Why bolted joints come loose under vibration, and what actually holds them

Bolted joints rarely fail because the bolt breaks. They fail because the joint loses preload. Transverse vibration lets the clamped surfaces slip against each other, and once slip begins the clamping force bleeds away. To stop bolts loosening under vibration you need a locking element that works by friction and mechanical locking at the same time. Original SCHNORR serrated safety washers do both, and in Junker testing they held their pretension force after 2,000 cycles.

Every maintenance engineer has seen the same failure. A joint that was torqued correctly at build comes back six months later with the nut finger tight, the paint cracked around the washer face, and the bolt head polished where it has been moving. Nothing is broken. The joint simply stopped clamping.

That is preload loss, and it is the mechanism behind most vibration related bolt failures. Understanding it is the difference between choosing a locking device that works and choosing one that only looks like it does.

What loosening actually is

A bolted joint is a spring. Tightening stretches the bolt, and that stretch is the clamping force holding the assembly together. Vibration, thermal cycling and fluctuating loads all attack that stretch. When the load acts across the joint rather than along the bolt axis, the mating faces move a few microns at a time. Each movement releases a little tension. Enough cycles and there is no tension left to release.

This is why a joint can loosen without anything rotating in an obvious way, and why a locking device that relies only on spring pressure against the nut face has so little effect once slip starts.

How a serrated safety washer holds the joint

The original SCHNORR safety washer was developed as a reliable and economic bolt locking device built on the principle of a disc spring. A serrated safety washer has a conical, closed ring form with a trapezoidal cross section and serrations on both faces, and its outside diameter is matched to the head diameter of pan head and socket head cap screws.

That geometry does two jobs at once. The serrations bite into the bearing surface and the nut or bolt head, giving mechanical locking. The conical spring form maintains contact pressure across the interface, giving locking through friction. Because the ring is closed, it develops a high degree of pretension without the burst open effect of a split washer.

• Uniform axial loading eliminates bending torques and deformation of the bolt stem.
• Serrations protect the mating components rather than gouging them.
• The washer maintains initial bolt tension instead of relying on friction alone.
• Hardness is controlled at 39 to 45 HRC (380 to 450 HV30), so the bite is repeatable.

What the Junker test shows

The Junker test is the standard way to compare locking devices, because it applies exactly the transverse movement that causes real world loosening. The Junker test results for SCHNORR safety washers are straightforward:

• Connections secured with SCHNORR safety washers maintained pretension force after 2,000 cycles.
• Locking performance was superior to split lock washers, detent edged washers, ribbed washers and spring washers.
• The original SCHNORR safety washer showed a locking effect equivalent to a wedge-locking washer.

That last point matters commercially. If you are specifying a wedge-locking washer alternative, the comparison is on equal footing for locking effect, with a wider choice of materials and finishes and a part that can be reused.

Torque to 100 per cent of the specified preload

No locking device compensates for an undertightened bolt. Testing shows that bolts tightened to only 75 per cent of the required initial force gave poor results, with a proportion of joints coming loose, and the effect was more pronounced again at 50 per cent. Tighten to the full initial force corresponding to the minimum required bolt tension.

Three practical notes follow from that. With a type S or type VS washer, the initial torque needs to be around 10 per cent higher than for a bolt with no securing device. Zinc plated washers do not need that increase, because the extra friction from the serrated surface is offset by the lubricating effect of the coating. And the scatter in initial torque is no greater with safety washers than in a joint with no securing device at all.

Can they be reused?

Yes. When a joint secured with SCHNORR safety washers is undone and retightened, the securing effect does not diminish. The initial torque drops very slightly, which means the same applied torque produces a marginally higher initial force. The washers can be used twice or more without restriction, which is useful on assemblies that are opened for scheduled inspection.

Where this shows up in service

SCHNORR components sit in joints where loosening is not an acceptable outcome: turbochargers, shock absorbers, brake force amplifiers, stationary railway clamps, ski lift cable clamps, emergency brakes in elevators, offshore drills and safety valves. The common thread is dynamic load and a consequence of failure that is measured in downtime or safety, not in parts cost.

If you are working through a joint that keeps coming back, the practical next step is to check the preload first, then the locking element. Our engineering team in Ann Arbor can help you match a washer type, material and finish to the joint.

• Bolted joints loosen under vibration through loss of preload caused by transverse slip between the clamped faces.
• SCHNORR serrated safety washers secure joints through a combination of friction and mechanical locking, based on the disc spring principle.
• In Junker testing, joints secured with SCHNORR safety washers maintained pretension force after 2,000 cycles.
• Their locking performance is superior to split lock, detent edged, ribbed and spring washers, and equivalent to a wedge-locking washer.
• Bolts must be tightened to 100 per cent of the initial force for the minimum required bolt tension. At 75 per cent, a proportion of joints came loose.
• Initial torque should be about 10 per cent higher than an unsecured bolt, except with zinc plated washers, which need no increase.
• SCHNORR safety washers can be reused two or more times without loss of securing effect.