Hydraulics goes a long way to improve the function, power, controllability and safety of material handling equipment.
By Josh Cosford, Contributing Editor
Material handling involves the moving, transporting, lifting, and dumping of products, components or bulk ingredients, and mostly within the production and distribution of finished goods. Much of the equipment used for material handling is human-powered, such as carts, dollies and wheelbarrows, but you wouldn’t be reading this fluid power article if we stopped there.
Hydraulics goes a long way to improve the function, power, controllability and safety of material handling equipment. Without hydraulics, many of the material handling machinery you know and use daily would not be possible. In fact, some of the most straightforward hydraulic machines in the world exist in material handling. Conversely, an airport baggage handling system is a vast and complex series of lifts, robots and conveyors, but is beyond the scope of this article.
Simple hydraulics facilitate human-powered machines
Even the smallest production or distribution facility likely uses fluid power daily. Incoming and outgoing shipments too large to hand-bomb get strapped and wrapped to a wooden pallet. The simple pallet jack lifts and moves wooden skids around the shop floor using the most basic single-acting hydraulic bottle jack. Also called a pallet truck (or affectionately the pump cart), this forked, human-powered cart slides neatly under the pallet where it lifts anywhere up to 5,000 lb or more.
The pallet jack employs a single-acting bottle jack mounted atop the pivoting steering wheels. A lever remotely operates the two-position valve within the handle, and when flipped forward allows pumping action to occur. The simple up and down reciprocating action of the handle sends fluid into the ram, lifting the pallet a few inches off the ground to facilitate easy transport. Many variations of the pallet jack provide a full selection balancing price with productivity. Some units employ electrically driven hydraulic pumps for easy and rapid lift, while others gain electric wheel drive to aid operators with heavy loads.
It is more ergonomically correct to load a pallet at waist height rather than bending down to place parts or boxes, potentially injuring one’s back. A pallet jack equipped with a scissor lift helps the operator avoid injury by lifting the pallet to a manageable height 2 or 3 ft up. A scissor lift combines the effectively short effort arm with the power density of a hydraulic cylinder to move heavy loads high. The scissor lift is a common tactic in material handling, of which we’ll discuss more shortly. I should also mention the telescopic pallet jack, which uses a two-stage telescoping bottle jack to lift without any scissor action.
Reaching heights with forklifts
Of course, pallets in a distribution center are prolific, to say the least, and they are often organized and stored atop towering rack systems where no simple pallet jack can lift. High lifting is where the venerable forklift comes in, using myriad configurations to satisfy every industrial or distribution environment imaginable.
The walkie-style stacker takes the pallet truck to the next level, providing lift options up to 7 or 8 ft, but still requires a standing operator to manipulate the movement of the lift across the concrete. Although operated using hydraulic levers or electronic joysticks, it’s short profile allows the stacker to fit into tight spaces and omits the extensive training required for self-propelled, riding forklifts (although, without fail, you provide safety training to all warehouse staff). Stackers have their limits in both height and weight capacity, however.
Naturally, the forklift is the next branch in the pallet handling family tree … it’s also where the hydraulics proliferate. Where the pallet truck had a single bottle jack, the forklift runs with a minimum of three hydraulic functions. Depending on the number of lift stages for the mast, there could be one, two or three lift cylinders on a forklift (employing counterbalance valves, of course). Each mast type combines the cylinders with a chain system that increases the movement distance while sacrificing force, like what happens in a scissor lift. Like always, the power of hydraulics easily overcomes the mechanical disadvantage.
If a forklift control system only allowed for lift, the operator would be busy salsa dancing the truck back and forth as she tries to place pallets upon barely visible shelves up high and accurately. To make the task easier, a side-shift cylinder mounted to the carriage moves the forks laterally six to ten inches. The cylinder uses a double rod construction to supply equally consistent velocity and force regardless of left or right movement.
Finally, a hydraulic tilt operation moves the mast angle forward or backward to not only ease removal and placement of pallets on high racks but also pulls the load backward to improve the center of gravity. A center of mass is ideal when located as close to the center of the forklift as possible, which is especially true for heavy loads being transported up high.
If you are lucky enough, your forklift also has a hydrostatic transmission. A single axial piston pump with an overcenter-capable swashplate provides both forward and reverse operation by merely tilting the swashplate in one direction or the other. Of course, it would be pilot- or electro-proportional valves operating the swashplate angle, but you get the idea. Regardless, the pump powers the wheel motors in either direction, as asked by the operator, in a smooth, controlled fashion. Horsepower control on expensive machines supplies full power balanced across the entire speed range, which is especially helpful on high capacity forklifts.
Horsepower control is an ingenious hydraulic pump function that allows maximum power to be distributed regardless of load pressure. When pressure is low, the pump will generate full rated flow. As pressure increases to the level of the horsepower set point, the pump automatically reduces pump flow inversely proportional to the increase in pressure.
Should the forklift start from stop carrying a full load, the pump provides full pressure to generate maximum torque but with low flow. As the forklift gains momentum, the pressure to maintain acceleration drops, allowing that unused portion of horsepower to now be transmitted as flow. Horsepower control is a great way to give your machine maximum possible flow while always being capable of high pressure, high torque (or linear force) action.
Modern machines specifying anything above the entry-level choice employ digital control and diagnostics. Electronic controls have replaced hydraulic joysticks and levers, and for the most part, electronic joysticks are less expensive than their remote pilot-operated joystick counterparts. The proportional valves, drivers and controllers previously left electrohydraulic control outside the budget for many machines. But the downright low price of entry in today’s age makes electronics economical for anything this side of a logsplitter.
Material handling applies to more than equipment for handling pallets, of course. Conveyors are common but rarely employ hydraulics. A bin tipper, however, is a hydraulically powered machine that lifts and tips any bin that needs tipping (shocking, I know). A bin tipper may lift and tip a bin of garbage into a larger dumpster, or perhaps a load of bulk ingredients into an industrial mixer. Two cylinders may operate the bin tipper; one to lift and another to tip. However, some smart designs use just one cylinder combined with levers to lift then tip sequentially.
Naturally, a great place to use a bin tipper is atop a forklift. In a machine shop, for example, bins collect chips as a by-product of machining metal parts. These bins are wheeled, and also manufactured with an auxiliary mechanism that allows tipping when you operate an optional valve, or can be done manually with the pull of a pin. Although not always hydraulic powered, I thought it was serendipitous as a combination of material handling accoutrement.
Fluid power makes a natural partner for the material handling environment. From off-the-shelf pallet handling equipment to custom manufactured, automated product handling machines, every corner of a manufacturing, distribution or general industrial environment can take advantage of fluid power.