In machine design, indexing is the movement from an initial position to a new position, starting and ending in rest. What applications require such motion? Machine tool changers, beverage cappers, and medical-product stampers are just a few examples. Mechanical cam-driven indexers are motion control devices that use physical parts to convert constant rotary motion into this intermittent, dynamically controlled indexing motion. It is true that electronic cams are used in some systems. However, electronic approaches are sometimes unnecessary. In fact, as we will discuss, mechanical cams frequently offer superior performance and accuracy in a simpler, more robust design.
Basics of cam motion
Mechanical cam shape determines the motion characteristics of going from one position to another. Motion type is totally flexible for any application, and is limited only by the physical properties of the mechanisms used. Modified sine motion has evolved as the industry standard: Its acceleration curve combines a low acceleration factor with a gradual transition from acceleration to deceleration. Modified sine with constant velocity motion is obtained by inserting a period of constant velocity (known as zero acceleration) into the middle of a modified sine motion; it features a low velocity factor. Modified trapezoid motion is obtained with the insertion of two periods with constant acceleration and features a lower acceleration factor. Finally, asymmetric modified sine motion is obtained with acceleration and deceleration periods that are not symmetrical. Longer deceleration periods feature a lower negative acceleration factor. Each motion curve/profile can be specific for the required machine.
Three types of cam mechanisms — paradromic, barrel, and globoidal — have become predominant for mechanical indexing functions because of their speed, precision, and reliability. All can be designed to output different stops or rest positions per revolution, with alternating indexing (moving) and dwell (or rest) periods. (An index cycle is the specification that sums these indexing and dwell periods.)
In addition, cams can be designed to turn on the same axis as motor-shaft input, or they can provide output rotation at a right angle to input. (Paradromic designs supply the former, while barrel and globoidal designs generally supply the latter.) Also, globoidal cams have the ability to provide oscillating output from constant rotary input, providing yet another unique mechanical motion profile.
Index rate (the number of cycles per minute in continuous run mode) and cam types are two other considerations: Here, the cam type is frequently overlooked or misunderstood in designing speed for the camshaft and in critical accuracy applications.
In a single-cycle cam, one revolution of the camshaft produces one index cycle on the indexer-output. In a multi-cycle cam, one revolution of the camshaft produces multiple index cycles on the indexer-output. (Note that multi-cycle cam indexers exhibit an inherent margin of error that must be factored into overall machine design.) Camshaft rpm is multiplied by the cam's cycles — so to illustrate, a double cycle cam running at 60 rpm has a rate of 120 indexes per minute.
There are two different ways to run a cam system: continuously or intermittently. Running the camshaft continuously produces a fixed relationship between the index and dwell periods. Connecting the index drive to the machine line shaft assures exact synchronization with other machine functions. The index period is the minimum available for the required motion characteristics, but overlapping of machine motion can substantially enhance indexer performance.
For example, with modified sine motion, the first 15% of the index period produces only about 2% of output displacement. Selecting a 120° index period in a case where an index period of 90° is required causes a 15° overlap at the beginning and a 15° overlap at the end — which substantially reduces torque requirements, caused by the inertia load, by more than 50%. Some manufacturers can supply displacement charts to assist designers in specifying the proper timing for an application.
In contrast, by stopping the camshaft while in dwell period, dwell time can be varied before restarting the next index. This cycle-on-demand operating mode is best suited for machine operation and improves production process flexibility. The dwell period is the minimum available or determined by the time required to stop the transmission.
For optimum timing, adjust the phase-cam to stop the camshaft before it reaches the middle of dwell period position; when the index cycle is restarted, the camshaft reaches the nominal speed rolling on the remaining part of the dwell period without load. This allows the cam to enter the next index period at constant speed and is the optimal condition to drive the mass load on the indexer. Some manufacturers can also supply 0-to-180° adjustable phase cams with either a micro-switch or a solid-state proximity sensor for cycle control.
To minimize the possibility of problems and maximize cam life, capacity values should be conservative — 1.75 or more. Application software can assist designers in selecting the proper indexer; some allows for particularly thorough selection to ensure that products perform trouble-free for the life of the machine.
For accuracy, some manufacturers also record and supply reports on their proprietary technology and follower-wheel accuracy in accordance with an ISO 9001-certified quality system, to ensure that accuracy values are true as stated. Make use of these reports when possible. Another accuracy-related tip: Upon installation, never bore fixtures with the dial plate attached to the indexer, as errors are not random but station dependent. Multi-cycle cams naturally accumulate an additional margin of error, which is stated in product literature, and this must be considered in certain applications.
Cam system input
When selecting the input for a cam system, consult a factory-trained sales engineer. Play it safe: They can assist in choosing the best indexer and input drive system to meet specific requirements. Due to the cyclic nature of the loads in indexing applications and because a complete reversal of load is imparted to camshafts during the acceleration/deceleration phases of the index period, power transmission elements must be selected to meet torque carrying, rigidity, and zero or minimum backlash requirements. After all, speed fluctuations of the camshaft drastically degrade performance and load capacity of the index drive.
