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Nuts and Bolts

Probably one of the first things you learned when starting out working on cars was the old "righty tighty lefty loosey" saying. Always good for a laugh or two to repeat it when your buddy is under the car and trying to loosen something from the backside and he is really tightening it. Well just like everything else in life seems like the more you learn the less you know. Below is an overview of how fasteners work. The purpose of the information is not meant to help you to pass an engineering class but simply to pass on some basic information about how and why fasteners do what they do.

Basically we use nuts and bolts to hold things together. Many times instead of using a nut, the bolt simply threads into the component, securing the items together. Seems simple enough.

Many times the big question is how tight to install the fastener? If you over tighten the fastener you can damage the threads on the bolt, nut or the threads on the component. If you do not tighten the fastener enough then it can fatigue and fail due to loading and off-loading.

What happens when you use a simple nut and bolt to fasten two pieces of metal together?

Basically you install the hardware and begin to tighten the nut against the components. Naturally the nut should spin freely on the bolt until the nut begins to put pressure on the component. Further tightening of the nut will draw the bolt head and the two components together against the nut and actually begin to compress the metal components. As the tension is applied to the bolt it will begin to stretch. The bolts ability to rebound like a spring is what will provide the "clamping force" and will hold the components together.

The metal bolt has a certain amount of elasticity to it and will actually spring back to its original size when the tension is released. If you over-torque the bolt then you will reach a point where the bolt will NOT return to the original length. Once you pass this "Elastic Limit" then you have reached the "Yield Zone". If you have passed this point before, it will usually start in your stomach and works its way up to your mouth as you begin to realize that what you were tightening up just got a lot easier to turn.

Once the bolt has reached the "Yield Zone" the bolt will stay stretched. If you were to stretch it even more then the bolt will break and separate. Nuts tend to loosen when operated under heat, vibration and heavy loads. At 50 - 60% of it "Elastic Limit", the nuts and bolts tend to stay together.

The amount of tension that the nut, bolt and components are under is called the "Preload" The amount of tension that the component puts on the fasteners is called the "Load".

For the fastener to do its job we need to make sure it is under the correct "Preload". There are several ways we can measure this.

  • Measuring the amount the bolt has stretched
  • Turning the fastener a predetermined amount or "Torque Angle"
  • Use a torque wrench

Measuring the amount that the bolt has stretched is by far the most accurate, but in many cases is impractical. Turning the fastener a set amount of degrees after reaching contact or after a set torque setting is also method but again is impractical except for certain applications. Many times cylinder head bolts will be torque to a setting and then the bolt turned a set number of degrees. The most used method for setting the "Preload" is to use a torque wrench. There are several different kinds of torque wrenches available to day. Torque values are readily available in repair manuals and will help to install the fastener in the desired "Preload" range.

Note: Regardless of the method used to set the "Preload", there are many factors that will affect the settings. When a measurement is given in a manual they are factoring in all of the variables. If you change one of the variables for example the type of bolt material or the material of the components, you are changing the equation. For this reason when working with critical components it is best to use the applicable directions and settings as provided by the manufacture.

Physical characteristics of a fastener

Bolt Terms

Bolt Strength

The SAE established grades from 0 to 8 for steel bolts.

SAE Bolt Designations

ASTM Bolt Designations

AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM)

Standard Metric Bolt Dimensions and Grades

Thread specifications

Thread Terms
We currently use Metric threads and American threads. Both conform to the same profile, which is a series of equilateral triangles with the top crest removed and the roots rounded off

Metric and American threads both conform to the same profile, a series of equilateral triangles with the crests chopped off and the roots rounded where they engage the next thread.

The thread depth is 54.127% of the thread pitch, the distance between the threads. The radius of the rounded root is 14.434% of the pitch. Because of the parameters of the equilateral triangle another way to look at the thread designs would be that the 1/8th of the height of each triangle is cut off the top and ¼ of the height off the bottom. This would leave 5/8 of the height available.

Metric Fasteners Thread Designations
A metric fastener designation will begin with the capital M or MJ and then is followed by the diameter and thread pitch width in millimeters and then the length in millimeters. For example a M12 X 1.25 X 20 Would be a Metric 12mm diameter with 1.25mm thread pitch and the overall length is 20 millimeters.
Note: If the designations for the thread pitch are left out then it defaults to a coarse pitch thread that would be a 1.5 mm thread pitch.

To find the thread pitch in a metric bolt simply count the number of threads in a section and divided the section in millimeters, by the number of threads. For example 8 threads within 10 mm would be 10 divided by 8 would be 1.25 mm distance between threads.

American Fasteners Thread Designations
An American fastener designation will list the diameter in inches and then the number of threads per inch. Last will be the length in inches. For example a 1/4 X 20 X 1 Would be a ¼" diameter with 20 threads per inch and 1 inch long.

