Measuring Overdrive on a Low Tension Spiral Conveyor

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In a low-tension, friction-driven spiral cage conveyor system, overdrive is the difference in relative speeds between the cage drum and the belt. This is also commonly referred to as “slip” because it allows the belt to “slip” on the drive bar caps as it climbs the cage while it’s driven. In all instances of a low-tension, friction-driven spiral cage, the cage itself must travel faster than the belt. A healthy overdrive level will typically reduce tension in the belt and increase the life expectancy of the drive bar caps while providing a more reliable production schedule.

To determine overdrive, you should first obtain a drive bar count and then run it through a quick calculation as is described below.

Obtaining the Drive Bar Count

To begin, position yourself at a stationary object on the spiral cage such as a post of the frame on the outside of the spiral conveyor.
Next, locate an identifiable marker on one of the vertical drive bars (or place a distinct mark on one of the drive bars). Be sure that this marker can be clearly seen as the cage rotates past you.
Allow the drum to rotate, and when you see your identifiable marker align radially with the stationary object on the conveyor, place an object (such as a tape measure) on the belt at the point of alignment at that point of alignment. (Note: if the application uses circulation fans that are running as you are taking the overdrive measurement, be sure to select an object to place on the belt that is heavy enough so that the airflow will not remove or re-position it before you can complete the next step.)
Let the cage continue to rotate for a complete revolution, and when the identifiable marker rotates past your stationary point, begin counting how many drive bars pass until the object you placed on the belt also passes the stationary point.

Calculating Overdrive

Once you have the drive bar count, there are two calculations for reporting overdrive: overdrive in inches, and tier spacings of overdrive. Both calculations are used to determine if the overdrive on your spiral cage system is at a healthy level of operation.

Overdrive in Inches

Overdrive in Inches = (Drive Bar Spacing) x (Number of Drive Bars Counted)

Tier Spacings of Overdrive

Tier Spacings of Overdrive = (Overdrive in Inches) / (Tier Spacing in Inches)

The range of normal overdrive values can vary based on whether your system is an Up-Go or a Down-Go configuration (see the table below), though this can change according to the particular application. If you find your overdrive is outside of the recommended range, contact your belting supplier to help determine if some adjustments to your spiral system are needed.

Knowing your level of overdrive is important to ensuring your spiral conveying system is in good working order. For assistance with your spiral conveyor, contact our technical support group at 800-638-9560. Also, you can find more information on our turn and spiral conveyor belt products on our product page.

Tom Perdue

Tom Perdue is an Engineering Manager in Cambridge, MD. He is an industry veteran bringing extensive knowledge to our metal conveyor belting portfolio, including spiral cage systems.