March, 2013. This is Part 3 of a 3-part white paper series (Click Here to Read Part 1 and Part 2) which provide an overview of the latest automation technologies available for high volume full pallet distribution transport, storage and retrieval operations such as in the Fast-Moving Consumer Goods (FMCG) industry. Highly effective automation technologies are available from global material handling vendors and our expectation is that these technologies will increasingly be deployed on North American soil.
Part 3 of this white paper reviews the concepts listed below:
- Pallet Shipping and Sortation Solutions
Automated Truck Loading Systems
- Pallet Conveyor Systems
- Pallet Conveyor Transfer Cars
- Overhead Monorail
- Inverted Monorail
- Laser Guided Vehicles (LGV)
- Skate Conveyor Technology
- Chain Conveyor Technology
- Automated Truck Loading Systems (ATLS)
- Laser Guided Vehicles (LGV)
Pallet Shipping and Sortation Solutions
In Part 2, we discussed the different types of automated storage and retrieval systems that are typically deployed in high bay warehouses (HBW). Once pallets of finished goods have been stored in the HBW, the warehouse control system that manages the ASRS machines is responsible for prioritizing the sequence to release outbound orders that need to be shipped to customer locations or to other distribution centers within the logistics network. In a multi-aisle high bay warehouse, the cranes bring forward the pallets to the shipping end of each aisle and from this point the pallets need to be sorted/transferred to their assigned shipping doors in advance of trailer loading. First we discuss the most commonly deployed options to automatically sort pallets to the assigned shipping doors and the options that exist to automatically load outbound trailers without human intervention.
1. Pallet Sortation Systems - Pallet Conveyor Systems
In the past, the most common approach to horizontally transferring pallets from a high bay warehouse (HBW) to the assigned shipping doors was to deploy a pallet conveyor system. The concept was usually based on placing an oval conveyor racetrack at the shipping end of the HBW. Pallets are passed from the ASRS to conveyor infeeds that transfer pallets to the oval conveyor loop. Once pallets enter the loop, they are navigated to the designated dock door lane, either by traveling around the loop or via a series of shortcuts designed to reduce transfer time. Pallet conveyors are the slowest form of horizontal pallet transfer, with speeds in the range of 70 feet per minute (0.35m/s), therefore the pallet conveyor sortation system must be intelligently designed to minimize the horizontal travel distance between each ASRS output aisle and each shipping door.
Some automation solutions have more than one crane in operation per ASRS aisle to double up the throughput capacity of the HBW within the same square footage. This concept typically has one ASRS crane working the north end of the aisle and the other ASRS crane working the south end of the aisle. If the shipping dock is on the south end of the HBW, then pallets being output from the north crane are transferred via conveyor to the south end of the HBW. Often these pallets emerge from the HBW on an elevated conveyor system such that a vertical elevator is required to lower the pallets to floor-level prior to loading.
Pallet conveyor transfer systems have been around for decades so this is a low risk technology from that standpoint. There are issues that need to be considered as follows:
- Pallet conveyors will likely require higher levels of maintenance over the long term because conveyor chains need to be lubricated and inspected for stretching. This type of maintenance is low in complexity but it implies that the conveyor system needs to be shutdown during inspection periods.
- Pallet conveyor systems are not fail-proof and a system failure can result in the entire sortation system coming to a halt which can be devastating in a high volume throughput environment. More often than not conveyor stoppage is not the fault of the equipment itself, rather it is due to poor quality pallets. The pallet will always be the weakest component of any automated distribution system. Broken pieces of wood can jam the conveyor system and stop it from moving forward. A maintenance operator usually has to back up the pallet to get at the problem and remove the broken wood.
- Typically, within 7 - 10 years of continuous use over a 3-shift operation, a pallet conveyor system will require an expensive overhaul which usually involves replacing chains and drive motors. This can mean a system shutdown needs to be planned for an extended period of time. It also means a significant capital re-investment in a system overhaul needs to be budgeted as part of the calculation when evaluating this type of solution.
