Equipment Focus: Hydraulic Magnets

March/April 2015

Hydraulic magnets aren’t the right tool for every job, but they can be an efficient and cost-effective choice for those with certain work demands and base machines.

By Kent Kiser

At first glance, they look like extra-thick, double-decker lifting magnets. In reality, they are lifting magnets with a hydraulically powered generator on top, and they give users another way to generate electricity for their magnets. These self-contained, all-in-one magnet gen sets—on the market for roughly 10 years—transform the hydraulic power of the base machine into electrical lifting power. Users and manufacturers praise this equipment for its easy installation, lifting capacity (though not speed) that’s comparable to other gen set options, and ability to turn a single-function base machine into a multifunctional asset. Although these units aren’t the right tool for every magnet lifting need, they could be just the ticket for those who can benefit from their specialized nature.

The Hydraulic Power Factor

There are three common ways to drive the generator for an electro lifting magnet: connect it via a belt to the shaft of the carrier’s engine, use a separate diesel or gas engine, or tap into the carrier’s hydraulic system. Hydraulic-based generator systems come in two basic configurations: those with the hydraulic generator mounted on the base machine and those with the gen set attached to the magnet. This article focuses on the latter type, generically called hydraulic magnets.

Most hydraulic magnets attach to the base machine’s stick with lifting chains, but some models have a plate on top that attaches to the carrier using a quick-coupler linkage. Once mounted, these magnets have two hose connections for the hydraulic fluid inflow and outflow. The incoming fluid first passes through a flow-control valve that compensates for any overage of flow. The fluid then enters a small piston or gear motor that is coupled to the shaft of a generator, giving it power to turn the generator at its rated revolutions per minute and torque. The generator, in turn, puts out alternating-current electricity, which a rectifier converts to 230-volt direct-current electricity to energize the magnet. A voltage regulator controls the generator’s electrical output, preventing power surges should the generator’s rpms get too high.

The operator activates a control valve to start the flow of hydraulic fluid to the gen set from the cab using a button, lever, joystick, or foot pedal, depending on the user’s preference and installation. Once the circuit is activated, the flow and pressure power the drive motor and generator, which then charge the magnet so it can begin attracting material. When the user turns off the circuit, the generator slows down and the material drops.

Hydraulic magnets come in a variety of sizes, with the magnet’s diameter based on the application as well as the operating weight and lifting capacity of the base machine. “The generator size is based on the power requirement of the magnet,” says an Ohio-based magnet manufacturer. “The generator is simply providing power to match the magnet’s rated ampacity.” Manufacturers design hydraulic magnets with diameters from 28 to 68 inches, requiring generators with power output from 3 to 30 kW, for use on carriers weighing 12 to 30 tons. Although the vast majority of these magnets get installed on excavators, “you can use these hydraulic systems wherever you’ve got hydraulic power and need a magnet,” the Ohio magnet manufacturer notes, including on truck cranes, backhoes, skid-steers, rough-terrain forklifts, and wheel loaders.

While most hydraulic magnets are circular units designed for sweeping and lifting tasks, one New York-based magnet manufacturer offers quick-coupling magnets equipped with three teeth on one side and two teeth on the opposite side. The teeth, combined with the unit’s direct-mounting feature, allow the operator to use the magnet for sorting material and raking the ground to dislodge embedded scrap, the vendor says. 

A Demo Contractor’s Dream

Vendors of hydraulic magnets list several reasons why these tools are more popular among demolition contractors than among traditional scrap processors. For one, demolition operators often use excavators equipped with an auxiliary hydraulic circuit to power attachments such as hammers and concrete pulverizers. Thanks to that circuit, the user can connect the hydraulic magnet to the excavator stick, attach the hydraulic hoses, and start operating. “This ‘plug-and-play’ convenience factor is really what sells this product,” the New York magnet vendor says. This technology, the Canadian magnet producer adds, allows users to “easily and quickly move the magnet assembly from one machine to the other,” so the demolition contractor can share one hydraulic magnet among several excavators rather than install a magnet generator on each machine. That also saves the contractor from having to dedicate one machine to magnet work, a machine the contractor would have to haul wherever magnet work is needed. Instead, the contractor only needs to transport the hydraulic magnet and attach it to the excavator that’s on site.

