One-hole vs two-hole lugs: vibration and rotation

What the second bolt is for

A one-hole lug is held by a single stud. Friction between the palm and the terminal resists rotation, and that friction comes from clamp load. A two-hole lug is held by two studs, so the palm cannot rotate about either one without moving the other. Orientation is set by the hole pattern rather than by friction.

That is the whole mechanical argument, and it is enough to explain why two-hole lugs dominate transformer, switchgear and busbar connections. The second bolt is not there to carry twice the current. It is there to keep the palm in place and to share the clamping load across a larger area.

The cost is real: a wider palm, a second hole, a second bolt, a second torque operation and a terminal that must have the matching hole spacing. On a small terminal block there may be no room for any of it.

There is also a tooling cost that buyers often miss. A two-hole lug needs a two-stud fixture or a second bolt access, which changes both the assembly station and the service procedure. If the second bolt sits behind a cable or against a wall, the maintenance team may not be able to reach it with a torque wrench, and a joint that cannot be checked is not really a two-bolt joint.

What rotation does to a joint

A lug that rotates even a small amount changes the contact area under the palm. The contact spots that carried the current move, and the new spots start with oxide and contamination between them. Contact resistance rises, the joint runs hotter, and thermal expansion adds more movement.

Once movement starts, the failure can accelerate. Repeated micro-motion frets the plating, exposes base metal and creates oxide debris. In a humid or salty environment the debris holds moisture. The joint does not fail in one event; it drifts upward in resistance until the temperature rise becomes visible.

Rotation is not only a current problem. A lug that turns can pull the cable against an enclosure edge, reduce phase clearance, or load the barrel in a direction the crimp was not designed for. On a vertical busbar, a heavy cable hanging from a loose one-hole lug can rotate the palm under its own weight.

Where a one-hole lug is the right call

A one-hole lug is compact, cheap and fast to install. It suits terminals with a single stud, small cable sizes, and joints where the cable is supported so it cannot apply a turning moment. Control panels, earth bars and low-current terminations are typical.

It can be reliable under vibration if the clamp load is maintained. That means a flat, clean terminal surface, the correct torque, and a washer that keeps tension under thermal cycling. A spring or Belleville washer is often more useful than a second bolt on a small joint, because it maintains load as the joint expands and contracts.

The risk is that the installer cannot see clamp load. A torque wrench records the applied torque, not the tension that remains after a year. On a critical joint with a single bolt, add a torque mark and a re-torque check to the maintenance schedule.

Where a two-hole lug earns its cost

A two-hole lug fixes orientation and spreads the clamp load. On a transformer terminal or a switchgear busbar, that prevents the palm from turning when the cable is pulled during installation or service. It also keeps the phase spacing stable, which is an insulation requirement as much as a mechanical one.

Two studs also change the failure mode. With one bolt, a loss of preload frees the joint. With two, the palm is still located by the second stud, so a partial loss shows up as a resistance rise rather than an immediate movement. That gives maintenance a chance to find the problem before the joint overheats.

The hole spacing must match the terminal exactly. A lug with the right stud diameter but the wrong spacing will not seat, and forcing it distorts the palm. Ask for the terminal drilling drawing, not just the stud size, and check the palm width against the available land.

Two-hole lugs also help where the joint sees cyclic load. Two bolts share the preload, so a small loss of tension in one does not immediately free the joint. The palm is larger, which reduces contact pressure for a given torque, so the torque value must come from the lug manufacturer rather than from the single-hole table.

Vibration, washers and torque

Vibration loosens a bolted joint when the clamp load drops enough for the parts to slip. The thread does not unwind on its own; the joint loses preload through embedding and movement, then the bolt can turn. A second bolt reduces the chance of slip because the palm is constrained in two places.

Hardware matters as much as the bolt count. A flat washer spreads the load and protects the palm. A spring washer or a Belleville washer stores elastic energy so the clamp load survives thermal movement. Thread locking compound helps, but it does not replace correct torque and a flat surface.

Torque values must come from the lug and stud supplier. A value copied from a bolt table can crush a thin palm or leave a thick one under-tightened. Record the torque, the wrench calibration and the date on the inspection sheet, and mark the fastener so a later check can see movement.

The electrical side of the choice

Both patterns can carry the same current if the contact area and the material are the same. The palm of a two-hole lug is larger, so the current has more path, but the improvement is modest compared with the effect of a clean, flat joint at the correct torque.

Where the two-hole pattern helps electrically is stability. A joint that cannot rotate keeps its contact spots, so the resistance stays closer to the value measured at commissioning. That is why a two-hole joint is easier to defend in a temperature rise test or a routine maintenance programme.

Do not use a second bolt to compensate for an undersized palm or a plated surface that cannot take the current. If the joint is hot at the design current, the answer is a larger contact area, a different plating or a larger conductor, not another fastener.

There is a practical test for the choice. Ask whether the palm can rotate by hand after the joint is torqued. If the answer is no, and the cable is supported, the second bolt adds little. If the palm can move, or the cable can pull it, the second bolt is doing real work and should not be removed to save space.

What to put on the drawing

Specify the stud size, hole diameter, hole spacing, palm width, palm thickness and palm angle. Add the terminal drilling reference and the torque value with its source. If the joint is in a high vibration location, state the washer type and whether a re-torque check is required.

On the inspection sheet, record the torque, the wrench calibration, the hole spacing and a visual check for palm flatness. For a critical joint, add a contact resistance measurement at commissioning so a later rise can be compared against a baseline.

• Stud size and hole diameter for each hole
• Hole spacing and its tolerance
• Palm width, thickness and angle
• Terminal drilling drawing reference
• Torque value and the standard it comes from
• Washer type and thread treatment
• Contact resistance baseline, if the joint is critical