Basic Mechanisms II Engineering System Mechanisms Content Cams

Basic Mechanisms II Engineering System : Mechanisms

Content • • Cams Eccentrics Ratchets Inclined plane Crank/Slider Screw (Bearings)

Cam – Specially shaped piece of metal or plastics – Edge/profile of the cam guides the motions of a follower – Cam converts the input motion (rotary/reciprocating) into a reciprocating output motion of the follower.

• Cam motion – One complete rotation of the cam is called a cycle. – Crown : follower is in contact with the highest point of the cam – Heel : lowest part of the follower is in contact – Dwell : when the follower is stationary

– Stroke : the distance between the highest and lowest points on the cam profile

• Types of Cam & Followers Point follower Sliding & oscillating follower Roller follower Angled foot follower

Flat follower Knife follower Edge follower Sliding yoke follower A follower can be kept in contact with a cam by its own weight, or by the forces acting through a lever or linkage, or by a spring

• Profile shapes vs motions – Most cams are designed to have a smooth curved shape so that the motion transmitted to the follower is smooth and without sudden changes. – – Pear-shaped cam Circular cam Heart-shaped cam Uniform acceleration & retardation cam

• Pear-shaped cam – Often used for controlling valve – Follower remains motionless for about half a revolution of the cam – Pear shape is symmetrically, so the period for the rise and fall motion are same

• Circular cam – Eccentric cam – Cam profile is a circle – The center of rotation is offset from the geometric center of the circle – Produce simple harmonic motion

• Heart-shaped – Follower moves with a uniform velocity – For motions needs to be uniform or steady e. g. winding wire evenly on the former of a solenoid

• Uniform acceleration & retardation cam – Follower gains & loses velocity at a constant rate – Used to control the motion of linkages and levers in complex machinery

• Examples of cams – Flat plate cam – Box cam – Swash plate cam – Barrel or cylindrical cam

• Flat plate cam – The edge is shaped and the profile can be used to guide the motion of a follower – Used in automatic machine tool e. g. cutting tool on automatic lathes

• Box cam – A groove can be milled in the face of cam discs. – As the cam rotates, a follower located in the groove has its motion guided by the groove. – e. g. winding mechanisms of a fishing reel

• Swash plate cam – A disc is fixed to a rotating shaft and the disc is attached an angle to the axis of rotation of the shaft – e. g. pump mechanisms

• Cylindrical or barrel cam – Consisting of a rotating cylinder with a helical (screw shaped) groove in its curved surface – A follower with a tapered roller end is located in the groove – As cylinder turns, the follower moves in straight line parallel to the axis of the rotating cam – e. g. to guide thread on sewing machines

Eccentrics • A circular cam is often called an eccentric cam • The axis of rotation of the cam is offset from the geometric center of the circular disc • Transmit simple harmonic motion to the follower

• Eccentric cam is used on cam shaft of a motorcar to drive the petrol pump

Ratchet – A wheel with sawshaped teeth round its rim is called ratchet – The ratchet wheel usually engages with a tooth-shaped lever called pawl – The purpose of ratchet & pawl is to allow rotation in one direction only and prevent rotation in the opposite direction

• A ratchet wheel can also be used to arrest motion – e. g. A fishing reel uses a ratchet to prevent the fishing line unwinding

– The ratchet wheel and pawl can be used in a different manner. – The pawl is designed to rotate about he same center as the ratchet wheel. – An oscillating input motion is given to the pawl and it transmits motion to the ratchet wheel. – The ratchet wheel is rotated in steps pf one or more teeth by each input of motion from the pawl.

Inclined Plane • Inclined plane is a sloping surface used to gain mechanical advantage when raising a load

Exercise Calculate the velocity ratio & mechanical advantage of the inclined plane

• Effort=weight x sinθ=mgsinθ=h/s So effort E=mgh/s • Mechanical advantage =Load/effort=s/h • Velocity ratio =distance moved by effort distance moved by load =s/h

• Wedge – Wedge is two inclined planes back-to-back – A wedge is often used to raise a heavy load through a small distance – To cleave apart the fibers of material

Screw Thread • A disguised form of inclined plane? • Screw thread is a helical or spiral groove cut in the surface of a cylindrical shaft. • As the thread winds round the shaft • Screw is used to transmit motion and force

• Screw thread terms – Crown : Top of the thread – Pitch : Distance between two adjacent crowns – Lead : Distance moving in one complete revolution of the screw shaft. – Thread angle – Major/minor diameter

– A screw made with one helix or thread running around is single screw thread. – A screw with more than one thread running round it is known as multiple start thread. Multiple start threads transmit more rapid motion.

• Screw thread forms – V-thread : vee-shaped groove of the thread form • Used in threaded or tapped hole or with a nut • Fastening devices, e. g. set screws, nut & bolts (because large amount of friction between the sides of the screw and nut)

British Standard Whitworth Thread (BSW) British Association thread (BA) ISO metric thread

– Square thread • Gets its name from the profile shape of the thread form • Used for moving parts of machines e. g. valve spindle, lifting jack • Not as strong as the corresponding v-thread but friction is less.