Cranes – Design, Practice, and Maintenance phần 10 pdf

Miscellaneous 295

plate of the hopper and the side plates, should be flattened out in order

to prevent the clogging of the material in the hopper in sharp corners.

It is strongly advisable to build a sturdy grid in the top of the hopper

as it cushions the shocks when the material is dropped into the hopper

by the grab. It also catches lumps, wood, and all other rogue material

and prevents clogging in the chutes of the conveyor-belt system in the

unloader and behind the unloader. The walls of the hopper can wear

out rather quickly through the abrasion, particularly by wet material.

Abrasion resistant plates can be built-in to prevent this wear as far as

possible. If required a dust-suppression system can be built-in as well

as vibrators to prevent clogging of the material.

9.8 Apron feeders; conveyors

To unload the hopper a number of mechanisms can be used, for

example:

– apron feeders (for ore and similar materials);

– heavy duty belt conveyors (for coal);

– vibrating feeders.

Each mechanism has its own advantages and disadvantages. Apron

feeders and conveyors in an unloader can only offer a very limited

distance for transportation. The heavy load from the material in the

fully-loaded hopper resting on the feeder or belt, and the extra force

which is needed to draw the material out of the hopper, require con￾siderable power.

Problems with hoppers and conveyors can be overcome as follows:

– A wide feeder with enough body to take up the impact of the

material in the hopper should be used.

– A grid in the top of the hopper should be fitted to cushion the

shocks of the material dropping out of the grab opened above the

hopper.

– Construction of skirts between the hopper and the conveyor must

be very carefully engineered in order to prevent damage to the

conveyor or apron feeder.

– Enough power must be available to drive the conveyor and apron

feeder. The brake-out-force from the material out of the hopper

can be considerable.

296 Cranes – Design, Practice, and Maintenance

Fig. 9.8.1 Hydraulic drive for conveyor underneath a hopper

An approximate method of calculation of the required horsepower for

a conveyor underneath a hopper is given.

Main characteristics

Capacity: QG2500 t
hr

Material: Coal, density dG1 t
m3

Conveyor speed: ûG0,6 m
sec

Length of the loaded part of the

conveyor: LG8 m

Opening width of the hopper: BG1,8 m

Height of the load on the HG Q

B · d · û · 3600

m

conveyor:

HG 2500

1,8 · 1 · 0,6 · 3600

HG0,64 m

Miscellaneous 297

1. Resistance by the hopper and N1G2 · µ · L · d · H2 · û · 103

102 · η conveyor skirts (kW):

N1G2 · 1 · 8 · 1 · 0,642 · 0,6 · 103

102 · 0,9

N1G43 kW

2. Resistance through ‘drawing N2Gµ1 · B · L1 · û · 103

102 · η out the material’ out of the

full loaded hopper (kW):

N2G0,5 · 1,8 · 6,5 · 0,6 · 103

102 · 0,9

N2G38 kW

3. Resistance through the moving N3G f · Gm · L11 · û

102 · η conveyor itself (kW):

where:

fGresistance coeff. of the

moving conveyor: fG0,65

GmGweight of the moving part

per metre of the upper

and under strand: GmG240 kg
m

L11Gcentre to centre length of

the end drums: L11G9,5 m

ûGconveyor speed: ûG0,6 m
sec

ηGtotal efficiency: ηG0,9

N3G0,65 · 240 · 9,5 · 0,6

102 · 0,9

N3G10 kW

The necessary motorpower to

drive the conveyor is: NGN1CN2CN3 kW

NG43C38C10G91 kW