Global Material Handling System Integrators

The Beauty of Zero Pressure Accumulation Conveyor

It may not seem like a big deal to keep products from touching each other as they are conveyed until you are standing under a conveyor and glass bottle cases begin crashing into each other and raining down their contents on everyone. There definitely is an art and science to conveying very fragile, light products in between large, heavy products. The level of difficulty also grows substantially as the conveyor speed and throughput increase. In the olden days (25 years ago), a pressure sensor roller with mechanical linkages to each zone was used to engage or disengage accumulation zones. This was a pretty “tried-and-true” technique unless you got very light product that wouldn’t depress the pressure sensor roller, or the springs and linkages wore over time. In the mid-nineties, photo eyes with distributed logic began to replace the physical pressure sensor rollers as photo eyes and micro controllers dropped in price. This was a great advantage because now you were no longer limited by the weight of product to depress a mechanical sensor. Of course there were new challenges that occurred with the photo eye technology like black or very dark totes not being seen by the photo eye or vibration causing the photo eye to go out of alignment with their reflector causing all product flow to stop. Throughout the last 10 years, the electronic photo eyes in combination with the distributed controllers have continued to advance. Beyond standard zone release accumulation, a whole host of new techniques have been introduced:
  • Semi-slug release: where you release a small group of products in a slug to improve throughput.
  • Dynamic zoning: where larger products can take up two zones that operate together where small products only take up a single zone.
  • Sleep mode: to turn off the conveyor mechanically when no product is being conveyed. This saves mechanical wear, reduces noise, and saves electrical power.
  • Interaction with WCS: Also, specific conveyor zones can interact with the overall warehouse control system to change modes and report back how full particular conveyor lines are. This is nice so you can dynamically re-assign the work flow to other areas of the system that are not as busy.
Depending on the speed of the conveyor, accumulation zones can use brakes or go without brakes. At speeds below 150 feet per minute, brakes are a nice option to ensure that the heavier products don’t coast into the downstream zone as they are stopping. The brakes can be used on the full zone or just the first few discharge rollers. For faster speeds (greater than 150 feet per minute), brakes are often removed and the product coasts to a stop and can lightly touch each other. In the last 5 years, belted accumulation conveyor has grown in popularity for several reasons:
  • With a belted surface completely under the product, there is less of a chance it will get stuck or jam in comparison to pure roller conveyor.
  • Cartons are less likely to rotate on a belted surface than a roller surface. This helps maintain a “narrow side leading” product profile down the conveyor, which makes fixed bar codes on the carton more readable.
  • Belted zones can accommodate very small products (less that 4” by 6”), where roller conveyor has a tough time with these small products.
  • Belt accumulation also does a better job of preventing side-by-side products within an individual accumulation zone. Side-by-side conditions normally result in a no-read situation that requires the cartons to either recirculate or be diverted to an exception handling station to correct the issue.
Zero Pressure accumulation zones are now used on inclines, declines, and spirals to gain additional accumulation linear footage. Accumulation conveyor is the primary tool to buffer and regulate work imbalances between upstream operations and downstream operations, for instance between picking and packing. Zero Pressure accumulation conveyor is a beautiful thing, and it is one of the most widely used types of conveyor in the world to regulate and buffer material flow.

Author: Ahmed Arif


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