spiral conveyor system is a great way to efficiently use plant floor space in food processing and other applications. Often,

applications that require a lot of product dwell time (such as freezing, cooling, proofing, etc.) utilize a spiral configuration to keep throughput levels high without cutting the process short and sacrificing product quality. There are a number of different types of spiral conveyor systems, each with its own set of considerations, advantages, and potential trouble spots. In this whitepaper, we will discuss three of the most popular spiral conveyor systems in use today: Low Tension, Direct Cage Drive, and Edge Drive.

Low Tension SystemThe construction of a low tension spiral cage system includes a conveyor belt that wraps around an inner rotating drum (also referred to as a “cage”). The drum makes contact with the inner edge of the conveyor belt, helping to drive it forward through a combination of friction and traction around the perimeter of the drum. Low tension spiral systems

have been in use for decades and, for good reason, have long been the de-facto standard for the construction of spiral systems. These systems can be fairly easy to commission and maintain once installed. There are

Spiral Conveyor Systems

just a few parameters to set or adjust, those typically being the amount of slip between the rotating drum and the inside edge of the conveyor belt, the amount of belt in the take-up, and the location of sensors used to monitor the position of the belt in the system. Sometimes it can be helpful or even necessary to apply lubrication to the support rails of the conveyor to limit or reduce the belt edge tension in large or heavily loaded systems.

With or without lubrication, maintaining cleanliness of the spiral conveyor is extremely important as contaminated support rails and/or drum can cause significant increases in belt tension which will negatively affect the overall or expected service life of the belt. If severe

...have long been the de-facto standard for the construction of

spiral systems. These systems can be fairly easy to commission and

maintain once installed.

ABy: Tom Perdue, Product Development Manager

enough, neglecting to regularly clean the conveyor can cause possible damage to the spiral conveyor structure itself.

Belt edge tension on a well-built and maintained low tension cage system can be expected to be fairly constant throughout most of the tiers within the spiral, though variations in tension can be expected to occur at the lower and upper tiers due to “edge effects” of the system. Depending on the length of the infeed and discharge sections, along with the return path of the belt, belt edge tension within the first few tiers can be appreciably higher or lower than the general belt tension within the stack. It can generally take up to 3-5 tiers of belt wrap for the edge tension to equilibrate within the spiral. Similarly, as the belt nears the exit tier of the spiral, the edge tension can begin to increase as the system is transitioning between the engagement between the belt and the rotating drum to a straight linear pull of the belt by the sprockets within the take-up system. These variations are normal and should be anticipated in this type of spiral conveyor.

Configurations

When the term “configuration” is used in reference to a spiral conveyor, it typically refers to some combination of the overall direction of the belt within the spiral (up or down) and the return path (offset). For a low tension system, the three common infeed/outfeed configurations are as follows:

• Up-Go System: Designed so the product enters the spiral cage at a low elevation and discharges at a higher elevation.

• Down-Go System: Product enters the spiral at a high elevation, travels down and is discharged at a lower elevation. This system often has a higher tension as compared to an Up-Go system.

• Up-Go, Down-Go System: There is more than one spiral cage in

this configuration. Starting from the bottom, the product enters the first spiral, travelling up. At the top, the product transfers to another spiral, travels down and discharges at the bottom. This type of system is typically implemented when the product requires a long period of time in the application, or throughput volumes require it. Over the years, advances in system controls have permitted the successful operation of Up-Go/Down-Go systems using only one belt and one take-up, thus eliminating the transfer between the two spirals.

The return path that the belt must take depends not only on whether the system is an Up-Go or Down-Go spiral, but also on the system offset. The offset is the angle in which the product offloads relative to where it is loaded. The system offset is generally dictated by the layout of the spiral within the flow of the overall manufacturing or processing line, as sometimes the spiral is used to redirect the product in another direction as it exits the spiral conveyor. There are four typical types of offsets, as shown below. Generally, the straight through setup (no offset) is best for achieving the lowest belt tension levels.

Cage Drive Bar Caps and Overdrive

For a low tension spiral conveyor operating with a metal belt, the drive bar caps are an important component for an effective system. The drive bar caps are installed on the cage structural uprights and provide the important contact interface between the rotating cage and the edge of the belt. Depending on the equipment manufacturer, a variety of drive bar caps shapes, sizes, and center spacings are used. Some caps are flat faced, others come with beveled edges, while some may have vertical grooves in

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Straight Through

180° Offset 270° Offset

90° Offset

much overdrive can cause excessive product movement, belt surging, or a high rate of surface wear on the drive bar caps.

Common Trouble Spots

As with any system, understanding its trouble areas can improve preventive maintenance and avoid unplanned downtime. With a low tension spiral system, common areas of concern include:

• Elevated Tension Levels: An overloaded system, worn components (drive bar caps, support rails, etc.), low overdrive, and/or a dirty system can cause belt tension levels to increase. High belt tension, if not corrected, can lead to premature failure of belt and sprockets.

