Bolted Joints: Fastener Grades, Torque, and Threadlockers

A bolt holds a joint together by stretching. When you tighten it, it pulls on the parts like a very stiff spring, and that pull, called preload or clamping force, is what keeps the joint closed under load and vibration. Torque is just the way you get there. So the goal is never to make a bolt "tight." The goal is to reach a specific clamping force, with the right fastener, in the right order. Using the wrong grade, skipping a pass or tightening an oiled bolt to a dry number can make a joint fail, even though it looks fine.

What You Will Be Able to Do

• Read a thread callout and identify SAE and metric grades from head markings.
• Explain torque specs and why dry and lubricated bolts differ.
• Tighten a multi-bolt joint in a star pattern and in passes, and apply a torque-angle spec.
• Use a torque wrench correctly and keep it in calibration.
• Choose and apply locking nuts and threadlockers without substituting.

Threads

Thread pitch is the distance from one thread to the next, measured from crest to crest. Inch and metric callouts express it differently.

UNC is coarse thread and UNF is fine thread. Coarse threads go together quickly, tolerate dirt and resist stripping in soft material. Fine threads are a bit stronger and resist vibration loosening a little better, but they cross-thread more easily. A thread pitch gauge tells you what you have. Never force an inch fastener into a metric hole, or the reverse. Some sizes feel close at first, then bind and strip.

Starting a Thread

Start every threaded fastener by hand for at least two full turns before you use a tool. If you feel resistance, back it out and try again. A cross-threaded fastener can look tight and hold nothing.

Check that you have enough thread in the joint. In steel, a rule of thumb is thread engagement of at least one bolt diameter. Softer materials such as aluminum or cast iron need more, often 1.5 to 2 diameters. On a nut, at least a thread or two should show past it.

Fastener Types

Set screws have different tips. A cup point grips best, a cone point is for a permanent position, and a flat point does the least damage to the shaft.

Thread-Cutting and Thread-Forming Screws

Self-tapping screws come in two kinds.

• Thread-cutting screws have a cutting edge that removes material and leaves chips.
• Thread-forming screws push the material aside to form a thread without removing any, so there are no chips and the fit is tight.

Thread-forming screws are the usual choice for plastics, because they make a strong thread with no loose chips. A very hard, brittle plastic may crack from that pressure, so those parts may call for a thread-cutting screw instead. Either way, set the driver's torque so you do not strip a plastic boss.

Reading Fastener Grades

Grade tells you how strong a bolt is. Two bolts can look the same and be very different in strength. Installing the wrong grade is a dangerous error, so check the head marking against the drawing. Treat an unmarked bolt as low strength.

SAE Grades for Inch Fasteners

Strength figures for Grades 2 and 5 drop a bit on larger bolts. Grade 2 is low carbon steel, Grade 5 is medium carbon steel, and Grade 8 is alloy steel, with Grades 5 and 8 quenched and tempered.

Metric Property Classes

Class 8.8 rises to 830 MPa on bolts above M16. The numbers on the head mean something. The first number times 100 gives the rough tensile strength in MPa. The number after the dot gives the yield strength as a fraction of the tensile strength. So class 10.9 has a rough tensile strength of 1,000 MPa (its required minimum is 1,040 MPa), and it yields at about 90 percent of that.

For comparison, Grade 8's 150,000 psi is about 1,034 MPa, so class 10.9 sits level with Grade 8, not below it. Similar strength does not make them interchangeable. Follow the drawing, and get engineering approval for any substitution.

Match the nut to the bolt as well. A high-strength bolt with a weak nut will strip the nut's threads.

Stainless Steel Fasteners

Stainless fasteners are marked by material group. A2 is 304 stainless, for general corrosion resistance. A4 is 316 stainless, for marine and chemical settings. A number after the dash, as in A2-70, gives the strength. Stainless is generally weaker than Grade 8, so never swap it into a strength-critical joint without approval. Stainless threads can also gall, which means they seize together as you tighten. Go slowly, and use anti-seize only if the spec allows.

Torque: What the Number Means

A torque specification is the turning force needed to reach the right clamping force. It is given in foot-pounds (ft-lb), inch-pounds (in-lb) or newton-meters (N·m). There are 12 in-lb in 1 ft-lb, and 1 ft-lb is about 1.36 N·m.

Most of your effort never reaches the bolt's stretch. About half of the torque is used up by friction under the head or nut, and about 40 percent by friction in the threads. Only about 10 to 15 percent goes into stretching the bolt. Small changes in friction therefore make big changes in clamping force.

