2026-09-02

How to Use Lock Washers: Assembly Order, Torque, and Type Choice for Bolted Joints

Zhejiang Chance & Union Import and Export Co., Ltd.

Content

1 What a Lock Washer Does and What It Cannot Do
2 How to Use Lock Washers in the Right Order: A Five-Step Assembly Sequence
3 How to Use Lock Washers by Type: Split, Internal Tooth, and External Tooth
4 Torque and Surface Details That Decide Whether the Lock Holds
5 Where Lock Washers Are the Wrong Tool, and What to Use Instead
- 5.1 A lock washer fits when
- 5.2 Choose another method when
6 Frequently Asked Questions About Using Lock Washers

A maintenance crew at a mid-size packaging plant re-torqued the same conveyor drive bracket every six weeks for two years. A new lead then swapped the plain flat washer under the nut for a hardened tooth lock washer, and the bracket held torque through the next thirteen months of daily operation. Same bolts, same wrench, same vibration.

That result is the practical case for learning how to use lock washers with discipline. Bolted joints rarely loosen because the bolt is weak; they loosen because rotation steals clamp load a few degrees at a time. This guide covers assembly order, type selection, torque behavior, and the joints where a lock washer is the wrong tool.

Written for maintenance leads, design engineers, and sourcing teams who specify fastening hardware.

Transverse vibration, the side-to-side kind, strips clamp load out of a bolted joint faster than any axial load, and it is the exact condition lock washers are specified against.

What a Lock Washer Does and What It Cannot Do

A lock washer works by adding spring tension or biting friction between the fastener and the joint surface, so the nut or bolt head resists the back-rotation that vibration causes. Resistance to rotation, not raw strength, is the entire value of the part.

Definition

A lock washer is a hardened washer engineered to preserve clamp load by applying spring force or gripping friction between a bolt head or nut and its mating surface, resisting loosening under vibration, thermal cycling, and dynamic loads.

Two mechanisms dominate the category. Spring-type washers, such as split helical washers, act like compressed springs pushing back against the turning fastener. Tooth-type washers, internal or external, dig hardened teeth into both mating surfaces so rotation has to climb or shear metal. Both approaches only matter once the joint already has correct preload.

Torque creates clamp load; the washer's only job is to help the joint keep it. If the initial torque is wrong, no washer will rescue the assembly.

How to Use Lock Washers in the Right Order: A Five-Step Assembly Sequence

Install the lock washer on the rotating side of the joint, directly under the nut or bolt head that turns during tightening, so its spring or teeth fight the element most likely to back off. On metric assemblies, a hardened split washer made to DIN 127 is the workhorse version of that arrangement:

Metric Spring WasherProduct Overview The Metric Spring Washer is a precision-engineered, split-type fastener accessory designed for metric-sized bolted connections. It provides spring ten...View Product →

Check the fit first. The washer's inner diameter should match the bolt size closely; an oversized washer rocks under load and loses bite.
Build the stack in order. Bolt through the joint, flat washer on the head side if the face needs protection, then the lock washer on the nut side against the joint, and the nut on top. To protect a machined or painted face, put the sacrificial flat washer between the lock washer and the joint, never between the nut and the lock washer.
Snug the joint before final torque. Turn the fastener until the washer compresses; on a split washer, the gap between the cut ends should narrow but stay visible.
Torque to specification. Use a calibrated wrench. Oiled threads reach higher preload at the same reading, so follow the lubrication condition called out in the spec, not just the number.
Verify in service. Re-check torque after the first hours on vibrating equipment, and replace any flattened split washer or tooth washer with rounded teeth.

TIP

On soft aluminum or finished faces, let the lock washer bite into a sacrificial flat washer instead of the part itself. You keep the locking action and the surface.

How to Use Lock Washers by Type: Split, Internal Tooth, and External Tooth

Match the washer to the motion and the surface. Split washers add axial spring for light-duty joints, external teeth give the strongest bite on the outer face, and internal teeth suit small heads and electrical bonding work.