It is important that the sizing of the indexer takes into account torque capacity needed to decelerate and stop the mass once in motion. The inertial load takes a greater capacity to stop the motion than to start.
Most mechanical cam systems include a motor-driven lineshaft. Cams on this lineshaft turn with its rotation, and interface with other following elements to provide varied motion. When supporting the maximum load, lineshafts connected either at the input or at the output of the index drive must be free of all torsion deflection. When shaft connections are used, torsionally rigid couplings are recommended. Pulleys or sprockets must feature the largest practical diameters possible and not be smaller than the indexer's cam diameter.
Double rigid idlers are recommended for eliminating the slack in timing belts or chains; double-enveloping worm gear reducers are recommended to minimize backlash at the camshaft and for greater shock resistance. Usually extended, ground camshafts and adapter mounting plates allow direct indexer mounting.
Ac motors are suitable for both fixed speed and cycling cam applications; they are the simplest and most commonly used. If an index cycle is performed in less than 0.8 sec (for an index rate equal to 75 cycles/min. in continuous run mode) a brake motor may be used to prevent the camshaft from rotating out of the dwell period. Electromechanical brakes may be suitable up to 40 cycles per min., though they tend to be noisy and brake pads wear out over the long term.
Electromagnetic, dynamic brakes are more suitable up to 120 cycles per min. They are noiseless and do not wear. For cam systems that require variable speed or fast cycling, ac motors and VFDs feature variable speed, load compensation, current and torque limiting, and solid-state dynamic braking.
For systems that require even more control, servomotors with an axis controller may be required. A constant-lead cam transforms the indexer into a zero-backlash reducer that (when fitted with a servomotor and axis controller) allows programmable stops and kinematic characteristics with high accuracy through program compensation. Lineshaft and electronic sensors work together to provide cam-motion reference: The electronic signal from a speed or position transducer (often an encoder or resolver) to a servo motion controller provides information to drive the actuator (servomotor) producing cam motion. This motion is often linear, though it can be rotary and follow a prescribed path — such as the sinusoidal trails mentioned earlier.
Indexers are normally lubricated with an internal mineral base oil bath reservoir. Cam-system sealing is designed to allow indexers to work in any position without lubricant leakage. Different lubricants and sealing gaskets of specific compounds are available under specific requirements for use in high-temperature environments, in the presence of aggressive chemicals, pharmaceutical and food industry applications, and for radioactive conditions.
Mechanical cam-driven systems vs. electro-cam control systems
Modern-day cam-driven systems can perform complex functions utilizing rotary, linear, oscillating, and cyclic motions. Cam systems provide inherent advantages for which mechanical systems are known — simplicity, speed, accuracy, repeatability, and reliability. Modern cam motion systems are also a viable technology that meets ever-increasing need for higher precision and production rates, allowing machinery to run faster and more accurately than possible in the past.
Mechanical cam-driven systems: For repetitive motions requiring millions of strokes, low maintenance, and high degrees of accuracy and repeatability, nothing can beat the inherent properties of mechanical cam-driven systems, which provide:
Inertia and weight capacity
They can handle the highest-inertia systems.
They are extremely repeatable and give consistent results.
They move heavier loads at faster speeds than most electronic systems.
High reliability is because of fewer components to fail.
This is good for systems requiring accurate repetitive cycles.
Cost effectiveness and simplicity
They can be maintained and operated by less skilled factory personnel, but are impervious to tampering. Synchronized motions are mechanically fixed for high volume production with repeatable results 24/7.
Electronic-cam control systems: For lighter and low-inertia systems that require flexibility, electronic cam control systems are another choice. Also:
Inertia and weight capacity
These are good for lighter low-inertia systems. Heavy loads and high-inertia systems are difficult to handle with full electronic control systems.
They require more sophistication in the form of servos, amplifiers, PLCs, and feedback-loop programming — creating more complex controls to program and maintain.
Speed is acceptable for low inertia or lightweight systems.
Reliability. The system is more complex with components to fail or become obsolete over time.
Good for systems requiring flexibility for various functions.
They are costlier to design, procure, setup, and maintain.
One manufacturer is reenergizing traditional cam motion technology by transforming existing mechanical cams with state-of-the-art Flexible Manufacturing Systems or FMSs into precise engineered products for high-speed motion. “Through implementation of FMS over the last 15 years, we've moved beyond the status quo of traditional ‘old school’ cam-driven machinery, which had been stagnant and excluded from new innovation for many years,” says Robert Zaruba. “The complex tasks of defining optimal cam motion and machine cycle times, diagramming, and engineering are done onsite — so customers receive a simple mechanical motion control device that combines the latest in speed, precision, and reliability.”