Precision Link vs Rotary Indexer

Precision Link Conveyors vs. Rotary Indexers

High-speed automation systems are typically available in two types of configurations: Precision link conveyors or Rotary indexing tables. Today, both linear systems and rotary systems can be outfitted with either fixed cam driven motion or programable servo driven motion. There are specific considerations that must be weighed before choosing fixed or programable part conveyance. More on this later in the article.

The choice between a linear or rotary approach for an automated assembly system depends on several factors. Small assemblies with few processes are generally suited to a rotary layout. When the number of stations and part size increases, the rotary dial becomes large and is often an inefficient use of floorspace. A linear layout using a precision link conveyor offers a number of advantages.

Precision Link Conveyor

The most common linear indexing system is a precision link conveyor powered by a barrel cam indexer or by a servomotor. This type of conveyor consists of precision machined links that are joined together by a precision ground shaft. The “link chain” rides in tracks, constrained by precision bearings between two large sprockets.

Advantages of a Precision Link Conveyor:

Precision link conveyors are better for complex assemblies.
Higher production speeds with motion dynamics are possible on precision link conveyors.
When 100% inspection and testing is required choose the Precision Link Conveyor
Precision link conveyors are the best selection when 40 to 80+ stations are needed.
Layout of work flow in the plant is optimized with the precision link conveyor.
The Swanson precision link conveyor is modular and therefore is expandable.
Linear systems offer better station access for set-up, change-over, and maintenance.

In a facility with narrow aisles, a linear system is often a better fit. This type of system optimizes space by placing stations on both sides of the conveyor and allowing tool mounting in the center area. The Swanson conveyor is modular and easily expandable, like inserting leaf’s in your dining room table to make it larger. Manufacturers often use rotary systems because the assembly being produced is simple, or the production volume is low. If the assembly process requires more steps or if product volume increases, the manufacturer can begin with a small Swanson Agile precision link conveyor and expand it to meet future volume demands.

Rotary Indexer

Advantages of a Rotary Indexer:

A typical Rotary Indexing system consists of a circular multi-position dial plate indexed by a barrel cam or by a servomotor. Rotary assembly dials are generally used on applications with 8 or fewer parts/processes. For an example, the first station could load a part. The second might load a second part and perform a gluing task. The third station could load a “snap on” part. The fourth station could drill a hole. The fifth inspects the assembly, and the sixth offloads the complete product.

Traditionally a dial plate operates at 30 parts per minute, or slower. It can run higher production rates with multiple tooled stations. The more parts or processes, obviously the more stations that are needed. Rotary indexers are usually used on applications requiring between 8 and 16 stations. They can have up to 36 stations, but this is unusual.

In assembly processes that are more involved, the precision link conveyor system has been the first choice for manufacturers. Advances in technology now enable the movement of the rotary indexer or the precision link conveyor to be controlled by either traditional fixed cams or servomotors. In fact, both drive technologies can be used together for an operation that requires a part to be moved 6 inches one day, but 12 inches the next day.

Both fixed and flexible drive technologies offer distinct advantages, but only if manufacturers optimize important operational parameters. If there will be large, unbalanced loads during the dwell period, a fixed cam system may be the best option. Servomotors can be adversely affected by imbalanced loads. With a rotary table driven by a constant lead cam and a servomotor, the indexing motion can be changed from 90 degrees to 120 degrees simply by changing the program. A table driven by a barrel cam would have to be replaced or rebuilt.

Drive considerations for both precision link and rotary indexer

In manufacturing operations that require an indexing motion, the dominant control motion technology has been based on the barrel cam indexer. In recent years, however, servo motor driven technology using a constant lead cam has grown in prominence. The constant lead cam is giving manufacturers a flexible alternative to the conventional fixed barrel cam drive. In this section we will explain the benefits (and potential drawbacks) of each system to help you make a more informed decision.

Defining terms – Barrel Cam index drive

A barrel cam indexer is a mechanical automation component that has been manufactured to produce an accurate and repeatable intermittent motion. The cam has two distinct operating sections:

Index cam section: The number of degrees on the 360° cam that are used for moving the part fixture into its assembly or process position.
Dwell cam section: The remaining number of degrees on the 360° cam that are used to maintain the part “in-station” where no movement occurs.

In other words, if the indexing operation calls for the cam to rotate 90 degrees and then stop, it is cut in such a way that those operational parameters are achieved every time. The motion cannot be changed. The barrel cam has been the technology of choice for indexing operations for decades. As such, it has gained a loyal following among machine builders that favor a mechanical design. It is a “tried and true” technology that is both robust and low maintenance.

Advantages of barrel cam index drive:

Since indexing systems that use barrel cams are mechanical in nature, machine startup is less involved because the cam controls all movements. Simply power up the conveyor and the cam reliably delivers what it has been designed to provide. No programming, decision making or system debugging is required.
Because of the way the cam is machined, when it comes to a stop, it is locked into that position, without the chance that ancillary movement will occur, i.e. minimal “lash”
Fixed indexing systems are robust and can easily handle high load capacities.
Maintenance is simple. All the operator has to do is change oil in the cam box after a predetermined number of hours of operation and grease appropriate movements.
On high speed assembly machines the barrel cam can also be used to actuate process tooling. This feature results in extremely reliable equipment with minimal down time.

