A Comprehensive Guide to Powered Roller Accumulation Conveyor Systems

A Comprehensive Guide to Powered Roller Accumulation Conveyor Systems

At first glance, an accumulation conveyor system looks similar to a powered conveyor section designed to simply transport material from one location to another. What sets the accumulation conveyor apart from other powered conveyors is that they assist in the control of the rate of flow of products through a production process. Rather than a continuous flow of products like other powered roller conveyors, the accumulation conveyor starts and stops the movement of loads automatically, allowing for storage and metering or batch advancing of loads. This is often achieved using accumulation zones, sensors, air valves and brakes. Some common applications that incorporate accumulation conveyors are manual workstations, robotic palletizers, sorters, and merges. 
 
Method of Operation 
 
The control of the movement of material on an accumulation conveyor is with sensors mounted on the conveyor line.  Once these sensors are activated, the signal is sent to the motor powering the conveyor in the accumulation zone.  This motor can be an external drive as well as a motor within the roller as is the case with a motorized roller conveyor line. The sensor most often used is a photoelectric sensor.  Photoelectric sensors use light to detect the presence of objects.  These sensors emit and receive light at certain wavelengths.  Another type of sensor that offers cost savings is the sensor roller.  This roller is positioned slightly higher than the surface of the roller conveyor line.  When the load moves over the sensor roller, it is pushed down and activates a limit switch.  Switching the drive on and off as pressure is applied or removed from the roller sensor.
 
An accumulation zone is a group of rollers that are controlled together, started and stopped as a group.  Zone lengths can vary but should not be less than the maximum load length. The typical zone lengths for conveyors not intended to handle pallets are 24” or 36” long. The rule of thumb for conveyors designed to handle pallets is at least 12" of additional space between loads to prevent any contact during accumulation.
 
Accumulation Methods 
 
The basic types of roller accumulator conveyor systems are low/adjustable pressure, zero pressure, and index (slug) accumulation.
 
Low Pressure or Adjustable Pressure Accumulation
Low-pressure accumulation uses a conveyor that continuously drives the loads forward, creating line pressure. The discharge end of the conveyor would include some method of holding back the accumulated loads such as a roller brake, brake belt, or meter belt conveyor. Accumulation begins when the hold back device at the end of the conveyor prevents the load at the discharge end from advancing. The loads back up behind the first load now located in the discharge position.  Since this method of accumulation requires loads to come in contact, this accumulation method is suited for loads that are consistent in size and weight. Totes often have tapered ends or handles which may not be suitable for with this type of conveyor.  If the boxes are not consistently sized, there is a likelihood that the cartons can be shifted or turned as they come in contact with smaller boxes downstream in the accumulation zone. If the load contains cardboard boxes, the cartons need to be sturdy enough that the contents are not crushed or damaged.  The total load weight in the accumulation zone is limited due to the drive pressure that is needed to move the accumulated load out of the zone. Since there is no space between loads in this accumulation method, it offers the highest possible storage density. 
 
 
Animation of Pressure Accumulation Type  
 
 
 
Zero Pressure Accumulation
Zero pressure accumulation method uses accumulation zones of a pre-determined length and a way to apply and release drive pressure within these zones. In each zone, a sensor to turn off the rollers within that zone.  The accumulation zones are configured so that the loads on the conveyor line do not make contact with each other.  This spacing prevents loads that would be otherwise damaged if they came in contact with adjacent boxes or pallets. For carton handling applications, the load is allowed to coast into the accumulation zone by cutting power to the drive, in pallet handling applications, the drive is powered until the load reaches the sensor.  The drive chains in a pallet handling roller accumulation system are engaged/disengaged using an air or electric clutch. 

Zero Pressure Accumulation Type
 
 
Index (Slug) Accumulation
Index (slug) accumulation is a good solution for low volume and high-density accumulation on a conveyor. The total length of conveyor is broken into at least three equal length drive zones with an input and discharge drive zone at the entry and exit. Loads are index advanced onto the first of the three equal length conveyors. Upon filling the first of the three equal length conveyors the entire contents of the first conveyor will be advanced as a group to the furthest downstream completely empty conveyor.
 
