Overhead Cranes for Special Circumstances
Cranes equipped with hoists and suction lifters have, quite literally, taken a large part of the workload off stonemasons’ shoulders. But not every crane is ideal for every business. Here’s what you need to know.
The range of crane solutions for stone-processing businesses is enormous: It ranges from lightweight cranes with a maximum load of 250 kilograms for light slabs and workpieces to permanently installed or rail-mounted double-girder gantry cranes, which can have a lifting capacity of 500 metric tons or more. While the latter are primarily used outdoors for handling raw blocks weighing over 25 metric tons, the machine shops of somewhat larger stonemasonry businesses typically feature overhead cranes for the further transport of stone processed into slabs (slices and rough slabs). These cranes, available in single-girder or—for upper transport areas—double-girder configurations, typically run on elevated crane tracks along the hall walls. One of these most common crane types is in service, for example, at Heuger Fliesen & Naturstein GmbH in Glandorf. With a load capacity of two metric tons and a span covering the entire width of the hall, the Scheffer bridge crane is ideally suited for loading and unloading at the large machines of this Münsterland-based natural stone and tile specialty store, which operates its own manufacturing facility.
In addition, overhead cranes offer solutions for special hall configurations where the crane runway cannot be mounted on columns but must instead be attached directly to the hall ceiling. Wall-mounted overhead cranes (cantilever cranes) are used on their own crane runway below other types of overhead cranes on the second level of the facility—a suitable solution for serving multiple workstations simultaneously.
An interesting feature among overhead cranes is the semi-gantry crane, in which the upper girder travels along a conventional crane runway; the lower girder requires no floor-mounted guide rails, thus eliminating any protruding edges. Since this type of crane places only minimal loads on the building structure, it is suitable for retrofitting in older facilities. From an economic standpoint, the semi-gantry crane is characterized by comparatively low investment costs and, not least, by simple and quick installation.
Stationary overhead rail systems—available as single- or double-rail systems as well as single- or double-girder cranes—are designed specifically for on-site lifting and linear point-to-point transport, in contrast to overhead cranes that travel in two directions.
Column-mounted and wall-mounted jib cranes for smaller businesses
Smaller businesses, which typically have lower-ceilinged facilities, usually use column-mounted and/or wall-mounted jib cranes. They operate at the push of a button and can easily and gently handle loads of up to several metric tons, though in most cases they are limited to 500 kilograms or one metric ton. It makes no difference whether they stand on their own column or are mounted on walls or pillars. The former simply increases the working radius from 180 to up to 270 degrees or even 360 degrees. Whether loading machines, changing heavy tools, or lifting workpieces onto workbenches—swing cranes make lifting operations easier, more economical, and safer.
Finally, light-duty gantry cranes offer a cost-effective solution wherever lifting capacity is not required on a permanent basis and is often needed at different locations.
The various cranes are usually equipped with an electric chain or wire rope hoist, or with a spur gear block and a crane hook as the lifting device. This is where the various load-handling and slinging devices are attached, such as crossbeams, sling chains, lifting straps, vacuum lifters, and plate clamps.
Plate clamps for friction-based lifting
Plate clamps—which are clamps that close under load and generate their holding force through pressure and friction—are used both in conjunction with forklifts and in crane operations. They are attached to the forklift via a forklift boom, allowing the freely suspended panel to be rotated in front of the lift mast.
Transporting multiple panels simultaneously with the panel clamp is not permitted—for the following reasons: Even if there are sufficient friction coefficients between the panel and the clamping surface for the outer panels, the middle panel(s) are held in place solely by friction between the panels themselves. The holding force can be significantly lower in this case. Furthermore, the lateral compressive force exerted by the clamp halves increases as the load weight increases. If there are slight irregularities between the panels—for example, due to foreign objects or uneven saw cuts—a panel may break and fall off due to the high pressure. When transporting multiple panels with the panel clamp, there is therefore a smaller safety margin than when securing a single panel.
Variable Vacuum Lifters
Vacuum lifters are becoming increasingly common for transporting irregularly shaped panels as well as for intermediate transport within the machining process. Since vacuum lifters, like panel clamps, do not provide a form-fitting connection between the load and the lifting equipment—the load is held solely by suction forces—the load must never be moved over people.
In addition, it is essential to observe the different load capacity specifications for loads perpendicular to the suction surface (tear-off force) and parallel to it (slip force).
Unlike a panel clamp, a vacuum lifter equipped with multiple suction cups is highly likely to hold both panel sections together even if a hairline crack in the panel goes undetected. Furthermore, the load is distributed over a large area rather than concentrated at a single point, so tearing of the material is unlikely. Another advantage is that a slab can be placed flat directly onto the saw table without effort; a tilting table is not required.
Since a slanted stone slab may tip over when lifted with the vacuum lifter, the lifter should always be positioned above the slab’s center of gravity so that the bottom edge of the slab does not swing backward.
The Golden Rules ofTransport
– Never use lashing straps as lifting slings.
Lifting slings must meet different requirements and are specially manufactured.
-Slinging with an inverted angle of inclination is prohibited.
The slings may slip out from under the load.
-There are specific regulations for extending slings.
Lifting straps and round slings must not be knotted or extended by tying them together; they must only be connected using appropriate shackles. Special connecting elements must also be used with all other lifting slings!
-Always use slings of the same type and pay attention to the material as well! Chains, as well as polyester and polyamide lifting straps, stretch differently under load, which can cause the load to slip out.
-When lashing, always maintain the “natural” lashing angle of 120 degrees.
-Never attempt to retighten the sling after it has been secured.
The resulting frictional heat can damage the sling.
-Always follow the specific instructions for use for each lifting sling.
-Ensure that the lifting sling cannot be damaged during lifting. Pay particular attention to adequate edge protection.
-Never allow lifting straps or round slings to run loosely over the crane hook.
The load could tip over.
A more detailed report on transport options and workplace safety can be found in STEIN 1/2018.












