What is a U-Bolt
author: www.qishine.com
2026-04-15
A U-bolt, named for its "U" shape, is commonly referred to as a "riding bolt" in industrial piping installation and automotive repair. It is a non-standard fastener with threads on both ends, typically used with nuts. Its primary function is to securely fasten pipes, hoses, cables, or cylindrical objects to supports, steel plates, or walls. Due to its simple structure and stable fastening performance, it can be found in construction plumbing, fire protection piping, automotive suspension systems, and agricultural machinery.
U-Bolt Assembly Drawing
I. Key Parameters
When selecting or designing a U-bolt, the following key parameters directly determine its applicability and safety:
1. Dimensional Parameters
Inner Width (P): The straight-line distance between the inner sides of the two legs of the U-bolt. This is the most important parameter for selection, determining the maximum outer diameter of the pipe or object the bolt can clamp. If the inner width is too small, the bolt cannot fit over the pipe; if too large, it cannot fit tightly, leading to fastening failure.
When selecting or designing a U-bolt, the following key parameters directly determine its applicability and safety:
1. Dimensional Parameters
Inner Width (P): The straight-line distance between the inner sides of the two legs of the U-bolt. This is the most important parameter for selection, determining the maximum outer diameter of the pipe or object the bolt can clamp. If the inner width is too small, the bolt cannot fit over the pipe; if too large, it cannot fit tightly, leading to fastening failure.
Inner Height (H): The vertical distance from the inner apex of the U-bend to the ends of the bolt legs. This determines the height of the object the bolt can accommodate and whether there is sufficient space for washers and nuts.
Nominal Thread Size (M): The diameter of the threaded portion, e.g., M8, M12, M20. It determines the matching nut specification and the bolt's load-bearing capacity.
Thread Length (L): The length of the threaded section must be long enough to ensure that after passing through mounting holes and washers, the nut can be fully tightened with some allowance.
Pipe Outer Diameter (Applicable Pipe Size): This is a guiding parameter in engineering applications, indicating the specific outer diameter of the pipe the U-bolt is intended for, e.g., 21.7mm (for DN15/1/2-inch pipe), 26.9mm (for DN20), etc.
U-Bolts Dimensions: Inside width, Height, Thread Length, Diameter
2. Material and Surface Treatment Parameters
Material: Common materials include carbon steel (e.g., Q235), alloy steel (e.g., 40Cr, 35CrMo), and stainless steel (e.g., SUS304, SUS316). Carbon steel is low-cost, suitable for general environments. Stainless steel offers high corrosion resistance, ideal for humid or chemical plant conditions. Alloy steel provides very high strength, used in heavy-load scenarios like automotive leaf springs and petroleum industry piping.
Material: Common materials include carbon steel (e.g., Q235), alloy steel (e.g., 40Cr, 35CrMo), and stainless steel (e.g., SUS304, SUS316). Carbon steel is low-cost, suitable for general environments. Stainless steel offers high corrosion resistance, ideal for humid or chemical plant conditions. Alloy steel provides very high strength, used in heavy-load scenarios like automotive leaf springs and petroleum industry piping.
Surface Treatment: For rust prevention and aesthetics, common treatments include electro-galvanized (general indoor use), hot-dip galvanized (high outdoor anti-rust requirements), or Dacromet (corrosion protection for high-strength bolts).
3. Mechanical Performance Parameters
Performance Grade: e.g., Grade 4.8, 8.8, 10.9. Grade 4.8 is ordinary low-strength, used for general pipe fixing. Grade 8.8 and above are high-strength, used in areas subject to dynamic loads and shear forces, such as automotive suspension.
Performance Grade: e.g., Grade 4.8, 8.8, 10.9. Grade 4.8 is ordinary low-strength, used for general pipe fixing. Grade 8.8 and above are high-strength, used in areas subject to dynamic loads and shear forces, such as automotive suspension.
II. Manufacturing Process Sequence
The manufacturing of a U-bolt is a typical metal cold-forming process. The main sequence is as follows:
1. Step 1: Raw Material Inspection & Cutting
Select the appropriate coil steel (carbon or stainless steel), straighten it, and cut it into a straight bar using an automatic cutting machine according to the calculated unfolded length.
The manufacturing of a U-bolt is a typical metal cold-forming process. The main sequence is as follows:
1. Step 1: Raw Material Inspection & Cutting
Select the appropriate coil steel (carbon or stainless steel), straighten it, and cut it into a straight bar using an automatic cutting machine according to the calculated unfolded length.
Impact: Material composition directly affects strength after heat treatment; cutting length error affects the final inner height (H).
2. Step 2: Thread Processing at Both Ends (Critical Control Point)
Use a thread rolling machine or threading machine to cold-form threads on both ends of the straight bar.
Note: U-bolts are typically threaded before bending because rolling threads on a straight bar is more efficient and accurate.
Use a thread rolling machine or threading machine to cold-form threads on both ends of the straight bar.
Note: U-bolts are typically threaded before bending because rolling threads on a straight bar is more efficient and accurate.
3. Step 3: Bending Forming
Place the straight bar (now with threads on ends) into a dedicated hydraulic U-bending machine or stamping die. Through die pressing or roller rotation, the middle section is bent into the standard U-curvature. This is the core of the manufacturing process.
Place the straight bar (now with threads on ends) into a dedicated hydraulic U-bending machine or stamping die. Through die pressing or roller rotation, the middle section is bent into the standard U-curvature. This is the core of the manufacturing process.
