Std. 29  Mechanical Power Transmission Systems

 

In our class, we will discuss 3 types of mechanical power transmission systems.

Belts & Pulleys (Sheaves)

  • Pulleys or sheaves are used on the driver and the driven device to allow the belt to transfer power across a distance.

  • Most common belt type is V-belt.  Best value belt considering cost vs. life of belt.  These belts wedge themselves into the sheave and make contact on both side walls.  Can sometimes be reinforced to reduce stretching due to wear and heat.  V-belts are characterized by the trapezoidal shape of the cross-section of the belt.

  • Timing Belts are the most expensive belt that we will discuss but it is also the most efficient and has the least loss of power due to slipping.  It is also frequently used where exact positioning is needed, i.e. robotics.  Timing belts are characterized by the “teeth” on the belt that line up with matching grooves in the sheave.  These teeth and the matching grooves are what keep the timing belt from slipping, making it more precise in positioning, and more efficient.

  • Flat belts were the first type of belts used and are commonly used for conveyor belts.  They are commonly fit on sight where they will be used with a welding process to join the ends of the belt into a continuous loop.  These are the least expensive of the belt types we studied.  These belts are commonly used for conveyor belts that can be found at the grocery, on treadmills, in airports, package sorting facilities, and in many factories.

  • If the distance between the driver and driver is too great, then a double pulley can be used in between the devices to allow one belt from the double pulley to the driver and another belt to go from the double pulley to the driven device. 

  • The speed and torque transferred from the driver to the driven device is impacted by the ratio of the diameter of the driven pulley divided by the diameter of the driver pulley.

  • The direction of rotation of the driven device’s pulley is the same as the direction of rotation of the driver’s pulley, unless the belt is crossed, in which case the direction of rotation of the driven pulley is opposite of the direction of rotation of the driver pulley.  Both devices must be in the same plane.

Chains & Sprockets

  • The teeth of the sprocket mesh with the open spaces in the chain to transfer mechanical power from the driver to the driven device across a distance.  This is an efficient method of power transfer because there is minimal friction.  Efficiency is typically expressed as a percentage found by the ratio of power in to power out.

  • Chains, unlike belts, are adjustable in length because they have a master link that allows the ends of the chain to be disconnected and links added or removed.

  • The roller chain is the most common type of chain used.  The roller chain is so named because of the roller that rolls on the shaft of the pin connecting links.  This roller also reduces the friction to almost none as the chain slips onto and off of the sprocket.  An example of this would be a bicycle chain.  They are the most economical of the chains when there are no special circumstances to be considered.

  • The double chain, or double roller chain, is used like the roller chain but with larger loads.  It has to be used with a double sprocket as well. 

  • The silent chain is used in situations where noise is a factor to be considered.  These chains have inverted teeth.

  • The speed and torque transferred from the driver to the driven device is impacted by the ratio of the number of teeth on the driven sprocket divided by the number of teeth on the driver sprocket.

  • Just like belts, the direction of rotation of the driver sprocket is the same as the direction of rotation of the driven sprocket.  Both devices must be in the same plane.

Gear Systems

  • Gear systems are used when the driver gear and the driven gear are close together.  They are also used in circumstances where the driver and the driven are not in a shaft to adjacent shaft alignment.  The teeth of the gear mesh with the teeth of the next gear. 

  • The speed and torque transferred from the driver to the driven device is impacted by the ratio of the number of teeth on the driven sprocket divided by the number of teeth on the driver sprocket.

  • For gears to be used together they must have the same diametral pitch.

  • Spur gear has teeth on the edge of the gear. 

  • A worm gear has teeth on the face of the gear and it is paired with the worm to allow the driven device and the driver to be in different planes.

  • A worm is s gear round that has spiral teeth around the circumference of the gear.  It is paired with a worm gear.  Using these two gears together allows the driven device and the driver device to be in different planes.

  • Bevel gear has teeth on the face of the gear pointing in toward the shaft of the device it is attached to.  Using bevel gears allows the driven device and the driver to be at 90° angles to one another.

  • Gear systems allow for the driven device and the driver device to exist in different planes and be at angles not parallel to one another.

  • In gear systems, if the driver has a CW (clock-wise) direction of rotation, the gear that it contacts will rotate in the opposite direction, or CCW (counter-clock-wise).  Furthermore, if there are an even number of gears in the system, then the driven device will always rotate in the opposite direction as the driver.  However, if there are an odd number of gears in the system then the driven device will rotate in the same direction as that of the driver.

 

Driven device is the device that supplies the motion.  The driven device is the one that needs/uses the motion.

Contact

615-792-5641

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