Friction Coefficient of Fasteners and Its Influencing Technologies

Friction Coefficient of Fasteners and Its Influencing Technologies

Torque Calculation Formula for Bolted Connections in VDI 2230

The formula for calculating tightening torque is as follows

Tightening Torque Formula:

MA=Fv⋅(d2⋅μt+0.5⋅d⋅μg)M_A = F_v \cdot (d_2 \cdot \mu_t + 0.5 \cdot d \cdot \mu_g)MA=Fv⋅(d2⋅μt+0.5⋅d⋅μg)

Where:

• MAM_AMA: Tightening torque      (Nm)

• FvF_vFv: Bolt preload (N)

• d2d_2d2: Nominal thread      diameter (m)

• μt\mu_tμt: Thread friction      coefficient

• ddd: Average diameter of      the bolt head or nut contact surface (m)

• μg\mu_gμg: Friction coefficient of the nut bearing surface or bolt head bearing      surface

Preload FvF_vFv:

Preload is typically calculated based on the bolt material's yield strength and a design safety factor, ensuring it meets the mechanical requirements during assembly and operation:

Fv=α⋅As⋅RpF_v = \alpha \cdot A_s \cdot R_pFv=α⋅As⋅Rp

Where:

• α\alphaα: Tightening factor      (commonly 0.6–0.8)

• AsA_sAs: Effective stress area      of the bolt

• RpR_pRp: Yield strength of the      bolt materia

Friction Coefficients μt\mu_tμt and μg\mu_gμg:

• The      friction coefficients are determined by surface properties such as      roughness and lubrication.

• Typical      values:

• μt\mu_tμt: 0.1–0.2

• μg\mu_gμg: 0.08–0.18

Importance of the Friction Coefficient:

The friction coefficient is dimensionless and derived from physical measurements, influenced by the type and geometry of contact surfaces. It plays a critical role in determining the required tightening torque to achieve a consistent preload. Variations in the friction coefficient necessitate adjustments in torque values for consistent assembly outcomes.

Surface Treatment Processes Influencing Friction Coefficients:

The surface treatment of fasteners is a key factor in controlling the friction coefficient. Below are processes that affect friction coefficients:

1. Electroplating and Sealing

Sealing agents, also known as brighteners, consist of organic corrosion inhibitors, surfactants, and water-soluble polymers. They form a dense and uniform film after curing, enhancing corrosion resistance, surface smoothness, and brightness.

Process Flow:
Electroplating → Washing → Brightening (for zinc plating) → Washing → Passivation → Washing → Sealing → Drying

Key effects:

• Sealing      agents with added lubricants can adjust the friction coefficient.

• Friction      coefficients should ideally range from 0.09–0.15 for optimal material      utilization.

• Factors      affecting friction coefficient:

• Concentration       of lubricant in the sealing agent

• Centrifuge       speed, drying time, and loading capacity

• Curing       temperature and handling during the process

2. Zinc-Aluminum Coating (Top Coating)

Zinc-aluminum coating, also known as environmentally friendly Dacromet, provides a multilayer corrosion-resistant coating of fine zinc and aluminum flakes.

Process Flow:
Degreasing → Mechanical Blasting → Base Coating (2x or 3x) → Baking → Top Coating (1x or 2x) → Baking → Post-Treatment

Key effects:

• Top      coating with lubricants adjusts the friction coefficient.

• Excessive      centrifuge speed or baking temperature can thin the coating and increase      friction.

3. Phosphating

Phosphating forms a phosphate conversion layer through chemical reactions, often followed by oiling for lubrication.

Process Flow:
Degreasing → Washing → Pickling → Neutralizing → Phosphating → Rinsing → Drying → Oiling

Key effects:

• Phosphating      ensures consistent torque-preload relationships, widely used in structural      steel connections and high-strength bolts to prevent hydrogen      embrittlement.

• Adding      lubricants to the oil ensures stable friction coefficients.

4. Electrophoretic Coating

Electrophoretic coating involves depositing charged paint particles onto a surface under an electric field, forming a protective film.

Process Flow:
Pre-Treatment (e.g., pickling, blasting, or phosphating) → Electrophoresis → Rinsing → Sealing (to adjust friction) → Baking

Key effects:

• Friction      coefficient depends on the paint properties and pre-treatment processes.

• Post-treatment      sealing with lubricants adjusts the coefficient as needed.

By understanding and controlling these processes, manufacturers can ensure fasteners' consistent performance, meet the desired assembly requirements, and enhance product reliability.