General Design Information > Cam-Mechanism Design > Cam Types (Follower-Rollers only)

Force-Closed and Form-Closed Cam Types

Your design must make sure the Cam-Profile and Follower-Profile are in contact.

You have two design options: Force-Closed Cam or Form Closed Cam.

This topic shows the schematics of cam types with a Follower-Roller. and not a Flat-Faced Follower. Follower-Rollers are far more common as there are many, commercially available, standard sizes and types of roller bearings intended for Cam Systems.

Force-Closed Cams

An external force makes sure the Follower-Roller is in contact with the Cam-Profile.

These examples use a Spring as the external force.

Other external forces that you can use are: Gravity (slow speed), Air, and Hydraulic-Cylinders.

RECIPROCATING FOLLOWERS (also called Translating 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 Follower (also called Swinging-Arm 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-Profile to manufacture

•Less manufacturing precision

•Less expensive to manufacture

•Zero backlash (play) - when the Follower-Profile does not lift off the cam!

•Reduced Scuffing (with Follower-Roller): The surface-speed of the Cam-Profile and the Follower-Roller will be equal to each other at low speeds, which 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 Follower-Roller cannot accelerate or decelerate quickly 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 Follower-Profile will lift off the Cam-Profile.

•Alternatively, it is possible to use an air-cylinder to actively lift the Follower in response to a sensor detecting a jam in the machine.

Disadvantages:

•Cam-Lift: You must calculate the Spring's (or Cylinder's) parameters (Free-Length and Spring-Rate) to make sure the Follower-Profile does not lift off the Cam-Profile at all operating speeds.

•Cam-Force: A Spring exerts a greater force on the Cam than a Form Closed Cam, which may reduce the operating life of the Cam-Profile and Follower-Profile.

•Machine Frame Stiffness: A Spring and the Follower-Part exerts a greater force on the machine-frame.

•Residual Energy after Stop/E-stop: At stand-still, the Spring exerts a torque on the cam when the pressure-angle is not zero, which tends to turn the cam.

•Cam-Shaft Speed Fluctuation:The Spring Force tends to increase the torque fluctuation on the cam-shaft as the Spring changes its length.. The torque fluctuation is greater if the pressure-angle is high.

The speed of the cam-shaft may fluctuate: the spring resists the Cam-Shaft rotation during the 'rise', and assists its 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 (Body-Closed Cams)

An Internal Force makes sure Follower-Profiles remains in contact with Cam-Profiles.

There are three design types.

1: One Follower-Roller

One Follower-Roller is in a Cam-Groove (also called a Cam-Track).

The two side walls of the groove are called Cam-Flanks. It is the most common Form-Closed Cam.

Its disadvantage is that there must be Backlash between the Follower-Roller and the Cam-Flanks.

Reciprocating and Oscillating Followers - 2D Cams

2D-Cams

Linear Rib Cam, Reciporcating or Translating Cam Follower Output, Form or Body Closed

Rotary Rib Cam, Reciporcating or Translating Cam Follower Output, Form or Body Closed

Rotary 'Complimentary' Cam, Reciporcating or Translating Cam Follower Output, Form or Body Closed

Translating Cam Drive

Rotating Cam Drive

Translating Reciprocating Cam

Translating / Reciprocating Follower

Translating/Reciprocating Cam

Rotating / Oscillating Follower

Rotating Cam

Translating / Reciprocating Follower

Rotating Cam

Rotating / Oscillating Follower

Reciprocating and Oscillating Follower - 3D Cams

3D-Cams

Rotary Barrel Rib Cam, Oscillating or Swinging Arm Cam Follower Output, Form or Body Closed

Rotary Globoidal Rib Cam (for or Body Closed), Oscillating or Swinging Arm Cam Follower Output

Rotary Barrel Rib Cam, Oscillating Cam Follower Output, Form or Body Closed

Cam Driving a Coupler of a Four-Bar Mechanism

Rotating Cam

Multi-turn Barrel (Cylindrical) Cam

Rotating Cylinder

Reciprocating Follower

Rotating Barrel or Cylinder Cam

Rotating / Oscillating Follower

Rotating Barrel/Cylindrical Cam

Rotating / Oscillating 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 manufacturing accuracy as the Conjugate Cam Type.

