Wire cages and guards: open area, stiffness and finish

What a cage or guard has to do

A wire cage or guard usually has four jobs: keep something in, keep something out, let air or light through, and look acceptable on the product. Those jobs pull against each other. A finer mesh keeps more out and blocks more air. A thicker wire is stronger and heavier. A tighter frame is stiffer and uses more material.

The design review should start with the job that cannot be compromised. A fan guard needs open area and finger protection. A machine guard needs rigidity and a safe edge. A decorative cage needs a consistent aperture and a clean finish. Once the primary job is clear, the mesh and the frame can be chosen against it rather than by habit.

Open area is a geometry calculation

For a square mesh, the open area is the clear aperture divided by the pitch, squared. With a 2 mm wire on a 20 mm pitch, the clear aperture is 18 mm and the open area is about 81 per cent. With the same wire on a 10 mm pitch, the aperture is 8 mm and the open area falls to about 64 per cent. Doubling the pitch does not double the airflow, but it moves it a long way.

The calculation is a starting point. Welded mesh flattens slightly at the intersections, and a coating adds thickness to every wire. A heavy powder coat can reduce the aperture by a noticeable amount on a fine mesh. If the open area is a performance requirement, calculate it on the coated wire and confirm the figure with the supplier.

Stiffness comes from the frame as much as the mesh

A flat panel of welded mesh is flexible in bending and weak in shear. Most of the stiffness in a guard comes from the frame, the edge treatment and the way the panel is fixed to the product. A rolled edge, a formed channel or a wire rim turns a floppy panel into a stiff one without changing the mesh.

Triangulation is the other cheap move. A diagonal brace, a corner gusset or a second plane of mesh stops the panel from racking. If the guard is a box, the corners do the work, and a box with open corners will distort even if the mesh is heavy. Look at the load path from the fixing point to the middle of the panel, and put material along that path.

• Roll or form the perimeter instead of leaving a cut edge
• Add a diagonal brace where the panel can rack
• Use a heavier wire at the frame and a lighter wire in the field
• Fix at four points, not two, and spread the fixings
• Keep the mesh plane flat so the load does not start with a buckle

Joints are where guards fail

A welded wire guard has dozens or hundreds of joints, and the quality of those joints decides the life of the part. A weld that is undercut, cold or missing will let a wire move, and a moving wire frets the coating and eventually breaks. The joint also creates a heat affected zone that can soften the wire and reduce its strength.

Specify the weld type and the inspection. For a safety guard, a visual check for missing welds and a push test on a sample may be enough, backed by a periodic destructive test. For a guard that carries load, add a pull or shear test on a coupon made with the same settings. If the guard is assembled with clips or rings rather than welds, inspect the clip closure and the ring tension.

Finish: galvanising, powder and PVC

Hot dip galvanising gives a thick zinc coating and good outdoor corrosion resistance, but it adds a rough surface and can block a fine aperture. The coating thickness is not uniform, and the process heats the part, which can relax a formed shape. Design the aperture and the drainage with the coating in mind.

Powder coating gives a smoother, coloured finish and a controlled thickness, usually much thinner than galvanising. It needs a clean, phosphate or conversion coated surface, and it can bridge a tight inside corner. A PVC dip gives a soft, thick coating that is useful on a handle or a guard edge, but it can hide a broken weld and it changes the aperture significantly.

For outdoor parts, the surface preparation matters as much as the coating. A galvanised part with a poor pre-treatment will fail at the cut edges and the welds first. Ask for the coating standard, the average thickness and the test method, and check the coating on a weld and a cut edge rather than on a flat face.

Safe edges, burrs and handling

A cut wire end is sharp. On a guard, the ends should be rolled into the frame, capped, or finished so that no wire end is exposed. A raw end also rusts first, because the coating is missing from the cut face. A plastic cap is cheap but can come off; a formed return or a welded rim is more durable.

Burrs on the mesh matter for handling as well as safety. A burr on a wire that will be coated can cause a coating holiday, and a burr on a guard that will be handled can cut a glove. Specify a deburr or a tumbled finish where the part is handled, and inspect the edges as well as the face.

What the drawing should state

A cage drawing should give the wire diameter with tolerance, the mesh pitch, the aperture, the overall dimensions, the frame section and the fixing points. Add the weld type, the coating standard, the coating thickness and the colour. State the load or the impact the guard must resist, and the test that proves it.

If the guard is part of a safety function, say so. That changes the inspection level, the traceability and the change control. A guard that is only decorative can be sampled; a guard that protects a hand cannot. The supplier needs to know which one is being quoted.