Washers: Selection, Standards and Joint Function: How Helical Spring Lock Washers Work

How Helical Spring Lock Washers Work

Helical spring lock washers provide four distinct mechanical functions:

1. Good bolt tension per unit of applied torque for tight assemblies
2. Hardened bearing surfaces to create uniform torque control
3. Uniform load distribution through controlled radii, section, and cutoff geometry
4. Protection against looseness resulting from vibration and corrosion

The helical (split) geometry creates a spring action that maintains axial force on the bolt even as the joint settles. The sharp edges at the split bite into both the bolt head and the clamped surface, creating mechanical resistance to rotation.

Available Series

Helical spring lock washers are available in four series:

Important: Non-carbon-steel materials are available only in the Regular series. If your application requires corrosion-resistant or non-ferrous lock washers, you must use Regular series dimensions.

Designation Convention

These washers are designated by the following data in sequence: Product name; nominal size (number, fraction, or decimal equivalent); series; material; and protective finish, if required.

Example: Helical Spring Lock Washer, 0.375 Extra Duty, Steel, Phosphate Coated

Complete Helical Spring Lock Washer Dimensions — Regular, Heavy, and Extra Duty Series

$$T = \text{mean section thickness} = \frac{t_i + t_o}{2}$$

Where $t_i$ = inside thickness and $t_o$ = outside thickness.

Note: For sizes over 1-1/2 to 3 inclusive (Regular and Heavy) and over 1-1/2 to 2 inclusive (Extra Duty), see ANSI/ASME B18.21.1-1994 standard directly.

Hot-Dip Galvanizing Considerations

When carbon steel helical spring lock washers are to be hot-dipped galvanized for use with hot-dipped galvanized bolts or screws, they must be coiled to limits one inch in excess of those specified for minimum inside diameter and maximum outside diameter.

Critical warning: Galvanizing washers under 1/4 inch nominal size is not recommended. The galvanizing process adds sufficient material thickness to potentially compromise the washer's spring characteristics at small sizes.

Hi-Collar Helical Spring Lock Washers

Standard: ANSI/ASME B18.21.1-1994

All dimensions in inches.

the practitioner specified Hi-Collar washers for every counterbored socket head cap screw installation at the grain facility. "Regular spring lock washers sit flush or below the surface in a counterbore," he explained. "The Hi-Collar version has an extended collar height that provides proper spring action even when the bolt head is recessed."

Complete Hi-Collar Helical Spring Lock Washer Dimensions

$$\text{Mean section thickness} = \frac{\text{inside thickness} + \text{outside thickness}}{2}$$

Hi-Collar vs. Regular: The Critical Difference

Compare the 1/2-inch size across series:

The Hi-Collar washer is narrower in width but thicker in section — giving it a taller profile that projects above the counterbore surface while maintaining effective spring force. The smaller OD makes it compatible with the tighter clearances found in socket head cap screw counterbore geometries.

Tooth Lock Washers — Internal, External, and Combination

Standard: ANSI/ASME B18.21.1-1994

All dimensions in inches.

When the practitioner encountered the facility's electrical panel enclosures — sheet metal housings where helical spring washers would have been overkill — he reached for tooth lock washers instead.

"Tooth lock washers work on a completely different principle than spring locks," the practitioner explained. "They don't maintain spring force. Instead, they bite into the mating surfaces with hardened teeth that resist rotation. The more torque you apply trying to loosen the fastener, the deeper the teeth dig in."

Types and Construction

The ANSI standard covers three tooth configurations and two construction types:

Tooth Configurations:

• Internal teeth — teeth face inward toward the bolt shank, hidden under the bolt head for clean appearance
• External teeth — teeth face outward, providing maximum grip area and holding power
• Internal-external teeth — teeth on both sides, maximum resistance for critical applications

Construction Types:

• Type A — standard construction for general applications
• Type B — modified construction for specific requirements

Tooth Lock Washer Function

These washers serve to lock fasteners (bolts, nuts) to the component parts of an assembly, or increase the friction between the fasteners and the assembly. They are available in carbon steel and are designated in a manner similar to helical spring lock washers.

Internal Tooth Lock Washers — Complete Dimensions

Internal tooth washers provide a clean, professional appearance because the teeth are hidden beneath the bolt head or nut. They are the preferred choice for finished surfaces, electrical connections, and applications where external teeth could snag wiring or create clearance issues.

External Tooth Lock Washers — Complete Dimensions

External tooth washers provide the highest holding power of any tooth lock washer configuration. The outward-facing teeth create a larger effective grip diameter, producing more resistance to rotation per unit of thickness. They are the go-to choice for structural connections, heavy equipment, and any application where maximum lock security outweighs appearance.

Note: External tooth lock washers are not standard in No. 2 size or in sizes above 1 inch.

Internal-External Tooth Lock Washers

Standard: ANSI/ASME B18.21.1-1994

All dimensions in inches.

These are the heavy artillery of the tooth lock washer family. With teeth on both the inside and outside diameters, they provide dual-surface bite — gripping both the bolt/nut bearing surface AND the workpiece simultaneously.

the practitioner specified internal-external tooth washers for every vibration-critical electrical grounding connection at the grain facility. "When you need both the holding power of external teeth AND the clean bore fit of internal teeth, this is what you use."

Internal-External Tooth Lock Washer Dimensions

Each size offers multiple OD options, giving engineers the ability to match the washer to the specific clearance and grip requirements of their application.

How to Read the Multi-OD Options

The multiple OD options exist because different applications demand different washer footprints for the same bolt size. Consider the 1/4-inch internal-external tooth lock washer:

• 0.760 OD — minimum footprint for tight clearance applications
• 0.900 OD — moderate coverage for general assembly
• 0.985 OD — wider grip for softer materials
• 1.070 OD — maximum coverage for sheet metal and panel work

In all cases, the teeth perform the same locking function. The larger OD simply distributes the tooth-bite over a greater area, which reduces surface marring and increases total holding force.

The Washer Selection Decision Framework

After six months of rebuilding the grain facility's fastener program, the practitioner distilled his experience into a decision framework that any maintenance technician could follow.

Quick-Selection Matrix

The Bearing Pressure Decision Rule

the practitioner taught every technician a simple rule of thumb for selecting between Narrow, Regular, and Wide series:

$$P_{\text{bearing}} = \frac{F_{\text{clamp}}}{A_{\text{washer}}}$$

Where:

• $P_{\text{bearing}}$ = bearing pressure under the washer (force per unit area)
• $F_{\text{clamp}}$ = bolt clamp load (determined by torque and friction)
• $A_{\text{washer}}$ = washer bearing area = $\frac{\pi}{4}(OD^2 - ID^2)$

The rule: If $P_{\text{bearing}}$ exceeds 60% of the clamped material's compressive yield strength, move to the next wider series.

The Vibration Decision Rule

For any joint subject to dynamic loading (vibration, thermal cycling, impact, or oscillating loads), the practitioner required at minimum a helical spring lock washer. His decision tree:

1. Static load only, no vibration? → Plain washer sufficient
2. Mild vibration, general machinery? → Regular helical spring lock washer
3. Moderate vibration, process equipment? → Heavy helical spring lock washer
4. Severe vibration, high-frequency equipment? → Extra Duty helical spring lock washer
5. Electrical connections requiring metal-to-metal contact? → Tooth lock washer (external or internal-external)
6. Appearance-critical with vibration? → Internal tooth lock washer

the practitioner's Transformation: From Failure to Fastener Authority

Six months after the initial conveyor failure, the practitioner completed his audit of every bolted joint in the grain processing facility. The results spoke for themselves:

• Zero fastener-related failures in the six months following implementation
• Maintenance labor hours reduced by 34% — technicians stopped re-torquing bolts that kept loosening
• Spare parts inventory rationalized — instead of bins of random "flat washers," the tool cribs stocked specific Type B sizes in each series, plus appropriate lock washers for every application class
• New technician training time cut in half — the decision framework eliminated guesswork

The operations manager who'd originally questioned the cost of "upgrading washers" became the practitioner's strongest advocate. "I used to think a washer was a washer," he told the corporate engineering team during the quarterly review. "Now I understand that the right washer — specified correctly, installed properly — is the difference between a machine that runs and a machine that self-destructs."

the practitioner's final lesson to the facility team was the simplest of all:

"Every bolted joint tells a story. The washer is what determines whether that story has a happy ending. Learn these tables. Understand the differences. And never, ever, skip the washer."

Your Next Step

You now have the complete dimensional reference for every inch-series washer type covered by the American National Standards — Type A plain washers, Type B plain washers in all three series, helical spring lock washers in four series, and tooth lock washers in every configuration.

Here's what to do with this knowledge:

1. Audit your most critical bolted joints this week. Identify any connection that uses the wrong washer type, the wrong series, or no washer at all. Start with vibration-prone equipment and connections into soft materials.

2. Build a washer specification chart for your facility. Use the decision framework above to map each application class to its correct washer type and series. Post it in every tool crib and maintenance shop.

3. Calculate the bearing pressure on your three most heavily loaded joints. Use the formula above and compare the result against the clamped material's compressive yield strength. If you're above 60%, you need a wider series.

4. Review your lock washer usage on all rotating equipment. Any bolted joint within three feet of a motor, gearbox, pump, or conveyor drive should have an appropriate lock washer — no exceptions.

What washer specification challenge are you facing right now? Whether it's selecting the right series for a soft-material joint, choosing between helical spring and tooth lock washers for a vibration application, or tracking down a non-standard Type A size for an unusual geometry — the answer is in these tables.

Save this guide. Print the reference tables. And the next time someone tells you a washer is "just a washer" — show them what the practitioner showed the grain facility: the right washer is the cheapest structural insurance policy in engineering.

All dimensional data sourced from ANSI/ASME B18.22.1-1965 (R1998) for plain washers and ANSI/ASME B18.21.1-1994 for helical spring and tooth lock washers. Nominal washer sizes are intended for use with comparable nominal screw or bolt sizes. Tolerances shown are for metal washers only. Consult manufacturers for current stock availability and non-metallic washer tolerances.

The Assembly That Nearly Ended a Career

The hydraulic press frame was supposed to hold 200 tonnes. It held 200 tonnes exactly once.

the practitioner, a structural assembly engineer at a mid-size fabrication shop, had specified every bolt, every nut, every torque value with painstaking care. The M24 high-strength bolts were Grade 10.9. The nuts were hardened. The torque wrench was calibrated that same morning.

What the practitioner hadn't specified were the washers.

The procurement team had substituted soft metric plain washers for hardened ones. The difference seemed trivial on the purchase order — same nominal size, same inside diameter, same bolt compatibility. But under 200 tonnes of clamping force, the soft washers embedded into the flange surface like coins pressed into wet clay. The preload dropped. The bolts relaxed. And the frame shifted 3 mm during the second press cycle — enough to crack a weld and shut down the entire production line for eleven days.

Eleven days of lost production. Over a washer.

This guide exists so that never happens to you. What follows is the most comprehensive reference on metric washers available anywhere — covering ANSI standards, ISO comparisons, British Standards, spring washers, material specifications, designation systems, and the complete dimensional tables you need to specify the right washer every single time.

Why Washers Matter More Than You Think

A washer is not "just a flat piece of metal with a hole in it." That description is technically accurate and catastrophically incomplete.

Here is what a properly specified metric washer actually does in a bolted joint:

• Distributes bearing load across a wider area, preventing localized stress concentrations that lead to surface damage and fastener loosening
• Provides a uniform bearing surface so torque-tension relationships remain predictable and repeatable
• Prevents surface marring of the work material, particularly critical when assembling painted, plated, or precision-machined surfaces
• Minimizes embedment in high-strength joints where bolt preload can literally push a nut into softer base material
• Bridges clearance holes and slots when the bolt hole is oversized or slotted for adjustment, ensuring full bearing contact under the nut or bolt head
• Controls friction at the bearing interface, which directly affects how much clamping force you get per unit of applied torque

Every one of these functions depends on selecting the correct washer type, series, material, hardness, and dimensions for your specific application. Get any one of those parameters wrong, and the washer becomes either useless or actively harmful.