An important requirement of a design is to make sure the cam and cam-follower are always in contact.
There are two designs that make sure there is contact between the cam and the cam-roller.
Note: This topic shows cam types with cylindrical cam-follower, only.
An EXTERNAL FORCE makes sure the cam-follower remains in contact with the cam profile. Examples of 'external forces' are: Gravity, Springs, Air and Hydraulic-Cylinders. The cam designs illustrated below use a spring. Reciprocating Cam-Follower [Translating Cam-Follower]
Oscillating Cam Follower - also called Swinging Arm Cam-Follower.
Force-Closed Cams: Advantages and Disadvantages Advantages:
In the case of a machine jam, you can:
Disadvantages:
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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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 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. The disadvantage is Backlash between the cam-roller and the cam. Reciprocating and Oscillating Cam-Followers - 2D Cams
Reciprocating and Oscillating Cam Follower - 3D Cams
Groove Cams: Advantages and Disadvantages Advantages:
Disadvantages:
There are three damaging consequences when the cam-roller moves from one flank to the other flank:
See Also: Positive Action from Cam to Follower |
The design is very similar to an 'indexing cam', but these cams 'oscillate' and do not 'index'. See 'Indexing Cams' below. There are usually two cam-rollers, held in cam-follower part, that act on different cam-flanks. The motion of the two cam-rollers is identical. It is possible, with precision, to reduce backlash and ideally pre-load the cam-rollers against the cam. If this is possible, the rollers roll and almost eliminate skidding between the cam-rollers and cam-flanks. It is possible to increase the diameters of each cam-roller when you are certain that the cam-roller will not lose contact with the cam. The inertia of the roller increases rapidly, and thus its acceleration, becomes more onerous. 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:
Disadvantages:
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 |
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 cam and output shafts. The output-shaft has several cam-rollers that are equally spaced around a 'turret'. 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-Rollers are nearly always circular and they usually roll. However, other shapes are possible. In this topic we use the term follower for the element that should remain in continuous contact with the cam. Followers can have a number of different shapes, each with advantages and disadvantages. The cam-follower arm may be a swinging-arm-follower or a translating-arm-follower.
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Cylindrical & Barrel (Crowned) Cam Rollers The most common cam-roller shape. Advantages:
Crowned-Rollers [also called Barrel- 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 Follower See Tutorial 6A2 Advantages:
Disadvantages:
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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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