2. Pallet Sortation Systems - Pallet Conveyor Transfer Cars
An alternative to the oval conveyor race track concept discussed above is the use of a pallet transfer car to move single pallets (or pallet pairs) from the ASRS aisles to their designated shipping lanes. The transfer car is basically an extended component of the conveyor system and its purpose is to move pallet loads at a higher speed than a conveyor to increase throughput capacity. The transfer car is basically responsible for moving pallets quickly from every ASRS output aisle to every dock door staging lane.
With this type of system, the transfer car is similar to a public bus whereby the bus travels back and forth along a straight road all day long. The transfer car stops to pick up pallets on one side of the road and then drops the pallets off on the opposite side of the road.
At the end of each ASRS aisle, a short conveyor is used to transfer outbound pallets to the transfer car pickup point. These conveyors are usually long enough to hold a small buffer of output volume which is queued up for pick-up by the transfer car. The transfer car runs along a straight floor-mounted track that is perpendicular to these output conveyor lanes. The car moves back and forth along the track so that it picks up pallets on one side and drops the pallets off on the opposite side of the track. The car transfers pallets to conveyor staging lanes in front of each shipping door.
- Transfer cars are a popular way to move pallets to assigned shipping lanes when a pallet conveyor system is the automation strategy being used for pallet sortation on the shipping dock.
- Transfer cars have a throughput limitation in that they can only move a limited number of pallets per hour. Thus the sortation system needs to be designed such that there is enough transfer cars to handle the peak hourly shipping requirements of distribution system.
- Transfer cars represent a potential single point of failure. If a transfer car breaks down then there is no way to navigate outgoing pallets from the HBW to the shipping doors unless the transfer car is removed from the floor rail and replaced with a redundant unit. Probably not a bad investment for such a mission critical piece of equipment. The conveyor buffers at the end of each ASRS aisle are usually short so if a pallet transfer car breaks down then the ASRS cranes needs to be shut down to stop output from overflowing.
3. Pallet Sortation Systems - Overhead Monorail Systems
We discussed the use of overhead monorail systems in Part 2 of this series and we indicated that these systems are most suited to longer horizontal transport distances to move pallets from the production lines to the input points of high bay warehouse (HBW). Therefore it may come as a surprise that overhead monorails can also be used for short-distance racetrack loops to move pallets from the HBW to the shipping staging lanes.
- The overhead monorail carts travel in a one-way direction around a racetrack loop that is positioned on the shipping side of the HBW.
- Pallets are output at each ASRS aisle such that they are conveyed to pick-up points along the racetrack.
- Electric monorail carts pick up (and drop off) pallets as single unit loads (or as pallet pairs). Thus the overhead monorail cart has the advantage of supporting high volumes of output within a small oval loop.
- If ever an overhead monorail cart fails for any reason, then the problem can usually be resolved within several minutes. The monorail system is shutdown and the stopped cart is manually pushed off to a run-off lane. The run-off lane is sometimes called a hospital lane since a “sick” cart can be switched off of the main loop and transferred to the hospital lane for maintenance. Suffice to say that once the problem cart is removed from the main loop, the system can be started up again and the remaining healthy carts can continue working. Thus the overhead monorail system has the benefit of not having a single point of failure.
- It is important to note that these shipping loops usually span the width of the HBW and the purpose is to move outbound pallets from the HBW to the shipping staging lanes assigned to each dock door. The monorail carts are captive to the loop thus they start and stop as a serial process. If too many empty carts are on a line then this can actually slow down the entire monorail system. Thus it is important to stage empty carts in the run-off lane during slower times to prevent excessive queuing in the system.
- The typical speed that overhead monorail carts travels at is in the range of 295 feet per minute (1.5 m/s) which is 4 times faster than the typical pallet conveyor system.
- Overhead monorails are fast and highly reliable horizontal transfer systems to move and sort pallets from the HBW to shipping. Perhaps the only downside of this technology is that it is relatively expensive in terms of the up-front investment.
4. Pallet Sortation Systems - Inverted Monorail Systems
The inverted monorail system is similar to the overhead monorail system except that transfer carts run along a floor-mounted rail rather than being supported by an overhead rail system. The floor-mounted inverted monorail system is typically designed as a racetrack loop that is positioned at the end of the high bay warehouse (HBW). Outbound pallets are transferred by ASRS to the loop where they are transferred to the monorail carts either as single unit loads or as pallet pairs. The inverted monorail is a fast and highly reliable system to sort pallets on the shipping dock.
