When it comes to high-density, dynamic pallet storage solutions, two of the most commonly used systems are pallet flow and pushback rack. While both rely on gravity to move pallets within the rack, they function quite differently and serve distinct purposes depending on the application.
Pallet flow rack— also known as gravity flow — may hold up to 20 pallets in a single storage lane. It incorporates gravity wheels or rollers mounted at a slight downward slope from the charge (entry) side to the discharge (exit) side. A forklift aligns each pallet load with entry guides at the beginning of the lane, then gravity pulls it down toward the pick face. Facilities handling large quantities of the same fast-moving products often use this storage solution.
Pushback rack features a series of nested carts that roll over inclined steel rails in each storage lane. A single pallet load rests on each cart in a series. Forklifts load pallets into the system from the front. As each new pallet enters the system, it pushes the one behind it deeper into the system. These systems typically hold up to four pallets per lane.
Both pallet flow and pushback rack system designs incorporate a sloped structure to facilitate movement. The incline typically ranges from 5/16-inch to ½-inch per foot, depending on factors such as pallet quality, load weight, and the specific application.
First-In/First-Out Versus Last-In/First-Out
One of the most fundamental differences between pallet flow and pushback rack is their inventory management method, explained Donald Toohey, Director of Engineering at UNARCO Material Handling. The company is a member of the Rack Manufacturers Institute (RMI).
“Pallet flow operates on a First-In/First-Out (FIFO) basis. That means the first pallet loaded is also the first one retrieved,” he said. “In contrast, pushback racking follows a Last-In/First-Out (LIFO) method, where the last pallet placed in the system is the first one removed.”
This makes pallet flow ideal for perishable goods or products with expiration dates, Toohey continued. Conversely, pushback is a better fit for applications where stock rotation is less critical.
Common Depth Configurations for Pallet Flow and Pushback Rack
The vast majority of pushback systems range from 2- to 4-pallets deep, though deeper lanes are possible. However, most facilities do not exceed four pallets per lane due to space constraints and efficiency concerns, Toohey noted.
“Pushback is particularly useful when storing products against a wall,” he added. “That’s because it only requires a forklift access aisle on one side, unlike pallet flow, which requires access at both ends.”
When deep storage lanes are necessary, pallet flow is generally a better choice because it offers more clearance. “Pushback systems rely on carts that stack within the storage lane. As the lane depth increases, so does the height of the stack of carts. That reduces the available vertical space,” Toohey explained.
Additionally, because pushback rack is a LIFO system, the entire lane must be empty before replenishment. This can result in wasted space and inefficiencies, commonly referred to as “honeycombing.”
Brakes, Speed Controllers, and Safety Considerations
A critical difference between the two systems is the use of brakes and speed controllers. In pushback racking, a pallet only moves one position at a time during system loading and unloading. This reduces the need for braking mechanisms.
Pallet flow lanes utilize brakes to control the movement of pallets as they travel down a lane. Without the brakes, the pallet load could gain too much speed and potentially impact the discharge end with too much force. This could cause products to fall into the aisle or result in damaged goods. However, in certain cases, 2- or 3-deep pallet flow lanes may not require brakes as the pallets do not travel an extensive distance. This depends on load weight, stability, and whether the unit load has stretchwrap applied.
Pallet Quality and Size Critical to Rack Function
Pallet quality is a crucial factor in determining the efficiency of either system. Damaged or inconsistent pallets may not move smoothly or become jammed within the racking.
Pushback systems are generally best suited for uniform pallet sizes, as the carts’ design specifically accommodates one pallet type, said Toohey. “In contrast, some pallet flow system designs can handle multiple pallet sizes. That can make it a more versatile option for warehouses that store a variety of products,” he added.
Further, in certain applications, pallet flow rails without brakes can be repurposed as pushback lanes. This can enable an operation to adapt to handling multiple pallet sizes or when cart stack height becomes a limiting factor.
Pallet Flow and Pushback Rack Structural Considerations
The location of building columns within an operation can also influence the choice between pushback and pallet flow systems. Toohey noted that while areas with structural interferences can support pushback racks, pallet flow requires unobstructed lanes to function properly.
“When configuring a pick module, operations will frequently use pallet flow on the picking levels,” he added. “However, at the in-between, non-pick levels, pushback systems can be used instead.”
System Cost Factors
Cost is a key factor when selecting a storage system. If comparing a pushback rack system to a deep lane pallet flow system with speed controllers, pushback racking is significantly less expensive, said Toohey.
“In fact, pushback lanes generally cost around 65% of the price of a comparable pallet flow lane,” he noted. “The additional components required for controlled movement in pallet flow, such as speed controllers and brakes, contribute to the higher cost.”
Learn More About Pallet Flow and Pushback Rack
Looking for more insight into specifying pallet flow and pushback rack in your operation? RMI offers standards and publications, including the recently updated “Considerations for the Planning and Use of Industrial Steel Storage Racks.” Additionally, the members of RMI are available for consultations, insights, and recommendations. For more information, visit mhi.org/rmi.
