Self-Pierce Riveting explained

What Is Self-Pierce Riveting (SPR)?

Self-Pierce Riveting (SPR) is a mechanical fastening technique for joining two or more layers of material without a pre-drilled hole. A semi-tubular rivet is driven through the upper layers, flaring out within the bottom layer to create a mechanical interlock — crucially without breaking through the bottom surface.

Macro cross-section of a Self-Pierce Riveting (SPR) joint illustrating the mechanical interlock formed between two sheet metal layers without penetrating the bottom sheet.

It is a cold joining process widely used where materials are difficult to weld, such as in automotive aluminium structures.

The SPR Process Steps

Clamping: Sheets are tightly clamped between a die and blank holder.
Piercing: The rivet is driven at high force, penetrating the upper sheets.
Flaring: The rivet shank flares inside the bottom sheet, creating an interlock.
Release: The punch is retracted, leaving a strong, permanent joint.

The bottom sheet remains intact, making SPR ideal for structural integrity and sealing.

New Developments in Servo-Driven SPR Systems

Traditionally, SPR systems were hydraulic — applying steady force — or inertia-driven, using a spinning flywheel to deliver a high-energy impact. Recent advances introduced servo SPR systems, offering:

Permanent Magnet Synchronous Motor driving the rivet via:
- Belt drive.
- Planetary Roller Screw Mechanism (PRSM).
- Integrated Clamping Mechanism and C-Frame.

Servo systems allow precise control over:

Setting velocity.
Motor current limit.
Stroke offset.

This results in finer control of joint quality and energy efficiency.

System Dynamics and Modelling Insights

A physics-based model has been developed combining analytical and empirical approaches:

Full system dynamics model (motor, belt, roller screw, clamp, C-frame).
Joint force-displacement relationships derived from experimental data.

The model predicts:

Head height.
Cycle time.
Energy consumption.

Validation against experimental data shows less than 5% error in force prediction and head height within ±0.25 mm.

Materials Suitable for SPR

Aluminium alloys (e.g., 5754, 6000 series).
High-Strength Steels (HSS).
Magnesium alloys.
Dissimilar materials (e.g., aluminium to steel).
Composite materials (with adapted rivet designs).

Advantages of SPR

Advantage	Details
No Pre-Hole	Rivet pierces materials directly.
Cold Process	No thermal distortion or heat-affected zone.
Dissimilar Materials	Suitable for aluminium-steel joining.
One-Side Access	Only the punch side needs access; die is positioned underneath.
Automation Friendly	Highly adaptable to robotic assembly lines.
Environmentally Friendly	No fumes or sparks, minimal energy use (particularly with servo systems).
Fatigue Resistant	Excellent fatigue performance, enhanced with hybrid adhesives.

Limitations of SPR

Limitation	Details
Material Ductility Required	Brittle materials may crack during insertion.
Die Access Needed	Die must be positioned under sheets — limits some applications.
Thickness Limitations	Optimal for total stack thickness of 1–10 mm.
Rivet Corrosion Risk	Requires coatings or careful material choice to prevent galvanic corrosion.

Process Parameters Impact

The SPR process is sensitive to:

Setting velocity: Higher velocities reduce cycle times but affect energy consumption.
Motor current limit: Balances force application and system wear.
C-frame stiffness: Influences force absorption and joint quality.
Stroke offset: Shorter offsets improve cycle time and reduce energy use.

Key Equations in SPR

Rivet Force (Static Model)

$F_{\text{rivet}} = f(\delta)$

Where:

$F_{\text{rivet}}$ is the force exerted by the rivet punch.
$\delta$ is the rivet insertion displacement.

Polynomial Fit Example:

$F_{\text{rivet}} = a_{10} \delta^{10} + a_9 \delta^9 + \cdots + a_1 \delta + a_0$

Material Stack Compression Force

$F_{\text{clamp}} = K_{\text{mat}} \left( \delta_{\text{clamp}} \right)$

Where:

$K_{\text{mat}}$ is the effective stiffness of the material stack.