- Each ASRS aisle outputs pallets to short transfer conveyors that move the pallets to the pickup stations positioned along the inverted monorail loop. Pallets are picked up as single unit loads (or in pairs) and transferred to the monorail carts that receive their power from a floor-mounted rail. The carts run along the rail which is basically an oval loop.
- The carts transfer each pallet to its designated staging lane whereby the pallet is moved from the cart to a conveyor lane in front of each dock door.
- If ever a cart breaks down, the system must be shut down to permit the extraction of the cart from the rail. Since the carts ride on a closed rail system, a rail attachment must be liberated and a forklift with an overhead boom lift is needed to lift the cart of the track. The cart is then transferred to a maintenance area such that the inverted monorail system can start-up again. This has the benefit of providing a system without a single point of failure.
- The inverted monorail offers the fastest velocity of the horizontal transfer systems at 590 feet per minute (3.0 m/s) which makes it an excellent choice for environments where throughput requirements are on the high side such as in the beverage industry.
- When things are slow, the empty monorail carts simply queue at one end of the loop to await their next mission. There may be several carts queued similar to a taxi stand until such time that a transfer task is created.
- Perhaps the only downside to the inverted monorail system is that the transfer cart resides underneath the pallet load hence there is a possibility that leaking bottles can drip onto the equipment resulting in corrosion over time. Site visits to older installations are a good way to see if this is really an issue or not. We have not seen instances where this has been a serious issue.
5. Laser Guided Vehicles (LGVs)
The use of LGVs to move pallets from the HBW to their assigned shipping lanes is less common, but nevertheless represents a viable alternative for some companies depending on the layout of the facility.
Laser guided vehicles can move single pallets (or pallet pairs) from the HBW to designated staging lanes for shipping. Some suppliers manufacture LGVs that are equipped with sonar technology so that the LGV can also perform the trailer loading function as well. This way the same vehicle that performs pallet retrieval and horizontal transport can also perform the loading function.
- A fully loaded LGV with a dual-pallet mast carrying a payload of 2500 KG can move at about 335 feet per minute (1.7 m/s).
We will discuss more about LGVs within this white paper as we review this equipment as an option for automatic trailer loading.
Automated Truck Loading Systems
Once pallets have been staged to their designated shipping lanes in the correct loading sequence, it is possible to automatically load trailers without human intervention. There are several different options that can be considered and there is no shortage of details that need to be understood when deciding on the right automatic truck loading system (ATLS) for your business. We will discuss multiple approaches to automated truck loading and our perception of where each technology fits.
1. Automated Truck Loading Systems - Skate Technology
The concept of skate technology is that an entire truckload of say 24 pallets can be automatically loaded into (or unloaded from) a trailer in a single movement such that the truck can be turned around in 10 minutes. Skate technology requires that the floor of the trailer is modified with a subfloor to allow a pair of skates to enter into and retract from the trailer in a single motion whilst carrying the pallets. Here is how it works:
- Let us say that 24 Pallets are staged on a pair of pallet conveyors in front of a shipping dock door such that we have 2 lanes x 12 pallets deep.
- When the truck with the trailer backs up to the door, the back wheels of the trailer are firmly locked into place using an external kingpin system (or other type of system) designed to prevent the trailer from moving forward during the loading process. It is critical to have the trailer exactly aligned to the dock for the system to work.
- Once the trailer is ready, it is time to start the automated skate loading process. Pallets rest on top of the skates that inflate and deflate with the use of an air bladder. Inflating the bladder causes the skates to lift up such that the pallets are elevated slightly upwards. Deflating the bladders causes the skates to lower such that pallets can be lowered to rest on the trailer floor.
- The loading process starts by inflating the air bladders which reside within the skate channels. With the bladders inflated, the skates move forward into the trailer by navigating into two channels within the subfloor of the trailer. The skates can transfer 24 pallets into the trailer without any friction because the pallets are elevated slightly above the subfloor of the trailer due to the inflated air bladder.
