A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming
rotary motion into linear motion or vice-versa.
It is often a part of a rotating wheel or shaft that strikes a lever at one or more
points on its circular path.
The cam can be a simple tooth, as is used to deliver pulses of power to a steam hammer,
for example, or an eccentric disc or other shape that produces a smooth reciprocating
motion in the follower, which is a lever making contact with the cam.
Overview The cam can be seen as a device that rotates
from circular to reciprocating motion.
A common example is the camshaft of an automobile, which takes the rotary motion of the engine
and translates it into the reciprocating motion necessary to operate the intake and exhaust
valves of the cylinders.
Displacement diagram
Certain cams can be characterized by their displacement diagrams, which reflect the changing
position a roller follower would make as the cam rotates about an axis.
These diagrams relate angular position, usually in degrees, to the radial displacement experienced
at that position.
Displacement diagrams are traditionally presented as graphs with non-negative values.
A simple displacement diagram illustrates the follower motion at a constant velocity
rise followed by a similar return with a dwell in between as depicted in figure 2.
The rise is the motion of the follower away from the cam center, dwell is the motion where
the follower is at rest, and return is the motion of the follower toward the cam center.
However, the most common type is in the valve actuators in internal combustion engines.
Here, the cam profile is commonly symmetric and at rotational speeds generally met with,
very high acceleration forces develop.
Ideally, a convex curve between the onset and maximum position of lift reduces acceleration,
but this requires impractically large shaft diameters relative to lift.
Thus, in practice, the points at which lift begins and ends mean that a tangent to the
base circle appears on the profile.
This is continuous with a tangent to the tip circle.
In designing the cam, if the profile is treated as a large base circle and a small tip circle,
joined by a common tangent, the relationship can be calculated, given the angle between
one tangent and the axis of symmetry, while is the maximum distance between tip and cam
base, and is the radius of the base and r that of the tip
and r = [R(1+sinφ)-Lsinφ]/1-sinφ Plate cam
The most commonly used cam is the plate cam which is cut out of a piece of flat metal
or plate.
Here, the follower moves in a plane perpendicular to the axis of rotation of the camshaft.
Several key terms are relevant in such a construction of plate cams: base circle, prime circle,
pitch curve which is the radial curve traced out by applying the radial displacements away
from the prime circle across all angles, and the lobe separation angle.
The base circle is the smallest circle that can be drawn to the cam profile.
A once common, but now outdated, application of this type of cam was automatic machine
tool programming cams.
Each tool movement or operation was controlled directly by one or more cams.
Instructions for producing programming cams and cam generation data for the most common
makes of machine were included in engineering references well into the modern CNC era.
This type of cam is used in many simple electromechanical appliance controllers, such as dishwashers
and clothes washing machines, to actuate mechanical switches that control the various parts.
Cylindrical cam
A cylindrical cam or barrel cam is a cam in which the follower rides on the surface of
a cylinder.
In the most common type, the follower rides in a groove cut into the surface of a cylinder.
These cams are principally used to convert rotational motion to linear motion parallel
to the rotational axis of the cylinder.
A cylinder may have several grooves cut into the surface and drive several followers.
Cylindrical cams can provide motions that involve more than a single rotation of the
cylinder and generally provide positive positioning, removing the need for a spring or other provision
to keep the follower in contact with the control surface.
Applications include machine tool drives, such as reciprocating saws, and shift control
barrels in sequential transmissions, such as on most modern motorcycles.
A special case of this cam is constant lead, where the position of the follower is linear
with rotation, as in a lead screw.
The purpose and detail of implementation influence whether this application is called a cam or
a screw thread, but in some cases, the nomenclature may be ambiguous.
Cylindrical cams may also be used to reference an output to two inputs, where one input is
rotation of the cylinder, and the second is position of the follower axially along the
cam.
The output is radial to the cylinder.
These were once common for special functions in control systems, such as fire control mechanisms
for guns on naval vessels and mechanical analog computers.
An example of a cylindrical cam with two inputs is provided by a duplicating lathe, an example
of which is the Klotz axe handle lathe, which cuts an axe handle to a form controlled by
a pattern acting as a cam for the lathe mechanism.
Face cam A face cam produces motion by using a follower
riding on the face of a disk.
The most common type has the follower ride in a slot so that the captive follower produces
radial motion with positive positioning without the need for a spring or other mechanism to
keep the follower in contact with the control surface.
A face cam of this type generally has only one slot for a follower on each face.
In some applications, a single element, such as a gear, a barrel cam, or other rotating
element with a flat face, may do duty as a face cam in addition to other purposes.
Face cams may provide repetitive motion with a groove that forms a closed curve, or may
provide function generation with a stopped groove.
Cams used for function generation may have grooves that require several revolutions to
cover the complete function, and in this case, the function generally needs to be invertible
so that the groove does not self intersect, and the function output value must differ
enough at corresponding rotations that there is sufficient material separating the adjacent
groove segments.
A common form is the constant lead cam, where displacement of the follower is linear with
rotation, such as the scroll plate in a scroll chuck.
Non-invertible functions, which require the groove to self-intersect, can be implemented
using special follower designs.
A variant of the face cam provides motion parallel to the axis of cam rotation.
A common example is the traditional sash window lock, where the cam is mounted to the top
of the lower sash, and the follower is the hook on the upper sash.
In this application, the cam is used to provide mechanical advantage in forcing the window
shut, and also provides a self-locking action, like some worm gears, due to friction.
Face cams may also be used to reference a single output to two inputs, typically where
one input is rotation of the cam and the other is radial position of the follower.
The output is parallel to the axis of the cam.
These were once common is mechanical analog computation and special functions in control
systems.
A face cam that implements three outputs for a single rotational input is the stereo phonograph,
where a relatively constant lead groove guides the stylus and tone arm unit, acting as either
a rocker-type or linear follower, and the stylus alone acting as the follower for two
orthogonal outputs to representing the audio signals.
These motions are in a plane radial to the rotation of the record and at angles of 45
degrees to the plane of the disk.
The position of the tone arm was used by some turntables as a control input, such as to
turn the unit off or to load the next disk in a stack, but was ignored in simple units.
Linear cam A linear cam is one in which the cam element
moves in a straight line rather than rotates.
The cam element is often a plate or block, but may be any cross section.
The key feature is that the input is a linear motion rather than rotational.
The cam profile may be cut into one or more edges of a plate or block, may be one or more
slots or grooves in the face of an element, or may even be a surface profile for a cam
with more than one input.
The development of a linear cam is similar to, but not identical to, that of a rotating
cam.
A common example of a linear cam is a key for a pin tumbler lock.
The pins act as the followers.
This behavior is exemplified when the key is duplicated in a key duplication machine,
where the original key acts as a control cam for cutting the new key.
History An early cam was built into Hellenistic water-driven
automata from the 3rd century BC.
Cams were later employed by Al-Jazari, who used them in his own automata.
The cam and camshaft appeared in European mechanisms from the 14th century.
See also Reciprocating motion
References
External links Cam design pages Creates animated cams for
specified follower motions.
How round is your circle?
Contains various linkages.
Kinematic Models for Design Digital Library - Movies and photos of hundreds of working
mechanical-systems models at Cornell University.
Also includes an e-book library of classic texts on mechanical design and engineering.
Introduction to Mechanisms - Cams Classification, nomenclature, motion, and design of cams;
information for the course, Introduction to Mechanisms, at Carnegie Mellon University.