Dry Versus Lubricated

Oil, anti-seize or plating lowers friction, so more of your torque goes into stretching the bolt. At the same torque setting, a lubricated bolt clamps much harder than a dry one, and it can go past its limit.

Engineers estimate torque with T = K x D x F. T is torque, D is the bolt diameter, F is the clamping force you want, and K is a nut factor that covers friction. A textbook K is about 0.20 for dry steel and about 0.15 for lightly oiled steel.

Worked example with a 3/8-16 Grade 5 bolt. Its proof load, the force it can hold without staying stretched, is about 6,590 pounds. A common target clamp is about 75 percent of proof load, so F is about 4,940 pounds.

• Dry: 0.20 x 0.375 x 4,940 = 370 in-lb, or about 31 ft-lb.
• Oiled: 0.15 x 0.375 x 4,940 = 278 in-lb, or about 23 ft-lb.

Now see what happens if someone oils the bolt but still uses 31 ft-lb. F = 370 / (0.15 x 0.375) = about 6,580 pounds. That is right at the bolt's proof load, with no margin left.

The rules to follow on the floor:

• If the spec says dry, keep the threads clean and dry.
• If the spec names a lubricant, use that lubricant, in the amount it says.
• If you add a lubricant or anti-seize, the torque has to come down by the amount the engineering documents give. Never guess the reduction.
• Do not add oil to "make it easier."

If you are not sure, ask your supervisor or check the engineering spec.

Tightening Patterns

On a joint with several bolts, tightening one pulls the parts together and loosens its neighbors. If you go around in a circle at full torque, one side clamps hard and the other side ends up loose, and a flange can warp or a gasket can leak.

For most flanges and covers, use a star (crisscross) pattern, going back and forth across the joint. Number the bolts clockwise around the circle.

Bring the joint up in passes. For a final torque of 40 ft-lb on six bolts:

1. Run all bolts in by hand, then snug them in star order.
2. Go through the star order at 50 percent, which is 20 ft-lb.
3. Go through at 75 percent, which is 30 ft-lb.
4. Go through at the final 40 ft-lb.
5. Make a last check pass around the circle at 40 ft-lb, until no fastener moves.

Most specs require at least two passes, and critical joints can call for three or four. Some gasketed joints use a circular pattern, starting at 12 o'clock and working clockwise. Always follow the pattern and percentages in your instruction if they differ.

Torque-Angle Tightening

Some joints are tightened with a torque-angle spec, written as, for example, "30 ft-lb plus 60 degrees."

1. Torque the fastener to the first value. This pulls the joint together and seats it.
2. Mark the fastener or use an angle gauge.
3. Turn the fastener the extra angle in one smooth motion.

Turning a set angle moves the nut a set distance along the thread. One-sixth of a turn on a 1.5 mm pitch moves it 0.25 mm. So the bolt stretches a precise amount, and friction has less effect. Clamping force comes out more even from bolt to bolt than with torque alone. Many of these bolts are tightened into their yield range, so they are one-time use. Check the instruction, and replace them when it says to.

Using a Torque Wrench

1. Set the torque and lock the handle, if it has a lock.
2. Seat the socket fully on the fastener.
3. Hold the wrench at the grip mark. Pulling from another point changes the lever length and the torque.
4. Pull smoothly and steadily, at right angles to the handle and the fastener. Do not jerk.
5. Stop the moment it clicks or reaches the set value. Do not pull again to "make sure," because that adds torque.

Use a calibrated torque wrench for final torque on critical fasteners. Impact tools can over-torque a fastener easily, and they have no way of showing you the torque.

Adding a crowfoot wrench or extension in line with the handle changes the torque that reaches the fastener. Set the wrench using: setting = target x L / (L + E), where L is the wrench length and E is the added length. Example: for 50 ft-lb with a 16-inch wrench and a 2-inch crowfoot, set 50 x 16 / 18 = 44.4 ft-lb. A crowfoot turned 90 degrees to the handle needs no correction.

Wrench Care and Calibration

A torque wrench is a measuring tool, so treat it like one.

• Do not use it to loosen fasteners. Doing that can damage its calibration.
• Store a click wrench at its lowest marked setting to relax the spring. Do not wind it below the scale.
• Do not use it as a breaker bar or add a cheater pipe.
• If you drop it, have it checked before you use it again.
• Keep it clean, in its case.

