Flexible busbars: strip count, bend life and installation

What a flexible busbar is doing

A flexible busbar is a stack of thin copper strips. At each end the strips are consolidated into a solid terminal by welding, crimping or clamping. Between the ends they are free to slide and bend, so the conductor can accommodate movement between two points that are not rigidly connected.

The free section is the part that gives the flexibility, and it is also the part that fatigues. Every time the ends move relative to each other, the outer strips see a strain cycle. The design question is not simply how much current the stack can carry. It is how much current the stack can carry while surviving the movement it will see for the life of the equipment.

Strip count and current

For a given strip width and thickness, more strips mean more copper and more current capacity. They also mean a stiffer stack. A ten-strip link is not twice as flexible as a five-strip link; it is roughly twice as stiff in bending, and it needs more force to form the loop.

The current does not divide perfectly evenly between strips. The end consolidation, the strip length and the proximity of the strips affect how the current shares. A stack with a poor weld at one end can leave the outer strips carrying less current and the inner strips running hotter. The rating should come from a test or a calculation for the actual stack, not from multiplying the current of one strip by the count.

Strip thickness and bend radius

The strain on the outside of a bent strip is approximately the strip thickness divided by twice the bend radius. A 0.2 mm strip on a 5 mm radius sees about 2% strain. The same strip on a 2 mm radius sees about 5%. The second case is much harder on the copper, even though the difference in radius looks small.

That relationship explains why thin strips are used for flexible links. Halving the strip thickness halves the strain for the same radius, or allows the same strain at half the radius. It also explains why a stack of thin strips can bend where a solid bar of the same total thickness cannot.

The radius has to be measured to the inside of the bend, and it has to be held through the whole loop. A loop that is tight at one point and slack elsewhere concentrates the strain at the tight point. The first bend after the terminal is the critical one, because the strip is constrained at the weld and free immediately after it.

Bend life and fatigue

Copper fatigues when it is cycled beyond a certain strain. The number of cycles to failure depends on the strain amplitude, the strip thickness, the surface quality and the mean stress. A link that moves a few millimetres once during assembly is a different duty from a link that flexes every time a door opens.

For a static installation, the movement is mainly thermal expansion and a small amount of vibration, and the cycle count is low. For a moving part, the cycle count can be in the hundreds of thousands or millions, and the design has to be tested for that duty. A bend life test should use the actual stroke, the actual radius and the actual strip stack, because a test on a different geometry does not transfer.

The test result is a distribution, not a single number. Ask for the test conditions, the number of samples, the stroke and the failure criterion. A link that survives a million cycles at a small stroke may fail early at a larger stroke, even if the radius is the same.

• Define the stroke and the direction of movement.
• Define the cycle count for the service life, with a margin.
• Hold the bend radius through the full stroke.
• Test the actual stack, not a strip sample.
• Inspect the strip edges for cracks at the end of the test.

Installation geometry

A flexible link should be installed with a loop that sits in the plane of the strips. Bending in that plane keeps the strips sliding over each other. Twisting the stack separates the strips, opens gaps and concentrates stress on the edges. A twisted link can look acceptable at installation and fail after a few hundred cycles.

Leave enough free length for the loop to move without pulling on the terminals. The loop should be supported so that its weight does not hang on the joint, and so that it does not rub against a sharp edge or another conductor. A support that clamps the loop too tightly turns it back into a rigid link and moves the strain to the terminal.

The terminal end needs a flat, clean contact face and the correct torque. A flexible link is often used to bridge a tolerance gap, so the installer may be tempted to pull it into place. That preload is a permanent stress on the strips and the joint. The link should reach the terminal without being stretched.

1. 01

   Set the loop plane

   Keep the loop in the plane of the strips and avoid any twist between the terminals.

2. 02

   Set the loop length

   Give the loop enough free length to move through the full stroke without loading the terminals.

3. 03

   Set the radius

   Use a radius that keeps the strain within the fatigue limit for the required cycle count.

4. 04

   Support the loop

   Add a guide or support that carries the weight without clamping the strips.

5. 05

   Protect the edges

   Route the loop clear of sharp edges and use a sleeve where contact is possible.

Terminations and sleeving

The termination is where a flexible link becomes rigid. The strips are welded, crimped or clamped into a block, and the transition from free strip to solid block is a stress concentration. A weld that is too brittle, a clamp that is too loose or a block that is too short will shorten the life of the link.

Sleeving protects the strips and the surrounding equipment, but it also changes the thermal behaviour. A tight sleeve traps heat and can prevent the strips from moving freely. A loose sleeve can rub. The sleeve material has to suit the temperature and the environment, and the design has to leave room for it at the terminal.

For a high-current link, ask for a millivolt drop check across the terminal and a visual check of the strip edges. For a moving link, add a bend cycle test on a sample and inspect for broken strips at the end.

What to put on the specification

A flexible busbar specification should state the current, the ambient, the conductor material, the strip thickness and width, the strip count, the stack height, the terminal type, the plating, the insulation and the movement duty. The movement duty is the item most often missing, and it is the one that decides the bend life.

Add the installation envelope: the loop length, the minimum radius, the plane of movement and the support points. If the link is part of an assembly that will be fitted on site, state how the installer should position it. A good link can be ruined by a bad installation.