Brief Study of HEMM- Bucket Wheel Excavator


The dramatic output in the volume of the bulk material during the past few decades has generated an increase need of Heavy Earth Moving Machinery (HEMM) in the field like mining project, Irrigation project ports, big cities etc throughout the world.

Heavy Earth Moving Machinery (HEMM) comprise of machines which cut and extract earth, blast rocks, sand and other loose materials and then transfer them to a predetermined position and finally dischange the material on dumping points through dumpers, belt conveyors, etc.

Heavy Earth Moving Machinery (HEMM) are the combinations of various mechanisms designed to perform the special type of work as mentioned above.

The mechanisms are
▪️A prime mover (diesel engines or electrical motors).
▪️A power transmission system including clutches, gear boxes, chain sprockets, etc.
▪️Control systems, and
▪️Mechanical linkages

BUCKET WHEEL EXCAVATOR 

 Introduction  
Bucket wheel excavator is a multi bucket continuous excavator. Continuous digging with this machine is done with evenly spaced bucket on the periphery of the rotating wheel. Once the material is dug by the bucket, it flows in a continuous stream via a transfer to the conveyor system which in turn transport it to the discharge point. The wheels are adjustable in r.pm and mount 6 to 18 bucket of equal size.

The bucket wheel excavator were first developed to handle only the easy digging materials such as brown coal in germany, but with the advancement in the electric drives. metallurgy of bucket and teeth and introduction of high speed steel cord belts the BWE's are finding its applications to dig medium to medium hard digging materials such as hard pottery clays. oil shales, etc.

 Classification 
The BWE's can be classified into different type based on certain criteria

Type of buckets
1. Cell type
2. Cell-less type. (more popular)

Propel mechanism
1. Crusher 3 point support. (most popular)
2. Walking pad type
3. Rail mounted

Type of cutting boom
1. Crowding type
2 Non-crowding type. (most popular)

Principle of Operation

A bucket wheel excavator generally comprises a bucket wheel which can be rotated to bring successive buckets into engagement with the ground structure to be excavated so that excavated material is torn from that structure, raised to a higher level and dumped from the bucket. The buckets can be provided with teeth or the like to cut into the material. At least one bucket-wheel belt is provided to carry away the material dumped from the buckets and to guide this material onto the bucket-wheel conveyor belt, a bucket-wheel chute can be provided

The bucket-wheel conveyor belt, in turn, can dump the excavated material from the chute into a receiving chute for a dumping conveyor belt which displaces the excavated material still further to an end of the dumping conveyor belt which can overhang a dump truck or other heavy motorized vehicle for carrying away the excavated material. The dumping conveyor belt can also have a drum over which the belt passes and at which the excavated material is discharged into the truck positioned therebelow.

Application 

Bucket wheel excavator finds it application when:
▪️The formation are soft to medium hard
▪️The formation are free of geological disturbances
▪️The formations of free of boulders
▪️The formation are such that it will not clog the wheel, transfer point belts etc because of stickiness or pressure of buried vegetations or such other things
▪️Selective mining of inter bedded ore and waste is necessary
▪️Continuous working is possible.
▪️Reserve is large and amenable to high rate of extraction

Working/ Cutting/ Digging
Using BWE, mining is either done by Lateral block method or full block method of excavating

Cutting Geometry 

 In Lateral Block method  the machine travels parallel to the highwall with the crawlers offset Long deep benches or terraces are cut along the highwall, starting at the top. The angle of swing into the highwall in the from 10 to 90 degree to the direction of the travel. This generally requires a machine equipped with a long boom.

 In Full Block Method  the wheel travels parallel to the highwall, removing a block width which a function of the wheel cutting radius and the swing arc. Optimum block width is normally achieved with a swing angle of 800 degree to the highwall and 45° to 50° on the side.

Different types of Bucket

 Cell Type Bucket 
Here each bucket is provided with Individual cell chutes, lined with wear plates. The chute receives material from the bucket and discharges it on to an inclined plate and ultimately to the conveyor belt.

 Cell-Less Type Bucket 
There is no individual chute here. The material is discharged on to a common hopper, which is not an integral part of the bucket.

Their Comparison
In cell type bucket, discharge capacity is less as material acceptance is more and discharging time from each bucket is less. Hence the cutting speed are limited.

Whereas in cell-less type bucket, the material is allowed to discharge into the ring chute, which ultimately discharges on to a fixed chute. This design permits greater cutting and bucket speed.

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