- When the skates reach the end of the trailer, the air bladder deflates thereby lowering the pallets such that they end up resting on the floor of the trailer. The reverse process is used to automatically unload a trailer. This written explanation does not do justice to explaining the process so the best way to understand this concept is to watch a video which can be seen here.
- This type of system lends itself well to a company that has a large fleet of trailers within a closed loop system. The cost of the system relates to the number of doors being automated which can be a few. The cost of modifying the trailers with a subfloor is relatively inexpensive because no motorized equipment is installed on the trailer with this type of technology. Pallets can be loaded on the short or wide side and double-decked pallets can be handled as well.
2. Automated Truck Loading Systems - Chain Technology
- An alternative approach to skate technology is the use of a chain conveyor system which moves pallets onto and off of trailers through the use of a powered chain conveyance system that is installed inside of each trailer. If pallets are standardized in size then this type of system can be deployed.
- On the shipping dock, pallets are staged onto two chain conveyor lanes with 12 pallets of depth totaling 40’ - 48’ (12.2 - 14.6 meters). The conveyors consist of 2 to 3 chains per pallet - heavier pallets require 3 chains.
- A similar chain conveyor system is installed inside of the subfloor of the trailer with 2 to 3 chains per pallet lane, again depending on the weight of the pallet.
- When the trailer arrives, the driver plugs the trailer into a external power outlet on the outside of the building. This is required to provide power to the chain conveyor system inside the trailer. Once the trailer is locked into place, the chain conveyors are activated and the pallets are automatically loaded (or unloaded) 24 pallets at a time by moving pallets from one conveyor system to another. The entire process takes about 10 minutes. This type of system can be seen here.
- Since the investment cost for this type of system is related to the number of trailers that need to have powered conveyor systems installed, this type of technology is most suited to smaller fleets that operate within a closed loop system.
3. Automated Truck Loading Systems - Slat / Belt / Roller Track Technologies
- There are three other types of automated trailer loading technologies that we will spend less time discussing because they are suited for more specialized applications:
- Slat conveyor technology refers to the modification of trailers with a subfloor that consists of a powered slat conveyor system that can be used for a mix of pallet sizes, slip sheets and/or non-palletized cargo. The entire floor is covered with a flat moving slat system similar to a horizontal escalator such that goods that are inducted into the trailer automatically slide forward towards the nose of the trailer.
- Belt conveyor technology refers to the modification of trailers with a subfloor that consists of a powered conveyor belt that spans the width of the inside of the trailer. The belt creates a continuous flat surface that is suitable for automatic loading of parcels and other non-palletized goods.
- Roller Track technology refers to the modification of trailers with a subfloor that consists of roller tracks with wheeled rollers to enable automated loading of air cargo pallets.
4. Automated Truck Loading Systems - Automated Truck Loading With Mobile Loading Machines
An alternative approach to deploying an automated truck loading system that requires a trailer subfloor modification is the use of a mobile automated truck loading system (ATLS). It is important to note that this equipment does require a slight trailer modification. A pair of standard 12” high (300mm) steel kick plates need to be installed along the entire length of the inside trailer sidewalls at floor level. This is to provide a good rolling surface for the lateral guide wheels that the mobile loading equipment uses when navigating in and out of the trailer.
- Outbound pallets are staged on conveyors at each shipping door to await the automatic trailer loading process.
- A rail-mounted loading machine travels back and forth in front of a group of dock doors (e.g. 3-4 doors per machine). When it is time for the trailer to be loaded, the machine travels to the dock door where the next loading process will take place. This way a single mobile loading machine is shared across multiple doors.
- The loading machine is similar to a dual-masted forklift vehicle except that it is captive to the mobile ATL machine. The loading machine starts by picking up a pair of pallets from the staging conveyors. The forklift machine then drives into the trailer with a pair of pallets until it reaches the first empty row where the pallets can be let down inside the trailer. Lateral wheels on the mobile forklift machine rub against the steel kick plates that are installed alongside the inside of either side of the trailer walls which is how the forklift machine navigates once inside the trailer.
- In effect, the mobile forklift machine must move in and out of the trailer a total of 12 times to load 24 pallets inside the trailer. As such there is a fair amount of movement going on with this type of solution as compared to the skate/chain technologies which only require a single loading movement to get the same job done. In our opinion, this is the main disadvantage of this type of solution. More movement inherently implies more maintenance and more opportunity for something to stop working.