Hydraulic magnets also are a good fit for demo contractors based on the type of scrap handling they do. “This equipment is most well-suited to the demo type of cleanup application,” says a Kansas equipment dealer that handles hydraulic magnets. “They’re sweeping up after they’ve torn down a building, picking up rebar, and collecting scrap their torchmen have cut. They’re not taking scrap from one big pile and loading trucks or rail cars all day long.”

Among traditional scrap processors, hydraulic magnets are fine for lower duty-cycle work, such as “facilities that aren’t doing a lot of loading or unloading with a magnet, or mobile scrap units that go out and do cleanups at factories and farms,” the New York vendor says. They aren’t as well-suited for the rigors of high-volume scrapyards, however. “Hydraulic magnets aren’t a fast-acting system, so most of your typical scrapyards don’t use them,” the Canadian magnet maker says. Why the difference in response speed? Hydraulic magnets rely on the flow of hydraulic fluid and generator speed—which regulate the electrical current—to control the magnetization/demagnetization cycle. “When you turn off the flow of hydraulic oil, the magnetic field has to collapse as the generator winds down; then when you turn the flow back on, there’s a delay getting the generator back up to speed,” the Kansas equipment dealer explains. That short delay isn’t as problematic in demo scrap cleanup work as it might be for a scrap­yard crane operator who has to load tons of scrap in a matter of hours.

Also, hydraulic magnets are heavier than lifting magnets alone, which reduces the weight the base machine can lift when fully extended and how far it can safely reach when loaded. The magnet and the hydraulic gen set together can weigh from 800 to 7,000 pounds, depending on their size. “If you’re working in a small range—such as with demolition cleanup work—then the reduced lifting range matters less than if you need the max lifting capacity at max reach,” the Kansas equipment dealer says.

Another factor limiting hydraulic magnet use in traditional scrapyards is that most new, purpose-built material handlers “come from the factory with a gen set already built on the carrier,” the New York manufacturer says. “At that point, it’s cheaper and more sensible for the owner just to buy the magnet.”             

Going With the Flow

Hydraulic fluid is the lifeblood of these magnets, so it’s no surprise that flow issues are their most critical concern. The amount of flow a hydraulic magnet requires—in terms of gallons per minute—depends on the size of the generator and the amount of energy it must generate to power the magnet. Typically these generators need to spin in the 3,000 rpm range. Larger generators require higher flow, which produces the necessary torque to spin the hydraulic motor and generator. Most hydraulic magnets on the market have a flow capacity between 5 and 66 gpm and a pressure rating from roughly 1,500 to 3,700 psi.

A consistent, uninterrupted hydraulic flow to the attachment and back to the fluid tank is essential. “You need priority flow to the magnet so other functions don’t rob hydraulic flow from the magnet, or it could drop its load,” the Kansas dealer says. Further, “too much variation in the flow could cause a disruption in power,” the New York vendor adds. “It’s a concern, but it’s a concern that’s not difficult to overcome if the system is designed properly.”

On the inflow to the gen set, some hydraulic systems can’t provide fluid at a constant flow rate, so the flow control valve compensates, allowing more fluid to enter when the flow is lower and bypassing the fluid when there’s an overflow. In modern excavators, operators can set the amount of hydraulic flow that goes into such attachment lines, which is a beneficial feature. If 50 gpm flows into a gen set designed for 30 gpm, then it must bypass 20 gpm; but if the operator can direct only 35 gpm to run through the system, then it must bypass only 5 gpm. The less fluid the system must bypass, the less heat it generates and the fewer potential flow problems it faces. “We always try to limit the total amount of hydraulic oil that goes into the system,” the Canadian vendor says. “That can be difficult to do in older machines not equipped with an electronic control monitor.”

Hydraulic flow generally isn’t a problem if the base machine has a unidirectional auxiliary hydraulic circuit or a bucket hydraulic line with a free-flow return line. If the line is bidirectional on the return—meaning that fluid can reverse direction to accommodate machine or attachment operations—too much back pressure could develop in the line, which could “take out the seal on the hydraulic motor,” the New York vendor says. Although it’s possible to relieve high pressure on the return by using a bleed line, it’s preferable to have an unimpeded flow back to the hydraulic fluid tank, sources say. “You want the return line to bring the fluid back as quickly as possible to help the magnet drop its load, but if you add a lot of back pressure to the return line, then the discharge process takes longer,” the Kansas equipment dealer says. The Canadian vendor notes that even though its hydraulic motor and generator can handle a bidirectional flow and generate power in both directions, “it’s just easier to install as a unidirectional setup.” As a result, the company installs a low-pressure check valve in the hydraulic tank line to prevent fluid from backing up into the motor.

