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Advanced Mechanical Design - Mining Drill Head Gearbox Design - Mechanical Engineering Assessment Answer
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The overall drill string consists of numerous drill rods, a drill hammer and drill bit. Each drill rod is 6m long and inserted into drill string once the gearbox and hydraulic motor have traversed down the mast, allowing the hole to drilled a further 6m and the process is repeated until the desired drill depth Is achieved. The mast drives the drill string into the ground with a pull down (red arrow) load of 15kN. To assist with cutting, a hammer is located above the drill bit and increases the drill bit load. The hammer consists of a 15kg mass that oscillates in a sinusoidal pattern with a frequency of 15Hz and an amplitude of 100mm. The drill bit (which has equivalent diameter to the drill rods) consists of small cutting teeth that are situated on the drill bit perimeter and has an average coefficient of friction of 0.5 with the ground being cut.
The 6m long drill rods are tubular steel with an outside diameter of 120mm and an inside diameter of 110mm and it is expected that the drill rig can drill a hole up to 500m deep. Whilst drilling, the walls of the hole drags against the side of the rotating drill string, providing a 25N friction force per unit area (m2) of drill string surface. It is assumed that the hammer and drill bit are negligible weight, however their combined length is 2m. To improve drill productivity, the drill bit speed should be as high as
possible.
Hydraulic motor (TB0390) and gearbox are attached to one another, where the gearbox is then mounted to the mast (as illustrated in Figure 1). As a result, the gearbox and hydraulic motor should be designed as one single device that is then attached to the mast, where the gearbox is a structural member that ensures attachment of both the drill string and hydraulic motor to the mast. No details of the mast are provided, hence you can specific where you would like to mount the gearbox to the mast with provisions to do so and sized attachment fasteners accordingly. Rotating motion between components should be isolated with rolling element bearings that are adequately sized. There is no requirement for coaxial alignment between the hydraulic motor and drill string.
The drill rig has a hydraulic power supply that offers a maximum of 100 Bar oil pressure and a maximum of 57 L/min oil flow rate. Along with the gearbox design, please provide any operating conditions (such as hydraulic performance) in order for the operator to maximise productivity. The drill rig is designed to operate as reliably as possible to avoid any downtime, however, this must be within reason to avoid unnecessarily expensive components and the entire gearbox should not need servicing for a minimum of 10,000 hours.
Mechanical Engineering Assessment Answer
TASK: Design an entire gearbox assembly that is mounted to a drill rig mast and drives a drill string, as illustrated in Figure 1 below. The drill string is driven by a hydraulic motor via the gearbox assembly. The overall gearbox design should include, load calculations, gear design, shaft design, fastener sizing, bearing selection (and any accompanying components such as seals and clips, etc.) and the design of any other fixtures, housing(s) and/or couplings to ensure a completely operating gearbox. Fastener calculations are required to justify the size of any main structural bolts used in the design. It is a design requirement that any rotating structural components in the design are located by rolling element bearings.
The overall drill string consists of numerous drill rods, a drill hammer and drill bit. Each drill rod is 6m long and inserted into drill string once the gearbox and hydraulic motor have traversed down the mast, allowing the hole to drilled a further 6m and the process is repeated until the desired drill depth Is achieved. The mast drives the drill string into the ground with a pull down (red arrow) load of 15kN. To assist with cutting, a hammer is located above the drill bit and increases the drill bit load. The hammer consists of a 15kg mass that oscillates in a sinusoidal pattern with a frequency of 15Hz and an amplitude of 100mm. The drill bit (which has equivalent diameter to the drill rods) consists of small cutting teeth that are situated on the drill bit perimeter and has an average coefficient of friction of 0.5 with the ground being cut.
The 6m long drill rods are tubular steel with an outside diameter of 120mm and an inside diameter of 110mm and it is expected that the drill rig can drill a hole up to 500m deep. Whilst drilling, the walls of the hole drags against the side of the rotating drill string, providing a 25N friction force per unit area (m2) of drill string surface. It is assumed that the hammer and drill bit are negligible weight, however their combined length is 2m. To improve drill productivity, the drill bit speed should be as high as
possible.
Hydraulic motor (TB0390) and gearbox are attached to one another, where the gearbox is then mounted to the mast (as illustrated in Figure 1). As a result, the gearbox and hydraulic motor should be designed as one single device that is then attached to the mast, where the gearbox is a structural member that ensures attachment of both the drill string and hydraulic motor to the mast. No details of the mast are provided, hence you can specific where you would like to mount the gearbox to the mast with provisions to do so and sized attachment fasteners accordingly. Rotating motion between components should be isolated with rolling element bearings that are adequately sized. There is no requirement for coaxial alignment between the hydraulic motor and drill string.
The drill rig has a hydraulic power supply that offers a maximum of 100 Bar oil pressure and a maximum of 57 L/min oil flow rate. Along with the gearbox design, please provide any operating conditions (such as hydraulic performance) in order for the operator to maximise productivity. The drill rig is designed to operate as reliably as possible to avoid any downtime, however, this must be within reason to avoid unnecessarily expensive components and the entire gearbox should not need servicing for a minimum of 10,000 hours.
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