Draglines and mining shovels are two of the largest machines used in surface mining and heavy earthmoving. Both move huge amounts of rock or overburden, but they do it in very different ways. A dragline uses a long boom, hoist rope, drag rope, and bucket to reach far across a pit.
A shovel uses a powered dipper and crowd mechanism to dig directly into a bank or pile and load nearby haul trucks.
Key Facts
- Productivity can be estimated by Q = bucket volume x fill factor x cycles per hour.
- A dragline bucket is pulled toward the machine by a drag rope and lifted by a hoist rope.
- A mining shovel digs by pushing a dipper into material using crowd force and then lifting it with hoist force.
- Draglines are best for long reach overburden removal, while shovels are best for truck loading and selective digging.
- Cycle time is the time for one complete dig, swing, dump, and return cycle.
- Power required for lifting can be estimated by P = Wv, where W is load weight and v is lifting speed.
Vocabulary
- Dragline
- A large excavating machine that uses ropes to drag and lift a bucket across the ground.
- Mining shovel
- A powered excavator that uses a rigid dipper or bucket to dig into material and load it into haul trucks.
- Overburden
- The soil and rock that must be removed to expose a valuable mineral or coal seam.
- Cycle time
- The time needed for a machine to complete one full loading or digging cycle.
- Fill factor
- The fraction of a bucket's rated volume that is actually filled during a digging cycle.
Common Mistakes to Avoid
- Treating draglines and shovels as the same type of excavator is wrong because draglines use suspended rope-controlled buckets while shovels use rigid digging attachments.
- Ignoring cycle time is wrong because a large bucket does not guarantee high productivity if each cycle takes too long.
- Assuming draglines are mainly truck loaders is wrong because they are usually designed to cast material over long distances rather than load trucks efficiently.
- Comparing only machine size is wrong because reach, digging force, pit layout, material type, and haulage plan often matter more than height or mass.
Practice Questions
- 1 A shovel has a 40 m^3 bucket, a fill factor of 0.85, and completes 70 cycles per hour. Estimate its hourly production in m^3.
- 2 A dragline bucket holds 80 m^3 at a fill factor of 0.75 and completes 25 cycles per hour. How many cubic meters of material does it move in 8 hours?
- 3 A mine must remove overburden from a wide bench and place it directly on a spoil pile without using haul trucks. Explain whether a dragline or a shovel is the better choice and why.