 
Animation of Index or Slug Accumulation Type
 
 
Release Modes 
 
The five basic release modes in roller accumulating conveyor are referred to as slug, singulation, semi slug, dynamic release, and index release.  The application will often determine which release mode is ideal in terms of throughput and control of product on the conveyor line.
  
Slug Release
This method releases all the zones at the same time.  The result is, there are no gaps between loads and offers the highest throughput. Ideal for accumulating conveyors that are upstream from a sorter or merge. Similar to releasing runners at the beginning of a marathon, cartons are packed close together as they move downstream. This release mode is not to be confused with “slug” or index accumulation.
 
 
Animation of Slug Release Mode
 
 
 
Singulation Release
This method is used in pallet and carton handling applications to ensure gaps between loads.  This release mode is similar to the on-ramp traffic lights that we have here in Chicago. They allow traffic onto the expressway in a carefully metered flow. The method is ideal for outfeed to an unload station allowing an operator to manually move the load off the line.  The lower throughput is not ideal for applications where the accumulating conveyor is feeding into a sorter or to a robotic palletizer.  
 
Animation of Singulation Release Modes
 
Semi-Slug Release
In semi-slug, zones are released in a sequence.  This sequence uses a slight delay to create gaps.  This mode is used in applications that require gaps between loads and a higher throughput than what singulation release mode offers. 
 
 
Animation of Semi-slug accumulation conveyor release mode
 
 
Dynamic Release 
Zones are released at the same time similar to the Slug discharge method but the drive pressure is reduced. The result is a feed of product without the high line pressure. 
 
Index Release 
As loads are removed from the discharge zone loads will index release onto the discharge zone. 
 
 
Animation of index release mode
 
Recommended Accumulation Release Modes Based on Downstream Application
 
Application  Release Mode
Lines feeding a manual operation Singulated 
Recirculating lines on a sorter Singulated
Lines feeding a merge Slug
Lines that feed another accumulation conveyor Singulated
Induction to a sortation system Dynamic 

Source:  McGuire P.E., Patrick. Conveyors Application, Selection, and Integration. Boca Raton. CRC Press. 2010
 
The method to start and stop roller accumulator conveyors varies based on the way that the rollers are powered. Following is a description of how each conveyor accomplishes this task. 
 
Belt Driven Live Roller (BDLR) Accumulation Conveyors 
BDLR roller accumulation conveyor systems have a movable subframe that moves a drive belt up and down and engages the rollers to turn the conveyor line on or off. 
 
Chain Driven Live Roller (CDLR) Accumulation Conveyors 
CDLR roller accumulation conveyor systems use a clutch to engage or disengage the drive chains to move the load in the accumulation zone. 
 
Padded Chain Driven Live Roller Zone Accumulator (PCZAC) 
Padded Chain Driven Live Roller (PCZAC) uses a friction drive to power the carrying rollers, but the capacity for this conveyor type is significantly increased over the belt driven live roller (BDLR) sections. 
 
Lineshaft Roller Accumulation Conveyors 
Accumulation zones can be added almost anywhere on a line shaft conveyor including curves.  The lineshaft accumulating conveyor uses an air-operated roller brake to stop the rollers from turning. 
 
Motorized Roller Accumulation Conveyors 
Motorized roller conveyors are powered by compact motors built into some of the rollers.  These powered rollers are connected to adjacent rollers using urethane bands, chains, or poly-v belts to create the accumulation zone.  The power is turned off and on to the motorized rollers to move product in and out of the accumulation zone.
 
Accessories available for Accumulation Conveyor Systems 
Accessories are available to enhance the function of some accumulator conveyors.  Some common accessories are roller brake modules, case stops, guide rails, slave drives, and interfaces to control an upstream conveyor.