Plate clamp basics for fab shop crane operators

A plate clamp comprises a stationary lower jaw and an upper jaw that moves in a cam action. The closer the load is to the maximum (but not over the maximum), the more gripping power is sent to the teeth and the more secure the load is.

When fab shop clients call Peter Cooke about why a plate clamp rigging setup under an overhead crane supposedly overloaded and failed, he can pretty much guess what happened right away. President of National Rigging & Crane Training LLC, a training and consulting firm near Buffalo, N.Y., Cooke knows that if the plate clamp had really overloaded, the fab shop would know it, and it probably would have no reason to call him. With plate clamps, the problem often isn’t about a load being too heavy; it’s about the load being too light.

Versatile and simple in design, plate clamps—including those dedicated to plates, structural beams, and other shapes—can speed material handling in a variety of metal fabrication operations, especially in the heavy, industrial, and structural arenas. When suction lifts and magnets aren’t sufficient or available, plate clamps offer an efficient alternative to slings. On some workpiece geometries, plate clamps eliminate the need to create lifting points by drilling holes to attach some type of hitch.

“Plate clamps are very safe and efficient,” Cooke said, “and can speed many operations over conventional riggings with slings. But learning to use them is like driving a car. If you know and follow the rules of the road, driving can be safe. If you don’t, getting behind the wheel can be dangerous. The same thing goes when working with plate clamps. Not playing by the rules can put you in serious danger.”

To use plate clamps efficiently, crane operators need to know how to select, inspect, and use them safely. If they do all three, operators can safely and reliably speed the flow of material from one fabrication process to the next.

Selecting a Plate Clamp

When Cooke receives a call from operators asking why their plate clamp “failed” when the load clearly was below the maximum limit (the working load limit, or WLL), Cooke sometimes follows up with a question that crane operators aren’t expecting: Did the plate clamp rip apart? Almost without exception, operators say no, of course it didn’t. The operator lifted a seemingly light load, and then the thin plate just dropped out. What gives?

“Traditionally, we oversize things in rigging,” Cooke said. “With shackles, if you need to lift an object that’s less than half a ton, you could go with a half-ton shackle or a 1-ton shackle. In fact, the best practice is to oversize and go with the 1-ton shackle to make sure nothing can go wrong. But that’s not the case with plate clamps.”

Unlike most other rigging equipment, plate clamps have both an upper and lower load limit—that is, both a maximum and minimum load. In fact, a plate clamp is safest when the workpiece weight is close to, but does not exceed, the maximum load. If the load is too light, the plate clamps lose their grip.

To understand why this happens, it helps to know how plate clamps work. As the lift starts, the teeth bite into the plate. As Cooke put it, “The teeth become one with the object.” The greater the load, the greater that biting force becomes. At just below the load limit, the teeth’s contact with the plate is the most secure it can be. Go heavier than the load limit and, eventually, the clamp will rip itself apart and create an extraordinarily dangerous situation. However, lifting a load that’s lighter than the minimum load lessens the teeth’s gripping force, making it more likely for the clamps to drop the load.

Why this happens goes back to how exactly the teeth penetrate the plate. Lifting a lighter load exerts less force on the clamp. Combine this with a clamp designed for heavier loads (it has wider teeth, which creates a larger gripping area), and the teeth don’t have enough force behind them to exceed the material’s yield point and penetrate the material surface. When the teeth fail to penetrate, the plate clamp can lose its grip.

“The stronger the grip, the deeper the teeth bite,” Cooke said. “The weaker the grip, the less the teeth penetrate. That’s why we have minimum load requirements for plate clamps.”

That contact between the teeth and plate makes the lift secure. This means they’re not designed to clamp two or more plates sandwiched together. If the clamp’s teeth aren’t in direct contact with the material surface, the lift isn’t secure.

A crane operator must be aware of what happens to the load during the lift sequence, including the instance of no load when lifting a plate from the flat to vertical position using a vertical plate clamp.

As Cooke explained, it’s always best practice to choose a plate clamp as close to (but not exceeding) the maximum load as possible. It’s analogous to using a pair of pliers versus a 14-mm wrench to loosen a 14-mm bolt. Pliers can do the job in a pinch, but they aren’t as secure or efficient as using a 14-mm wrench. Similar thinking goes for plate clamps.

Beyond the weight of the load, crane operators also must consider material characteristics. This includes hardness. Harder material has a higher yield point, which means it will take more force for the plate clamp’s teeth to penetrate the metal surface.

