Screw Conveyor Design

Conveyor Design Steps:

1. Establish conveying requirements
2. Identify the material and the corresponding material code
3. Determine conveying capacity, conveyor size and speed
4. Calculate required horsepower — select motor size
5. Determine the recommended size of components
6. Check the torsional ratings of components
7. Check deflection, thermal expansion and abrasion

Step 1: Establish conveying requirements

To properly design a conveyor to meet your needs it is important to know several parameters surrounding the application. Fortunately, to begin, you only need to know a few. These are:

• Type of material to be conveyed

• Required flow (lbs per hour or cubic feet per hour)

• Distance material will be conveyed

Step 2: Identify material and corresponding material code

The type of material being moved can have a significant affect on the size and type of conveyor needed. The following charts will help you classify your material and will help in selecting the proper conveyor components.

You can easily find out code of material using table A and table B

TableA: Material Classification Code

TableB: Material Characteristics

Step 3: Determine conveying capacity, conveyor size and speed

For screws with standard, full pitch flights the conveyor’s speed is:

 N = screw rpm (not greater than the max recommended speed)

For the calculation of conveyor speeds where special types of screws are used, such as short pitch, cut flights, cut and folded flights and/ or ribbon flights, an equivalent required capacity must be used, based on factors in Table C. The equivalent capacity then is found by multiplying the required capacity by one or more of the capacity factors that are involved.

Equivalent Capacity (ft³/hr) = Required Capacity x CF₁ x CF₂ x CF₃

In this case:

L= Length of screw conveyor in ft.

Table C

Table D

Step 4: Calculating Horsepower

(Horizontal Conveying)

The horsepower required to operate a horizontal screw conveyor is based on proper installation, uniform and regular feed rate to the conveyor and other design criteria. The horsepower requirement is the total of the horsepower to overcome the friction (HP_f) of the conveyor components and the horsepower to transport the material

(HP_m) multiplied by the overload factor (F_o) and divided by the total drive efficiency (e), or:

 The following factors determine the horsepower requirement of a screw conveyor operating under these conditions.

L	=	Total length of conveyor, feet
N	=	Operating speed, rpm
C	=	Capacity required, cubic feet per hour
D	=	Density of material as conveyed*, lb/CF (See Table B)
Fd	=	Conveyor diameter HP factor (See Table L)
Fb	=	Hanger bearing HP factor (See Table M)
Fm	=	Material factor (See Table B)
Ff	=	Flighting modification HP factor (See Table J)
Fp	=	Paddle HP factor (See Table K)
Fo	=	Overload HP factor (See Table H)
e	=	Drive effic. (expressed as a decimal) (See Table G1)

It is generally accepted practice that most power transmitting elements of a screw conveyor be sized and selected to safely handle the rated motor horsepower. If, for example, a screw conveyor requires 3.5 horsepower as determined by the above formula, a 5 horsepower motor must be used and it is desirable that all the power transmitting elements be capable of safely handling the full 5 horsepower.

*Some materials, such as cement, will aerate when conveyed making their apparent density much lower than when static. This is factored into the densities shown in Table B.