Choosing rigid chain for power

Oct. 1, 2008
Linear motion is challenging wherever industrial workflow logistics must be efficient, machines are deployed, goods stored, and loads are moved or lifted.

Linear motion is challenging wherever industrial workflow logistics must be efficient, machines are deployed, goods stored, and loads are moved or lifted. One technology, rigid chain, combines the mechanical strengths of conventional chain with those of jacks — for flexibility as well as the ability to push and pull loads.

How does rigid chain work? Each link in a rigid chain includes a hook-like extension called a shoulder that interlocks with those of neighboring links when force is applied on forward thrust, allowing power transmission. At their cross axes, the links are flexibly connected as in ordinary chain, allowing the rigid chain to bend and coil for efficient storage.

Locking and unlocking

Power is applied to rigid chain much like it's applied in other chain and synchronous belt systems — — through a pinion on a drive shaft. As shaft rotates pinion, its teeth engage with rollers on the links' cross-axes, moving the chain forward or backward link by link. Both shaft and pinions are integrated in a chain drive housing. As chain moves out of this housing, it pushes attached load forward along a stroke path. Similarly, when the pinions rotate the other way, the rigid chain pulls the load back towards the housing.

Inside the drive, the chain's path through the housing is defined by guide and reaction plates, which counter thrust resistance and direct the links precisely onto the stroke path. Plate operation is simple: On the cross axis of each chain link, between the drive rollers, sits a big central roller. These central rollers run between the edges of the guide and the reaction plates, which serve as rails to hold the chain links on the track.

Once a chain link goes around the pinion and exits the curve, it reaches a point at which it comes into line with the stroke path. This is where forward thrust is applied to the link and subsequent pieces of chain.

At the chain's end, a special front link shifts the target of thrust above the axis of articulation to create a moment that locks the link shoulders. Thus, the shoulder axes are used for pushing while the axis of pulling remains at the height level of the cross-axes, on the axis of articulation.

Incorporating rigid chain

Though there are differences in design of components and overall structure, rigid chain's basic operation is the same for vertical applications and lift systems as it is for horizontal motion. For the sake of simplicity, let's review how to design with the latter.

On August 8, 2008, 8:08 pm, the moment had finally come: Inside the Bird's Nest, floating platforms lift smoothly, hundreds of dancers, light, and rhythm fill the stadium space — and the 2008 Olympic Games in Beijing have officially begun.

  1. Determine total load

    The first step in selecting a rigid chain system is to calculate all system forces — including the weight of items moved, friction, acceleration, and deceleration forces. Total force Ft to be applied to the chain is the sum of friction force Ff acceleration or deceleration force Fa and external forces Fe. Friction force is:

    Ff = W × f, Newtons

    where W = Load's weight force

    f = Friction factor

    Depending on surface properties and what kind of device is used (rolling or sliding, for example) the following values account for the most common situations: For dry sliding steel on steel f = 0.3. For sliding steel on plastic of PE/HD 500 f = 0.2. For roller wheels f = 0.05. For roller balls, f = 0.025. For caterpillar-type roller assemblies, f = 0.07.

    As with any mechanical solution, the design process must include consideration for the effects of acceleration or deceleration — for example, impact or shock loading when using a mechanical or emergency stop device.

  2. Consider drive hosing and chain storage options

    Chains can be stored in either vertical or horizontal magazines. Location of this chain storage affects overall system layout and chain selection. Housings can be designed for various drive and directional requirements.

    A chain drive housing consists of a drive shaft (to which an external drive is connected), pinions to apply force to the chain, and a guide and reaction plate to keep it on its track. One type directs the chain around a 90° curve, while another directs it around full 180° turns. One chain needs only one drive — however, it may be necessary to change chain direction on the way from storage magazine to drive. For this reason, chain redirection housings are similar to drive housings, without the drive shaft and pinions.

  3. Design the system

    A standard rigid-chain system may include just one chain working by itself or two chains working in parallel. For single-chain systems, force should be applied at the center of load for balance and to minimize eccentric loading. Under heavy load or long stroke, rigid chain must be guided.

    When load is large, dual-chain systems are often the most effective way to achieve optimal stability and positioning accuracy. Force is distributed over the two chains and the extra stability may make it unnecessary to guide the load. The two parallel chains, linked with a pushing bar that serves as a yoke, allow for more tolerance when contacting the load. Usually this bar suffices to keep a load on its track.

