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Cam Types & Cam-Roller Shapes

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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 follower remains in contact with the cam profile.

Examples of External Force are: Gravity, Springs, Air, and Hydraulic-Cylinders.

The external force that these cams illustrate is a spring.

Reciprocating Cam-Followers also called Translating Cam-Follower

2D-Cams			3D-Cams

Sliding / Translating Cam	Rotating Cam Drive
'Shear' or 'Slice' Cam	Roller on External Cam	Roller on Internal Cam	Cylinder Cam

Oscillating Cam Follower - also called Swinging Arm Cam-Follower.

2D-Cams			3D-Cams

Translating Cam Drive	Rotating Cam Drive
'Shear' or 'Slice' Cam	Disc with Roller on External Cam	Disc with Roller on Internal Cam	Barrel or Cylinder Cam Globoidal

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. At high speeds, or where the radius of the cam varies rapidly, there may be some scuffing where the surface speeds do not match. This reduces or eliminates scuffing wear on the cam-profile.

In the case of a machine jam, you can:

•	Configure the mechanism so that the 'fall' of the cam moves the tool into the assembly area, and the cam-follower remains on the cam due to only the spring force. If there is a jam, 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.

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.

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.

1: One Cam-Roller

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

2D-Cams

Translating Cam Drive		Rotating Cam Drive
Translating Reciprocating Cam (Slot Cam) Translating Reciprocating Cam-Follower	Translating Reciprocating Cam (Slot Cam) Swinging Oscillating Cam-Follower	Rotating Groove Cam Reciprocating Translating Cam Follower	Rotating Groove Cam Swinging Oscillating Cam Follower

Reciprocating and Oscillating Cam Follower - 3D Cams

3D-Cams

Rotating Cam Drive
Multi-turn Barrel (Cylindrical) Cam Rotating Cylinder	Reciprocating Cam-Follower Rotating Barrel or Cylinder Cam	Oscillating Cam-Follower Rotating Barrel or Cylinder Cam	Swinging Oscillating Cam-Follower Rotating Globoidal Cam

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:

o	Scuffing: 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.

o	Impact 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.

Over-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.

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

2D-Cams

Translating Cam Drive	Rotating Cam Drive
'Shear' or 'Slice' with Rib Cam.	Rib Cam	Internal Groove Cam	External Complementary Cam (Conjugate)

Oscillating Cam-Follower - Swinging-Arm Cam-Follower

2D-Cams

Translating Cam Drive	Rotating Cam Drive
Rib Cam.	Rib Cam	Internal Conjugate Cam	External Conjugate Cams
3D-Cams			2D-Cam

Rotating Cam Drive
External Rib Globoidal Cam	External Rib Globoidal Cam	External Rib Barrel Cam	Conjugate-Cam Coupler Follower.	Cam Inverted Slider-Crank

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.

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:

2D-Cams

3D-Cams

Rotary Parallel Indexing Cam, Form or Body Closed

Rotary Linear Indexing Cam, Form or Body Closed

Rotary Barrel Indexing Cam, Form or Body Closed

Rotary Globoidal Indexing Cam, Form or Body Closed

Parallel Flat Cam
Rotary Input/Output

Barrel Cylinder Cam

Rotary Input/ Linear Output

Barrel Cylinder Cam Form

Rotary Input/Output

Globoidal Cam

Rotary Input/Output

Indexing Terminology:

Number-of-Stops	Number of times the input shaft rotates to rotate the output shaft by 360º - or - The number of times the output shaft indexes to rotate the output shaft by 360º
Index Angle	The angle the output-shaft rotates each index. (360/Number-of-Stops)
Index-Period	The rotation, in degrees, of the input shaft to complete the INDEX motion segment. To Index is to move from one position to a new position.
Dwell-Period	The rotational angle, in degrees, of the input-shaft while the output shaft is stationary. 'To Dwell' is to stay in a fixed position.
Index-Period + Dwell-Period = 360
* It is possible that the output shaft has two index and dwell periods in one rotation of the cam-shaft.
Indexer Type	Type 1 = The input shaft rotates one time to move (index) the output shaft one time Type 2 = The input shaft rotates one time to move (index) the output shaft two times. Type 3 = The input shaft rotates one time to move (index) the output shaft three times.

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.

Typical Cam-Profile Shapes for Swinging-Arm-Followers

Cam with Circular or Round Cam-Follower

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.

Cam with Oscillating Flat-faced Cam-Follower

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.

Cam with Ovoid-Cam-Follower

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.

Cam with Knife-Edged Cam-Follower

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.

Tutorials and Reference Help Files for MechDesigner and MotionDesigner 14.2 + © Machine, Cam, Mechanism, and Motion Design Software by PSMotion Ltd