4. Step 4: Heat Treatment (For High-Strength Bolts)
For high-strength U-bolts Grade 8.8 and above, quenching and tempering are performed after bending to relieve bending stresses and achieve the required hardness and tensile strength.
For high-strength U-bolts Grade 8.8 and above, quenching and tempering are performed after bending to relieve bending stresses and achieve the required hardness and tensile strength.
5. Step 5: Surface Treatment
The finished bolts are cleaned, derusted, and then electro-galvanized, hot-dip galvanized, or black-oxidized to enhance corrosion resistance.
The finished bolts are cleaned, derusted, and then electro-galvanized, hot-dip galvanized, or black-oxidized to enhance corrosion resistance.
6. Step 6: Inspection & Packaging
Inspect threads using go/no-go gauges, measure inner width (P) and height (H) using calipers or fixtures, and finally package with matching nuts.
Inspect threads using go/no-go gauges, measure inner width (P) and height (H) using calipers or fixtures, and finally package with matching nuts.
III. Which Parameters Deeply Affect Production Manufacturing?
As seen from the process flow, certain design parameters directly determine manufacturing difficulty, cost, and yield rate:
1. Thread Length (L) & Position: Determines Process Sequence & Scrap Rate
Impact: When rolling threads on the straight bar, the reserved length of the plain shank must be precisely calculated. If threads are too close to the bend area, the enormous bending force during Step 3 can flatten or distort the thread profile near the bend apex, preventing nut installation and causing product rejection. Therefore, a sufficient straight transition section is typically required between the bend start point and the thread end.
As seen from the process flow, certain design parameters directly determine manufacturing difficulty, cost, and yield rate:
1. Thread Length (L) & Position: Determines Process Sequence & Scrap Rate
Impact: When rolling threads on the straight bar, the reserved length of the plain shank must be precisely calculated. If threads are too close to the bend area, the enormous bending force during Step 3 can flatten or distort the thread profile near the bend apex, preventing nut installation and causing product rejection. Therefore, a sufficient straight transition section is typically required between the bend start point and the thread end.
2. Material Hardness & Diameter (M): Determines Die Life & Machine Tonnage
Impact: Larger diameters (e.g., M30 and above) and harder materials (e.g., Stainless Steel 304, Alloy Steel) require significantly higher hydraulic tonnage for bending and demand extremely high wear resistance from the bending dies. Insufficient die strength can easily lead to excessive spring-back of the U-bolt's inner width, resulting in dimensional non-conformance.
Impact: Larger diameters (e.g., M30 and above) and harder materials (e.g., Stainless Steel 304, Alloy Steel) require significantly higher hydraulic tonnage for bending and demand extremely high wear resistance from the bending dies. Insufficient die strength can easily lead to excessive spring-back of the U-bolt's inner width, resulting in dimensional non-conformance.
3. Bending Radius (R-angle): Determines Appearance & Stress Concentration
Impact: The radius at the bottom of the U cannot be infinitely small. If the design requires too small an R-angle, the outer fiber elongation of the steel during bending becomes excessive, leading at best to flattening or cracking on the outside, and at worst, direct fracture. Manufacturing engineers calculate the minimum bending radius based on the material's elongation rate.
Impact: The radius at the bottom of the U cannot be infinitely small. If the design requires too small an R-angle, the outer fiber elongation of the steel during bending becomes excessive, leading at best to flattening or cracking on the outside, and at worst, direct fracture. Manufacturing engineers calculate the minimum bending radius based on the material's elongation rate.
4. Inner Width (P) Tolerance Requirement: Determines Inspection Cost
Impact: Due to physical spring-back after bending, the opening of the U typically opens slightly wider than the die size. If the drawing requires extremely tight tolerances on inner width P (e.g., within ±0.2mm), an additional cold-sizing step or angle compensation using a servo bending machine becomes necessary in the process, significantly increasing the manufacturing cost per part.
Impact: Due to physical spring-back after bending, the opening of the U typically opens slightly wider than the die size. If the drawing requires extremely tight tolerances on inner width P (e.g., within ±0.2mm), an additional cold-sizing step or angle compensation using a servo bending machine becomes necessary in the process, significantly increasing the manufacturing cost per part.
Qishine is ideal choice for EPC companies and project contractors.
Website: www.qishine.com
Email: qishine@qishine.com
Tel: 0592-5225595
WhatsApp: +86 15960259563
+86 15985833169
Website: www.qishine.com
Email: qishine@qishine.com
Tel: 0592-5225595
WhatsApp: +86 15960259563
+86 15985833169
What is the difference between eyebolts
What is a Heavy Hex Nut
Related Article
Insights from a Case Study on Exporting B8M Fasteners.Recently, during our final outgoing inspection of a batch of B8M stainless steel fasteners destined for Portugal, we discovered varying degrees of burrs on the edges of the bolt hex heads.
How to Efficiently Remove Burrs from Bolt Edges
Insulating Gasket Procurement Pitfalls: From "Price Traps" to "Supplier Selection"
What pitfalls should you watch out for when purchasing insulating gaskets
As a classic modified PTFE gasket material from Garlock, Gylon 3500 is widely used in petrochemical, chemical, and other industries for its excellent resistance to strong acids, solvents, and anti-creep properties.
How to Buy Genuine Gylon 3500 Insulating Gaskets at Competitive Prices in China
