Originally Posted by stedman2101

This is my first post to this forum and so I have a general and specific question to ask. I am not an engineer or similar, but an industrial chemist. My questions:

1 Why is the motor on a screw conveyor located at the discharge end?

2 I need to build a mini conveyor quickly (within a week or so). I have looked at various on-line design tutorials in vain.

Specifically I want to charge 4-15 kgs/h of ground unslaked lime, particle size 100% < 1.2mm, 92% < 75 micron, 80% < 53 micron,

Rate of 4-15 kilograms per hour into a vessel.

I cannot find any information for screw conveyors with a diameter of under 4" (100mm) NB - and there was only the one reference.

All the others had nothing for conveyors with a diameter of less than under 6"!

My estimate is that I will need a pipe approximately 25-40mm NB.

This would run at around 4-5 rpm.

I have no means of determining the power required or torque produced, nor the best design of screw.

I'm also worried about flooding.



The on-line calculator on this site would not accept a flow of less than 0.1 TPH - 100 kgs/h or 10-20 times bigger than I need.

Additional information:

Horozontal conveyor

Bulk density of the unslaked lime taken as 0.95-1.05 kg/L

Conveyor length 600mm

Feed hopper holds 25L (approx 25 cms diameter x 30 cms high)

Assumed the screw is only likely to run 30% full.

Any suggestions would be very welcome.

About your screw conveyor:

Lime is very, very, very, corrosive!!!!!!

This is a huge problem with lime manufacturers and lime users

On question one;

The Motor is located at the discharge end as the torque required is better

used up on this end as the load is constantly being discharged, and it

reduces any binding to an absolute minimum as the auger has less material

at the head end due to the augers screw angle/flighting.

On question two:

The only other serios option is to use a coal stoker screw and tube in combination

with a weight in loss/tippy dump scale.

1. In this way you can have the auger on a timer

2. You have a speed control on the auger to only meter so much material per hour.

3. the weigh in loss feeder/tippy dump scale can be used to control the volume and

weight delivered and the scale could be programmed to stop the auger from delivering

the lime.

You will be much better off using a weigh in loss feeder/tippy dump feeder

to meter the material into the vessel.

The only problem will be corrosion as the weigh in losss hoppers will need to be washed out with hot water and soap. ■

The difference between a discharge end drive motor and a feeding end drive motor is as follows:

Feeder end drive motor:

-The axial bearing on the screw shaft must be at the drive end. This is to prevent that the lengthening and shortening of the screw shaft has to be absorbed by a flexible coupling of the drive.

-The reaction force in the screw shaft is pulling from the axial bearing. Thus the shaft is under tension and there are no buckling forces and the supporting bearings can be spaced wider apart.

Discharge end drive motor:

-The reaction force in the screw shaft is pushing towards the axial bearing. Thus the shaft is under compression and because of buckling forces the supporting bearings have to be spaced closer to each other.

Both options will work, providing that the construction is designed properly.

Have a nice day

Teus ■

There is no problem using screws down to tiny sizes, as far as the screw is concerned, nor concern about driving from the feed end, which is most common with small screw feeders. Feed screws do not run 30% full but should fill as completely as possible and with material in a consistent density for accurate dispensation. Therein lies the rub because very fine powders are almost invariably cohesive when settled and prone to flushing when dilated, so 'powder state control' is the key to reliable performance, which means that the emphasis rests more with the supply hopper construction and its means of loading, rather than the screw geometry, although progressive extraction over the exposed length of the screw is a must for efficient filling and hopper flow.

There are two basic options, design a gravity flow system based on mass flow or adopt an agitated hopper for crude flow generation. A conical hopper is bad news for flow as the hopper strain is Pi times the diametrical strain,which is why they tend to arch and rathole. A mass flow interface with inibit ratholing and secure flow through the smallest practical orifice, i.e. in excess of the critical arching span that is calculated from material testing. I feel sure that it would be cheaper and quicker to purchase a proven machine from a reputable supplier than to design from scratch with the limited experience shown in the enquiry. It is more awkward to design a 25 litre hopper feeding 4 Kg/hr of this material than dispensing 4 Te/hr from one that has 25 Te capacity. Power and which end the drive is, are almost irelevant for the size and duty of screw that is needed for this duty, as the torque and end thrust experienced by the screw for metering this rate should be trivial. If a correctly designed screw buckles, the outlet is blocked. I hate plugging my book - 'Guide to the Design, Specification and Application of Screw Feeders' in this context as it is intended as a guide to purchasers, rather than be a design manual for those under pressure to re-invent development work thaat has progressed from Archimedes. ■

I feel that my last reply was less than supportive of the enquirer and would suggest the following design for his urgent needs.

Fit two flights 50 mm o/d x 25 mm pitch and about 30 flights 50mm o/d x 50 mm pitch on a 12 mm diameter shaft 600 mm long for the screw with sufficient projection past the 25 mm pitch end to fit double bearings to support the cantilever extension from the hopper end to allow a direct end outlet.

Make the hopper inlet 125 mm long on a tube nom 60 mm bore and construct a flared hopper section 200 mm high from the tube centreline to a cross section 200 mm wide x 125 mm long along the screw axis. (i.e. vertical side walls. To this, make a hopper extension flaring at 90 degrees to the lower section 40 mm tall, to a rim cross section 200 mm wide x 400 mm long along the tube axis. This hopper will hold about 40 litres when trimmed level, to give about 2.7 hours supply at 15 Kg/hr or 10 hours usage at 4 Kg/hr.

Filled gently, this hopper should generate mass flow and give sufficient time for the powder to stabalise to a consistent density at the point of extraction.

The drive speed will vary from nom 1 to 4 rpm, so a small timing belt on the shaft to give a 4:1 reduction allows an inverter rated 0.25 kw geared motor to have a nominal speed of 20 rpm for this light duty. Note that at this low screw speed the discharge will fluctuate due to the geometry of the screw flight and the cohesive nature of the fine powder will make it drop off the end in irregular surges. If it is essential to secure short-term uniformity of the feed rate the machine will need additional consideration. Contact me if you have a problem securing the screw flights in stainless steel and best of luck ■