• Gouged or Excessively Worn Drive Bar Caps: Drive bar caps can become gouged or deeply cut if one or more splice rods on a metal belt is installed incorrectly, or if the rotating drum is not properly

the area of the cap that contacts the belt edge. The purpose of the various shapes is to provide a smooth, but effective engagement with the inside belt edge to propel it forward with the least amount of contact pressure (normal force) between the belt edge and the caps. The drive bar caps are made from UHMW (Ultra-High Molecular Weight Polyethylene), a common material used for other conveyor components such as wear strips, sprockets and rollers.

One exception where cage bar caps cannot be used are in those spiral systems that operate in extremely low temperatures (cryogenic). In that case, the temperature is too low for the plastic caps and the metal belt contacts the metal uprights of the spiral cage directly. Additionally, for many low tension systems operating with plastic belts, no cage bar caps are used as the metal-to-plastic interface is obtained with the metal structural uprights of the spiral cage alone.

Overdrive, often referred to as slip, is recognized as a key indicator of a system’s belt tension level. It is important as it allows the belting to slip on the drive bar caps as it climbs or descends the cage. Overdrive, by definition, is a measurement of the amount to which the inner drum rotates as compared to the belt. This value can be reported as inches (or mm), but is often normalized based on the width of the belt or the vertical tier spacing. An appropriate amount of overdrive in a system of this type is typically 3-5 tier spacings, though other overdrive settings are sometimes used. Too little overdrive can result in unnecessarily high belt tension; too

centered and is contacting a structural part of the conveyor. Excessively worn drive bar caps can be the result of excessive slip between the drum and the belt or rough edges on the inside edge of the belt. Cut bar caps will often generate loose plastic shavings; worn bar caps will generate plastic dust. Bar caps that have been damaged usually will not provide optimal drive to the belt and may prohibit the belt from engaging or functioning properly on that system.

• Surging or Pulsing: Belt surging is a type of slip-stick pattern of movement and usually is observed at the system infeed section. It is typically a function of the infeed length, the belt tension at that location, the belt speed, and the elasticity of the belt. For freezer applications, sometimes belt surging is seen at room temperature, but not at operating temperature, or perhaps vice versa. Regardless, it is best to address the surging when the system is at operating temperature, and this is most often

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As with any system, understanding its trouble areas can improve preventive maintenance and avoid unplanned downtime.

steps have been taken to resolve many of the technical issues inherent with a direct cage drive system. In this design, the belt’s inner edge is driven, and the cage bar caps are specially shaped to make the cage essentially function as a large sprocket, making it useful in situations where a positive engagement to the belt is beneficial, such as dirty or slippery environments. Because of the usage of an inner cage, this system can be retrofitted from a low tension system much easier that of other spiral systems, thereby increasing its appeal to a lot of users.

With a direct cage drive system, there is no overdrive because the belting is engaged all the way around the cage. The belt must operate with some amount of edge tension in order to keep the belt tightly engaged with the rotating drum. However, because of the positive engagement between the drum and the inside belt edge, these systems can operate at a much lower belt edge tension, thereby increasing the effective service life of the belt. The key to a successful operation of this type of system lies in the proper initial engagement of the belt with the vertical drive bars. This requires a special setup of the system including special components to guide the belt properly as it feeds from the infeed into the

done by reducing overdrive to slightly increase the tension of the belt. Pulsing is sometimes seen within the spiral tiers themselves where the belt seems to grab and release from the drive bar engagement. It can be caused by a bad belt edge, bad drive bar caps, or a cage that is out of round. Sometimes there are easy corrections for this such as locating a defective component or repairing a section of belt. Where that is not the case, it may be necessary to adjust the overdrive setting or if the problem is severe enough, consider adding lubrication to the conveyor rails. For a low tension system, only the middle and outer rails should be lubricated. The very inside rail should not be lubricated as it is too close to the rotating drum creating a risk of oil getting on the surfaces of the drive bar caps, which would likely cause very high belt tension.

Direct Cage Drive SystemA direct cage drive system is similar to the low tension system, except there is positive engagement between the rotating drum and the inside edge of the belt, thus no overdrive. Systems of this type have been in existence for many years, but their application base has been very small due to operational limitations. But this situation has changed significantly in the past few years as some innovative

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first tier of the spiral. This is often done by tapering the toothed section of the drive bar cap to ease the belt into position as it begins to wrap around the rotating cage.

Configurations

Permitted configurations for the direct cage drive system are essentially the same as with the low tension system since they both depend on the inner edge drive provided by a rotating center drum.

Common Trouble Spots

Drive Bar Cap Engagement: As discussed above, the initial proper engagement between the drive bar cap and the inside edge of the belt is critical for the proper operation of this type of system. Adjustments to the system including replacement of components may be needed at times. Otherwise, the system may operate erratically and/or operate with excessive belt tension at certain locations if not throughout, thus negating the benefits this system was intended to provide.

...in the past few years as some innovative steps have been taken to resolve many of the technical issues inherent with a direct cage drive system.

