How SOLIDWEDGE™ Clamping Force Is Calculated

Clamping force is the output of a wedgelock mechanism, and it drives two of the most critical performance requirements in a conduction-cooled VPX system: mechanical retention under shock and vibration, and thermal contact quality at the chassis cold wall interface. But clamping force is not a simple function of how hard you tighten the drive screw. Wedge geometry, the number of ramp interfaces, and friction at every contact surface all interact to determine the final output. This article walks through the mechanics behind SOLIDWEDGE™ wedgelocks and demonstrates how to estimate clamping force for a given configuration.

Why Does Clamping Force Matter?

In a rugged VPX chassis, the wedgelock is the primary structural connection between the module and the cold wall slot. It has to perform two jobs simultaneously.

The first is mechanical retention. When a system experiences shock or vibration, clamping force is what keeps the module seated in the slot. Insufficient clamping allows the card to move, which can cause connector fretting, intermittent faults, and mechanical fatigue. Military and aerospace applications routinely specify severe shock and high-g vibration environments, and the wedgelock clamping force must be sufficient to hold the module through those events with margin.

The second is establishing an efficient thermal pathway from the module to the chassis. Higher clamping force creates more contact pressure at the wedgelock-to-cold wall interface, which reduces interfacial thermal resistance and improves heat transfer. For high-power modules dissipating 50 to 100 watts or more, inadequate clamping force leads directly to elevated component temperatures and reduced system reliability. The same force that retains the card mechanically also drives conduction cooling.

How Does a Wedgelock Generate Clamping Force?

A wedgelock converts rotational screw torque into lateral clamping force through a system of angled ramp interfaces. When the drive screw is tightened, it pulls the drive wedge axially along the length of the wedgelock. The drive wedge rides against inclined ramp surfaces shared with adjacent wedge segments. As the drive wedge advances, those inclined surfaces push the wedge segments outward, pressing them against the chassis cold wall on one side while the card heatframe makes direct contact on the other.

The wedge angle and friction conditions at every interface determine how efficiently axial input force converts to lateral clamping output. This conversion is not 1:1, and understanding it quantitatively is necessary for predicting real performance across varying surface conditions and torque specifications.

Converting Drive Screw Torque to Axial Force

Before analyzing the wedge ramps, the drive screw torque must be converted to an axial input force. The relationship between torque and axial force depends on screw diameter and friction between the screw and the drive wedge:

F_IN = Tk × D

• T = Applied drive screw torque (in·lb)
• k = Friction coefficient between screw and drive wedge (≈ 0.25 for typical hardware)
• D = Screw major diameter (in)

For a #6-32 drive screw (D = 0.138 in) torqued to 10 in·lb:

F_IN = 100.25 × 0.138 = 290 lb

This is the axial input to the wedge ramp system. Note that a larger screw diameter reduces F_IN for the same torque because the thread friction acts over a larger moment arm. Torque specifications must account for screw size to achieve a target clamping force.

The Wedge Ramp Force Balance

With F_IN established, the mechanics of a single wedge ramp interface determine the relationship between axial input and lateral clamping output. The force system is captured in two free-body diagrams.

The first shows the drive wedge itself. Axial input force F_IN drives it forward along the wedgelock body. At the inclined ramp surface, normal force F_N acts perpendicular to the face and ramp friction force F_f2 (coefficient μ₂) acts along it. Cold wall friction F_f1 (coefficient μ₁) acts at the base. The combined reaction of those forces is lateral clamping output F_OUT.

The second shows the mating wedge segment pair. Each segment receives axial input on one face and produces clamping output F_OUT perpendicular to the chassis cold wall. Friction force F_f1 acts at each cold wall contact surface, opposing lateral expansion on both segments.

Summing Forces Perpendicular to the Cold Wall

With the ramp surface inclined at angle θ, summing forces perpendicular to the cold wall:

F_OUT + μ₂F_Nsinθ − F_Ncosθ = 0

F_N = F_OUTcosθ − μ₂sinθ

Summing Forces Along the Wedge Axis

Summing forces along the wedge axis:

F_IN − μ₁F_OUT − F_N(sinθ + μ₂cosθ) = 0

Combining the Two Equations

Combining these equations:

F_OUT = F_IN × cosθ − μ₂sinθsinθ + μ₁cosθ + μ₂cosθ − μ₁μ₂sinθ

In simplified form:

F_OUT = F_IN × 1 − μ₂tanθtanθ + μ₁ + μ₂(1 − μ₁tanθ)

This is the clamping force contribution from a single ramp interface. The ratio F_OUT / F_IN is the mechanical efficiency of one ramp, and it is entirely determined by wedge angle and the friction coefficients at the two contact surfaces.

The Complete SOLIDWEDGE™ Clamping Force Equation

A SOLIDWEDGE™ wedgelock has multiple ramp interfaces, and each one contributes independently to total clamping output. Multiplying the per-interface result by N, the number of ramp interfaces, gives the total clamping force for the entire wedgelock:

F_OUT = N × F_IN × 1 − μ₂tanθtanθ + μ₁ + μ₂(1 − μ₁tanθ)

When μ₁ and μ₂ are equal (a reasonable approximation when all contacting surfaces are similar materials and finish) and θ = 45° (so tanθ = 1), the equation simplifies to:

F_OUT = N × F_IN × 1 − μ1 + 2μ − μ²

This simplified form is straightforward to evaluate for any friction coefficient and ramp count.

