A U-bolt is a metal fastener in the shape of the letter U. It has a pin or screw bolt closure that fits across the two ends, making a closed clamp. The curved shape of the bolt allows it to fit around pipes or tubing easily. As a result, U-bolts frequently support and restrain pipework or tubes. In this article, you will learn about the purposes of u-bolts, types, material, dimensions, torque specifications, and service recommendations.
U-bolts are a jack-of-all-trades in construction. It’s a versatile fastener that sees use as a foundation and roof framing fastener, pipe and conduit holder, and for motor and engine shaft components.
U-bolts provide the clamping force required to clamp the leaf spring, top plate, axle seat, axle, and bottom plate firmly together and are critical to the overall stability of spring operation.
These bolts are also ideal for holding things in place that would otherwise be difficult to bind to a specific surface due to their distinctive design.
Here are some common uses:
- Body mounting
- Piping support
- Conduit support
- Framing fasteners
- Foundation anchors
- Roof anchors
- Wire fasteners
- Cable fasteners
- Motor fasteners
- Affix part to pole, beam, or wall
- Suspension mounting
Type of U-Bolts
U-bolts come in a variety of sizes and finishes to sustain a variety of loads. Three basic types of bends used on U-bolts depends on the suspension design and the shape of the mating parts: round bend, square bend, and forged top.
Round bend U-bolts have a broad range of industrial applications. Typically they secure pipe or steel round bar attachment to a steel post or fixture. U-bolts embedded in concrete as anchor bolts. Round bend U-bolts also use for leaf spring truck and trailer applications.
Round U-bolts possess an almost perfect semi-circle shape while the semi-round versions possesses a slight arch. Semi-round bends are frequently used in vehicle and spring-loaded applications.
Square bend U-bolts fix an item to a box section structure or clamp square or rectangular objects. This square bend style may also suspend metal channels and ducting. They also anchor in concrete or secure heavy machinery.
Further varieties exist for specialty applications. These have square developed, round developed, or semi-round generated. Furthermore, each of these bend kinds uses sophisticated material. Forged top primarily used where additional clearance requires between the U-bolts and, for example, the frame.
Materials of U-Bolts
The load-carrying capacity of the unit is usually the deciding element in material selection for U-bolts. The evaluation of the forces experienced in the joint, including standard shock and cyclic fatigue, to determine the ability required.
Engineers should select a nominal diameter to provide a sufficient capacity with a safety margin. A big diameter may be less expensive than a smaller heat-treated alloy option.
During assembly, torque requirements may require a higher material grade than the load requirements. A 5/8-11 bolt has a tensile strength of 90,000 PSI, for example. The bolt and nut’s material hardness and compatibility are crucial.
The makeup of the washer has an impact on torque readings as well. A range of coatings, including zinc plating, paint, and other coatings, provide corrosion protection.
Plated U-bolts, especially those made of high-tensile material, see heat treatment after zinc plating. This process prevents hydrogen embitterment at the radii and thread roots. For corrosion resistance or increased aesthetics, stainless steel grades are also available.
U-bolts are available in a variety of materials. However, they typically are formed from non-corrosive, long-lasting metal. Some of the most frequent materials used to make U-bolts are listed below.
In electrical systems, stainless steel U-bolts support conduits or pipes.
For food processing, industrial, and marine applications, 302 stainless steel provides high corrosion resistance. It has non-magnetic characteristics as well as exceptional durability at all temperatures.
304 Stainless Steel
The corrosion resistance of 304 stainless steel is excellent in the presence of oxygen. Chemical, food processing, and paper are some of the typical industries that use 304 stainless U-bolts.
309 Stainless Steel
309 provides high strength and oxidation resistance in continuous service applications. This alloy endures temperatures of up to 2000 degrees Fahrenheit (1093 C). In terms of strength and corrosion resistance, it outperforms 304 stainless steel.
316 Stainless Steel
Superior corrosion resistance makes 316 stainless steel ideal for chemical, food processing, coastal and offshore marine applications. The oxidation resistance and mechanical qualities of the 316 stainless are excellent. Paper, mining, pharmaceuticals, and petroleum are just a few of the industries that employ them. 316 provides an excellent option for offshore installations that see continuous salt exposure.
B-8 Stainless Steel
B-8 provides excellent corrosion resistance and features the non-magnetic properties of stainless steel. This variant provides a tensile strength of 75,000 psi and is made out of the same material as AISI type 304 stainless steel. It undergoes a carbide solution annealing process.
B-8M Stainless Steel
Similar to B8, the 8M variant adds Molybdenum. Hex bolts, studs, and other fasteners are standard in the B8M specification because it has superior corrosion resistance than B8.
PH 17-4 Stainless Steel
PH 17-4 is a martensitic, chromium-nickel-copper stainless steel with excellent strength and moderate corrosion resistance. Stainless 17-4 PH properties provide required resistance in chemical and food processing as well as nuclear components.
Carbon steel is a common fastener material because of its cost-effectiveness, high tensile strength, and ability to bear high loads. It takes heat treatment well, with slight work hardening and brittleness. It’s simple to produce and can hold tight tolerances for threading and fastening arrangements.