Energy Consideration

$W = \int_0^{\delta_{\text{max}}} F_{\text{rivet}}(\delta) , d\delta$

The work done is the energy absorbed by deformation and friction.

Motor Dynamics

$J_{\text{M}} \ddot{\theta}_{\text{M}} = T_{\text{e}} - R_1 K_{\text{b}} \left( \theta_{\text{M}} - \theta_{\text{L}} \right) - R_1 C_{\text{b}} \left( \dot{\theta}_{\text{M}} - \dot{\theta}_{\text{L}} \right)$

Where:

$J_{\text{M}}$ is the motor inertia.
$T_{\text{e}}$ is the electromagnetic torque.
$R_1$ is the pulley radius.

PRSM Force

$F_{\text{L}} = \frac{2 \pi}{P_{\text{h}}} \left( R_1 R_2 K_{\text{b}} \left( \theta_{\text{M}} - \theta_{\text{L}} \right) + R_1 R_2 C_{\text{b}} \left( \dot{\theta}_{\text{M}} - \dot{\theta}_{\text{L}} \right) \right) - F_{\text{fric}}$

C-Frame Deflection

$m_{\text{c}} \ddot{x}_{\text{d}} + C_{\text{c}} \dot{x}_{\text{d}} + K_{\text{c}} x_{\text{d}} = F_{\text{rivet}} + F_{\text{clamp}}$

Where:

$m_{\text{c}}$ , $C_{\text{c}}$ , and $K_{\text{c}}$ are mass, damping, and stiffness of the C-frame.

Head Height Prediction

$\text{Head Height} = Z_2 - \left( x_{\text{pu,max}} - x_{\text{n,max}} \right)$

Where:

$Z_2$ is the initial punch-clamp offset.

Energy Consumption

$E_{\text{total}} = \int_{t_0}^{t_{\text{end}}} V_{\text{DC}}(t) \cdot I_{\text{DC}}(t) , dt$

Where:

$V_{\text{DC}}$ is DC bus voltage.
$I_{\text{DC}}$ is DC bus current.

Comparison to Other Joining Methods

Method	Heat Input	Pre-hole	Dissimilar Materials	Automation
SPR	No	No	Excellent	Excellent
Resistance Spot Weld	Yes	No	Poor (Al–Fe)	Good
Mechanical Fastening	No	Yes	Good	Moderate
Adhesive Bonding	No (cure heat)	No	Excellent	Moderate
Friction Stir Welding	Solid-state	No	Good	Moderate

Applications of SPR

Automotive: Body-in-White (BiW) structures, EV battery enclosures, doors, and hoods.
Aerospace: Lightweight structural panels.
White Goods: Appliance structures where dissimilar metals are used.
Electronics: Precision casings and lightweight frames.

Future Trends in SPR

Hybrid SPR + Adhesive Systems: Combining mechanical fastening with adhesive bonding.
SPR in Composites: Modified rivets and process parameters for fibre-reinforced polymers.
Die-less SPR: Eliminating the need for die access.
Smart SPR Systems: Integrating AI and machine learning for process monitoring and control.

Conclusion

Self-Pierce Riveting is a critical technology for modern lightweight and multi-material structures. Innovations like servo-driven SPR systems enable precise, efficient, and sustainable joining, with significant productivity and energy savings.

Lightweight aluminium Body-in-White (BiW) frame of an electric vehicle, showcasing precision assembly with Self-Pierce Riveting (SPR) for improved efficiency and crash performance.

As industries move towards electric vehicles and sustainability goals, SPR — particularly with advanced control systems — will become even more indispensable.

Product Development Engineers Ltd

What Is Self-Pierce Riveting (SPR)?

The SPR Process Steps

New Developments in Servo-Driven SPR Systems

System Dynamics and Modelling Insights

Materials Suitable for SPR

Advantages of SPR

Limitations of SPR

Process Parameters Impact