Configurations

• One of the key features of the edge drive spiral system is the flexibility it offers in terms of configurations. Not only can this system operate in the standard Up-Go or Down-Go configuration, but it can be configured to operate as an Up-Go/Down-Go on the same spiral, or operate using two separate belts on the same conveyor, but running at different speeds. None of this is possible using a system with a rotating drum.

• Single Asymmetrical Belt Configuration - With this design, only one edge of the belting is intended to engage with the sprockets. This means that the

Edge Drive Spiral SystemEdge drive spiral systems are positively driven conveyors that are unique in that the belt is only driven from the outside edge through the engagement with one or more sprockets located on each tier. The sprockets are driven by a shaft that is positioned vertically on the outside next to the spiral system, while the belt itself is supported against an outermost wearstrip. Because the belt on each tier is driven, there is no need for a separate take-up tower due to the fact that each tier or part of a tier retains its own slack belt, similar to a catenary. Though this type of system has its share of complexities, the conveyor itself is usually considered to be of a simpler design overall when compared to a rotating drum system due to the absence of the drum and the take-up, along with the motors/gearboxes/drives/controls that are associated with those components.

belting only goes in one direction on the spiral, and that direction must be specified before the belt is manufactured. Because the belting requires fewer components with this system, it is generally less expensive and is the most common construction. However, it is also limited in the versatility it can offer.

• Single Symmetrical Belt Configuration – The key benefit of this system is the ability of the belt to travel both up and down through the same spiral, and thus operate within a much smaller floor footprint. With this configuration, both edges of the belt engage with drive sprockets, but it moves in a clockwise rotation going in one direction and counter clockwise in the other. To complete this path, the belt must be looped around during the transition between travel directions such that the outside edge becomes the inside, and vice versa. On a functioning system of this type, the belt is moving in opposite directions on every other tier on the spiral. In a typical Up-Go and Down-Go configuration, the product enters the spiral at the bottom while the belt is driven on one edge by the sprockets to the top of the spiral. At this point, rather than discharging the product and returning to the infeed, the belting swings out and loops back into the spiral. A separate shaft of sprockets then takes over the belt engagement and the product

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...unique in that the belt is only driven from the outside edge through the engagement with one or more sprockets located on each tier.

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moves in a Down-Go fashion, but in opposite direction from before, to the bottom of the spiral. The versatility afforded with this system does include more components than the asymmetrical system, and therefore can be more costly. But this configuration can drastically decrease the necessary floor space when compared to a more traditional spiral conveyor.

• Multiple Belt Configuration - In this configuration, multiple belts (typically feeding from multiple production lines) travel into and out of the system. The Multiple Belt Configuration, though generally more expensive, offers the greatest level of versatility of any spiral system and this versatility can help save on energy costs, capital expenditures and facility footprint. Multiple production lines feed into one freezing, cooling or proofing system, and travel in one or both directions transferring to the next step in the production line. Additionally, belts in this system can travel at different speeds to accommodate a variety of product dwell times.

Common Trouble Spots

• System Setup – for an edge drive system to operate properly, it must be properly setup from the start. A proper setup means having the correct amount of belt on each tier or partial tier between the points of sprocket engagement, along with the proper distance between the outer edge of the belt and the sprockets. If the sprocket is located too far from the belt, the belt can skip a tooth during attempted engagement. If the sprocket is too

close, it will pinch the belt between the sprocket tooth and the support rail causing unnecessary flexing of the components and wear. For a proper overall system setup of the belt, there must be adequate belt between each drive sprockets. There should be sufficient belt to allow the belt tension to completely unload as the belt disengages from the sprocket at every drive location. That generally means having an additional 1-3 pitches of belt extra at each location. If there is too little belt, there is the likelihood that the belt will be out of phase with the sprocket and skip a tooth. If there is excessive belt at any location, the belt may ‘tent-up’ and could possibly disengage from the outer rail.

• Edge Breakage: Some belt designs have very light weight edge components. Because they are not terribly robust, these pieces can fatigue or snag leading to breakage. When selecting a belt for an edge drive system, consider the edge of the belt carefully to ensure it can withstand being driven in that manner.

• Light Weight Conveyor Design: Some edge drive systems are made with very light weight structural components. While this may be economical initially, it can lead to conveyor and/or belt damage or poor operation. This is especially true at the engagement point between the belt and the sprocket on each tier. As the sprocket engages the belt, the tension on the belt acts against the outer support rail, forcing it inward if possible. In addition, there is a natural rejection force component between the belt and the sprocket

teeth that also pushes the support rail inward toward the center of the spiral conveyor. If the rail structure is insufficient to counteract these forces, the belt could jump the sprocket at this engagement point, eventually affecting the setup of the entire system. Be sure to select components with the necessary rigidity and operational life that will meet the necessary system requirements.

In closing, there is much to consider when determining what spiral conveyor system is best overall for any given application. Fortunately, there are more excellent choices available today than ever before. When the time is right to make a decision, be sure to consult with your conveyor belting supplier to ensure the system and belting selected is the best fit for your product, floor plan, and budget.