Question

1. Using pictures and/or diagrams, describe in detail the digging forces generated by both the Electric and Hydraulic mine shovels. (2.5page minimum)

please type your answer, please do not write on the paper and post it, please answer the question in detail

Answer 1

1. Digging forces generated by the Electric mine shovels :

The digging force of the ERS is the resultant of crowd and hoist forces acting on the dipper. Crowd force pushes the dipper into the material and keeps it in the material meanwhile hoist force (bail pull) pulls the dipper upwards at bench. Crowd force governs the depth of cut of the dipper during the digging segment of the ERS cycle time (CT). Therefore, crowd force represents dipper penetration whereas hoist force represents breakout force. Depth of cut (penetration) is a function of resultant of crowd and hoist forces and rake angle of dipper. If it is too shallow it is waste of energy and poor bucket filling performance, if it is too thick, the ERS stalls. Therefore, an optimum cutting thickness has to be achieved.

Figure 1. Digging forces generated by ERS

Usually, ERS operators optimize this by experience. However, the new generation ERSs’ have dig optimizing systems which optimise the crowd and hoist (bail pull) forces to have an optimum resultant dipper digging force. Thus, the risk of stalling and boom jacking during operation are reduced to a minimum level. Depth of cut (slab thickness) is adjusted by the magnitude of crowd force. Thicker cuts (depth of cuts) may slow the bail speed and may increase the CT and in turn; bigger magnitudes of crowd force may generate Boom Jacking (BJK), see Figure 2. The thinner the cut, the less the energy consumed per cycle.

Figure 2. Boom jacking and caused vibration on boom and boom suspension ropes

1.1. OPTIMIZATION OF DIGGING FORCES OF ERS

Good operators manage applying balanced crowd and hoisting movements so that a smooth digging is achieved at the bench face; in case of excessive crowd force application, there is a danger of stalling and/or boom jacking of ERS because of thick cutting depth of dipper. Coordination (optimisation) of crowd and hoist is extremely important for a safe and sound operation. Boom jacking generates severe vibration which is detrimental on the boom structure and the suspension cables. Dipper breakout force is the resultant of crowd and bail pull forces. In order to prevent BJK crowd force should not be bigger in magnitude than the component of ERS dipper breakout force in the direction of the acting crowd force. BJK should be avoided, because it is stressful on the ERS boom and boom suspension ropes. The new generation ERSs’ are equipped with boom jacking warning systems; for example, OptiDig® which optimizes crowd and hoist forces during digging operation ie optimising the cutting depth, thus preventing the boom jacking happen. In the last decade, studies on ERS digging kinematics and dynamics, digging trajectory, depth of cut etc increased. For the best and efficient digging, the equipment operator should adjust the crowd and hoist forces such that the optimum depth of cut is obtained, the dipper is filled in the shortest possible time with minimum energy consumed.

2. Digging forces generated by the Hydraulic mine shovels :

Hydraulic excavators are used in the construction and mining industry and are offered in a wide range of different sizes and versions, including mobile and tracked units. A variety of attachments are available to adapt for multiple purposes, such as hydraulic breakers, hammers and grabs or special shaped buckets. The entry into the quarrying industry starts with an operating weight of at least approximately 45 tonnes; this market has certainly seen an influx of a lot of manufacturers recently. The very large and mining-proof machines, however, can be delivered only from a handful of suppliers

In order to achieve the designed tearout and breakout forces of hydraulic backhoe keeping sharp dipper teeth is a must. Furthermore, maintaining sharp excavator teeth on the dipper will assist increasing the productivity of the equipment and reducing the stres on the equipment and wear and tear on the dipper. As it is known, dipper teeth have two-fold function; increasing the digging forces and protect the the lip and body of dipper. The dipper teeth have to be replaced with a new one when it is dull and worn out prior to wear of lip and other wear plates etc. Thus in the long run both dipper and equipment repair and maintenance costs and fuel costs can be minimised.

2.1. Tearout Forces

Digging force of hydraulic backhoes composed of tearout (penetration) and breakout forces, See Figure 3. Tearout force is generated by the dipper arm (stick) cylinder and dipper cylinder and breakout force is generated by dipper cylinder (actuator), See Figures 3 and 4

Figure 3. The hydraulic cylinders generating tearout and breakout forces

In order to generate penetration of dipper teeth or edge, dipper cylinder is

actuated. Dipper is rolled (pulled) toward the equipment until it is full. Upon filling the dipper, it is raised in a smooth manner high enough above the ground to clear the spoil pile or the hauler and dump the excavated material

Figure 4. Bucket-cylinder operation

Digging force components of a hydraulic backhoe consist of breakout force (BOF) and tearout force (TOF). Tearout force’s function is penetrating and crowding the dipper

2.2. Breakout Forces

In order to generate breakout force dipper stick cylinder is actuated. For a maximum breakout force generation stick cylinder to be perpendicular to dipper stick. The dipper is lowered to digging position, and it is rolled until the dipper teeth or the cutting edge flat on the ground, See Figure 5

Figure 5. Backhoe dipper digging and filling phases

Tearout forces of hydraulic backhoes are comparatively smaller than that of their breakout forces

2.3. Determination of digging forces

Figure 6. Shows the measurement of bucket curling force FB, arm crowd force FS, the other terms in the figure dA,dB, dC, dD, dD1, dE, and dF shows the distances as shown in figure 6.

Maximum radial tooth force due to bucket cylinder (bucket curling force) FB is the digging force generated by the bucket cylinder and tangent to the arc of radius dD1. The bucket shall be positioned to obtain maximum output moment from the bucket cylinder and connecting linkages. FB becomes maximum when distance dA reaches maximum,because rest of the distances in the following equation are

                   ……………………….. Equation (1)

Where,

Bucket cylinder force = (Working pressure) × (End area of bucket cylinder)

if the end diameter of the bucket cylinder = DB(mm) and the working pressure is p(MPa) and other distances are in mm then the equation can be written as:

                               ……………………….. Equation (2)

Equation (2) determines the value of the bucket curl or breakout force in N. Now let us determine the maximum radial tooth force due to arm cylinder FS. Maximum tooth force due to arm cylinder is the digging force generated by the arm cylinder and tangent to arc of radius dF. The arm shall be positioned to obtain the maximum output moment from the arm cylinder and the bucket positioned as described in the case of maximum bucket curl force (Max. bucket tangential force). While calculating maximum force FS occurs, when the axis in the arm cylinder working direction is at a right angle to the line connecting the arm cylinder pin and the boom nose pin.

                                         ……………………….. Equation (3)

Where,

dF= bucket tip radius(dD) + arm link length and DA= end diameter of the arm cylinder

The combination of the backhoe excavator’s arm crowd force FS and bucket curling force FB give this machine configuration more effective bucket penetration force per mmof bucket cutting edge than is available with other machine types such as wheel and track loaders.