Engineers designing motion-control devices have a long list of standard, off-the-shelf options to choose from, everything from balls screws to electromagnetics. But the list shrinks dramatically if the application involves long travel distances, high cycles, limited space, heavy loads, or harsh environments. Rigid Chain, however, can pick up the slack in these challenging applications. Unfortunately, most engineers are unfamiliar with Rigid Chain and its attributes, even though has been on the market for nearly 40 years.
Here’s a look at the inherent advantages of Rigid Chain and how engineers have used it in a variety of applications.
How Rigid Chain works Rigid Chain collapses and can be coiled clockwise or counterclockwise. The direction it bends or coils depends on its configuration. And when aligned and pushed in the right direction, it becomes as stiff as a steel column. The chain accomplishes this using articulated links with shoulders that self-support and lock into each other when pushing or lifting a load.
Rigid Chain used to lift loads needs special links because they will be pushed vertically as well as pulled to lower the load and braked to hold the load in an elevated position. So links in vertical applications need to be stronger to handle the additional stresses.
In general, Rigid Chain actuators include the chain, drive housing, and storage magazine. Sprockets inside the drive exert the push on the chain, much like conventional chains. But the load being lifted or pushed exerts a reaction force through the chain heel, creating a locking moment on the chain.
There are two general ways to use these chains: with guides to ensure it remains rigid, or without them, in which case the chain can be supported by a flat surface or have limited lengths or heights.
When either E or T guides are used on horizontal or vertical chain, strokes can be unlimited and thrust capacity is maximized. Guided chain is also more stable and unused links can be stored above or below the stroke path because the chain can be either shoulders up or shoulders down, which determines the direction the chain flexes.
When used without guides but supported by a flat plane, the maximum load depends on stroke length and position of the shoulders in relation to the plane. The shoulders can lie against the plane or opposite it.
If the chain is unguided and not supported, such as when it is used as a lifting column, the maximum load depends on the stroke, as well as cycle frequency, speed, and maintenance intervals. When unguided with the shoulders down, the chain is more stable and the return path of the chain is above the stroke path. Conversely, unguided chains with shoulders up get stored below the stroke path.
Rigid Chains in action Handling heavy loads: Stamping dies can be heavy, with some weighing in at 100 tons or more. They are also large, measuring as much as 25-ft long by 10-ft wide and 10-ft high. So moving them around a plant can be dangerous and time consuming. In many plants, Rigid Chain pulls dies out of presses while new dies are prepped on equipment outside the press. When the new die is ready, the chains push the new die into the press in a matter of minutes, limiting downtime. Without the chain drive, this process could take an entire 8-hr shift.
Rigid Chain is also used to move heavy, cumbersome nuclear waste and material. In these applications, motion control has to be precise, operated remotely, and reliable. Risks associated with maintenance in highly radioactive environments are great, so the motion-control aspect must operate smoothly and safely.
A recent application used a stainlesssteel Rigid Chain on a carriage that routinely moves up to 3 tons of nuclear waste. The load moves horizontally 200 in., with several stopping points along the way. At each stop, the carriage locks in place while the load is lifted. A single chain handles the entire process. For example, when the carriage locks in place at a stop, a redirection housing on the carriage shifts the chain so that it provides vertical thrust rather than horizontal motion.
The chain has to tolerate a highly acidic environment in an unmanned chamber. Corrosion protection, safety, and accessibility are largely taken care of by putting as much machinery as possible, including the pressure-sealed chain-storage magazine, outside the chamber. The chain moves in and out of an opening in the chamber’s wall to push or pull the carriage into position. The entire chain fits into a relatively small space and is powered by a single electric motor.
Low maintenance: Rigid Chain is entirely mechanical, so it is durable and requires little maintenance. This has made it attractive to auto companies for powering the commonly used, heavy-duty, high-cycle scissor lifts. Conventional scissor lifts were once notoriously unreliable and require lots of maintenance. Plus, they lacked accuracy and repeatability. And a failed scissor lift could shut down an entire auto plant. So now chain-driven scissor lifts have become more common in car plants, with some having performed over 7 million cycles.
Environmentally friendly: Sometimes devices have to move loads in clean-room conditions, which automatically eliminates hydraulic-based devices. And screw jacks are often ruled out due to the amount of maintenance they require. In one such application, a company uses a series of Rigid Chains to move a 7-ton platform up and down 2.75 m at 42 mm/sec. Workers stand on this platform stacking materials to be moved in and out of a fuel cell, so safety is also a consideration.
Compact storage: Rigid Chains can be coiled for storage. It can also be guided into storage compartments up to 180° from the transfer direction. This lets unused links be stored vertically or horizontally next to the used or loaded portion of the chain. These attributes are routinely exploited in the entertainment industry where stage-lifting devices once relied on hydraulics or screw jacks. Both of these require pits up to 25-ft deep to store the retracted cylinders. Rigid Chain can be stored in a more-compact magazine, eliminating the need for deep pits. This is less expensive and removes the possibility of hydraulic leaks and environment damage.
In one instance, Rigid Chain let a welding company cut storage requirements for a linear-motion subsystem by 66%. The chain positions a gauge box that holds four plates being welded together into a telescoping crane boom. The overall length of the actuator went from 100 to 36 ft as the need to store a cylinder was eliminated. The chain also had no problem withstanding weld splatter and abrasive dust.
In another application, four Rigid Chain lift columns installed on a cruise ship move an entire bar — complete with water and electrical hook-ups — up and down 35 ft between two different decks. The biggest design challenge in this job was transferring the load carried by the chain back into the ship’s structure without the drive and lift equipment taking up too much space, which is always at a premium on a cruise ship. Because the chain can be coiled, it could be stored in the space between decks. Each of the four columns carry 10 tons of dynamic and static load, and travel in guide tubes. The tubes carry all the loads, even those caused by the motion of the sea.
Reliable, precise and repetitive: Rigid Chains can position loads with resolutions of about 1 mm, loaded or unloaded, and will never lose this capability due to wear or age. For example, a company building satellite-simulator equipment needed a way to lift a 6,000-lb load on a 5.5 × 14-ft platform and hold the load in position — 13 ft up — for up to a week without power. They ended up with a scissor-guided platform lift with a vertical chain providing the lift. The chain attaches to the platform and lifts it directly instead of pushing horizontally on one leg of the scissor. Pushing the leg at a constant speed would lift the platform, but the vertical speed would continually increase the higher the platform rose. In this case, the scissors act as guides and are independent of the motion.
Immune to harsh environments: Rigid Chain can withstand a variety of harsh environments, including extreme temperatures. The standard chain material, semihard carbon steel, handles temperatures up to 200°C (392°F). Upgrading the material lets them operate in temperatures up to 900°C (1,652°F). And using coatings or stainless-steel chain links let them survive in corrosive environments.
For instance, chain drives are used to quickly lift 24-in.-diameter steel pipes fresh out of the mill and still red hot (1,600°F), and move them out of the way to keep up with production cycles. The chains have no problem with the temperatures and dirty steel-mill environment.