Mechanical Engineering Chapter 13 Observatory pp STEST ASTSE

4.Motion Transmission Systems  Motion Transmission is the mechanical function of relaying a motion from one part to another without altering the nature of the motion.  A Motion Transmission System is a set of components that perform the function of transmitting motion.  435

Some definitions:  Driver component: receives the force required to activate the system eg: crankset on a bike  Driven component: receives the motion and transfers it to another part Eg: rear gears on a bike  Intermediate component: located between the driver and driven component – not all systems have this. Eg: the chain on a bike  445

Characteristics of Motion in Transmission Systems  The most common rotational transmission systems are:  Gear Trains  Chain and Sprocket Systems  Worm and Worm Gear Systems  Friction Gear Systems  Belt and Pulley Systems  436

Motion Transmission Systems  See Figure on p. 437 for the main types

Construction Considerations for Motion TRANSMISSION Systems  Depending on the need, a system may only rotate in one direction.  That will affect the choice of system.  438

Gear Trains Gear Trains  The direction of rotation changes from one wheel to the next.  The system can be reversed.  438

Gear Trains

Gear Train Factors Gear Train Factors  Gear teeth: all the gear teeth in a system must be identical – same shape, direction, size and be equally spaced. E.g. Straight or helical  Gear type: the rotational axis of the gears can be positioned different ways (eg: car differentials)  Gear size: the higher the number of teeth, the lower the speed of rotation – or bigger diameter  slower speed.

Helical Gears

Car Differentials

Chain and Sprocket Systems  The direction of rotation of all sprockets on the same side of the chain is the same.  A sprocket on the other side of the chain rotates in the opposite direction.  It can be reversed.  The smaller the sprocket the faster it turns  Requires frequent lubrication  439

Worm and Worm Gear Systems  The direction of rotation depends on the direction of the threads on the worm screw shaft.  It is not reversible.  Worm must be the driver  440

Friction Gear Systems  The direction alternates from one gear to the next.  It is reversible.  The smaller the diameter of the gear, the faster its rotation  440

Friction Gear Systems  Friction gear systems are similar to gear trains except that motion is transferred by FRICTION and not by the GEAR TEETH.  They are less efficient because of slippage.  Factors that affect friction gear systems are: gear type (straight, bevel or spherical), gear size and choice of material.

Belt and Pulley Systems Belt and Pulley Systems  Similar to the chain and sprocket system.  The chain is replaced by a belt.  The sprocket is replaced by a pulley.  The choice of the belt material and the tightness of the belt affect the friction and hence the efficiency of the system.  The direction is the same for any pulley on the same side of the belt.  It is reversible.  The smaller the pulley, the faster its rotation  441

Belt and Pulley systems

Speed Changes in Motion Transmission Systems  A Speed Change occurs in a motion transmission system when the driver does not turn at the same speed as the driven component(s).  The speed change depends on the ratio of gears/threads of the driver compared to that of the driven component.  442

Speed Changes in a Pulley or Sprocket Transmission System  To increase the speed, the driven component should have a smaller diameter.  To decrease the speed, the driven component should have a larger diameter.  To keep the same speed, the two pulleys should have the same diameter.  E.g. Driver Diameter = 15cm Driven Diameter 5 cm Driven Diameter 5 cm So the driven pulley is 3 times FASTER  442

Calculating Gear Ratios  See p. 443

Speed Changes in Worm and Worm Gear Systems Worm Gear Worm Gear  The larger the diameter of the worm gear, the greater the decrease in speed.  To increase the speed, the driven component should have a smaller diameter.  442

Torque

Torque  Torque involves two forces of equal strength but applied in opposite directions which cause a component to rotate about an axis.  Engine Torque increases the rotational speed of components in mechanical systems; provided by the engine  Resisting Torque slows or stops the rotation of components in mechanical systems; caused by friction, air resistance, gravitational force.  444

Torque and Speed Change  If engine torque = resisting torque, No speed change  If engine torque > resisting torque, speed increases  If engine torque < resisting torque, speed decreases 445