    Simple rigid chain has two rows of link plates and thus two rows of shoulders. Duplex versions have three rows, while a joined version has four. Side plates can either be cranked so that they bulge at one end, or two component plates can be riveted together. Cranked links can also be reinforced by assembling two mirroring plates together — a cost-effective way to boost stability.

  4. Think about chain guides

    Every rigid chain transmits a certain maximum force, depending on its structural properties and stroke length. When pushing, the chain works much like a homogeneous bar. It reaches its limits of stability when stroke exceeds a certain length. Therefore, specified maximum force cannot be guaranteed for strokes longer than one meter, unless the chain is guided.

    At a greater distance from the drive, and with no reduction of force, an unguided chain may buckle sideways and upwards. In contrast, guided chains maintain their nominal capacity over any length.

    When space constraints make it impossible to use guides, installation with shoulders down provides better stability, because the interlocking shoulders receive additional support through their contact with the worktop.

  5. Choose the right chain

    The capacity of unguided chain depends on whether it can be coiled upward or downward, and therefore operate with shoulders up or down. Capacity also depends on stroke length. (Capacity charts are useful here.) If guides can be used or stroke does not exceed one meter, select a chain according to its maximum capacity.

    Standard chain material is semi-hard carbon steel, as it's suitable for ambient temperatures up to 200°C. For high-temperature applications (loading furnaces, for example), chain made of HT steel is recommended and can withstand up to 900°C. For corrosive environments, chain of stainless steel or with coated surfaces is suitable.

  6. Look up catalogue data

    Once an appropriate chain type, location, and storage method are selected, horizontal rigid chain catalogues detail information on dimensions, weights, guides, drives, and redirection housings. Technical drawings are also presented in printed catalogues and online. Size is determined by the chain's pitch — defined as the length of each link, measured between two consecutive cross axes. Standard pitches are 40, 60, and 90 mm.

  7. Calculate chain length

    This value is the stroke length plus a few additional links engaged by the drive pinions. If a chain's rear end is attached to a drive housing, a few additional links are required. Length L is usually expressed in number of links:

    where B = Blind stroke length (depending on load's start position)

    U = Useful stroke length, mm

    p = Chain pitch, mm

    X = Number of additional links that remain in the drive housing or are used for rear-end attachment

    For 90° and 180° drive housings with no rear-end attachment, three links are added for X = 3. For a 90° drive housing with rear-end attachment, X = 6. For a 90° drive housing including a 180° return with rear-end attachment, X = 10. If necessary, round the result to the next-higher integer.

  8. Choose an attaching device

    For push-only operations, there's no need to fix the load to the chain; provided that the load moves on a defined path, contact with the chain's front link is enough. On the other hand, if the load's course is not particularly stable, or the load needs to be pulled as well, a contacting or attaching device is required. The chain's front link has a pushing point and a point to which the load is attached. The latter is situated on the pulling axis and can be used to install a hook that engages automatically with an interface mounted to the load. Disconnecting may be automatic or manual.

  9. Calculate drive power

    Rigid chain systems can be driven with electric, pneumatic, or hydraulic gearmotors, as required by the application. Often, motors can be integrated into rigid chain designs.

    To calculate required motor power, first determine the application's drive moment and revolutions per minute. Drive moment M is based on total thrust force:

    where Ft = All forces in effect

    p = Chain pitch, mm

    0.8 = System efficiency constant

    The number of drive revolutions R is based on required speed:

    where S = Speed, m/min.

    6 = Number of pinion teeth

    p = Chain pitch, mm

    Output power is:

  10. Fill out an application questionnaire

    The last step in rigid chain selection is to complete an application form with performance requirements and as many details as possible, and submit it to the manufacturer. For special inquiries, a call to the manufacturer is also recommended.

For more information, call (586) 274-0774 or email [email protected].

Beijing Olympics: Stage technology breaks records

Serapid USA Inc. supplied LinkLift 100 lifting columns to elevate the stage area in the Beijing stadium, measuring 11,625 ft2. The stage is divided into 10 platforms, their unusual shapes following the ideas of UK-based event designer Mark Fisher. Beneath the imposing surface, the lifting equipment rises to 23 ft, carrying up to 595 tons. The platforms are independent of each other and freestanding, so the lifting column must do without the aid of external guides. A triple-scissor mechanism boosts stability and keeps lateral drift under 5 mm, even at full extension.

LinkLift is used in other stage installations and has won several design awards, one reason why Beijing Special Engineering and Design Institute used the recognized lift systems for their opening-ceremony production.

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