The assembly phase constitutes a critical and labor-intensive segment in modern manufacturing, particularly within the aerospace industry. Among the myriad of fastening methods, threaded connections remain indispensable. Hi-lock bolt-nut systems, characterized by their high strength-to-weight ratio and reliable clamp-up force, are extensively used in primary aircraft structures. However, the manual installation of these nuts is a repetitive, ergonomically challenging, and time-consuming task. The transition to automated assembly is therefore not merely advantageous but essential for enhancing productivity, ensuring consistent quality, and reducing worker fatigue. This article details the design and engineering of a specialized robotic end effector developed to fully automate the installation process of hi-lock nuts.
The hi-lock nut installation process presents unique challenges for automation. The nut itself is a two-piece component comprising a threaded collar and a hexagonal tail, connected by a breakneck groove designed to fracture at a precise torque. The installation sequence requires: (1) precise delivery and orientation of the nut, (2) alignment of the nut’s hex tail with a socket wrench, (3) alignment of a separate anti-rotation tool with the bolt’s internal hex, (4) axial feeding of the nut onto the bolt thread, (5) synchronized rotation and axial feed to run the nut down, and (6) final tightening until the breakneck shears, indicating proper preload. A robotic end effector must integrate mechanisms to perform all these sub-tasks reliably.
The core design philosophy for this end effector is modularity and precision. It is conceived as an integrated tool head mountable on a standard industrial robotic arm, which provides gross positioning. The end effector itself handles all fine motions and process-specific actions. Its architecture can be decomposed into four primary subsystems: the Nut Delivery and Orientation System, the Tightening Drive System, the Linear Feed System, and the Perception & Control System. The following sections provide a detailed analysis of each.
1. Nut Delivery and Orientation Subsystem
This subsystem is responsible for retrieving individual nuts from a bulk supply and presenting them correctly to the tightening drive. Reliability and speed are paramount. A pneumatic-based solution is employed for its cleanliness, simplicity, and rapid actuation.
The sequence begins with nuts being fed from a vibratory bowl feeder into a flexible tube. A pulsed air jet propels a single nut through the tube to a staging area. An optical sensor confirms nut presence. A dual-finger pneumatic gripper, with profiles matching the nut’s collar, opens to receive the nut. Once detected, the gripper closes to secure it. A linear pneumatic cylinder then advances the entire gripper assembly forward, extracting the nut from the delivery tube. Subsequently, a vertical pneumatic slide moves the gripper (and nut) down to the precise coaxial height of the socket wrench and anti-rotation tool.
The most critical step is the hex alignment—inserting the nut’s external hex tail into the internal hex of the socket wrench. A passive compliance mechanism is designed for this. The gripper assembly is mounted via a spring-loaded floating platform. As the cylinder retracts to insert the nut, if the hexes are misaligned, the gripper meets resistance, compressing the springs instead of forcing the nut. A limit switch on the floating platform detects this misalignment state and signals the socket drive motor to index slowly. When the socket rotates into the correct angular position, the hexes align, the springs push the gripper forward, and the nut slides fully into the socket. A vacuum port at the base of the socket is then activated to retain the nut, allowing the gripper to release and return to its home position. Key parameters for this subsystem are summarized below.
| Component | Type/Specification | Key Function |
|---|---|---|
| Nut Feeder | Vibratory Bowl with Escapement | Singulation and initial orientation |
| Delivery Method | Pneumatic Blow Feed | High-speed transfer through tube |
| Gripper | Pneumatic, Two-Finger | Secure nut handling |
| Horizontal Actuator | Pneumatic Cylinder (Stroke: 50mm) | Nut extraction and insertion |
| Vertical Actuator | Pneumatic Slide (Stroke: 30mm) | Height alignment |
| Alignment Mechanism | Spring-Loaded Floating Plate | Passive hex search and compliance |
| Retention Method | Vacuum Suction (≈ -60 kPa) | Hold nut in socket after gripper release |
2. Tightening Drive Subsystem
This is the core torque-applying module of the end effector. It must provide controlled rotation for tightening while preventing the bolt from spinning. It consists of two coaxial, independently managed drives.
2.1 Socket Wrench Drive: The internal hex socket is driven by a high-precision servo motor coupled to a low-backlash harmonic drive reducer. This ensures accurate angular positioning for the alignment phase and provides the high, controlled torque required for tightening. The torque output is given by:
$$ T_{socket} = \eta_{gear} \cdot K_t \cdot I $$
where $T_{socket}$ is the output torque, $\eta_{gear}$ is the gearbox efficiency, $K_t$ is the motor torque constant, and $I$ is the motor current. A rotary torque sensor is mounted in-line between the reducer and the socket to provide closed-loop feedback. The drive assembly is housed in a rigid bearing block to handle reaction forces.