Disadvantages of barrel cam index drive:

Barrel cam systems rely on hard automation to operate, meaning that if the cam is designed for a certain movement, that’s the only way it can ever be used.
If parameters of the indexing operation need to be altered (for example, if a new part is larger than the previous one), that can’t be accommodated without changing the barrel cam. The indexer will require major modifications.

Defining terms – Constant Lead Cam with servo motor drive

By comparison, the index characteristics of the constant lead cam are not determined by the way the cam has been cut. Rather, the characteristics are determined electronically by a programmable device, in most cases, a servo motor. Instead of having the indexing and dwell times built into the cam, the constant lead cams have a programmable indexing period. The dwell period is determined electronically by shutting down the drive motor.
The servo drive is always reading the motor’s encoder, comparing the current position against the programmed position and making corrections. This approach results in a programable index time and programable dwell time.
One potential negative results from the “life expectancy” of the drive components. In general the drive chain is robust, but they have a finite life due to the constant start/stop of the motor.

Advantages of servo motor drive:

Some automation systems require a certain task to be run constantly for a year or longer, but then it will need to be modified, for example from a 90-degree index to a 120-degree index. In this case, the servo motor can simply be reprogrammed with the new parameters, whereas the mechanical system with a barrel cam would have to be replaced or rebuilt.
A servo system can be configured to work with different part sizes just by selecting a different program. The motion is controlled by the servo motor.
The servo motor can monitor how much power is being consumed. That information can be fed to a PC that accumulates the data and can create a report on variables like power demand, uptime and downtime, displacement rate, velocity, and acceleration and deceleration time.
The use of a central programmable controller (to control the servo motor) makes the operation of these systems more predictive. That may sound counterintuitive at first, however, most machines have many axes of motion (six, eight, 10 or more). When their operation is tied to a central controller, that controller can sync and fine-tune the actions that are occurring at previous and succeeding workstations. This can’t be done in a fixed barrel cam system that has no central controller.

Disadvantages of the servo drive motor:

Flexible indexing systems can often have a higher upfront cost than their fixed system cousins. Fixed systems consist of an AC motor, position switch and variable-frequency drive. A servo motor is more sophisticated than the typical AC motor. Because of the flexible system’s feedback capabilities, it requires additional hardware, including power cables, drives that send signals to the motor, a programmable logic controller (PLC) that coordinates its operation through the amplifier to the motor, and a more complex electronic system that drives the system’s flexibility.
The technical skill level needed to set up the system is greater. Engineers must write a program that tells the constant lead cam when and for how long to accelerate, how long to stay at a certain velocity, when and for how long to decelerate, and when to come to a stop. In fixed systems, these parameters are machined into the cam.
The programming can take several days and some trial and error to find the setup that best meets the needs of the end user.
The servo motor’s continuous feedback loop, which is always seeking and correcting the position of the cam, may compromise the system’s precision. In this case, a braking mechanism can be installed on the servo motor to stop any ancillary movements.
Maintaining a flexible system can be more challenging. Any incorrect adjustment can alter the recommended operating pattern, which can have an adverse effect on production rates and even lead to damaging the indexer.

Final Thoughts

Manufacturers that are deciding between a precision link conveyor and a rotary indexer would be wise to consider some specific questions and criteria concerning the design and expectations for their operations. These include:

Will there be more than one type of part processed that requires different index motion parameters? This sounds like a servo motor process.
If the machine is only ever going to make one part during its lifetime, fixed barrel cam motion is the way to go. If it’s a project where the parameters will be changed at some point, a flexible servo motor is probably the better choice.
Does the part need to be rigidly held or locked in place during processing? Both the precision link conveyor or the rotary indexer will accomplish this, but for larger / heavier parts, you should select the precision link conveyor.
If there is a high number of parts in the assembly (more than 6-8) then you should consider the Swanson Agile Precision Link conveyor for this application.
Will there be any large, unbalanced gravity loads while in the dwell period? This is an area where barrel cam systems shine because they can handle heavier loads and the operation of servo motors can be affected by imbalanced loads.
Does the process require displacement, velocity or acceleration feedback during movement? If the accumulation of feedback by a central controller is required for process monitoring, then a servo motor system is the best choice.
Is there value in being able to more precisely control or restrict incidental movement while in the dwell period of the cam? With fixed barrel cam systems there is no chance for ancillary movement to occur.
Some operations may require the dwell time to be sped up or slowed down at certain times in the process, which makes it ideal for a flexible servo motor system.
Will there be technical support available in the field to program, reprogram, adjust and troubleshoot if a servo motor controls the system? If technical support is not readily available, a fixed barrel cam mechanical system is the better option.

Of course, there are many more questions to consider since every manufacturing operation is unique. For more information, call us at 888-654-9353.