Another flow-related issue pertains to the size of the base machine itself: It must have enough horsepower and hydraulic flow capacity to accommodate the demands of a priority circuit and the magnet’s power requirements. A general rule is 2 hp of machine power for every kilowatt the magnet needs, so a 5 kW hydraulic magnet needs at least 10 hp of machine power, and so on, according to the Ohio magnet manufacturer. “You don’t want a magnet system that runs slow or is unable to reach its full output because then you end up with a weak magnet,” the Kansas equipment dealer adds. 

Designed for Durability

Hydraulic magnets can perform as well at scrap-handling tasks as magnets that draw from other gen set options, these vendors say. The hydraulic gen set simply provides electricity to the magnet, so the lifting capacity and general performance characteristics are more a function of the operator’s skill, the size and characteristics of the magnet itself, and the material being handled. For instance, is the magnet 32, 48, or 60 inches in diameter? Does it have a deep-field design? Is the magnet lifting turnings, loose scrap, or heavy-melting steel? “It’s pretty much the magnet and its design that makes the performance determination,” the Ohio magnet manufacturer says.

That said, some hydraulic magnet vendors identify lifting capacities for their equipment to give potential customers a feel for the units’ capabilities. One manufacturer says its 48-inch magnet equipped with a 15 kW hydraulic gen set, for example, can lift 1,750 pounds of No. 1 HMS, 1,200 pounds of No. 2 HMS, and 600 pounds of steel turnings. Its 57-inch magnet with the same 15 kW gen set can lift 2,700 pounds of No. 1 HMS, 1,775 pounds of No. 2 HMS, and 850 pounds of steel turnings. Other vendors provide similar lifting tallies for their units.

One important performance-related consideration is how well hydraulic magnets hold up under harsh scrap-handling conditions. After all, having the hydraulic gen set on top of the magnet exposes it to considerable shocks and potentially puts it in harm’s way. “Certainly there’s more vibration than there would be if it were mounted on the machine, but we’ve taken that into account in how we design and mount the products,” the New York manufacturer says. One vendor, for instance, positions the generator on rubber isolation mounts, top and bottom, sandwiching it between the top and bottom plates of the gen set. Then the entire gen set is enclosed in a heavy-duty, all-weather steel housing up to ½-inch thick to shield the generator from damage and moisture. By sitting on top of the magnet, the hydraulic gen set housing protects the electrical cable that goes from the magnet into the generator housing base plate. The two hydraulic cables entering the top of the gen set are more exposed, however, so the Kansas equipment dealer suggests wrapping them with protective tape or using armored lines. The above safeguards can’t protect the hydraulic magnet, however, if the operator uses it improperly, the vendors agree. “You don’t want the stick to beat on top of the magnet unit, and you can’t pack down scrap into a truck with this type of setup,” the Kansas equipment dealer says. “It’s not a wrecking ball.”

If treated and operated properly, hydraulic magnets are virtually maintenance-free, manufacturers say. The use of generators with brushless alternators means there are no brushes to change and no contact tips to replace. The generator’s heavy-duty sealed bearings have a long life span before needing to be replaced. “We’ve had bearings run 25,000 hours on these generators, and they’ve never been touched,” the Canadian manufacturer says. The main maintenance issues pertain to the hydraulic system, including checking the hose fittings, the condition of the hoses, and the cleanliness of the hydraulic fluid filters on the base machine. “The biggest problem with failures is just dirty fluid,” the Ohio magnet manufacturer says. “Same as with your car oil, you need to keep the hydraulic fluid clean.”

Given the above information, if you feel a hydraulic magnet could be right for your needs, be prepared to spend $10,000 to $40,000 for the gen set unit, not counting the cost of the magnet or installation. “For specific users, this technology is gaining in popularity quite a bit,” the New York vendor  says. “It isn’t a product for everybody, but if you want to take a magnet on and off quickly, if you want to share a magnet across multiple machines, this could be a good product for you.”

Kent Kiser is publisher of Scrap and assistant vice president of industry communications for ISRI.