U-bolt bending
A U-bolt is a rod threaded at both ends and bent round a former so it wraps a pipe or a structural member. It is sized by what it has to fit: a U-bolt for 2 in. pipe has to clear the pipe's 2.375 in. (60.3 mm) outside diameter, not 2 in.
Cut, thread, bend, check.
- STEP 01
Cut the rod
Rod of the ordered grade is cut to the length of both legs plus the bend.
- STEP 02
Thread both legs
Each leg is threaded, rolled or cut, to the length the order calls for while the rod is still straight, with the thread kept clear of the bend.
- STEP 03
Bend round a former
The rod is bent cold round a former sized to the pipe or member: a half-circle for a round bend, two rounded corners for a square bend, a flatter curve for semi-round.
- STEP 04
Check
Inside width, length and thread length are checked against the order, and the threads are gauged.
- STEP 05
Nuts and plates
Each U-bolt goes out with its nuts, and with plates when the order asks.
- Quenched-and-tempered high-strength rod may need heating to bend and then heat treatment again, done by an outside Houston processor.
- A square bend is usually given an inside radius of at least twice the rod diameter; sharper corners may need hot bending.
Order it by what it has to fit.
Pipe sizes are nominal: NPS 2 pipe is 2.375 in. across the outside and NPS 4 is 4.500 in. A round U-bolt is sized from the outside diameter, with the width taken between the legs and the length from the end of a leg to the inside of the bend. Some makers measure to the centre of the bend or between leg centres, so a drawing settles it.
Bending stretches the outside of the bend and squeezes the inside. ASTM F1554, the specification for bent anchor bolts, asks for a bend free of cracks at 10× magnification that keeps at least 90% of the rod's cross-section, and caps the temperature of hot bending.
ASME B18.31.5 covers bent bolts in inch sizes, with tables for U-bolts of different bends; the threads are Unified threads to ASME B1.1.
Parts and features.
U-bolt bending: common questions.
How do I measure a U-bolt?
Width between the legs, length from the end of a leg to the inside of the bend, thread length and rod diameter. Some makers measure to the centre of the bend or between leg centres, so a drawing removes any doubt.
Should the inside width equal the pipe size?
No: the pipe's outside diameter. NPS 2 pipe is 2.375 in. across the outside and NPS 4 is 4.500 in., so the U-bolt is sized from those figures.
Is there a standard for U-bolts?
ASME B18.31.5 covers bent bolts in inch sizes, including U-bolts of different bends; it superseded IFI-136. The threads follow ASME B1.1.
Can U-bolts be coated?
Yes: galvanized or PTFE-coated by outside Houston processors, or supplied plain. Hot-dip galvanized U-bolts need nuts tapped oversize after coating (ASTM A563).
Before and after it.
Thread rolling
Threads pressed into the bar, not cut out of it
Die-head thread cutting
A ring of chasers cuts the thread in one pass, then springs open
Thread gauging and dimensional inspection
Threads are checked with Go and No-Go ring and plug gauges, and pitch diameter is measured by the three-wire method or on a thread comparator. Heads, flats, lengths and radii are measured with calibrated instruments and, for complex parts, on the CMM.
Coating thickness and salt spray
Coating thickness is measured with a magnetic or eddy-current gauge, or by X-ray fluorescence for thin platings, and checked against the class ordered. Salt-spray exposure checks corrosion protection on a sample from the lot when the specification calls for it.