When designing an engineered chain drive for a conveyor, not any sprocket will do. Choosing the right one optimizes the sprocket-chain interaction, ensures peak drive performance, and eases maintenance.
Engineered chain offers an alternative to roller chain in many heavy-duty applications, such as conveyors, drum drives, and elevators. These applications often require engineered chain drives because they are more durable and able to withstand adverse operating conditions.
On the other hand, these systems operate at lower speeds, which means that the engineered chain and sprockets can be less precise than roller chain types.
To ensure the best match of chain and sprocket for optimum performance, obtain both components from the same manufacturer, if possible. Also, make sure they are both manufactured to a well-accepted standard such as ANSI B29.1.
There are many types of sprockets to fit the various engineered chain versions, and the number of choices may surprise you. In general, these sprockets have hub configurations similar to those used with roller chain. Spokes or lightening holes are commonly used to reduce the weight of these sprockets, which are often larger and heavier than roller chain sprockets.
To accommodate special needs, the options go beyond those found in ANSI standards. Following are some of the specific types that are designed to handle different installation and operating conditions.
One type of sprocket is split into two 180-deg segments, Figure 1, to simplify installation and removal, particularly of large sprockets that are installed between bearings. This lets a technician place them on the shaft and bolt them together without removing bearings or disturbing the shaft alignment.
Sprockets with removable rim segments, Figure 2, enable replacing wear surfaces without removing the sprocket hubs or bearings, or moving the shafts. These come in both split and non-split versions.
Wide-flange, drum-type sprockets, Figure 3, support the wide offset chain commonly used in drag conveyor applications. These sprockets are generally made of cast steel or flame-cut steel plate.
Another type of sprocket, called a traction wheel, Figure 4, is used in bucket elevators instead of conventional chain and sprockets. It has a smooth OD, rather than teeth, and it functions by frictional engagement with the bushings in the chain links.
Elevator drive applications tend to cause rapid tooth wear due to a scrubbing action between conventional sprocket teeth and the chain rollers or bushings. Traction wheels eliminate this effect, so they last longer. Also, many of them have segmented rims to facilitate replacing wear surfaces.
In the event of an obstruction or overload, the chain slips on the OD of the traction wheel, thereby preventing damage to elevator components. Because slippage may generate heat and sparks, don‚Äôt use traction wheels in applications where an explosion might result, such as in grain elevators.
The frictional grip of a traction wheel must be sufficient to transmit the power needed. Therefore, make sure elevator height, material lubricity, and chain tension are within a range that ensures the necessary traction.
Teeth come in different shapes to accommodate various types of engineered chain and operating conditions. For example, the tooth profile for engineered drive chain, Figure 5, is approximately the same as for roller chain. But conveyor chain and engineered bar chain require different profiles. Sprocket tooth profiles for engineered chain are specified by ANSI Standard B29.10.
Some sprockets used with longpitch conveyor chains resemble bar chain sprockets but include pockets at the tooth corners to accommodate the chain rollers. Conveyor chains normally have attachments on the top side, which, if used with a drive chain sprocket, would interfere with the tops of the teeth. Therefore, make sure the sprocket matches the chain type.
Engineered chain drives often operate in contaminated environments, which requires a modified tooth form to accommodate foreign elements that get between the chain and sprocket teeth. Unlike a roller chain sprocket tooth, which has minimal pitch line clearance, the sprocket tooth for engineered chain is narrower to provide more clearance, Figure 6. This clearance between the tooth and the chain roller or bushing allows contaminants to pass through.
Accuracy. The operating characteristics and speed of roller chain and engineered chain call for different levels of sprocket tooth accuracy. Steel or cast-iron sprockets with machined teeth are typically required in high-speed roller chain or silent chain drives.
But the long chain pitches and slower speeds of engineered chain usually do not require the precision of machined teeth. Sprockets with cast or flame-cut teeth provide adequate life and cost less.
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Single or double cut. Sprockets for roller chain are either single-cut or double- cut to enhance wear life. With a single- cut sprocket, the pitch (distance between tooth centers) equals that of the chain links. Each tooth engages a chain roller during every revolution, and the number of effective teeth (those that engage the chain during one revolution) is the same as the number of teeth on the sprocket.
With a double-cut sprocket, the tooth pitch is half that of the chain. Therefore, the number of effective teeth is half the number of actual teeth. If the sprocket has an odd number of teeth, the number of effective teeth is fractional, and each tooth engages the chain only on every other revolution. This cuts tooth wear almost in half, nearly doubling sprocket life. The same is not inherently true for sprockets with even numbers of teeth. But you can achieve the same effect by periodically advancing the position of the chain on the sprocket by one tooth (a manual operation). This moves the teeth that initially did not engage into a position where they do engage after the advancement.
Engineered chain sprockets achieve the same effect with a ‚Äúhunting tooth‚ÄĚ design. The sprocket is made with an odd number of teeth and a tooth pitch of onehalf the chain‚Äôs pitch. Because the chain links engage only alternate teeth, each tooth on the sprocket engages the chain only on every other revolution, again effectively doubling sprocket life.
Sprocket materials include cast iron, an economical choice for moderate service; chill iron, in which rapid cooling of the teeth creates a hard surface that resists abrasion; and either cast steel or fabricated steel (with flame-cut teeth), which accommodates higher loads.
For corrosive environments, stainless steel sprockets are available. Sprockets made from cast urethane and other polymers are used in wastewater and foodhandling applications.
For long life, sprocket teeth may be flame-hardened, carburized, or carburized and induction-hardened. The effectiveness of the heat treatment depends on both the steel hardenability (carbon content and alloy), and the process. Flame hardening (followed by quenching) is the simplest and most common method but provides the least depth of hardened wear surface. To obtain more accurate control of heat distribution, induction hardening can be used.
For medium carbon steel, either method alone produces a shallow case depth (typically 1/8 in. at 40 Rc, depending on the process) that gives moderate wear protection. A combination of carburizing and induction hardening provides higher hardness (about 60 Rc) and depth (typically about 1‚ĀĄ8 to 1‚ĀĄ4 in.) for maximum wear resistance.
In most applications, sprockets limit the effective life of a chain because of the amount of chain elongation they can accommodate. Once the chain wears and elongates beyond about 3 to 5% for a drive chain, and 3 to 6% for a conveyor chain, it no longer engages the sprocket teeth properly. Instead, the longer effective pitch causes the chain to ride up on the sprocket teeth, Figure 7, eventually jumping teeth.
To determine chain elongation, count the number of chain pitches in a five or 10-ft section. Comparing the measured length with the original length (when new) gives the percent of elongation. For example, 20 pitches of a 3-in. pitch chain measure 60 in. when new. If the chain now measures 63 in., then 3 √∑ 60 = 5% elongation.
Sprockets must be replaced when wear extends through the hardened tooth areas. When replacing chain, replace the worn sprockets as well. Otherwise, the worn sprocket teeth will accelerate wear of the new chain.
For information on engineered chain sprockets from Rexnord Corp., circle 315 on the reader service card.
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Tom Beranek is a product specialist, Rexnord Corp., Engineered Chain Div., Milwaukee, Wis.