Introduction
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 (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 follower, which is a lever making contact with the cam.
A cam follower, also known as a track follower, is a specialized type of roller or needle bearing designed to follow cams. Cam followers come in a vast array of different configurations, however the most defining characteristic is how the cam follower mounts to its mating part; stud style cam followers use a stud while the yoke style has a hole through the middle.
The first cam follower was invented and patented in 1937 by Thomas L. Robinson of the McGill Manufacturing Company. It replaced using just a standard bearing and bolt. The new cam followers were easier to use because the stud was already included and they could also handle higher loads.
OVERVIEW
The cam can be seen as a device that translates from circular to reciprocating (or sometimes oscillating) 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.
The opposite operation, translation of reciprocating motion to circular motion, is done by a crank. An example is the crankshaft of a car, which takes the reciprocating motion of the pistons and translates it into the rotary motion necessary to operate the wheels.
Cams can also be viewed as information-storing and -transmitting devices. Examples are the cam-drums that direct the notes of a music box or the movements of a screw machine's various tools and chucks. The information stored and transmitted by the cam is the answer to the question, "What actions should happen, and when?" (Even an automotive camshaft essentially answers that question, although the music box cam is a still-better example in illustrating this concept.)
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 to the radial displacement experienced at that position. Several key terms are relevant in such a construction of plate cams: base circle, prime circle (with radius equal to the sum of the follower radius and the base circle radius), 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 (LSA - the angle between two adjacent intake and exhaust cam lobes). Displacement diagrams are traditionally presented as graphs with non-negative values.
History
An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The use of cams was later employed by Al-Jazari who employed them in his own automata. The cam and camshaft appeared in European mechanisms from the 14th century.A camfollower uses a cam, usually a flat piece of tooled metal, and follower system to replicate a specific motion. As the cam is rotated, pressure is applied to the follower, which tracks the shape of the cam by its edge and translates the movement of the cam into a movement pattern. The camfollower operates on a very simple principle and can be applied to a wide number of tasks, because although basic, it is highly versatile. Camfollower systems are used in a wide variety of daily applications, including motor vehicles, moving lawn ornaments, and pumping devices.
Camfollower systems usually take the form of a rotating rod that turns the cam or cams and followers mounted above the cams. A follower with a pointed head will more accurately hold and replicate the motion of the cam, but it will also wear down and require replacement more quickly. Followers with broader heads will not wear down as easily, but some accuracy will be sacrificed. It is important that an external downward force be applied to the follower, to make sure that it retains smooth and even contact with the cam. Without a stabilizing downward force on the follower system, the follower may have a tendency to wobble or jitter, potentially causing a malfunction in the device being moved by the camfollower and wearing the follower down more quickly.
Most cams are made in simple shapes designed to create basic motions – the more complex the shape of the cam, the harder the follower must work to replicate the motion. Many camfollower systems take the shape of a teardrop or have a single jagged inset to create a specific motion. The camfollower systems used on motor vehicles to control the pistons, for example, usually have a teardrop shape that causes a sharp upward motion, or displacement, as the follower reaches the apex of the teardrop, causing the attached piston to fire.
Construction
A cross-sectional view of a stud type cam follower
While cam followers appear to be very similar to roller bearings in construction they have quite a few differences. Standard ball and roller bearings are designed to be pressed into a rigid housing, which provides circumferential support. This keeps the outer race from deforming, so the race cross-section is relatively thin. In the case of cam followers the outer race is loaded at a single point, so the outer race needs a thicker cross-section to reduce deformation. However, in order to facilitate this the roller diameter must be decreased, which also decreases the dynamic bearing capacity.
End plates are used to contain the needles or bearing axially.
Types
Anti-friction element
The most common anti-friction element employed is a full complement of needle rollers. This design can withstand high radial loads but no thrust loads. A similar design is the caged needle roller design, which also uses needle rollers, but uses a cage to keep them separated. This design allows for higher speeds but decreases the load capacity. The cage also increases internal space so it can hold more lubrication, which increases the time between relubrications. For heavy-duty applications a roller design can be used. This employs two rows of larger rollers to increase the dynamic load capacity and provide some thrust capabilities. This design can support higher speeds than the full complement design.
Shape
The outer diameter (OD) of the cam follower (stud or yoke) can be the standard cylindrical shape or be crowned. Crowned cam followers are used to keep the load evenly distributed if it deflects or if there is any misalignment between the follower and the followed surface. They are also used in turntable type applications to reduce skidding. Crowned followers can compensate for up to 0.5° of misalignment, while a cylindrical OD can only tolerate 0.06°.The only disadvantage is that they cannot bear as much load because of higher stresses.
