Mechanical Power Transmission Systems: Torque and Speed

Torque Problem: Turning Fast with Little Torque

Power transfer mechanisms transform speed to torque. Synchronous devices keep input and output shafts in synchronization.

Power Transfer Mechanisms

• Belts (flat, round, timing)
• Chains (roller, ladder, timing)
• Plastic/Cable Chain (bead, ladder, pinned)
• Friction Driver
• Gears (spur, helical, bevel)

Belt Drives

• Flat Belts: Old design, low power devices, rotating power.
• O-Ring Belt: Moderate efficiency, cheap, requires proper tension alignment.
• V-Belt: Relies on friction, quiet, efficient, allows misalignment.
• Timing Belt: Flexible tooth, synchronous drive, used in wet conditions.

Chain Drives

• Plastic Cable Chain: Three forms (ladder, moving beads onto cable, spiral). Starts with steel cable, over molds rubber teeth.
• Ladder Chain: Below ¼ horsepower, simple, inexpensive, not very strong.
• Roller Chain: Functional to hundreds of hp, noisy, does not tolerate sand.
• Timing Chain: Silent, handles hundreds of hp, requires precision alignment.

Friction Drives

Power transferred by friction, resulting in low lifetime.

• C1 Drive: No gears or pulleys, features limited slip & differential clutch.

Gears

The most common form of power transmission. Gears are scaled, efficient, smoothly quiet, and vary in diameter, tooth size, shape, and face width.

Gears provide many forms of power transmission or control, including:

• Changing rotational speed or direction.
• Altering angular orientation.
• Multiplication & division of torque.
• Converting rotation to linear motion.

Gear Dynamics

• Backlash: The shortest distance between non-contacting surfaces of adjacent teeth.
• Gear Efficiency: Ratio of output power to input power.
• Gear Power: Gear load capacity determined by dimension & type.
• Gear Ratio: Determined by pitchline velocity.
• Undercutting: Recessing the bases of gear teeth to improve clearance.

Gear Classification

• External Gear: Teeth on the outside surface.
• Internal Gear: Teeth on the inside surface.
• Spur Gear: Cylindrical gears with teeth straight and parallel to the axis of rotation.
• Rack Gear: Has teeth on a flat surface.
• Helical Gear: Cylindrical gears with teeth set at an angle to the axis.
• Herringbone Gear: Double helical gears.
• Worm Gear: Crossed-axis helical gear. Used for high reduction ratios; inefficiency is caused by sliding contact.
• Pinion: The smaller of two mating gears.
• Bevel Gear: Teeth on a conical surface.
• Straight Bevel: Curved oblique teeth, handles high loads.
• Miter Gear: Mating bevel gears.
• Face Gear: Straight tooth surfaces; axes lie in a plane perpendicular to the shaft axes.

Harmonic Drive Speed Reducer

Components include the strain wave generator (prime mover), circular spline, and flexspline (nonrotating element enclosed in a ball bearing assembly that functions as the rotating input element). Repeatability & resolution (R) are excellent, but it has wind-up or torsional spring rate.

Building Blocks

• Flexible face gears
• Non-flexing face gears
• Wave generator

Gearheads

Used in servosystems to drive long trains of mechanics. Efficiency can exceed 90%. Can be mounted vertically, reducing cost.

Power Supply Note

Each motor is selected to supply half the required power to hoisting gears.