transmitted through the components selected.   The shafts, bolts and pipe all need to be sized appropriately for the drive horsepower and rpm.

Table M combines the various torsional ratings of bolts, couplings and pipes so that it is easy to compare the torsional ratings of all the stressed components of a standard screw conveyor.  The table conforms to the CEMC design standards (often more conservative than CEMA standards).

The lowest torsional rating figure for any given size of coupling will be the one that determines how much horsepower may be transmitted.  The torque produced (TQ) from the drive of the conveyor is a function of the size of the motor (HP) and the speed of the conveyor (rpm).

Deflection

It is also important to check the screw conveyor for deflection.  The amount of deflection the pipe of the screw experiences due to the weight of the screw is directly related to teh useful life of the pipe.  Deflection of the conveyor screw of standard length screws is not usually a problem. However, if longer than standard sections of screw are to be used without intermediate hangers, care should be taken to prevent the screw flights from contacting the trough. Deflection should be held to a minimum to increase the useful life of the screw.

The deflection at the mid span of the screw can be calculated from the following formula:

As a rule of thumb, applications where the calculated deflection exceeds 0.17 inches should be refered to our Engineering Department for recommendation.

Note: When screw deflections become excessive, additional stresses are applied to component parts. These additional stresses may result in fatigue and lead to early or premature screw failure.