•A slightly oversized machine cutter will give a small clearance between the Follower-Roller and Cam-Profile.

Disadvantages:

•There must be backlash - even if small - between the Follower-Roller and Cam-Profile.

•If there is a machine crash, then the machine must stop to prevent more damage, unless there is a Torque Limiter in the Output Transmission..

•As the follower starts to decelerate, the Follower-Roller will move from one cam flank to the other.

There are three damaging consequences when the Follower-Roller moves from one Cam-Flank to the other:

oScuffing: The Follower-Roller must change its rolling direction. The Follower-Roller and the cam-profile will scuff, until the Follower-Roller begins to roll again. A Follower-Roller with a large diameter skids more than a smaller Follower-Roller - it has a larger friction moment, but its inertia increases as the square of its radius.

oImpact Dynamics: There is impact between the Follower-Roller and Cam-Profile, which gives a 'shock' load to the Follower, output-transmission components and payload. The impact can significantly increase the contact-force and other forces in the machine components of the output-transmission.

oOver-run: The input-transmission and motor may overrun while the torque-load is zero as the backlash is traversed. There is a tendency to overrun even without backlash, as the torque-load reduces to zero when the acceleration reaches the 'crossover' point. Backlash makes overrun worse, sometimes significantly, if the Motion-Law has a rapid torque reversal - for example the Modified Trapezoid Motion-Law.

2: Two Follower-Rollers (also called Conjugate Cams)

There are usually two Follower-Rollers, held in the Follower-Part, that act on different Cam-Profiles. The motion of the two Follower-Rollers is identical.

Reciprocating Follower - Translating Follower

2D-Cams

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

Oscillating Follower - Rotating Follower

2D-Cams

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

Rotating Cam
External Rib Globoidal Cam	External Rib Globoidal Cam	External Rib Barrel Cam	Inverted Slider-Crank Cam

Conjugate Cams: Advantages and Disadvantages

Advantages:

When the Follower-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 machining of the Cam-Profiles and operating machine itself need greater precision.

•Usually, the Follower-Rollers will not lift off the cam in the event of a machine jam. Damage may occur to other machine components until the machine can stop.

It is possible to design the Follower-Rollers such that one of them lifts off the cam in the event of a machine jam.

3: More than 2 Follower-Rollers in one 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 Follower-Rollers that are equally spaced around a turret or star. The Follower-Rollers engage with the cam in sequence. Follower-Rollers replaces each other at the end of each index cycle. Each Follower-Rollers rotates to its original position after a number of motion cycles.

Commercial Indexers are pre-assembled, precision cam-boxes. The cam and the Follower-Rollers 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 the output-shaft 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 be 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.

Follower-Profile Shapes

In this topic we use the term Follower-Profile for the element that should remain in continuous contact with the Cam-Profile.

Follower-Profiles are nearly always circular - and given the name Follower-Roller - and they usually roll. However, Follower-Profile have a number of different shapes, each with advantages and disadvantages.

The Follower-Profile is a child to the Follower-Part, which is more usually termed Follower.

Typical Follower-Profile Shapes for Rotating-Followers

	Cylindrical & Barrel (Crowned) Follower-Profile - termed Follower-Roller The most common Follower-Roller shape. Advantages: •commercially available bearings, that are pre-assembled and optimized 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 Follower-Profiles that slide •oil lubrication can establish hydrodynamic lubrication to reduce wear. Crowned Follower-Rollers (also called Barrel Rollers) can tolerate misalignment to remove 'edge' effects on the contact-stress. However, the nominal contact-stress of Crowned Follower-Roller is greater. Generally, use a Crowned Follower-Roller if you cannot guarantee alignment between the Follower-Roller and Cam-Profile.
	Flat-Faced Follower-Profile See Tutorial 6A2 Advantages: •the contact-stress is less than a Follower-Roller. •there are no moving parts •the Cam-Profile and Follower-Profile can polish to give a better Film Thickness Ratio ( a ratio of Lubrication Film Thickness to Surface-Roughness) Disadvantages: •undercutting is more likely; negative radius of curvature is not possible. •possible negative entrainment velocity to give poor lubrication.
	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.
	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.