Force-Closed and Form-Closed Cam Types
An important requirement of a design is to make sure the cam and cam-follower bearing are always in contact.
There are two designs that make sure there is contact between the cam and the cam-follower: Force-Closed and Form Closed.
Note: This topic shows cam types with Cam-Roller Bearings, only.
Force-Closed Cams
An EXTERNAL FORCE makes sure the cam-roller remains in contact with the cam profile. External Forces include: Gravity, Springs, Air, and Hydraulic-Cylinders. The External-Force that cam-types illustrate below is a Spring. RECIPROCATING CAM-FOLLOWERS (also called Translating Cam-Follower)
Oscillating Cam Follower (also called Swinging-Arm Cam-Follower.
Force-Closed Cams: Advantages and Disadvantages Advantages: •Only one cam to manufacture •Less manufacturing precision to give an operating cam •Less expensive to manufacture •Zero backlash (play) - when the follower does not lift off the cam! •Reduced Scuffing: The surface-speed of the cam and the cam-follower bearing will be identical at low speeds. This reduces or eliminates scuffing wear on the cam-profile. At high speeds, or where the radius of the cam varies rapidly, there may be some scuffing (mismatching velocities) as the roller cannot accelerate or decelerate enough. In the case of a machine jam, you can: •Configure the mechanism so that the radius of the cam decreases as the mechanism moves towards where it is most likely to jam. Say, as the tool moves into the assembly-area, the cam-follower will lift off the cam profile. •Alternatively, it is possible to use an air-cylinder to actively lift the cam-follower off the cam-profile in response to a sensor detecting a jam in the machine. Disadvantages: •Cam-Lift: You must calculate the spring (or cylinder) parameters (Free-Length and Spring-Rate) to make sure the cam-follower does not lift off the cam-profile at all operating speeds. •Cam-Force: The contact-force on the cam-profile is more with a spring than with a Form Closed Cam. This may reduce the operating life of the cam. •Machine Stiffness: The increase to contact-force will also increase other forces on the machine frame, especially when the pressure angle is high. You may need a stronger and more rigid machine frame. •Residual Energy after Stop/E-stop: At stand-still, the torque on the cam from the spring will tend to turn the cam. •Cam-Shaft Speed Fluctuation The increase to contact-force will also increase the torque fluctuation on the cam-shaft. At slow speed, the torque fluctuation due to a spring may be very high, again, if the pressure angle is high. The speed of the cam-shaft may fluctuate because the spring resists the rotation during the 'rise', and assists the rotation during the 'return' segment (or vise versa, if arranged in the opposite sense). If the speed fluctuation is excessive (>10%) - the machine may damage itself - see Torque, Overrun, and Power. |
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Form-Closed Cams
Note: Form Closed Cams are also called Body Closed Cams, or sometime, Positive Drive Cams. An Internal Force makes sure the cam-follower remains in contact with the cam. There are three different design types.
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There is one cam-roller bearing in a Cam-Groove (also called a Cam-Track). The two faces of the cam-groove are the Cam-Flanks. It is the most common Form-Closed Cam. Its disadvantage is Backlash between the cam-roller bearing and the cam. Reciprocating and Oscillating Cam-Followers - 2D Cams
Reciprocating and Oscillating Cam Follower - 3D Cams
Groove Cams: Advantages and Disadvantages Advantages: •It is the easiest Form-Closed Cam to design and manufacture. •There is one set-up for the machine tool. •It is not as sensitive to machining or manufacturing accuracy as the Conjugate Cam Type. •A slightly oversized machine cutter will give a small clearance between the cam-roller and cam-profile. Disadvantages: •There must be backlash - even if small - between the cam-roller and cam-profile. •If there is a machine crash, then the machine must stop to prevent more damage. •As the follower starts to decelerate after it accelerates, the cam-roller will move from one cam flank to the other. There are three damaging consequences when the cam-roller moves from one flank to the other flank: oScuffing: The cam-roller must change its rolling direction. The roller and the cam-profile will scuff, until the roller begins to roll again. A roller with a large diameter skids more than a smaller roller - it has a larger friction moment, but its inertia increases as the square of its radius. oImpact Dynamics: There is impact between the cam-roller and cam-profile, which gives a 'shock' load to the cam-follower, output-transmission components and payload. The impact can significantly increase the cam contact-force and other forces in the machine components of the output transmission. oOver-run: The input-transmission and motor may 'overrun' as the load 'disappears' while the backlash is being traversed. There is a tendency to overrun even without backlash, because the torque reduces to zero as the acceleration reaches the 'crossover' point. Backlash exacerbates the overrun, sometimes significantly, if the Motion-Law gives a rapid torque reversal - for example the Modified Trapezoid Motion-Law. See Also: Positive Action from Cam to Follower |
