Gear-Shifting Mechanism

• Dog Clutch Features

  • Only one dog clutch required for two gears: forward and reverse.

  • Dog clutch can slide along a single shaft to engage forward gear, reverse gear, or remain in neutral.

• Transmission System Components

  • Input Shaft

  • Dog Clutch Shaft

  • Idler Gear Shaft

• Bevel Gears

  • Bevel gear fastened to the end of the dog clutch shaft.

  • Mates with another bevel gear connected to the system’s output shaft.

  • Pulley attached to the output shaft winds the cable suspending the weight.

• Dog Clutch Movement:

  • Moved along its shaft using two-pronged structures on either side of the clutch.

  • Prongs are rigidly connected at the bottom, sliding along tracks parallel to the shaft.

  • Moved using a single lever attached to a small shaft between the prongs of the outer structure.

• Brake Mechanism:

  • Lever engages the brake by rotating about the shaft, applying upward pressure on the dog clutch.

  • Rubber attached to the lever end provides frictional force to stop the dog clutch in neutral.

  • This action stops the rotation of the output shaft.

• Shaft and Stand Configuration:

  • All shafts held by stands attached to the baseboard with two screws each.

  • Stands house bearings for free shaft rotation while constraining translational movement.

  • Axial movement of shafts constrained using shaft clamps near the stands.

  • Thrust bearings placed between clamps and stands to reduce friction.

  • Identical stands keep all shafts at the same height, except for the idler gear.

  • Additional shaft clamps used to constrain axial movement of some gears, particularly the freely spinning gears on the dog clutch shaft.

• Gear Ratios:

  • Derived from the desired lifting and lowering speeds of the weight.

  • Necessary rotation speed of the output shaft calculated using the speeds and spool diameter.

Gear Ratio Determination

• Input Speed:

  • Kept constant at 60 RPM.

• Output Speed:

  • Ratio of input speed to output speed determines the respective gear ratios.

• Bevel Gear Interaction:

  • Designed with a gear ratio of 1:1 for simplicity.

  • This ensures that only the gears within the dog clutch system determine the overall gear ratio.

• Gear Diameter Calculation:

  • Diameters for the forward and reverse, driven and driving gears determined by the input-output speed relation.

Shaft Analysis

• Approach:

  • Segmented approach used to assess internal forces.

  • Shaft divided into distinct components: stands and gears.

• Segment Lengths:

  • First Stand: 0 in

  • Reverse Gear: 3.75 in

  • Second Stand: 8.75 in

  • Forward Gear: 9.625 in

• Calculations for Each Segment:

  • Internal shear force.

  • Bending moment.

  • Torque.

• Assumptions:

  • Components have negligible mass.

  • Frictionless connections.

  • No loss in horsepower.

  • 99% reliability.

• Force Calculation:

  • Transmission shifted into forward gear for calculations.

  • Forces considered: timing belt, forward driving gear, and stands.

  • Torque produced by the input calculated from horsepower.

  • Resulting torque divided by the radius of the forward driving gear to determine force on the gear.

  • Force assumed to be directly transmitted to the shaft as a shear force.

Fatigue Analysis

• The fatigue analysis was conducted on the same input shaft discussed previously.

• Results indicate that the shaft is not rated for an infinite life.

• After a certain number of cycles, replacement of the shaft will be necessary.

Machine Component Analysis

• Bevel gears and dog clutch endure the highest stress during performance.

• Both components are made of PLA.

  • Bevel gear: 100% infill ratio.

  • Dog clutch: ~20% grid infill percentage.

• Failure analysis focuses on the dog clutch.

  • Utilizes elastic deformation theory.

  • Assesses performance under worst-case loading condition.

• Loading scenario:

  • Static load of 2 kg (4.41 lb) applied to the output shaft.

  • Bevel gear has a 1:1 ratio.

    • Torque in output shaft equals torque in dog clutch shaft.

• Calculations include:

  • Torque in the output shaft.

  • Static loading to D-profile of the dog clutch.

Yield strength of PLA at 20% grid infill: 2.24 ksi.

• Factor of safety: ~2.

• Failure analysis findings:

  • Dog clutch is not supposed to fail.

  • Experiences minor yielding during performance.

• Assumption from the result:

  • Cross-section area or actual yield strength at the cross-section area is smaller than predicted.

Finite Element Analysis

• Chosen for the stand supporting the output shaft closest to the spool winding the wire.

  • Selected due to its simple load case.

  • Other stands have complex load conditions that are difficult to predict.

  • Incorrectly applied loads would render results unreliable.

• Load conditions:

  • Spool attached to the left end of the shaft.

  • Tension on the cable due to weight approximated at a 50° angle.

  • Simulation represents this tension as a bearing load of 20 N (force due to 2 kg weight).

  • Bearing load accurately models the force since the stand houses a bearing for the shaft.

• No axial forces modeled:

  • No axial forces expected on this stand.

  • Shaft collars constrain the axial motion of the shafts.

  • Bevel gears create an axial force, but the opposite shaft collar and stand pair manage this.

• 20 N bearing load corresponds to a static load:

  • Worst-case loading condition due to rotation direction of the output shaft.

  • Impact load not necessary:

    • Forward rotation creates a downward shear force reducing the reaction force needed.

    • In reverse, the bucket is "weightless" momentarily, delaying the impact load.

• Material properties

  • Custom properties provided:

    • 25% infill in a grid pattern.

    • Young’s Modulus: 2.76 GPa.

    • Yield strength: 28.1 MPa.

    • Poisson’s ratio: 0.38.

• FEA results:

  • Von Mises stress distribution shows stress concentrations around screw holes and sharp angles.

  • Maximum stress: 1.434 MPa, well below PLA’s yield stress at 25% infill.

  • Predicted safety factor: 19.6.

  • Stand expected to handle applied loads easily.