Selection summary for the three most common lock washer types.
Type	Use it for	Watch out for
Split helical spring washer	Light-duty metric and imperial assemblies where axial spring helps hold the nut	Flattens under over-torque; weak resistance to transverse vibration
External tooth lock washer	Strongest tooth grip; vibrating brackets, housings, and rigid faces under large heads	Teeth mar the surface; a poor choice where the finish must survive
Internal tooth lock washer	Countersunk and small-head screws; widely used under heads for electrical grounding	Smaller bite circle than external types; needs a surface hard enough to bite

External tooth washers carry their teeth on the outside diameter, which widens the bite circle and gives the highest rotational resistance in the tooth family. They are the default on vibrating brackets and sheet-metal housings where a large head bears on a rigid face:

External Tooth Lock WasherProduct Overview The External Tooth Lock Washer is a serrated fastener accessory with teeth extending outward, designed to grip both the fastener head and mating surfa...View Product →

Internal tooth washers turn the teeth inward, tucking under small or countersunk heads; they appear often in grounding and electronic assemblies, where the teeth also cut through light oxide films to improve electrical contact:

Internal Tooth Lock WasherProduct Overview The Internal Tooth Lock Washer features teeth oriented inward, making it ideal for use with small-diameter fasteners and applications requiring a low-...View Product →

For a deeper side-by-side, see our article on what sets external tooth lock washers apart from internal tooth designs.

Rule of thumb: teeth only work when they dig into the surfaces that actually move. A tooth washer biting a hardened underlayer does little; a tooth washer biting the joint face does a lot.

Torque and Surface Details That Decide Whether the Lock Holds

Hardness match, clean faces, and honest torque control decide whether a lock washer survives in service, because each of those details changes either the bite or the preload itself.

1969Year Gerhard Junker published the transverse vibration testing that still anchors bolt-loosening research

DIN 127The standard that defines the split helical spring lock washers used across metric assemblies

~120 CApproximate service ceiling for nylon-insert lock nuts, the common washer-free alternative

Clean and dry both faces. Grit or an oil film under a tooth washer behaves like a bearing and lets rotation continue.
Match hardness to the surface. A hardened washer on soft aluminum digs in and embeds, so add a larger flat washer under it or move up to a flanged fastener.
Respect lubrication. Unlubricated stainless pairs risk galling, while oiled threads change the torque-tension relationship; assemble to the condition the specification assumes.
Torque in stages on critical joints, and verify against the specification sheet rather than feel.

Over-torque does not tighten a lock washer, it flattens it. Once a split washer sits flush with no visible gap, its spring force is gone and it works as a spacer, not a lock.

Where Lock Washers Are the Wrong Tool, and What to Use Instead

Skip the lock washer on engineered structural joints, soft bearing surfaces, and high-temperature service, and spend the effort on preload control and the right locking device instead.

A lock washer fits when

Light and medium duty joints with real vibration, such as brackets, housings, and panels
Rigid faces that let teeth or spring ends grip properly
Service temperatures within the washer material's range
Maintenance-friendly assemblies where cost per position matters

Choose another method when

Structural connections where preload is engineered, such as heavy hex structural frames
Soft, painted, or thinly plated surfaces that cannot take a bite
Service above roughly 120 C, where nylon inserts fail and spring temper relaxes
Joints that need precise, verified clamp load, such as press frames and rotating equipment

Residual clamp load after severe transverse vibration

Wedge-locking washer pair90%

Nylon-insert lock nut65%

Tooth lock washer40%

Split spring washer15%

Plain nut on flat washer5%

Illustrative summary of published Junker transverse-vibration test results; residual clamp load shown as a share of initial preload.

The ranking is consistent across published tests: spring-only devices sit near the bottom once side-to-side motion starts, which is why engineers move to tooth forms or wedge pairs for serious vibration.

Outdoor and marine assemblies add corrosion to the same problem, so pairing the washer with suitable base hardware, such as stainless steel bolts and washers, matters as much as the locking element itself.

A lock washer is a retaining detail, not a structural guarantee. Specify it where vibration is real and the faces are hard, and invest the engineering budget in preload where the joint is critical.

Frequently Asked Questions About Using Lock Washers

Where does the lock washer go, under the bolt head or under the nut?

Under the element that rotates during tightening, which is usually the nut. If the assembly is tightened from the head side, put the lock washer under the head instead. The rule is simple: fight the side that turns.

How do you use lock washers with a flat washer?

Stack them in order: flat washer against the joint, lock washer on top, then the nut. Never put a flat washer between the nut and the lock washer; the nut would spin on a slick face and the locking element would lose its grip.

How tight should you torque a lock washer?

Tighten to the fastener specification for the joint, not to a feel. Stop when the split washer's gap narrows but stays visible; a deliberately flattened washer contributes no spring, and the clamp load at that point comes entirely from the bolt.

Are lock washers reusable?

Treat them as single-use parts. A split washer that has been fully compressed may not regain its temper, and a tooth washer with rounded teeth cannot bite again. The washer costs pennies; a loosened joint rarely does.

PREV：Bolt Head Types Explained: How to Choose Hex, Carriage, Flange and Eye Heads NEXT：How Do Industrial Precision Screws Improve Accuracy and Reliability in Modern Manufacturing?