To find the thread counts in an American bolt simply count the number of threads in an inch of threaded material.
Stress on the Threads
Because both the nut and the bolt stretch slightly under a load the stress is not distributed across the threads evenly. The first thread handles 34% of the entire load. The first three threads are responsible for almost ¾ of the entire load. Below is a table showing the amount of load the thread carries.

Because of the way the load is distributed there is little need for a nut to have more than 6 threads. In general course threaded nuts will have 5 threads in them and fine threaded nuts will have 8 threads. Typically you want to have at least two threads exposed past a nut. The reason behind this is because the first two threads of a bolt can be tapered to make assembly easy or they are just poorly formed when the bolt is manufactured. This could lead to the nut stripping out because the other threads were over loaded.

Demonstrating bolt stretching

To demonstrate the bolt stretching under a load I took some new 1/4" X 20 X 1" bolts and measured them. They came out to 1.115". I torqued the fastener to 10 lbs and it measured 1.156".

I had read on the ARP site that the bolt needed to be torqued 5 times before using. So I loosened the nut and re-torqued it again and then measured it again. Below are the results.

You can see there is a big change between the 1st time and the 2nd time. On something critical it looks like it might be beneficial to torque it and then loosen it a couple of times.

Affects of a lubricant on fasteners
What affect does oil or anti seize lubricant have on the fastener? I have read many conflicting opinions on the use of oil or anti seize lubricant on fasteners. Some anti seize manufactures claim you need to use 20% less torque when using their products. Others I saw were 50% and then others were reported to have said to use the same torque. Because of the totally different information I decided to take a look at the affects myself.

I got some new 1/4" X 20 X 1" bolts, nuts and some washers. I used three washers and torqued a nut on the bolt at 10 lbs of torque. I kept increasing the torque one pound at a time until the Bolt failed and twisted off. The average was that the bolt would fail at was 12 lbs.

Next I used WD-40 to lubricate the bolt and torqued it down to 8 lbs. I increased the torque in one half pound increments until it failed. The average failure was at 10.5 lbs.

Next I used anti seize lubricant and did the same thing. The average failure was at 10 lbs.

Under these conditions a reduction of around 15-20% probably would have been in order. Course who knows what the range would have been with different materials and different lubricants. It does prove however that it will make a difference.

Note: I used the average torque setting that the bolt failed at as my reference point. Had we been using a torque setting that was within the design of the bolt then I doubt the use of the oil or Anti seize lubricant would have caused the fastener to fail at the same torque setting. But my goal was to see if fastener would fail at a lower torque setting when using the lubricants.

Bottom Line…..
If you are working on something the quotes a torque specification then you need to torque it to that setting. If they instruct the use of oil or another product on the threads, then use it. If they do not then just make sure the threads are in good physical condition and clean. Any doubt about the condition of the bolt or the nut will not thread on easily, replace it with a bolt of the same quality.

Notes:

  • Allen wrench heads are normally stronger than the same size hex head bolt
  • People tend to over-tighten small fasteners and under-tighten larger fasteners.
  • Nuts are generally stronger than bolts so the bolt will generally break before the nut.

Torque Wrenches Overview

There are basically three types of torque wrenches.

  • Direct-reading bending beam
  • Rigid frame toggle action
  • Incased beam dial-indicating

Direct-reading bending beam torque wrench
The direct-reading bending beam type is very rugged. This type will remain accurate as long as the beam is not physically broken and the pointer aligns to Zero. Operating the direct-reading bending beam is pretty easy. When torque is applied the beam will bend at a known rate but the pointer will stay straight and point to the value on the scale showing how much torque is being applied. There is a pivot mounted in the handle that concentrates the applied force to a specific point on the beam. This ensure the greatest accuracy.

Rigid frame toggle action
The toggle action torque wrench, sometimes known as the clicker torque wrench, dominates the market due to the to it be inexpensive and ease of use. What the torque amount as set by the operator is reached, the torque wrench cams over and a distinct click is heard and felt by the user. The wrench has a coiled spring that the user changes the compression on by turning the handle. This spring pressure will hold the internal components of the torque wrench together until the pressure of the compressed spring is overcome. Once this internal force is overcome the surface of the load lever, toggle and the toggle slide bar break apart or toggles. The click is because the toggle action and the load lever are now contacting the frame of the wrench.

Incased beam dial-indicating
The incased beam dial indicating is just what the name implies. The dial indicator is used to measure the amount of deflection of the beam. Like the click torque wrench it usually has a ratcheting head. This style torque wrench is expensive, delicate and subject to wear.

Notes:

  • Allen wrench heads are normally stronger than the same size hex head bolt
  • People tend to over-tighten small fasteners and under-tighten larger fasteners.
  • Nuts are generally stronger than bolts so the bolt will generally break before the nut.
  • All adjustable torque wrenches are basically accurate to within 10 percent on measurements above 30% of the torque wrenches adjustable range. It is best to avoid the extreme ends of the torque wrenches scale.

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