Calibration is checked against a standard such as ISO 6789 or ASME B107.300. The allowed error depends on the wrench type and size, and about 4 percent of the setting is a common figure for click wrenches. The certificate shows what your wrench is rated for. A common rule is to recalibrate every 12 months or 5,000 cycles, whichever comes first, but your quality system sets the real interval.

If you find a torque wrench that is past its calibration date, tag it out of service and send it for calibration. Use a wrench that is in calibration in the meantime. If a wrench comes back out of tolerance, tell quality, because parts tightened with it may need to be checked.

Locking Methods

Vibration can loosen a joint. Preload is the first defense, and a locking method adds friction or a mechanical stop.

Why You Replace a Nyloc Nut

The first time a nylon-insert nut goes on, the bolt cuts its way through the nylon collar and leaves the collar permanently deformed. If you take it off and put it back on, the collar no longer grips as hard, so it has less locking force and the joint can loosen under vibration. Treat these as one-time parts and install a new one unless your procedure says otherwise. The same goes for fasteners your instruction marks as one-use.

Threadlockers

A threadlocker is an anaerobic adhesive, meaning it cures when it is cut off from air and in contact with metal, as in the gap between mating threads. It fills that gap, so the fastener cannot work loose from vibration, and it also seals the threads against leaks and rust.

Red is meant for joints you do not expect to take apart. In practice it is permanent unless you heat the fastener to about 500 F. The data sheet for one common red grade says about 250 C, or 482 F. Taking the joint apart while it is hot is how you get it open, and service work on a joint that used red is slow and difficult. That is why the choice is made in engineering, not at your bench. Heating with a torch is also hot work and needs your plant's fire controls.

Color is a handy shorthand but not a universal code. A different maker's color can mean something else, and green has been used for a high-strength product as well as a wicking one. Go by the product number on the drawing.

Applying Threadlocker

1. Clean the threads and remove oil. Some grades tolerate light oil, but do not count on it.
2. Check the instruction for whether the metal needs a primer. Stainless steel and zinc-plated parts can cure slowly.
3. Apply a few drops where the nut will engage the threads.
4. Assemble and torque to the spec within the time the product allows. Threadlocker also acts as a lubricant, so use the torque the spec gives for it.
5. Let it cure. Most reach handling strength in minutes to hours and full strength in about 24 hours. Cold slows the cure.

Do not use a threadlocker on plastic unless the product is approved for it, because it can crack some plastics. Read the SDS before you use it.

If You Have the Wrong Color

Say an instruction calls for medium-strength blue and the only bottle on the bench is red. Do not use it. Red in place of blue may mean a bolt that snaps during the next service. Blue in place of red may loosen in service. Get the correct product or tell your supervisor before you go on. The same goes for any substitution.

Common Mistakes

Check Your Understanding

1. A head shows six radial lines. Another shows "10.9". Which is stronger?

They are about the same. Six lines means SAE Grade 8, at about 150,000 psi or 1,034 MPa. Class 10.9 is at least 1,040 MPa. They are still not interchangeable without approval.

2. A spec reads 60 ft-lb dry. A coworker has already coated the threads with anti-seize. What do you do?

Stop. At 60 ft-lb the bolt will clamp much harder than planned and may stretch or break. Clean the threads and install them dry, or get the reduced torque from engineering.

3. Number the order for tightening a four-bolt cover, with a final torque of 20 ft-lb.

Use 1, 3, 2, 4 in passes. Snug all four, take them to 10 ft-lb, then 15 ft-lb, then 20 ft-lb, and finish with a check pass.

4. You are taking a bracket off and a Nyloc nut comes off with it. Your instruction says nothing special. What do you do on reassembly?

Use a new nut. Its nylon collar is deformed from the first use and will not grip as well.

5. An M10 x 1.5 bolt has a spec of 25 N·m plus 120 degrees. How far does the nut travel in the angle step?

120 degrees is one-third of a turn, so it travels 1.5 / 3 = 0.5 mm. That controlled travel sets the stretch.

Key Takeaways

• A bolt works by stretching. Torque is only a way of reaching the right clamping force.
• Three head lines mean Grade 5, and six mean Grade 8. Class 10.9 is about equal to Grade 8.
• Friction uses up most of your torque, so a lubricated bolt clamps harder at the same setting. Reduce torque only as the spec says.
• Tighten multi-bolt joints in a star pattern, in passes.
• Pull a torque wrench smoothly at the grip, stop at the click, never loosen with it, and tag it out if it is past calibration.
• Replace Nyloc nuts, and never swap one threadlocker for another without approval.