Guides / Design

Why your boom clamp slips, crushes, or will not come off

August 27, 2026

The clamp is the joint that holds the arm on. When it fails it fails in one of three ways, and each one comes from a different mistake on the drawing.

It slips. The bore was cut on nominal, the tube is undersize, the slot bottomed out before the clamp gripped.

It crushes the tube. The bore was undersize or the clamp is too narrow, so all the load went into a thin band of composite.

It will not come off. Corrosion product grew in the gap, or the clamp was hard anodised after boring and the tube was pressed in.

Start with the tube, not the model

Carbon tube is not precision stock. Pultruded and rolled tubes commonly run under nominal on outside diameter and are measurably out of round, and the variation between suppliers is bigger than the variation within one batch.

So the first job on a new airframe is to measure twenty tubes from the supplier you will actually buy from, in two planes, at both ends and in the middle. Now you have a real number and a real spread. The clamp is dimensioned from that, not from the tube’s catalogue diameter.

If you have not measured yet, say so on the RFQ. We will quote the clamp and hold the first article until the number arrives, which costs nothing, rather than cutting a bore to a nominal that does not exist.

How the clamp should work

A split clamp grips because the slot closes. That means three things have to be true:

1. The bore at rest is slightly larger than the largest tube you will use.
2. The slot gap is larger than the closure needed for the smallest tube.
3. The screws can produce that closure without yielding the clamp ears.

The common design error is a slot that is too small. The clamp faces meet, the screws torque up against metal, the torque wrench says everything is fine and the tube rotates in flight.

Bore it before you split it

The way to get a round, concentric bore in a split clamp is to machine it while the clamp is still solid, then separate the halves afterwards, usually by sawing the slot.

Two consequences for the drawing:

• The bore dimension before separation and after separation are different numbers. The saw removes material and the ears relax. Say which one the tolerance applies to. First article inspection should report both.
• The slot width on the drawing is the width after cutting, and the saw kerf is part of the design, not a detail.

A clamp bored after being split is machined into a shape that is no longer round when it closes, which is how you end up with a clamp that grips at two points and crushes there.

Contact area and crush

Carbon tube is strong along the fibres and weak across them. A narrow clamp concentrates load into a small band.

• Make the clamp wide enough to spread the grip. Something near the tube diameter is a good starting point.
• Break the edges of the bore. A sharp clamp edge is a stress riser cutting into the tube wall.
• Avoid point loading from a screw head sitting directly over the tube.
• If the tube is thin walled, consider an internal sleeve at the joint so the clamp squeezes something that can take it.

The corrosion question

Aluminium clamping carbon is a galvanic joint, and on an agricultural or coastal aircraft it is the joint that fails first. Options are an insulating sleeve, a sealed hard anodise, a titanium clamp or a sacrificial part. All of them change the bore dimension, so decide before the drawing is released. The detail is in aluminium and carbon fibre joints.

Material

7075-T6 for most clamps. The ears are thin and take bending every time the clamp is torqued, and the extra yield strength is exactly what that needs. 6061 works for lightly loaded clamps and for anything that has to be welded. Hard anodise on a 7075 clamp is close to standard on an aircraft that flies in weather.

What a clean clamp drawing says

• Nominal tube diameter, the measured range, and which supplier the range came from.
• Bore dimension and tolerance, with a note saying whether it applies before or after the slot is cut.
• Slot width and the closure available.
• Screw size, count and assembly torque.
• Sleeve or insulating layer, with its thickness, if there is one.
• Finish, with the bore masked or machined after coating.
• Clamp width and edge break.

That list turns a clamp from the part that comes back twice into the part that fits the first time. Send the model and the tube measurements through the RFQ page and we will reply within 24 business hours, including the question of whether the bore should be finished before or after anodising.

Frequently asked questions

Keep reading

• UAV component machining: what to put on the drawing
• Taking weight out of a machined drone part without losing the part