As manufacturing automation rapidly advances, conveyors are not only being used for transportation but also accurate material (or product) placement. They are often required to deliver product to a precise location consistently. When transporting product by conveyor, equipment choice will greatly impact the system’s ability to reliably deliver to a target position — either stationary (indexing) or dynamic in nature. This article explores some common conveyor types and compares the ranges of positional accuracy you might expect with each type.

Below, we will use results published by Dorner Manufacturing in 2019 for comparisons between crowned pulley and knurled v-guided conveyor performance, plus published data from various conveyor manufacturers. Multiple factors can impact the final product position. However, for this discussion, it is assumed that the material can be placed accurately and travel without slippage on the belt surface, and with no additional fixturing used. Conveyor Top Chain

Crowned spindle conveyors have been used to move coal and ore since their invention in 1892 by Thomas Robins. In 1913, Henry Ford introduced the conveyor belt on his assembly line at Ford Motor Company. At that time, conveyors relied primarily on crowned spindle design. The crown refers to a spindle with a slightly larger diameter at the drive surface center than at the edges. The physical phenomenon of conveyor belts following the largest diameter of the spindle and thus being self-centering or tracking is fascinating. This method of conveyor tracking has been accepted for over a century.

Some manufacturers add a v-guide strip to the back of the conveyor belt to allow for more accurate belt tracking. By applying such a belt to a conveyor with v-guide grooves in the drive, idler pulleys and the conveyor bedplate, the belt will be less likely to track or wander from side to side. This manufacturing process adds cost but has several advantages, including easier setup. The knurling or raised diamond pattern machined into the spindles can also reduce belt drift. However, this pattern is less practical when applied to a crowned pulley. The results for both tracking and drift can be significant. Notice that lateral and axial movement have decreased.

Precision conveyors are a somewhat new category of belted conveyors that can match the tracking characteristics of a v-guide belt while eliminating drift. This conveyor type can be either flat belt or fixtured. While more precise methods of product placement are mentioned later in this article, this conveyor style can be a lower-cost alternative. Adding a servo motor to a precision-belted conveyor can enhance its performance capabilities, such as higher speeds, higher accelerations, and more precise indexing to position. Notice that there is now no drift.

These conveyors are distinct in two ways. First, the product being transported is not riding on a flexible belt directly; it is generally nested in a fixture attached to the pallet, which acts as a carrier. The fixture helps ensure that the product alignment is consistent throughout the process. The second difference is that they are asynchronous — the individual pallets are held in place long enough to complete the process. The pallets can be released into a zone to stay in queue or proceed directly to the next lift-and-locate station.

The various styles of pallet conveyors include flexible chain, open-center dual strands, and open-center edge rollers. Each uses unique yet similar lift-and-locate devices to accurately position the pallet once it arrives at the lift-and-locate station. Notice now that both lateral movement and axial movement are very small.

Recently, we have seen the trending application of timing belt style conveyors, both flat belt and fixtured types. Precision belted conveyors with servo control, high speed and throughput capabilities, and eliminated belt drift are particularly useful in robotic applications. Precision timing belt conveyors are increasingly popular, where a mixture of high accuracy and throughput is critical. There will always be a need for pallet-style conveyors, as high-precision assembly work often requires more accuracy than any current belted style can offer.

This article first appeared in the April, 2023 issue of Motion Design Magazine.

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