What Variables Control Clamping Force?

The clamping force equation makes visible four independent variables that engineers can influence through design and installation choices. Each one has a distinct physical interpretation.

Number of Ramp Interfaces (N)

N appears as a direct multiplier with no interaction terms. Adding ramp interfaces increases total clamping force proportionally. SOLIDWEDGE™ wedgelocks are designed with specific ramp counts to achieve target clamping forces at standard torque specifications, and comparing wedgelocks by ramp count is one of the most direct ways to compare their clamping capability.

For applications that need more clamping force without changing the torque specification, SOLIDWEDGE™ SW7 7-segment wedgelocks produce higher clamping output than their SW5 counterparts with fewer segments. The additional ramp interfaces translate directly into more lateral force at the cold wall for the same screw input.

Wedge Angle (θ)

A shallower wedge angle increases mechanical advantage per interface. The N = 5, θ = 35° column in the estimation table shows multipliers 40 to 50% higher than the N = 5, θ = 45° column across the full friction range. However, shallower angles require more axial travel for a given lateral expansion, which imposes geometric constraints on the wedgelock body design.

WaveTherm's SOLIDWEDGE™ Max Force and Magnum Force series wedgelocks use 30° ramps throughout, producing significantly higher clamping output than standard 45° designs at the same torque input. For applications where maximum clamping force is the priority and the geometry allows for it, the shallower ramp angle is one of the most effective levers available.

Friction Coefficient (μ)

Friction is the variable with the most real-world variability, and it has the largest practical impact on clamping output. At μ = 0 (frictionless), a 4-interface wedge at 45° produces a multiplier of 4.00. At μ = 0.25, the same wedge produces a multiplier of 2.09. That is nearly a 48% reduction in clamping efficiency due to friction alone.

Surface finish and cleanliness directly affect μ. Worn or contaminated surfaces increase friction and reduce clamping force below what the torque specification would predict. This is why surface condition at installation matters, and why field-worn hardware can develop retention and thermal performance deficits even when the torque specification is being met.

SOLIDWEDGE™ wedgelocks are available with surface finish options specifically chosen to minimize friction and reduce long-term wear. BA (Black Anodized), BH (Black Anodized Hardened) and EN (Electroless Nickel) are the best options for maintaining low friction at the wedge interfaces over repeated installation cycles, helping preserve clamping performance in fielded systems.

Torque and Screw Diameter (T and D)

F_IN scales linearly with torque and inversely with screw diameter. Increasing the torque specification raises input force proportionally. Using a larger screw diameter reduces input force for the same torque because the thread friction acts over a larger moment arm. When sizing a wedgelock for a clamping force requirement, torque, screw size, and ramp configuration must all be evaluated together rather than treating any one parameter in isolation.

WaveTherm's Magnum Force wedgelocks use a #10 drive screw, and several SOLIDWEDGE™ variants use a #8 drive screw, both larger than the #6-32 standard. While a larger screw diameter alone would reduce F_IN at a fixed torque, these designs are paired with higher torque specifications that more than compensate, producing greater axial input force and higher overall clamping output.

Clamping Force Multiplier Reference Table

The table below gives the clamping force multiplier (F_OUT / F_IN) for common SOLIDWEDGE™ configurations across a range of friction coefficients. Multiply the value from the table by the calculated F_IN to estimate total clamping force.

μ	N = 2 θ = 45°	N = 3 θ = 45°	N = 4 θ = 45°	N = 5 θ = 45°	N = 5 θ = 35°
0	2.00	3.00	4.00	5.00	7.14
0.05	1.73	2.60	3.46	4.33	6.04
0.10	1.51	2.27	3.03	3.78	5.21
0.15	1.33	2.00	2.66	3.33	4.55
0.20	1.18	1.76	2.35	2.94	4.01
0.25	1.04	1.57	2.09	2.61	3.57
0.30	0.93	1.39	1.85	2.32	3.19
0.35	0.82	1.24	1.65	2.06	2.87
0.40	0.73	1.10	1.46	1.83	2.59

Example

Consider a SOLIDWEDGE™ configuration with 4 ramp interfaces at θ = 45°, a #6-32 drive screw torqued to 10 in·lb, and typical dry contact surfaces (μ ≈ 0.3).

Step 1: Calculate F_IN

F_IN = Tk × D = 100.25 × 0.138 = 290 lb

Step 2: Look up the multiplier

From the table: N = 4, θ = 45°, μ = 0.30 gives a multiplier of 1.85.

Step 3: Calculate total clamping force

F_OUT = 1.85 × 290 lb = 536 lb

That 536 lb is the total clamping force pressing the wedge segments against the cold wall, distributed along the contact length of the wedgelock. This force simultaneously secures the module against dynamic loading and establishes the contact pressure that drives conduction cooling.

What This Means for Your Design

Ramp count, wedge angle, and friction coefficient all factor into clamping output. When more force is needed, increasing ramp count or reducing wedge angle are the most direct paths to higher clamping performance, and because higher clamping force improves both conduction cooling and retention under shock and vibration, optimizing for one requirement improves the other at the same time.

SOLIDWEDGE™ wedgelocks are engineered with specific ramp counts and angles to achieve predictable clamping force output across a defined torque range. For engineers who need to verify retention margins or thermal contact performance, the mechanics described here provide the analytical basis for those calculations. For a closer look at how clamping force translates to measured thermal resistance, see our article on how wedgelock thermal performance is measured.