One of the most common questions our engineering team at Bossard receives is: "How much torque do I need for this specific bolt?" While it is a logical question, it highlights a fundamental misunderstanding in fastener application design. In reality, we never design a joint for a specific torque; we design it for a specific clamp load.

Clamp Load: The Foundation of Joint Design

When designing a joint, the primary goal is to ensure it can withstand the forces it will encounter during its service life. The necessary clamp load—the actual compression force holding the parts together—is the critical metric that must be determined first. This requirement typically comes from the customer, who identifies the needed load through Finite Element Analysis (FEA) and other engineering assessments.

Once the required clamp load is established, we can then determine the appropriate torque needed to achieve that load with a specific fastener.

The Risk of Relying Solely on Torque Charts

Many manufacturers rely on standard torque charts as a general rule of thumb. For example, a chart might suggest a specific torque for an 8.8 zinc-plated bolt. While these charts are a helpful starting point and work for many standard applications, they can be dangerous if used for critical joints.

If your application involves cyclical loading or high-load demands, a generic torque chart may not be sufficient. In these cases, you must take a closer look to ensure you are getting the most out of the joint and preventing potential failure.

The Variable of Friction

The reason torque is not a "one-size-fits-all" measurement is due to the coefficient of friction. Friction is present in the fastener finish and the joint itself, and it significantly impacts how much of the applied torque is actually converted into clamp load.

Many manufacturers have begun specifying the coefficient of friction for fastener finishes to gain better control over the assembly process. However, even with a specified friction range, the joint itself introduces new variables that can push the friction level outside of that expected range once the fastener is installed in production.

How Bossard Optimizes Your Joint

To move beyond guesswork, it is essential to understand how a fastener behaves within the specific environment of your production joint. At Bossard, we utilize our Applications Testing Lab to bridge the gap between theory and reality.

By testing the joint exactly as it exists in production, we can:

• Measure the actual clamp load achieved at various torque levels.
• Account for the unique variables of your specific application.

Determine the ultimate ideal torque to ensure maximum performance and safety.