2.2 Anti-Rotation Tool Drive: Nestled inside the hollow socket wrench is a hex key (anti-rotation tool) that engages the bolt’s internal drive. This tool is not actively rotated but must be held stationary during nut tightening. It is mounted on a splined shaft attached to a linear pneumatic cylinder. This allows the tool to move axially relative to the socket. During the final tightening phase, as the nut advances axially, the entire end effector feed system moves forward. The anti-rotation tool must stay engaged with the bolt. The pneumatic cylinder is actuated to allow the tool to “float” or even retract slightly relative to the socket housing, maintaining engagement without imposing axial drag. This decouples the tightening rotation from the feed motion for the bolt head.
| Component | Specification | Purpose |
|---|---|---|
| Socket Drive Motor | Servo Motor, 400W, with Multi-turn Encoder | Precise rotation & high torque |
| Reducer | Harmonic Drive, 100:1 ratio | Increase torque, reduce speed/backlash |
| Torque Sensor | Reaction Torque Sensor, 0-50 Nm range | Real-time torque monitoring for breakneck detection |
| Anti-Rotation Tool Actuator | Pneumatic Cylinder (Stroke: 20mm) | Axial compliance to maintain bolt engagement |
| Tool Holder | Splined Shaft | Transmit torque while allowing axial slide |
3. Linear Feed Subsystem
This subsystem provides the precise axial motion required to bring the nut to the bolt and to follow the nut’s axial travel during run-down and tightening. It moves the entire upper assembly (containing the delivery and tightening subsystems) relative to the end effector baseplate. Accuracy and stiffness are critical.
A ball screw drive, actuated by a second servo motor, is selected for its high precision, stiffness, and efficiency. The relationship between motor rotation and linear displacement is:
$$ \Delta x = \frac{P}{360} \cdot \Delta \theta_m $$
where $\Delta x$ is the linear displacement (mm), $P$ is the screw pitch (mm/rev), and $\Delta \theta_m$ is the motor angular displacement (degrees). The feed subsystem performs two key motions: a gross feed to close the gap between the nut and the bolt (guided by machine vision), and a fine feed during tightening that matches the nut’s axial advance per revolution, governed by the thread pitch $p_{thread}$:
$$ \dot{x}_{feed} = \frac{p_{thread}}{360} \cdot \dot{\theta}_{socket} $$
This coordinated motion, often referred to as “thread following,” prevents cross-threading and ensures smooth run-down. The system’s positioning resolution $\delta x$ is determined by:
$$ \delta x = \frac{P}{n \cdot R} $$
where $n$ is the number of encoder pulses per motor revolution and $R$ is the gear reduction ratio between the motor and the screw.
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Ball Screw Pitch | $P$ | 5 | mm/rev |
| Motor Encoder Resolution | $n$ | 16,384 | pulses/rev |
| Reducer Ratio (Motor to Screw) | $R$ | 1:1 (Direct Coupling) | – |
| Theoretical Positioning Resolution | $\delta x$ | 0.0003 | mm |
| Maximum Feed Force | $F_{max}$ | 800 | N |
| Maximum Feed Speed | $v_{max}$ | 200 | mm/s |
4. Perception, Control, and System Integration
The robotic end effector does not operate in isolation. It is part of a larger automated cell. A 6-axis industrial robot provides gross positioning, bringing the end effector into the vicinity of the target bolt. Final precision alignment is achieved through a combination of machine vision and force feedback.
A stereo vision system, mounted in the workcell, locates both the bolt and the tip of the end effector‘s anti-rotation tool. It calculates the 3D positional and angular offsets, providing correction data to the robot. This ensures the anti-rotation tool is perfectly aligned with the bolt’s internal hex before engagement. The vision system also measures the stand-off distance, informing the initial gross feed of the linear axis.
The control architecture is hierarchical. A central programmable logic controller (PLC) or industrial PC acts as the cell master, coordinating the robot, the vision system, and the end effector. The end effector itself has a dedicated motion controller managing its servos and pneumatic valves. Communication via Ethernet/IP or PROFINET ensures synchronization. The tightening process is a state machine, transitioning through stages (Nut Load, Align, Engage, Run-Down, Tighten) based on sensor inputs (vision data, torque, limit switches). The process terminates when the torque sensor detects the characteristic sudden drop indicating breakneck fracture.
| Sensor Type | Location/Purpose | Signal Used For |
|---|---|---|
| Stereo Vision Cameras | Fixed in workcell | Initial robot guidance, bolt/end effector pose measurement |
| Torque Sensor | In-line with socket drive | Monitor tightening torque; detect breakneck fracture |
| Optical Through-Beam | Nut delivery tube | Detect nut presence/absence |
| Limit Switches | On gripper floating plate, linear axes | Detect home positions, alignment completion, mechanical limits |
| Encoder (Socket Motor) | Integrated in servo motor | Angular position for hex search; speed control |
| Encoder (Feed Motor) | Integrated in servo motor | Precise linear position control |
5. Conclusion
The design of a robotic end effector for automated hi-lock nut installation is a multidisciplinary challenge integrating mechanical design, precision actuation, sensor integration, and real-time control. The modular end effector described herein successfully decomposes the complex manual procedure into a series of automated, reliable steps. The nut delivery subsystem handles part handling with passive alignment intelligence. The coaxial dual-drive tightening system provides precise torque application with necessary anti-rotation compliance. The high-resolution linear feed system enables accurate positioning and synchronized thread following. When integrated with a robotic arm and a machine vision guidance system, this end effector forms the core of a complete automated installation cell.
Such automation delivers significant benefits: drastic reduction in operator physical workload, elimination of variability in applied preload (directly impacting joint quality and fatigue life), and increased throughput due to consistent cycle times. The principles and subsystem designs embodied in this end effector are not limited to aerospace hi-lock nuts but can be adapted for other high-value, precision fastening applications across industries such as automotive, heavy machinery, and wind energy, wherever reliable, automated threaded fastening is required. The continued evolution of such end effector technology is pivotal for advancing flexible and intelligent manufacturing systems.