Cylinder Selection Process in Automotive Body-In-White (BIW) Tooling and Fixture Design

In the automotive industry, the Body-In-White (BIW) stage involves assembling the sheet metal components to form the car's structure before painting. The tooling and fixtures used in this stage are crucial for ensuring precision, repeatability, and efficiency. One key component in these fixtures is the pneumatic or electric cylinder, which provides the necessary force and movement for various operations such as clamping, positioning, and lifting. Selecting the right cylinder is vital for the overall performance and reliability of the tooling. This article outlines the key considerations and steps involved in the cylinder selection process for BIW tooling and fixture design.

1. Understanding the Application Requirements

The first step in selecting a cylinder is to understand the specific requirements of the application. This involves:

• Type of Operation: Determine the function of the cylinder in the fixture (e.g., clamping, lifting, positioning).
• Force Requirements: Calculate the force needed to perform the operation, considering factors like material properties, friction, and safety margins.
• Stroke Length: Identify the required stroke length, which is the distance the cylinder rod needs to travel to complete the operation.
• Speed and Cycle Time: Determine the desired speed and cycle time for the operation to ensure the cylinder can meet the production rate.

2. Choosing Between Air and Electric Cylinders

When comparing the force output between pneumatic and electric cylinders, several factors come into play, including the design, operating pressure, and application requirements. Here’s a detailed comparison:

A. Pneumatic Cylinders

Advantages:

• High Speed: Pneumatic cylinders can operate at high speeds, making them suitable for applications requiring rapid actuation.
• Simple Design and Maintenance: Pneumatic systems are generally simpler and easier to maintain.
• Initial Cost: Typically lower initial cost compared to electric cylinders.
• Safety: Pneumatic systems are safe to use in explosive or hazardous environments since they do not produce sparks.

Force Output:

• Operating Pressure: Typically, pneumatic systems operate at pressures ranging from 80 to 120 psi (pounds per square inch), but industrial systems can go higher.
• Force Calculation: The force output can be calculated using the formula:

F = PxA

Where:

• FFF = Force output
• PPP = Pressure (in psi or Pa)
• AAA = Piston area (in square inches or square meters)

For example, for a pneumatic cylinder with a piston diameter of 2 inches and an operating pressure of 100 psi:

A=π(D/2)^2=π(2/2)^2=π(1)^2=3.14 square?inches

F=100psi×3.14square?inches=314lbs

B. Electric Cylinders

Advantages:

• Precision and Control: Electric cylinders offer precise control over position, speed, and force, which is ideal for applications requiring high accuracy and repeatability.
• Energy Efficiency: More energy-efficient since they only consume power when moving.
• Integration: Easier to integrate with electronic control systems and automation setups.
• Cleanliness: No risk of fluid leaks, making them suitable for clean environments.

Force Output:

• Force Generation: Electric cylinders can generate a wide range of forces depending on the motor and screw mechanism used (ball screw, lead screw, etc.).
• Force Calculation: The force output is determined by the motor torque and the lead of the screw. The formula is:

F=(Txη)/L

Where:

• FFF = Force output
• TTT = Motor torque
• η = Efficiency of the screw mechanism (typically around 90-95%)
• LLL = Lead of the screw (distance the nut moves per revolution, in inches or mm)

For example, for an electric cylinder with a motor torque of 2 Nm, a screw efficiency of 90%, and a lead of 5 mm:

F=(2Nmx0.9)/(0.005m) = 1.8/0.005 = 360N

Comparative Analysis:

• Force Range: Pneumatic cylinders generally provide moderate force output compared to electric cylinders, which can be designed for both low and high force applications.
• Application Suitability: Pneumatic cylinders are well-suited for high-speed, repetitive tasks with moderate force requirements, such as material handling, clamping, and packaging. Electric cylinders excel in applications requiring precise control and high repeatability, such as robotics, CNC machinery, and automated assembly.
• Efficiency and Control: Electric cylinders offer better energy efficiency and control capabilities, making them ideal for complex automation tasks. Pneumatic cylinders, while less efficient, are simpler and often more cost-effective for certain applications.

3. Calculating Cylinder Sizing

Proper sizing of the cylinder ensures that it can deliver the required force and stroke without overloading. The key parameters to consider are:

• Bore Size: The diameter of the cylinder bore affects the force output. Larger bores provide more force. The force can be calculated using the formula:

F = P x A

where F is the force, P is the pressure, and A is the area of the bore.