Carbon steel is classified according to the amount of carbon in the alloy, which affects the steel’s qualities. Low carbon steel is flexible and malleable and possesses a carbon content of 0.05-0.3 percent. With a low tensile strength and can be carbonized to make it harder. It is the most cost-effective carbon steel alloy.
Medium carbon steel has 0.3-0.59% carbon, which provides the alloy with increased strength and good wear resistance. This variant sees common use in forged and automotive parts.
Solid high carbon steel and ultra-high carbon steel are used for springs and high-strength products. These alloys are tempered to very high hardness without brittleness and used for very high wear applications. Carbon content ranges from 0.6-0.99% for high carbon steel and 1.0-2.0% for ultra-high carbon steel.
Special alloys serve niche applications that require high corrosion resistance, high yield strength, and galling resistance. An example application is Alloy-60 U-bolts to secure pipe to pipe racks and for providing support in corrosive environments.
Made of a high-strength nickel-chromium-molybdenum alloy, Inconel provides outstanding corrosion resistance at high temperatures. Due to Inconel fasteners’ thermal stability, it is used in a wide variety of applications from the cryogenic to the 2200F range. Salt-laden and coastal conditions have little effect on Inconel.
This nickel-copper alloy has excellent toughness and corrosion resistance across a wide temperature range. Monel 400 fasteners are thought to be the most corrosion-resistant when compared to all unique exotic alloy fasteners. Widely utilized in the oil and gas industry for HCL and HF applications, and also a good fit for maritime applications.
Hastelloy C-276 is a favorite alloy since it is extremely corrosion resistant in a wide range of conditions. A better-wrought version of alloy C, built with wrought nickel-molybdenum-chromium alloy.
The Hastelloy family as a whole offers excellent corrosion resistance and resistance to both oxidizing and reducing substances. The Hastelloy C-276 alloy can be used in problem zones that are more likely to occur in process zones because of its versatility.
In as-welded form, Hastelloy 276 bolts and fasteners are appropriate for most chemical applications. However, it should be noted that in circumstances where the C-276 alloy weld joint is subjected to assault, C-22 weld filler materials should be considered.
Titanium, like platinum and gold, is towards the top of the galvanic scale. Its legendary resistance to all types of corrosion means that corrosion-related maintenance and replacement costs are nothing.
Beyond these parallels, titanium is so different that it almost beggars comprehension. Other metals pale in comparison to their strength-to-weight ratio. The mechanical characteristics of titanium fluctuate considerably in alloys that are only slightly different from one another.
The diagram below notes crucial u-bolt measurements.
The chart below references the above diagram.
In the metric system, U Bolt Dimensions range from M6 to M20 regarding thread diameter, with various lengths and specials also available.
The strength of a material is determined by its U-Bolt grade. The most frequent rates are 5 and 8, with grade 8 material having a yield strength nearly 40% higher than grade 5. This enables more significant torque levels and, as a result, better clamping force.
Grade 8 is the recommended choice for most applications and is always the safest choice if the original grade or use is unknown.
U-Bolt Torque and Clamping Force
An adequately torqued U-bolt stretches by design. The resultant clamping force acts as a spring clamp.
When mounting a U-bolt, it is practically impossible to evaluate the clamping force immediately. As a result, torque requirements are used as an additional measure of clamping force.
The basic formula for torque is T = (KDP)/12, where T = torque in foot-pounds, D = nominal diameter in inches, P = desire clamp load or tension in pounds, and K = coefficient of friction.
Dimensional tolerances for U-bolts vary. The U-bolt business expects equipment and tools to fulfill certain levels of conformity. In high-volume production, tighter tolerances may be impossible to achieve or negated by post-processing.
Cold forming can achieve a centerline width requirement of +/-.030 inches for a medium tensile U-bolt. Tumbling in a barrel during zinc plating might cause the legs to spring back unpredictably, requiring treatment to bring them back to specification. Loose tolerances offer no benefit in production or assembly, and they may influence other aspects of the U-bolt.
U-Bolts Service Recommendation
- Ensure that the correct size and material of U-bolts are used.
- Use recommended torque specifications.
- Only use clean and rust-free U-bolts.
- Lubricate U-bolts and washes with oil or anti-seize compound to reduce nut friction.
- Look for signs of movement within the area between the bolts.
- Be precise when tightening.
- Holes that host your U-bolt need to fit securely around each end of the bolt.
- Permanently attach the U-bolt at the proper depth.
- Consider using thermoplastic coatings when working with piping or tubing.
- Make sure to specify compatible metals.
- Check and retighten U-bolts as part of a preventive maintenance program.
- Tighten U-bolts to the snug-tight condition or manufacturer’s recommendation only.
- Do not reuse U-bolts.
- Avoid mixing metals.
- Broken or worn u-bolts, washers, and nuts should not be used.
- Do not use nuts or bolts that do not fit correctly together.
- Never use bolting that cannot be correctly identified.
- Do not mix U-bolts with different coatings on a particular flange. Different bolt coatings require different torques to achieve the same bolt tension, as some layers are more lubricating than others.
- Nuts should not be assembled with a hard stamp or pad against the flange. The machined face should always come into contact with the flange.
- Do not use bolting that lacks clear material grade marks.