Cooke added that the workpiece’s material type plays a role too. A plate clamp designed to transport carbon steel isn’t necessarily designed to carry stainless steel. “Stainless steel accelerates the rate of wear on a clamp’s teeth,” Cooke said, “and you’ll end up wearing a traditional clamp out quickly.”

If a shop needs to move stainless steel, and the use of suction grips isn’t an option, a crane operator can use a high-grip clamp, which has a smaller profile and a narrower mouth, with a cam action that creates a higher-force grip without the teeth becoming worn as quickly.

Another option for stainless, as well as for high-strength steel (especially when the material surface hardness exceeds around 32 RC) is a nonmarring clamp, which has a leather jaw. “That leather jaw applies a nice grip based on friction,” Cooke said. “Because this clamp doesn’t have teeth, it can live with lifting slightly lighter material.”

He cautioned, however, that dust, oil, or mill scale—any of which can create a slippery surface for the leather jaws of a nonmarring clamp—must be cleaned off before applying a nonmarring clamp to the material surface. (Note that mill scale can cause a traditional clamp not to engage properly as well.)

One final material consideration is its temperature. “You never want to use plate clamps to lift material that’s 250 degrees F or higher,” Cooke said. “Excessive heat will affect the integrity of the clamp.”

Plate clamps come in various configurations. There’s no mystery to it, really. Vertical plate clamps are designed to lift vertically; horizontal clamps are designed to lift horizontally. Other clamps are designed for lifting and flipping structural beams and other shapes. When crane operators use the wrong clamp for the wrong lift—such as using a vertical plate clamp to lift an I-beam—they can induce loads that those clamps were never designed to endure.

Operators also need to select equipment and set up rigging to prevent excessive side loads that could stress the teeth and create an unsafe lifting situation. For instance, many common vertical plate clamps do not have hinges on them, which introduces other factors when picking long workpieces that require two clamps. Attaching those clamps to a bridle sling or similar rigging equipment can introduce side loads that can be detrimental to the lifting operation.

Operating a crane isn’t just about pressing buttons. Operators need to know when and how to press those buttons and secure the load for the smoothest lift. All this requires training.

Similarly, when using four horizontal clamps to lift a large, wide plate in the flat (horizontal) position, operators should avoid connecting those plate clamps to a quad sling. Such a sling can twist the horizontal clamps, which can create an insecure lifting situation and wear the clamp teeth prematurely.

Instead, operators should use a spreader bar with two double-leg slings. Also, for any rigging setup using horizontal plate clamps, the two legs of the sling should be at least 90 degrees apart. Any narrower and the load can become unstable.

Vertical clamps with hinges can endure the side loads from, say, a double-chain sling to use two clamps to lift a long plate. That said, the angle of side loading affects each clamp’s load limit (from 50% at a near horizontal side load to 100% at the vertical position), so this rigging setup does add a dose of complications.

Where Plate Clamps Don’t Work

As Cooke explained, plate clamps are for lifting, not holding and positioning. Say an application involves lifting a long cut plate from a burn table, then placing it vertically over a weld fixture. That sounds simple enough, but what happens if the plate catches or binds on fixture components? The fixture takes part of the load, possibly reducing the load at the plate clamp well below the minimum. “There’s a good chance for that clamp to let go,” Cooke said, “because it’s clamping on to something with little or no weight.”

In these cases, some clamping redundancy might be in order. A crane operator might use both a vertical plate clamp for lifting but also secure the load with a pull clamp, which applies pressure to the work and maintains its hold as it enters a fixture.

A similar setup could apply when manipulating large plate at a press brake. Used alone, plate clamps won’t be a safe option, because they will effectively let go as soon as the brake tooling applies pressure. When the bend cycle completes, the plate could fall out of the loose clamps and drop. In these cases, pull clamps can maintain their hold on the plate throughout the bending cycle.

Operator Technique

Crane operators should never use a vertical plate clamp to lift horizontally (that is, with the plate in the flat position), but they can use a vertical plate clamp to lift a plate from the flat to vertical position. In these instances, however, the vertical clamp can safely hold only half its maximum load. Moreover, reorienting plates in this way adds another layer of complexity, mainly because the load’s weight changes as it rises from the flat to vertical position.

For instance, when the lift initiates on a 1,000-kg plate in the flat position, the vertical plate clamp’s load is only half the plate’s weight, since the floor is supporting the plate’s other edge and, hence, is taking half the load. For a brief instance, when the plate nears the vertical position, the floor is taking the plate’s entire load, which means that the vertical plate clamp is carrying no load whatsoever.

