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A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion. It is often a part of a rotating wheel (e.g. an eccentric wheel) or shaft (e.g. a cylinder with an irregular shape) 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 (back and forth) motion in the Cam follower, which is a lever making contact with the cam. A cam timer is similar, and these were widely used for electric machine control (an electromechanical timer in a washing machine being a common example) before the advent of inexpensive electronics, microcontrollers, integrated circuits, programmable logic controllers and digital control.
A common type is in the valve 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, the lift and the dwell angle are given. If the profile is treated as a large base circle and a small tip circle, joined by a common tangent, giving lift , the relationship can be calculated, given the angle between one tangent and the axis of symmetry ( being ), while is the distance between the centres of the circles (required), and is the radius of the base (given) and that of the tip circle (required):
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A once common, but now outdated, application of this type of cam was automatic machine tool programming cams.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 appliances controllers, such as dishwashers and clothes washing machines, to actuate mechanical switches that control the various parts.
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 a 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 the rotation of the cylinder and the other is the position of the follower 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 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 Inverse function 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 the 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 a 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 the rotation of the cam and the other is the radial position of the follower. The output is parallel to the axis of the cam. These were once common is mechanical Analog computer 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 tonearm unit, acting as either a rocker-type (tonearm) or linear (linear tracking turntable) 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 (normal to the groove faces). The position of the tonearm 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.
A common example of a linear cam is a key for a pin tumbler lock. The pins act as 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.
Watermills across medieval Europe powered many cam designs as detailed by Jean Gimpel: "In Europe, from the end of the tenth century the cam enabled millwrights to mechanize a whole series of industries which up to then had been operated by hand or by foot. In France, one of the first mills for making beer is mentioned in a document relating to the monastery of Saint-Sauveur at Montreuil-sur-Mer between '987 and 996; water-driven hammers seem to have been operating as early as 1010 at Schmidmiihlen in Oberpfalz in Germany. Hemp seems to have been treated mechanically in the Graisivaudan in 1040. The earliest mentioned fulling mill operating in France was in a village in Normandy about 1086. A tanning mill is mentioned in an 1138 document belonging to the chapter of Notre-Dame de Paris. Paper, which was manufactured by hand and foot for a thousand years or so following its invention by the Chinese and adoption by the Arabs, was manufactured mechanically as soon as it reached medieval Europe in the thirteenth century. This is convincing evidence of how technologically minded the Europeans of that era were. Paper had traveled nearly halfway around the world, but no culture or civilization on its route had tried to mechanize its manufacture."
The cam and camshaft later appeared in mechanisms by Al-Jazari, who used them in his automata, described in 1206. The cam and camshaft appeared in European mechanisms from the 14th century. Waldo J Kelleigh of Electrical Apparatus Company patented the adjustable cam in the United States in 1956 for its use in mechanical engineering and weaponry.
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