- On the flip side, this solution requires no subfloor modification to the trailer which means no loss of trailer height to accommodate the subfloor. For taller pallets (e.g. paper products) that require the full trailer height, this may be an important consideration.
- It is also important to keep in mind that this type of solution requires additional dock depth to accommodate the automated loading equipment system. The typical scenario is that sufficient dock depth is provided to stage a trailer load (i.e. 12 pallets of depth) onto a conveyor plus an additional 20’ (6.0m) for the ATLS system. Thus this type of system can only be installed in facilities that have sufficient dock depth to support the concept.
- You can see a detailed video of how it works here.
5. Automated Truck Loading Systems - Laser Guided Vehicles
The last option that we discuss for automated loading (and unloading) of outbound trailers is the use of laser guided vehicles (LGVs). This option is discussed last because it is the least mature solution of the solutions discussed in this white paper. While skate and chain conveyor systems have been around for 20 years, the use of LGVs to automatically load and unload trailers have only emerged over the last several years.
- Some LGV manufacturers can provide vehicles that are equipped with sonar technology which is used by the LGV to detect where it is relative to the walls of the trailer. While the LGV machine travels within the four walls of the facility, it uses its normal laser navigation system to detect its location in real time. However, when the vehicle enters the trailer, the laser navigation system no longer functions because the vehicle is surrounded by the inside walls of the trailer.
- The approach that we are familiar with is to have a sonar device on the left, right and front of the vehicle such that the device can detect reflected sound waves that bounce off the sidewalls of the trailer (and to the front) so that the vehicle can safely navigate inside the trailer.
- LGVs can be equipped with single or dual pallet masts to load one or two pallets at a time depending on the requirement.
- The benefit of using an LGV for trailer loading is that it is flexible. There is no need to modify the trailers in any way hence this type of solution can work with a non-captive trailer fleet. Also, the same LGV vehicle can be used within the distribution operation to perform other tasks such as the horizontal transportation of pallets.
- The disadvantage of this type of solution is that it is a relatively expensive technology for a trailer loading application. Also, the use of LGVs for trailer loading is relatively slow since the typical loading time is 45 minutes per trailer as compared to 10 minutes with a dedicated automated truck loading system.
- You can see a detailed video of how it works here.
Wrap-Up Discussion on Automated Truck Loading (ATL) Systems
Since the investment requirement to install any type of automated truck loading system is quite high, it is important to understand the cost justification for this type of solution.
The majority of the cost savings associated with the skate/chain/conveyor types of solutions are derived from the fact that a trailer can be turned around in a short time period which means a reduction in driver labor costs plus the cost of the trucking assets (truck and trailer). For companies that derive no benefit from a 3rd party carrier being used for shipping then clearly this type of application is of lesser benefit. This is why ATL technology that requires trailer modifications is generally used by companies that have some type of a closed loop trailer system. These companies generally place a high value on the speed to turn over a trailer either due to the nature of the business (e.g. parcel courier shipping) or because there is a high throughput requirement from a limited number of shipping doors which implies that high-speed dock door turnover is of critical importance.
The economic business case for automated LGV trailer loading is less obvious because the time required to load a trailer using LGV vehicles is not that much different than having humans do the work. Our perception is that companies would be inclined to use this type of technology in environments where the removal of the operator from the task is important because of the nature of the product, or because the company wants to reduce product damages that take place during the loading process. Also, the use of LGVs for trailer loading can be a natural extension of a distribution operation where LGVs are already involved in the pallet retrieval / transport process. Rather than having an LGV retrieve the pallet and then have an operator perform the loading function, it is just as easy to let the LGV perform the loading work.
This concludes Part 3 of our white paper on automation for distribution in the food and beverage industry. You can click here for Part 1 and here for Part 2.
Marc Wulfraat is the President of MWPVL International Inc. He can be reached at (1) (514) 482-3572 Extension 100 or by clicking here. MWPVL International designs automated distribution centers and automated material handling systems and can help your firm evaluate the most appropriate solutions for your manufacturing or distribution operation.