This is where crane operator technique and finesse come into play. A good crane operator lifts the plate to its vertical position and quickly lifts the plate off the ground, so that the entire load returns to the plate clamp. If the operator fails to do this, the plate might tip out of control. At worst, the operator loses the load.

Clamp Inspection

All elements of a plate clamp should be regularly inspected. Operators should remove debris and dirt, and before lifting, they should inspect the surface areas around the clamp’s teeth. Dirt around the teeth clog up the clamping action and prevent the teeth from attaining maximum penetration, which in turn affects gripping force.

The more worn a plate clamp’s teeth are, the less gripping force they have. According to industry standards, chipped teeth are safe to use only if the chip covers half the tooth’s width, and both adjacent teeth are undamaged.

Cooke added that a good crane operator will know which teeth engage what material thickness. The bottom teeth of the plate clamp remain stationary, but the upper jaw closes down onto a plate with a cam action. Different teeth will engage depending on the plate thickness, and operators can simply position the clamp and turn the cam to create a certain gap to see which teeth will engage. For instance, an operator can turn the cam until the gap between the upper and lower teeth is 0.25 in. By doing that, he knows which teeth will engage when lifting 0.25-in. plate and can ensure that those teeth are in good shape before beginning a lift.

If the teeth are in good shape and aren’t worn, they will penetrate as intended, which makes them secure to resist movement during the lift. That is, the teeth penetrate the plate and stay put until the lift is complete. Once the teeth’s grip moves slightly under load, clamp wear accelerates.

The Wear Triangle

Three elements cause wear: weight, movement, and lack of lubrication. So how does an operator minimize wear on a clamp? They can’t reduce the load, because that reduces the plate clamp’s gripping force and makes the lift less secure. They can’t add lubrication; the clamps need as much grip as they can get, and lubrication can have a reverse effect on the grip. This leaves the last strategy in the wear triangle: minimizing movement between the teeth and the load it’s lifting.

Clamping with old, worn teeth increases movement, which is why plate clamp inspection is so important. Properly applying the clamp and other strategies to reduce movement involve the basics of rigging and crane operation.

Lifting With Finesse

The first is knowing a load’s center of gravity, the load’s balance point. For complex loads with cutouts and odd shapes, crane operators might rely on crane scales at different pick points. The center of gravity will always be closer to the heavier end and farther from the lighter end. Operators can perform some math to figure this out, use software, or rely on callouts on engineering drawings if they’re available.

“This should be a fundamental part of every crane operator’s training,” Cooke said. He added that rigging a component with the center of gravity in mind keeps the load steady and safe, minimizing that movement between the teeth and the plate surface they’re biting into.

Another key component is stabilizing the initial lift as the crane lifts the load off the floor or worktable. If the operator just lifts the load abruptly without thought or finesse, it can swing, twist, create a hazardous situation, and induce some serious teeth wear in the clamp itself.

“When you look at crane operators in action, it looks like they’re just pressing buttons,” Cooke said. “They press up to go up and down to go down. But it’s not like that. They’re almost like a musician. They’re playing an instrument. To control the load, they’ve got to hit those buttons at the right time, the right sequence, and the right speed. That comes with experience.”

Mastering the technique gets easier if operators have crane hoists with variable-frequency drives (VFDs), though VFDs still require training. VFDs allow operators to lift at any speed they’d like and accelerate smoothly. Hoists without VFDs require extra operator technique, including pulsing to speed to perform the initial lift. “Operators hit the buttons methodically at a certain speed to ensure a smooth operation.”

Cooke equated a hoist with a VFD to a car accelerator, where the driver can slowly press the pedal to increase speed gradually. Operating hoists without VFDs (which are less expensive and most common) can be like having a car with an accelerator pedal with two speed settings, 30 and 60 MPH. To accelerate smoothly, the driver would methodically tap the pedal, “pulsing” the acceleration up to 30 MPH (the middle setting), then pulsing again up to 60 MPH. The same goes, roughly speaking, for a crane operator lifting a load with a non-VFD hoist.

Crane Efficiency and Safety

Many fabricators go to great lengths to minimize crane utilization. Many times, installing conveyors, carts, or other point-of-use material handling methods can make a world of sense. After all, incredibly fast cutting, bending, and welding mean less if the job sits on the floor forever, waiting for a crane to lift it to the next station.

On the other hand, certain material handling setups like conveyors do take up significant space that otherwise could be used for production. And crane operation—including smart use of plate clamps, which can be much faster than other rigging methods—can be made more efficient too. Crane operators just need the right tools and training to make it happen.