O-Clamps vs. C-Clamps: Choosing the Right Clamp for Stage Lights

In stage lighting, an O-clamp wraps all the way around a truss chord or pipe and holds a fixture from any angle, while a C-clamp (also called a hook clamp) hooks over a horizontal pipe and is tightened with a single bolt. Use O-clamps for moving heads, side-mounted fixtures and aluminum truss. C-clamps work well for PARs and other lighter fixtures hanging straight down from a steel pipe.

Both names here refer to hanging hardware for lights. They are not the hose clamps or woodworking C-clamps sold at hardware stores, and those should never hold a fixture overhead.

What an O-clamp is

An O-clamp is two curved halves that close around the tube and bolt together, so the clamp touches the tube around its whole circumference. Depending on the brand you will see it sold as a truss clamp, a half coupler, a wrap clamp or a quick-release clamp. A bolt or threaded stud on the outside attaches the fixture, usually through the fixture's yoke or an omega bracket.

Because the grip is all the way round, an O-clamp holds in any direction. You can hang a fixture underneath a horizontal truss, mount it on the side of a vertical upright, or stand it on top, and the clamp does not rely on gravity to stay seated. The wide contact area also spreads the force over the tube instead of pressing on one point, which matters on aluminum truss with thin-walled chords.

Most O-clamps for lighting are sized for 48–51 mm tube. That range covers standard 50 mm truss chords, 48.3 mm scaffold tube, and 1-1/2 inch schedule 40 steel pipe (1.9 in outside diameter). Smaller versions exist for 32–35 mm tube used on light-duty truss and stands.

What a C-clamp is

A lighting C-clamp is a cast or forged hook shaped like the letter C or J. You hang the hook over a horizontal pipe, then tighten a wing bolt that presses the pipe against the inside of the hook. The fixture's yoke bolts to the bottom of the clamp, and a second bolt lets you pan the fixture before you lock it.

The design dates back to theater pipe battens, and it is still the quickest way to hang and focus a row of PARs on a steel pipe. Its limits come from the same design:

• It is meant for loads hanging straight down from a horizontal pipe. Sideways or upward forces can rotate it on the pipe.
• The wing bolt presses on one small spot. Overtightened on aluminum truss, it can dent or mark the chord.
• If the bolt is not tight, a bump or vibration can let the clamp slide or rotate around the pipe.

O-clamp vs C-clamp: side-by-side

Which clamp for which fixture

Moving heads and beam lights

Use O-clamps. A moving head throws its weight around when it pans and tilts fast, and those forces push sideways on the clamp. If the head could reach the truss or nearby scenery at the end of its travel, set a pan and tilt limit in the fixture menu or console. Most moving heads mount through an omega bracket, a flat plate that locks onto the base with quarter-turn fasteners and takes one or two clamps. For a fixture in the class of ZQ02253 285W moving head light, use both clamp positions on the bracket if the manual allows it, and check the fixture weight in the manual against the clamp rating.

PAR lights and wash lights

Either clamp works. A single PAR with a standard yoke hangs well from one C-clamp on a steel pipe. On aluminum truss, or when the PAR is mounted sideways on an upright, use an O-clamp.

Small effects lights

Lightweight clamps sold as "party clamps" or mini clamps are made for small fixtures on smaller tube and stands. They are fine for a compact effect light within their rating, but they are not for full-size moving heads.

How to check a clamp before you trust it

1. Read the rating. A lighting clamp should have its working load limit stamped on it or listed by the maker. Add up the fixture, bracket, clamp and cable weight and stay well inside the rating. If a clamp has no rating at all, do not hang anything over people with it.
2. Match the tube size. Measure the tube's outside diameter. A 48–51 mm clamp on 35 mm tube will not close properly and can slip.
3. Check the bolt. Use the bolt size the clamp and fixture were designed for, commonly M10 or M12, or 3/8 in or 1/2 in in North America. Use a nut with a lock washer or a nylon insert so it cannot vibrate loose.
4. Tighten properly. Snug the clamp with a wrench so it cannot rotate by hand. Do not overtighten until the tube deforms. Follow the maker's torque figure if one is given.
5. Inspect for damage. Retire clamps with cracks, bent hooks, stripped threads or worn wing bolts.
6. Add a safety cable. Every overhead fixture needs a rated safety cable from the fixture's attachment point around the truss or pipe, independent of the clamp. Keep the drop short so a failed clamp cannot let the fixture fall far.

Common mistakes

• Using a woodworking C-clamp or G-clamp, a hose clamp or a cable tie to hold a light. None of them are rated for overhead loads.
• Hanging a C-clamp off a vertical truss upright or a diagonal brace, where the load pulls sideways and the clamp can spin.
• Mixing clamp sizes in one bag so nobody knows which ones fit the truss in use. Keep 50 mm clamps and 35 mm clamps separate and labeled.
• Clamping to things that are not designed to carry load, such as decorative pipe, curtain track, venue railings or ceiling grid. Ask the venue which points are rated.
• Skipping the safety cable because the clamp "is tight." Clamps are tightened by hand, and hands make mistakes during a rushed setup.

FAQ

Can I use a hardware-store C-clamp to hang a light?

No. Woodworking C-clamps and G-clamps are built to hold parts together on a bench, not to carry a load overhead, and they have no load rating for that use. Use a lighting C-clamp or O-clamp with a stated rating, plus a safety cable.

What size O-clamp fits truss?

Most 290 mm and 300 mm box and triangle truss uses 50 mm chords, and most O-clamps for lighting cover 48–51 mm. Light-duty truss and some stands use 32–35 mm tube and need the smaller clamp size. Measure the tube if you are not sure.

Do I need a safety cable if I use an O-clamp?

Yes. The safety cable is backup for the whole mounting, including the clamp bolt, the bracket and the fixture's own mounting points. Every fixture hung overhead gets one, whatever clamp holds it.

Is an O-clamp the same as a scaffold coupler?

No. Scaffold couplers join one scaffold tube to another and are part of the structure. A lighting O-clamp, or half coupler with a bolt, attaches a fixture to the tube. Some fit the same 48.3 mm tube, but only use the one made and rated for hanging fixtures.