Stud
Stud style cam followers usually have a standard sized stud, but a heavy stud is available for increased static load capacity.
Drives
The standard driving system for a stud type cam follower is a slot, for use with a flat head screwdriver. However, hex sockets are available for higher torquing ability, which is especially useful for eccentric cam followers and those used in blind holes. The only problem with hex sockets is that it eliminates relubrication capabilities on that end of the cam follower.
Eccentricity
Stud type cam followers are available with an eccentric stud. The stud has a bushing pushed onto it that has an eccentric outer diameter. This allows for adjustability during installation to eliminate any backlash. The adjustable range for an eccentric bearing is twice that of the eccentricity.
Yoke
Yoke type cam followers are usually used in applications where minimal deflection is required, as they can be support on both sides. They can support the same static load as a heavy stud follower.
Different type of cam and follower
The variety of different types of cam and follower systems that one can choose from is quite broad. The following is a list of these different types (each type is linked to more an image of the system, these images can be accesses by clicking on the name of the system):
Plate Cam with
· knife-edge follower
1.Clyindrical Cam with
1.Disc Cam with Knife Edge Follower
The diagram above shows an animation of a rotating cam and knife edge follower. As the cam rotates the the follower is pushed up and down. There is some external force pushing the follower back down, so that it remains in contact with the cam profile.The cam shown above is known as a plate cam.
Disc Cam with Roller Follower
The diagram above shows the an animation of a rotating cam and roller follower. As the cam rotates the the follower is pushed up and down. There is some external force pushing the cam back down, so that it remains in contact with the cam profile.
Disc Cam with Flat Follower
The diagram above shows the an animation of a rotating cam and flat follower. As the cam rotates the the follower is pushed up and down.
There is some external force pushing the cam back down, so that it remains in contact with the cam profile
This type of follower can be found in the cam and follower system used to open and close inlet and exhaust valves in an engine.
Drum Cam and Roller Follower
This cam and follower system is slightly different to the plate cams. This type of cam is cylindrical in shape with a profile machined onto it.
Applicationof Cam and Follower Systems in Use
The purpose of this section is to try and shine some light on where cam and follower systems are used. It also tries to show how dynamic, complex and exciting these uses are. To do this we will examine what is probably the most frequently used cam and follower system which is the cam and follower system in an engine, known to us as the cams and camshaft of an engine. This section concentrates mainly on cam and follower systems in relation to how they are used in engines.
The section is laid out in the following parts:
1. firstly we will discuss the function of cam and follower systems in an internal combustion engines
2. then we will consider the mechanisms that the cam and follower system interact with inside the internal combustion engine and
3. finally we will try to look at the how the cam and follower system interact with these other mechanisms.
1.Camshaft of an Engine incorporating a Rocker Arm
The diagram shown below is another typical cam and follower system that could be used in an engine. This system incorporates a rocker arm (shown in blue in the image). In this case the motion the cam imparts on the follower is translated to the valve through a push rod amd the rocker arm.
Camshaft with Plate Cam imparting on a Flat
2.Mechanism of an engine interaect with inside the engine
The other mechanism that the cam and follower system interact with in an engine is the actual mechanism that produces the power, i.e. the crankshaft, connection rod and piston
. 
3.The Four Stroke Engine
The four strokes of the four stroke cycle are:
4.The Exhaust Stroke
The final stroke is the exhaust stroke as as the name suggests this is the stroke where the burnt gases are expelled. Again the camshaft will have timed the opening of the exhaust stroke for this stage and as the piston moves up the cylinder once again the burnt gases are expelled through the open exhaust valve. Then the exhaust valve closes and the cycle begins again.
5.Another Common Use of a Cam and Follower System
Another common use of a Cam and Follower system is within a pump, such as an oil pump. In such pumps the cam and follower system is used to suck oil in through one non-return valve and push it out through on other non-return valve. The suck action is achieved by the system because the follower is cylindrically shaped and moves within a tight fitting cylinder so oil is sucked in and pushed out as the follower moves up and down. This is similar to the gaseous mixtures being sucked into and forced out of the engine cylinder as the piston moved up and down in the previous example.
REFRENCES
- Cam follower selection guide, http://www.rbcbearings.com/camfollowers/selguide.htm,
- McGill CAMROL Bearings, http://www.alliedbearings.com/mfg_prod/bearings/ept_brgs/camrolrevised2.pdf,.
- Robinson, Thomas L., "Bearing", US patent 2099660, published 1937-11-16.
- Difference from standard bearings, http://www.rbcbearings.com/camfollowers/difference.htm
No comments:
Post a Comment