LOOK NO HOLES

A few issues back we described the almost-infinite variety of ways in which that common load - a coil of wire - can be attached to a hook to be lifted. A load almost as common is the flat sheet of metal or of plywood or of glass, but unlike a coil, a sheet offers no convenient hole to act as a place to grip and lift. Devices for lifting sheets have to therefore attach themselves to the edges, with some sort of mechanical clamp or gripper or lifting ledge, or to the surface, using vacuum pads or, if the load is of steel, magnets. All these approaches work, some better than others according to the application. It is not surprising therefore that there are almost as many below-the-hook devices for lifting flat sheets as there are for lifting coils.

An early distinction can be made: do you want to lift your sheet in a horizontal orientation, parallel to the ground, or set vertically, upright like a windowpane? There are devices for either, and some that will do both.

Camlok, original inventors or the self weight clamping concept back in 1958, devotes no fewer than nine pages of its 65th-anniversary catalogue to vertical plate clamps, and a further fice to horizontal ones - which may be an indication of the relative use of the orientations. Vertical clamps grasp the sheet by the top edge. Working loads limits of their various models range from 50kg to 3,000kg: for moving long plates vertically they recommend using two clamps with a spreader beam between them. The same clamps can also be used for lifting and moving constructions - that is, your steel sheets after they have been welded together to form, for example, a vertical box-shape - and for profiles.

Vertical clamps, Cmlok's and others, need some mechanism to close the jaws. On Camlok's 92-series devices the jaw is opened and closed with a manual locking lever, except for its 92-500, the smallest in their range, with a 50-500kg working load limit, which uses a positive spring loaded cam. The safety lock spring loads the cam jaw, preventing the clamp from opening even when there is no load on the jaw. Jaw openings go up to 32mm, which would be the limiting thickness of the sheet you can lift with them.

Cmlok's CZ series vertical plate clamps are heavier-duty than their 92-series, with WLL up to 30,000kg. They again use a locking lever and spring-loaded safety lock. When lifting vertically, the teeth on such a clamp need to dig into the steel plate, to a certain extent at least, to give a grip: smooth-surface friction will not generally be enough for safety. For the teeth to dig in, the sttel must be slightly melleable. For that reason it is important to note that, for both Camlok ranges, the plate surface of the material being clamped must have a hardness level below HRC 30/Brinell 300. Similar clamps from other manufacturerers have similar requirements. Also note that such lifting will of course leave marks, probably visible ones, on the load.

Alternatively, you could use Camlok's HG (High Grip) plate clamp, which has an extremely high clamping force making the clmap suitable for plate with a surface hardness up to HRC 40/Brinell 375. There is also its TSB non-marking friction plate clamp (WLL 40-1,250kg), which does in this case work through the process of friction, achieved by a special material on the jaw and pad which is similar to brake linings. Another benefit of the TSB is that the same grab can be used for lifting different materials such as concrete, wood and stone, as well as hardened steel.

Camlok also manufacture the LJ non-marking plate clamps (WLL 25 - 1,500kg), which have rubber or leather pads according to application and are for structural steel plates, stainless steel, iron, timber and aluminium. They can also be used for steel plates with extremely hard surfaces. An obvious message from that huge variety of non-marking and hard steel devices is that they are highly specialised for particular applications and that it would be wise to take advice before choosing your option - which is exactly what Camlok recommends. Its sales team, they say, will be happy to advise.

There is a similary wide range in their clamps for lifting sheets horizontally. Here the gripipng issue is not so acute. The lower surface of the clamp can provide support without the help of friction. As long as the sheet is rigid, clamps on each side of the sheet can be joined by a two-legged chain sling which will provide the necessary inward force to keep them in place and to toggle the jaws closed. For handling larger plates it is recommended to use two such pairs in conjunction with a spreader beam.

The CH clamps have WLL from 1,000 to 10,000kg: the CH is their industry standard model. Camlok's horizontal-lift clamps are supplied as standard with smooth jawas. More specialised versions are availablej. There is the option of hardened steel serrated jaws for special applications; the RW/WH clamps have steel roller set into the jaws for non-marking applications. The ACH has adjustable jaws for lifting stacked plates; and so on.

Find the full article in the April-2024 issue of Hoist: