Holding It Together: Maintaining Bolt Preload with Belleville and Thermal Washers

Designing for Preload Stability at Cryogenic Temperatures

In cryogenic systems, bolted joints are more than a mechanical convenience. They are a critical part of both structural integrity and thermal performance. However, what works at room temperature does not always hold up at 4 Kelvin. As components contract due to extreme cooling, bolts can lose preload, joints can loosen, and assemblies that were tightly clamped during fabrication may begin to shift or separate.

This is especially important in coldmass systems, where superconducting coils, thermal interfaces, and support structures must maintain their position and pressure throughout cooldown and operation. Any loss of preload at a joint can reduce thermal conductivity, create unwanted movement, or introduce stress concentrations that compromise performance.

Re:Build DAPR addresses this challenge through a combination of preload modeling, material selection, and the strategic use of Belleville and thermal compensating washers. In this blog, we will explore how we maintain clamping force at cryogenic temperatures and why washer selection is key to reliable system design.

The Problem: Uneven Thermal Contraction

Every material used in a bolted assembly contracts when cooled. However, not all materials contract at the same rate. For example, aluminum can shrink significantly more than stainless steel, while certain alloys like Invar hardly contract at all. This mismatch in thermal contraction can dramatically reduce the clamping force in a joint.

Without a design that accounts for thermal mismatch, the preload in a bolt can drop to near zero during cooldown. That reduction can cause:

Joint separation or movement
Decreased thermal conductance across interfaces
Mechanical instability or fatigue
Loss of performance in pressure-sensitive components like thermal straps or epoxy bonds

To prevent these problems, we use components that actively compensate for thermal movement and preserve preload.

The Solution: Belleville and Thermal Compensating Washers

Belleville Washers

Belleville washers are conical spring washers that compress under load. Unlike flat washers, Bellevilles offer a measurable deflection when compressed, acting as a mechanical spring in the joint. As the bolted assembly contracts during cooldown, the washer’s stored energy continues to exert force on the joint, helping to maintain clamping force.

Benefits of Belleville washers include:

Preservation of preload through temperature changes
Flexibility to stack in different configurations for desired spring behavior
Usefulness in both static and vibrational environments

Bellevilles can be configured in series to increase deflection or in parallel to increase stiffness. Re:Build DAPR selects washer stacks based on the joint stiffness, bolt preload, and the expected thermal contraction of each material involved.

Thermal Compensating Washers

While Belleville washers address preload loss through mechanical spring action, thermal compensating washers tackle the problem through material science. These washers are made from materials with tailored thermal expansion characteristics that offset mismatch between the bolt and the structure.

Common options include:

Invar washers: Offer extremely low thermal expansion. These are ideal when the clamped material shrinks more than the fastener during cooldown.
Allvar washers: These innovative materials exhibit negative thermal expansion. As they cool, they contract more than typical metals, helping to increase clamp force instead of losing it.

By selecting a washer material that contracts in a way that compensates for the thermal behavior of the bolt and joint, preload can be preserved even in severe environments.

Modeling Preload at Cryogenic Conditions

To ensure our designs will perform as expected at 4 Kelvin, Re:Build DAPR performs a full preload retention analysis across temperature extremes. The process involves the following steps:

Gather material properties for each part of the bolted joint, including elastic modulus and thermal expansion coefficient.
Determine initial dimensions and preload at room temperature, including washer deflection and bolt tension.
Apply temperature change and recalculate lengths for each component at the final cryogenic temperature.
Solve for final preload by setting up a system of equations that balances spring forces and thermal contractions across the joint.

To manage the complexity of this system, we often use matrix-based methods such as Gauss Elimination to solve for the unknown values. The result is a clear picture of how preload will change during cooldown and whether the joint configuration needs to be modified.

Best Practices for Washer Selection and Joint Design

Through experience and testing, Re:Build DAPR has developed several key practices for designing cryogenic bolted joints:

Match washer force to bolt preload. The flat load of the Belleville washer should be close to the target preload to ensure optimal performance.
Use oversized flat washers. Underneath Bellevilles to protect softer clamped materials and distribute load evenly.
Stack washers appropriately. Use series stacking for higher deflection and parallel stacking for higher spring force.
Model thermal behavior of the full joint. Consider all materials in the load path, including bolts, washers, clamped parts, and any thermal interface materials.
Select washer materials strategically. Choose Invar or Allvar when standard materials would otherwise lead to significant preload loss.

Preload Integrity That Endures the Cold

At cryogenic temperatures, preload loss is not a possibility to be ignored. It is a certainty to be engineered around. Re:Build DAPR builds every bolted joint in a coldmass system to perform across temperature extremes, with reliable materials, validated calculations, and proven best practices.

Whether we are preserving contact pressure for thermal transfer, maintaining mechanical alignment, or preventing fatigue in critical systems, we rely on washer strategies that hold up under the coldest conditions.

Up Next

Want to know more about how Re:Build DAPR designs reliable Coldmass assemblies that stay clamped under extreme conditions? Stay tuned for Blog 4 in the series: Mind the Gap: Designing for Electrical Insulation in Cryogenic Vacuum Environments.

Have a Project in Mind?

Let’s Connect

Looking to connect with an experienced team?
Look no further than Re:Build Optimation! We are excited to connect with you.