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2: Two Cam-Rollers
There are usually two cam-rollers, held in the cam-follower part, that act on different cam-flanks. The motion of the two cam-rollers is identical. Reciprocating Cam-Follower - Translating Cam-Follower
Oscillating Cam-Follower - Swinging-Arm Cam-Follower
Conjugate Cams: Advantages and Disadvantages Advantages: When the cam-rollers (most common) are pre-loaded against the cam-flanks, the backlash can be removed and they roll. This means: •Impact does not occur at the cam-flank when backlash is crossed when, or shortly after, acceleration becomes negative after positive, and vice versa - the crossover •Scuffing wear is eliminated after acceleration crossover from positive to negative, or vice versa, as the roller bearing starts to roll again. Disadvantages: •More expensive to manufacture as the conjugate cams and machine require greater precision. •Usually, the cam-rollers will not lift off the cam in the event of a machine jam. Thus, damage may occur to other machine components until the cam shaft can stop. It is possible to design the conjugate rollers such that one of them will lift off the cam in the event of a machine jam. See Also: Positive Action from Cam to Follower |
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3: More than 2 Cam-Rollers in one Cam-Follower Part - Indexing Cams
Indexing: A motion that repeats and progresses in one direction. The motion 'index-period' followed by a 'dwell-period', to give an intermittent, or progressive, motion. Indexing cam assemblies nearly always have rotary input and output shafts. The output-shaft has several cam-rollers that are equally spaced around a turret or star The rollers engage with the cam in sequence. Each roller is replaced by another at the end of each index cycle. Each roller returns to its original position after a number of motion repetitions. Commercial Indexers are pre-assembled, precision cam-boxes. The cam and the 'roller turret' are inside the cam-box and not visible to the end-user. The input and output shafts are available, outside the cam-box, to connect to the drive-motor, gearing and machine with stiff couplings. Indexing Types:
Indexing Terminology:
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Cam-Bearing Shapes
* Cam-Bearings are nearly always circular - and given the name Cam-Roller - and they usually roll. However, other shapes are possible. In this topic we use the term cam-bearing for the element that should remain in continuous contact with the cam. Cam-Bearings can have a number of different shapes, each with advantages and disadvantages. The Cam-Bearing is a child to the Cam-Follower. The Cam-Follower may be a swinging-arm-follower or a translating-arm-follower.
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Cylindrical & Barrel (Crowned) - a Cam-Roller The most common cam-roller shape. Advantages: •commercially available bearings, that are pre-assembled and optimised to used with cam mechanisms •the steels are commercial with few inclusions, that are through hardened to approximately 63HRC. •they roll, thus there is less scuffing wear than cam-rollers that slide •oil lubrication can establish hydrodynamic lubrication to reduce wear. Crowned Cam-Rollers (also called Barrel Cam-Rollers) can tolerate misalignment to remove 'edge' effects on contact-stress. However, the nominal contact-stress of 'crowned' rollers is greater. Generally, use a crowned-roller if you cannot guarantee alignment between the cam-roller and cam. |
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Flat-Faced Cam-Profile See Tutorial 6A2 Advantages: •lower contact-stress than a cam-roller. •no moving parts. •cam and follower can 'polish' to give a better Film Thickness Ratio (ratio of file thickness to average surface roughness) Disadvantages: •undercutting is more likely; negative radius of curvature is not possible. •possible negative entrainment velocity to give poor lubrication. |
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Ovoid, Lozenge, or Multi-contour Follower This example, the follower is made from four(4) arcs. Use a Tangent constraint between each arc.
Disadvantage: These shapes are not commercially available. |
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Knife-Edged Follower A knife-edge is practically impossible! If a machine has follower with a 'knife-edge', the contact-stress will be extremely high. Thus, the follower will not be a 'knife-edge' for very long. However, they are found in copy-milling machines. In MechDesigner, you must use a 'small radius' to represent a knife-edge follower. The minimum radius you can dimension is 0.05mm.
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