• Rod Size: The diameter of the piston rod affects the cylinder's stability and load capacity. Larger rods provide better stability and can handle higher side loads.
• Mounting Style: The mounting style (e.g., flange, trunnion, clevis) should be selected based on the fixture design and space constraints.

4. Calculating Clamp Force and Holding Force

Calculating clamp force and holding force is essential for designing fixtures, particularly in automotive applications where precision and reliability are critical. Here’s a breakdown of the calculations involved:

• Clamp Force Calculation

Clamp force is the force exerted by a clamping device to hold a workpiece securely. It can be calculated based on various factors such as the applied torque, friction, and geometry of the clamping mechanism.

Using Torque:

For a simple bolted joint, the clamp force can be estimated using the torque applied to the bolt.

Fc = T\Kxd

Where:

F_c = Clamp force

T = Applied torque

K = Torque coefficient (usually ranges between 0.18 and 0.2 for lubricated bolts, and higher for dry bolts)

d = Nominal diameter of the bolt

Using Cylinder:

For pneumatic or electric cylinders:

Fc = PxA

Where:

Fc = Clamp force

P = Pressure (pneumatic) or force output (electric cylinder)

A = Piston area

The piston area A can be calculated as:

A = π(D/2}^2

Where:

D = Diameter of the piston

• Holding Force Calculation

Holding force is the force required to keep a workpiece in position against external loads such as cutting forces, vibrations, or other operational forces.

Using Friction:

For a clamping mechanism relying on friction:

Fh = μxFc

Where:

Fh = Holding force

μ = Coefficient of friction between the workpiece and the clamping surface

Fc = Clamp force

Using Fixture Design:

For fixtures designed to withstand specific loads, the holding force can be derived from the forces acting on the fixture:

Fh = W/(μxcosθ)

Where:

Fh = Holding force

W = Applied load (e.g., weight, cutting force)

μ = Coefficient of friction

θ = Angle of the fixture or clamping surface relative to the force direction

Practical Example

Example 1: Clamp Force Using Torque

Suppose you have a bolt with a nominal diameter d = 10 mm, and you apply a torque T = 50 Nm. The torque coefficient K = 0.2.

Fc = 50/(0.2x0.01)

Fc = 50/0.002

Fc = 25,000 N

Example 2: Clamp Force Using Cylinder

For a pneumatic cylinder with a diameter D = 50 mm, and an applied pressure P = 6 bar (600,000 Pa).

A=π(50/2)^2

A=π(25)^2

A=1963.5 mm^2 = 0.0019635 m^2

Fc=600,000x0.0019635

Fc=1178.1 N

Example 3: Holding Force Using Friction

Suppose the coefficient of friction μ = 0.3, and

the clamp force Fc=25,000 N.

Fh=0.3x25,000

Fh=7,500 N

5. Considering Environmental Factors

Environmental conditions can significantly impact the performance and lifespan of cylinders. Factors to consider include:

• Temperature: Ensure the cylinder materials and seals can withstand the operating temperature range.
• Contamination: In environments with high levels of dust, moisture, or chemicals, choose cylinders with appropriate protective features, such as seals and coatings.

- Vibration and Shock: For applications subject to vibration or shock loads, select cylinders designed to handle such conditions.

6. Evaluating Additional Features

Modern cylinders come with various features that enhance performance and functionality. Consider the following options:

• Cushioning: Built-in cushioning can reduce impact at the end of the stroke, prolonging cylinder life and reducing noise.
• Position Sensors: Integrated sensors provide feedback on the cylinder's position, enabling precise control and automation.
• Adjustable Stroke: Cylinders with adjustable stroke lengths offer flexibility in setup and can accommodate different production requirements.

7. Verifying Compatibility and Standards

Ensure that the selected cylinder complies with industry standards and is compatible with the existing equipment and systems. Check for:

• Standardization: Choose cylinders that adhere to international standards (e.g., ISO, NFPA) for interchangeability and ease of replacement.
• Compatibility: Verify that the cylinder's ports, fittings, and mounting dimensions match the fixture design and other components.

Conclusion

The cylinder selection process in automotive BIW tooling and fixture design is critical for ensuring efficient and reliable operations. By carefully considering the application requirements, cylinder type, sizing, environmental factors, additional features, and compatibility, designers can choose the right cylinder that meets the performance and durability needs of the fixture. Proper cylinder selection not only enhances the quality and consistency of the BIW assembly but also contributes to the overall productivity and cost-effectiveness of the manufacturing process.