The Difference Between Bolts And Screws

Bolts and screws are key fasteners in the construction and manufacturing industries. Choosing the wrong one can lead to joint failure, corrosion or breakage. This article explains and compares the two products from the aspects of raw materials, production process, structural characteristics, application scenarios, failure mechanisms and Installation . It can help you choose the right one to tighten it in place in five minutes.

Basic Definition

What Are Bolts?

Bolts are fasteners whose shanks can be fully threaded or partially threaded, and whose heads are designed with hexagonal heads, flange heads, etc. The materials are mainly quenched and tempered alloy steel, and the strength grade is clearly marked (such as ISO 898 10.9 grade = 1000MPa tensile strength). The installation requires precise control of the preload to transmit high torque. It is suitable for rigid connection scenarios with high loads and repeated disassembly such as steel structure bridges and heavy machinery, and needs to be used with nuts.

What Are Screws?

The screw thread is divided into mechanical thread that needs to be pre-drilled, or self-tapping thread that is an extruded hole. Its head is designed to adapt to electric drill drives such as cross drive and Pozi Drive. The material is usually made of low carbon steel such as C1022A or stainless steel. Its strength depends on the hardness and diameter of the base material. It is suitable for lightweight scenes with limited space such as thin metal plates, plastic shells, and electrical assembly.

Production Process Comparison

1.Bolts Process Key Points

When producing Bolt, carbon steel raw materials are first selected, pickled and phosphated to remove the oxide scale, and then coated with a saponified lubricating layer. Then the wire is cut in a multi-station cold heading machine, and after 6-8 continuous die stamping, the thread forming process requires two hardened rollers to roll the thread on the bare rod section in a radial extrusion manner, and the thread gauge is 100% fully inspected. Then in the processing step, Bolt uses heat treatment quenching, that is, quenching mesh belt furnace (980℃) plus high temperature tempering: 480℃×90 minutes (10.9 grade bolts), so that its tensile strength reaches 1040MPa. The final surface treatment can be electrogalvanized, hot-dip galvanized or Dacromet.

2.Screw Process Key Points

The first step in producing screws is to use low-carbon steel raw materials such as C1022, spheroidizing annealing (hardness 72 HRB), pickling and then coating with borax to reduce the friction coefficient of cold heading. Then a five-station cold heading machine is used for high-speed cold heading. Then, two tungsten steel rollers (60° tooth angle) are used for self-tapping thread rolling to radially extrude the polished rod synchronously. The metal fiber streamlines are complete and the strength is 20% higher than that of the cut thread. Then the processing step is carburizing quenching (920℃ propane atmosphere) plus + low-temperature tempering (220℃) to prevent the screw head from breaking. The final surface treatment can be phosphating and electrogalvanizing.

Core Structural Differences

Application Scenario Comparison:

Only The Right One Can Tighten

1.Bolts are suitable for high-load structural scenarios such as bridge steel frames, engine cylinders and other structural links that need to resist vibration. There are also links that can be disassembled repeatedly, such as mechanical assemblies or flanges, which can be matched with nuts for repeated disassembly and assembly.

2.Screws are suitable for thin metal plates or soft substrates, such as fixing wooden boards and plastic shells. Self-tapping screws do not require pre-drilling and can also be used in permanent fixing scenarios such as electrical circuit boards and gypsum boards. Compared with bolts, screws are more flexible and less restricted in space. Screwdrivers are easier to operate than wrenches.

Comparison Of Key Performance Parameters

Two Product Failure Scenarios

1.Typical Failure Of Bolts

• Bolt failure is reflected in the loss of preload. Under long-term vibration load, the preload attenuates by more than 50% due to micro-wear of the nut, causing a sudden increase in the alternating stress amplitude of the bolt, and finally cracks at the root of the thread and expands and breaks.

• Hydrogen embrittlement fracture is common in high-strength bolts (above grade 10.9). After hydrogen permeation (H content > 3ppm) by electroplating process, delayed fracture occurs within 48 hours, and the fracture shows typical rock candy-like intergranular characteristics.

2.Typical Failure Of Screw

• Screw failure is manifested in thread slippage: self-tapping screws in soft substrates have insufficient yield strength (<110MPa), and the thread engagement area undergoes plastic extrusion deformation, which is manifested as a sudden drop in the peak torque after the penetration, resulting in complete thread slippage and loss of fastening force.

• Screw head fracture Under repeated torsion loads, the cross slot head cracks due to stress concentration, and eventually the slot wall shear fractures

• Corrosion failure is concentrated in the defects of the galvanized layer or the pores of the phosphate film. The exposed steel substrate accelerates rust in a humid environment, and the effective cross-section of the thread is reduced by 30%, which causes fracture.

The Difference Between Bolts And Screws Installation Standards

Bolts Installation

The professional installation of bolts is based on ISO and ASME standards.

1. Before installing the bolts, confirm the construction scenario and select matching bolts such as 8.8/10.9, ensure that the threads are not damaged, and then select the appropriate gasket including size and gasket hardness.

2. Control of three elements of installation

• Calculation of preload

• Calculation of installation torque

• Tightening sequence

Flange connection: Tighten in 3 levels according to diagonal order (30%-70%-100% torque)

3. Selection of installation method

Screws Installation

The installation of screws is generally based on ASME B18.6.4, DIN 7500 and ISO 14589 standards, combined with actual usage scenarios and installation materials.

1. Core preparation before installation

Before installation, choose the right screws based on the construction materials, such as self drilling screws or self tapping screws, phosphating surface treatment or galvanizing

2. Key points for calculating installation torque

•  Maximum allowable torque (anti-slip teeth)

•  Recommended installation torque (80% safety threshold)

• Screw nominal diameter (mm)

3. Determine whether drilling is required

Determine whether drilling is required based on the type of screws and installation materials. For example, self-tapping screws need to be drilled on the substrate, and then an electric drill is used to drill vertically (angle <2°), and the impact mode is disabled. If it is a self drilling screw, no drilling is required and it can be directly drilled into the material

In summary

Bolts and Screws are usually used fastener in our life ,you just to know when they require the more powerful fastener choose the blots with nuts ,if require reliable fixing choose screws, so choose the right tools, the work will be much easier, If you want to learn more about various fasteners, or find the specifications of specific models, you are welcome to visit www.sinsunfastener.com at any time, where there are a lot of practical information and tips waiting for you.