The cam mechanism is a high auxiliary mechanism composed of three basic members: a cam, a follower and a frame.
The cam is a member with a curved profile or groove, generally an active member for constant-speed rotary motion or reciprocating linear motion.
A member that is in contact with the cam profile and transmits power and achieves a predetermined motion, generally reciprocating linear motion or swing, is called a follower.
The basic feature of the cam mechanism in the application is that the follower can obtain a more complex motion.
Since the motion law of the follower depends on the cam profile curve, it is only necessary to design the contour curve of the cam according to the motion law of the follower.
Cam mechanisms are widely used in a variety of automatic machinery, instruments and steering controls.
The reason why the cam mechanism is so widely used is that the cam mechanism can realize various complicated motion requirements, and the structure is simple and compact.
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The mechanism for propelling the reciprocating movement or swing by the rotary motion or the reciprocating motion of the cam.
The cam has a curved profile or groove, and has a disc cam, a cylindrical cam, a moving cam, etc., wherein the groove curve of the cylindrical cam is a space curve and thus belongs to a space cam.
The follower is in point contact or line contact with the cam, and has a roller follower, a flat bottom follower, and a tip follower.
The tip follower can keep in contact with any complex cam profile for any movement.
However, the tip is easy to wear and is suitable for low-speed mechanisms with low transmission force.
In order to keep the follower in contact with the cam at all times, a spring or gravity can be applied.
A cam having a groove allows the follower to transmit a determined motion as one of the operative cams.
In general, the cam is active, but there are also driven or fixed cams.
Most cams are single-degree-of-freedom, but there are also double-degree-of-freedom cone cams.
The cam mechanism is compact and ideal for applications requiring intermittent movement of the follower.
Compared with hydraulic and pneumatic similar mechanisms, it is reliable in motion and is therefore widely used in automatic machine tools, internal combustion engines, printing presses and textile machines.
However, the cam mechanism is easy to wear and has noise, and the design of the high-speed cam is complicated and the manufacturing requirements are high.
Classification of cam mechanisms
1) Disc cam
2) Moving cam
3) Cylindrical cam
By follower type:
1) Apex follower;
2) Roller follower;
3) Flat bottom follower
- Force locking: spring force, gravity, etc.;
- Geometric locking: equal-diameter cam, equal-width cam;
The structure is simple, compact and convenient in design, so it is widely used in machine tools, textile machinery, light industrial machinery, printing machinery, and electromechanical integration.
As long as the appropriate cam profile is made, the follower can be given any predetermined motion law.
- The cam is high in the secondary contact (point or line), and the point and line contact are easy to wear;
- The cam profile is difficult to process and the cost is high;
- The stroke is not big
Characteristics of motion
In the disc-actuated counter-moving disc cam mechanism with the roller, the cam revolves one revolution to perform the ascending-stop-down-stop four actions.
The relationship between the follower displacement s (or stroke height h) and the cam angle Φ (or time t) is called the displacement curve.
The travel h of the follower has a push and a return stroke.
The cam profile curve is determined by the shape of the displacement curve.
In some machines, the displacement curve is determined by the process.
However, in general, only the stroke and the corresponding cam angle are determined according to the work needs, and the shape of the curve is selected by the designer, and there are various motion laws.
Conventional cam motion laws include constant velocity, equal acceleration, equal acceleration, cosine acceleration, and sinusoidal acceleration.
The law of constant velocity motion has a strong rigid impact due to sudden changes in speed, and is only suitable for low speed.
Equal acceleration-equal deceleration and cosine acceleration also have sudden changes in acceleration, which will cause flexible impact, which is only suitable for medium and low speed.
The acceleration curve of the sinusoidal acceleration motion law is continuous without any impact and can be used for high speed.
In order to make the acceleration of the cam mechanism and its speed change rate are not too large, and consider the problems of momentum, vibration, cam size, spring size and process requirements, other various motion laws can be designed.
Apply a large number of useful motion curves, such as deformation sinusoidal acceleration, deformation trapezoidal acceleration and deformation velocity law.
It can also be combined into various motion laws by using an electronic computer.
It is also possible to use a motion law represented by a polynomial to obtain a continuous acceleration curve.
In order to obtain the most satisfactory acceleration curve, an acceleration curve can be given in numerical form, and then the displacement curve is obtained by the finite difference method, and finally the convex contour is designed.
Some automata usually work with several cams. In order to coordinate the movements of the various parts controlled by the cams, it is necessary to prepare a correct motion cycle diagram and reduce the surface roughness before the cam design.
The working conditions of the cam are air drying, clean lubricating oil, or lubricating oil with various additives.
The viscosity of the lubricating oil and the choice of the oil supply method should take into account the shape of the follower and the rotational speed of the cam.
The material of the cam and the follower should be matched properly, such as hard steel and cast iron, which are suitable for high-speed sliding.
Hard steel and phosphor bronze have low vibration and noise and can compensate for inaccuracies in the profile.
The effect of pairing cast iron and cast iron is acceptable.
However, the combination of hard nickel steel and hard nickel steel, mild steel and mild steel is not effective.
For geometric parameters, lubrication, materials and surface roughness, elastic fluid dynamic lubrication theory can also be used for comprehensive calculation to reduce wear.
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