Invar and ALLVAR Washers and Spacers: Benefits and Differences
Invar washers have long been the go-to solution for thermally stable joints in precision instruments, aerospace, and electronics. However, ALLVAR Alloy 30 washers offer superior thermal compensation, lower weight, and reduced preload loss. This guide explores the benefits of ALLVAR washers over Invar washers and provides a practical example to demonstrate their advantages.
What Are Invar Washers?
Invar washers, made from a nickel-iron alloy (36% nickel, 64% iron), are prized for their low coefficient of thermal expansion (CTE) of 1.5 ppm/°C. This minimal expansion makes them ideal for applications like cryogenics, aerospace, and electronics, where joint preload stability is critical. However, Invar washers have tradeoffs, including high density (8.1 g/cm³), magnetism, and limited corrosion resistance. Learn more about Invar here.
Introducing ALLVAR Washers
ALLVAR Alloy 30 washers are a revolutionary alternative to Invar washers. With a negative CTE of -30 ppm/°C, ALLVAR washers provide enhanced thermal compensation, reducing preload loss in bolted joints. They are non-magnetic, have a lower density (5.1 g/cm³), and offer superior corrosion resistance with no post-processing required.
Benefits of ALLVAR Washers
ALLVAR washers outperform Invar washers in several ways:
- Enhanced Thermal Compensation: ALLVAR’s -30 ppm/°C CTE compensates for CTE mismatches better than Invar’s 1.5 ppm/°C.
- Reduced Preload Loss: Minimizes joint preload loss in thermally dynamic environments.
- Lower Density: 5.1 g/cm³ vs. Invar’s 8.1 g/cm³, reducing weight.
- Thinner Washers: Up to 3.2x thinner than Invar washers for the same compensation.
- Significant Weight Savings: 5x less mass for spacers of the same diameters.
- Considerations: ALLVAR has a max operating temperature of 100°C (vs. Invar’s 200°C) and non-isotropic CTE, requiring specific orientation.
This example demonstrates how to calculate washer thickness for a thermally stable bolted joint, comparing invar 36 and ALLVAR Alloy 30 washers. A stainless steel A286 bolt clamps a graphite flange (4 mm thick, CTE 20 ppm/°C) and an aluminum flange (8 mm thick, CTE 24 ppm/°C) in a space application.
Using thermal compensation calculations like those found in SAE AIR1754B :
Equation for Thermal Compensation Washer Thickness when joining components of different materials:
Washer Thickness = [(L₁ × (α₁ – α_bolt)) + (L₂ × (α₂ – α_bolt))] / (α_bolt – α_washer)
| Component | Material | CTE (ppm/°C) | Thickness |
| Flange 1 | Graphite | α₁ = 20 | L₁ = 4 mm |
| Flange 2 | Aluminum | α₂ = 24 | L₂ = 8 mm |
| Bolt | A286 | α_bolt = 16 | – |
| Washer | Invar 36 | α_washer = 1.6 | – |
| Washer | ALLVAR Alloy 30 | α_washer = -30 | – |
Invar Spacer Thickness:
(4 mm × (20 – 16) + 8 mm × (24 – 16)) / (16 – 1.6) = 80 / 14.4 = 5.56 mm
ALLVAR Alloy 30 Spacer Thickness:
(4 mm × (20 – 16) + 8 mm × (24 – 16)) / (16 – -30) = 80 / 46 = 1.74 mm
ALLVAR washers are 3.2x thinner than invar washers for the same thermal compensation.
Weight Comparison:
| Material | Density (g/cm³) | Volume (cm³) | Mass (g) |
| ALLVAR Alloy 30 | 5.1 | 0.147 | 0.75 |
| Invar 36 | 8.1 | 0.472 | 3.82 |
ALLVAR washers provide a 5.1x mass reduction, combining lower density and smaller volume. Take this savings even further by reducing bolt length due to the thinner washers!
This example demonstrates how to calculate washer thickness for a thermally stable bolted joint, comparing invar and ALLVAR washers. A stainless steel A286 bolt clamps a two aluminum flanges (8 mm thick, CTE 24 ppm/°C). A slightly different equation is used than the one shown in Example 1:
Equation for Thermal Compensation Washer Thickness when joining components of the same material:
Washer Thickness = [(α_component – α_bolt)) / ((α_bolt – α_washer))] * (L₁ + L₂)
| Component | Material | CTE (ppm/°C) | Thickness |
| Flange 1 | Aluminum | α_component = 20 | L₁ = 8 mm |
| Flange 2 | Aluminum | α_component = 20 | L₂ = 8 mm |
| Bolt | A286 | α_bolt = 16 | – |
| Washer | Invar 36 | α_washer = 1.6 | – |
| Washer | ALLVAR Alloy 30 | α_washer = -30 | – |
Invar Washer Calculation:
(20 ppm/°C – 16 ppm/°C) / (16 ppm/°C – 1.5 ppm/°C) * (8mm+8 mm) = 4 / 14.5 * 16mm= 4.42 mm
ALLVAR Washer Calculation:
(20 ppm/°C – 16 ppm/°C) / (16 ppm/°C – -30 ppm/°C) * (8mm+8 mm) = 4 / 46 * 16mm= 1.39 mm
Once again, we see that ALLVAR washers are 3.2x thinner than invar washers for the same thermal compensation.
Weight Comparison:
| Material | Density (g/cm³) | Volume (cm³) | Mass (g) |
| ALLVAR Alloy 30 | 5.1 | 0.084 | 0.43 |
| Invar 36 | 8.1 | 0.265 | 2.15 |
ALLVAR washers provide a 5x mass reduction, combining lower density and smaller volume. Take this savings even further by reducing bolt length due to the thinner washers!
Why Choose ALLVAR Washers?
ALLVAR Alloy 30 washers enable improved joint preload, reduced weight, and smaller form factors, making them ideal for aerospace and precision applications. With a reduction in length, you can expand the savings further by using shorter bolts. Their negative thermal expansion pushes the boundaries of what’s possible compared to invar washers.
Want to better understand how ALLVAR Washers reduce preload loss? Learn more about how ALLVAR Alloy 30 washers can help reduce preload loss at CTE Mismatch Solved with ALLVAR Negative Thermal Expansion. If you would like more examples of determining the ideal thermal compensating washer thickness, please see Thermal Compensating Washers for CTE Mismatch – ALLVAR Alloys.
Not sure if ALLVAR or Invar is right for your project? Contact our materials experts to discuss your needs and optimize your design.
FAQ: Invar vs. ALLVAR Washers
What are invar washers?
Invar washers are nickel-iron alloy components with a low CTE (1.5 ppm/°C), used to maintain joint stability in aerospace and electronics.
How do ALLVAR washers reduce preload loss?
ALLVAR washers use a negative CTE (-30 ppm/°C) to compensate for thermal mismatches, minimizing preload loss, mass, spacer thickness, and bolt length compared to invar.
What are the limitations of ALLVAR washers?
ALLVAR washers have a max operating temperature of 100°C and non-isotropic CTE, requiring careful orientation during design.
Have more questions about thermal compensation washers? Share your thoughts below or contact us!
