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BEAD CHAIN DRIVE SYSTEMS

     
     
General Considerations      

Bead Belt drives using either metallic bead chain or plastic bead belt provide positive motion.  They differ from the usual chain drives in that they can transmit power in more than one plane.

Applications
These drives are used primarily in apparatus operating at slow speeds or intermittently.  Because they are, essentially, positioning devices rather than power drives, they cannot be considered as direct substitutes for medium or high speed gear and chain drives.  Bead belt sprockets are rarely operated at more than 100 rpm, with linear speeds not exceeding 25 fpm, except for short durations.

Typical applications of bead belt drives include instrument dials, chart-recorder drives, paper transports on office copiers, animation mechanisms for displays and toys, conveyor belts for drying ovens, timers and counters, window shade operators, vending machines, teaching devices, and photographic processing equipment.

Metallic Bead Chain
Bead chains consist of hollow metal balls connected by hollow dumbbell-shaped links.  They are available in brass, nickel-plated brass, carbon steel and stainless steel.  They are accurately sized for drive applications.

As with all chain devices, the pitch distance of bead chain tends to increase with use due to stretch, wear or realignment of beads and connectors.  This may require the use of idlers or other take-up devices on long chain assemblies.

Metallic bead-chain is usually limited to slow-speed applications where the number of cycles is limited.  Because cycle life is determined by the ability of the drive mechanism to operate with increasing length of loop, they are best used with manual drives which involve positioning rather than motor-driven drives requiring continuous motion.  Lubrication can improve life and performance; tension devices allow for increased chain length.

Plastic Bead Belt
Solid beads of plastic are molded onto a cord at uniform spacing to form a plastic bead-belt.  Thus, a bead belt is essentially a one-piece entity containing no loose or moving parts.

Physical Characteristics
Bead belt is available from stock with polystyrene beads molded onto polyester braided cord.  Other materials are available on special order.

Applications
A special advantage of plastic bead belt is that the increase of pitch distance is limited to that occurring in the base cord.  Because the cord is somewhat elastic, bead belt can be stretched slightly on installation.  The initial tension in the cord is retained for a considerable number of cycles.

Plastic bead belt is used where the required number of cycles exceeds the capabilities of metallic bead chain and where the higher strength of bead chain is not required.  It can also run at somewhat higher speeds than bead chain.

Another advantage of plastic bead belt is that it is electrically non-conducting.  Consequently, it finds use in applications where various elements of the drive must be electrically isolated from each other.

Sprockets
Although the basic designs for metallic bead chain and plastic bead belts are the same, they require slightly different sprocket design.  While bead chain and bead belt may have the same nominal pitch when extended, the metallic bead chain tends to decrease in pitch when wrapped around small diameters.  Because of the internal geometry of the hollow bead, the connector is forced farther into the bead as the chain is bent around the circumference of the sprocket.  Pitch distance therefore decreases as the chain enters the sprocket, and returns to nominal as the chain leaves the sprocket.

Plastic bead belt, however, behaves essentially as a cord; the amount of foreshortening is much less than for bead chain.  Thus, small diameter sprockets designed for one material will not fit the other.  As sprocket diameters increase, pitch diameters for the two materials become more alike.

Molded sprockets are usually made in two pieces with integral snap pins for assembly.  At present these are available in nylon only.  Stock sizes of machined brass sprockets have a minimum of 12 bead sockets for each size of metal bead chain.  Sprockets with fewer bead sockets would not permit the chain to wrap around the circumference because of foreshortening of the chain.  Although plastic bead belt can be used with smaller diameter sprockets, such sprockets are not stock items.

Sprockets up to approximately 3 " are generally stocked, but sizes up to 7" can be ordered.  The maximum number of bead sockets which can be produced on available equipment is 120, thus permitting direct ratios of 10 to 1(120 max to 12 min).  Although only brass and nylon sprockets are maintained in stock, other machinable materials are available.  Sprockets of aluminum, stainless steel and carbon steel are commonly manufactured.

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Last modified: 02/28/08