Bolt Strength and Preload

Hello,

For most applications you can safely apply a load that is equal to the "proof load" of the threaded fastener. The "proof load" is a little bit less than the "yield strength" of the bolt. For example, if you have a 17mm-36mm diameter SAE grade 8.8 bolt, the "yield strength" is 660MPa and the corresponding "proof load" is 600MPa (source SAE standard J1199).

The "yield strength" of a bolt is defined as the load at which the bolt plastically (permanently) deforms. Exceeding this load will cause a "permanent set" to the fastener meaning it will not return to the original length when the fastener is removed from the assembly.

The "ultimate strength" of the fastener is the point at which the bolt will fail. If a fastener is tightened to this load it will break. Take care to ensure loading and unloading of the assembly does not not cause the joint held together by the bolt to experience loads near the "ultimate strength" or the joint will fail.

Hope this helps!

They use aluminium bolts for jointing aluminium cables these are under ground power cables and have a clamp assembly that is fixed in place by these special bolts that are designed to shear off at the full torque setting. This means the bolt head is removed allowng the insulation to slide into place more easily. Just thought you might like to know.,

Again an answer relating not a wit to the question.

It may be well to note that a certain technique for tightening fasteners has been in use for a long time, and is very predictable concerning the amount of stress (thread deformation and fastener elongation) that is impressed upon a fastener during the tightening procedure.

Using a low initial rotational torque reading to indicate initial stress, the fastener is rotated an exact amount, say 30, 45, 60, or 90 degrees. This procedure allows a true prediction of the material deformation, as well as that of the fastener components. Not an answer, but just another idea for determining stress values without the benefit of a strain gauge.

One can also drill a hole through the length of the fastener so that one can truely measure the amount of TOTAL (not just threaded section) elongation, with the idea that most of the elongation will occur in the threaded section of the male member since it has the smaller (root) diameter.

Ing. Robert Forbus

Refer to Stress-Strain Curve, you have to select an "Allowable Tensile Strength" which represents a point on the "Elastic Portion of Curve". This means we use a "Safety Factor" which its value depends on the Code you follow. The mean value of allowable tensile strength equal to (approx.) 2/3 yield strength, also may equal to 1/4 ultimate tensile strength, whichever less. Another codes use another values, depend on factor of safety used.

So, it is not permitted to use neither "ultimate tensile strength" nor "yield strength" directly in your design calculations. And you have take into considerationthe allowable tensile strength of bolt material at "Design Condition", I mean design temperature, because it may be differs from its value at ambient condition.

To 1

The load magnitude you can apply depends on the kind of load: is it static? is it dynamic? which frequency? how long do you expect to use the assembly under specific conditions? and so on. So that a simple answer is not possible if the working conditions are not known. You mention a specification of preload and tightening torque. How do you obtain the actual (obtained at assembly) values especially for preload? Which dispersion do you alloow for torque? What kind of tools do you use?How do you specify the torque? Do you take into consideration the torsion at assembly? How do you compute the stress level at a given load?

Bolts are loosening even if the load does not free the fastened parts.

To 2 The two values are specified since both together give an indication not only for the limit but also for toughness and resilience. A bolt should not be as glass keep till it breaks it should show a plastic zone and this is indicated by the elestic limit.

If you have a safety assembly and dynamic loads do not consider it as a trivial problem as mostly done in machnie design. Better use either a serious book or the service of an expert. About 50%(in some fields more) of failures are due to wrong bolting (as well design as assembly).

Ok It is the tensile quality versus the size/load factor, the pitch of the thread the depth of the coupling hole/diameter of the bolt. The factors being that when you torque up a bolt the under side of the mating threwad and the upper portion of the bolts thread hold the two together by friction. As you increase the turning force these two surfaces lock together and compress against each other. The bolt and threads have to resist the torsional force so you dont strip the thread or shear the bolt. Size is proportional to the expected load to be applied and maintained. The greater these are the bigger the bolt needs to be and the higher its tensile strength. (resistance to stretching) and the courser the thread to spread the load. The more threads that inter lock the better (Loading is a factor of the total surface area shared between parts) the greater this is the less stress on any point of contact.

"The greater these are the bigger the bolt needs to be and the higher its tensile strength. (resistance to stretching) and the courser the thread to spread the load. The more threads that inter lock the better (Loading is a factor of the total surface area shared between parts) the greater this is the less stress on any point of contact."

It is not realy as you say those are generalities but not valid in every situation, many times increasing the bolt will lead to a failure. With respect to the number of threads inter locking, it is also not true since the load is not equaly ditributed on the threads so that going over a number brings nothing but cost. Look at a good book as Bickford or Ilgner there some on the market. In fact the distribution depends on a material factor the Young modulus of bolts and parts(nut). The same for the frontal contact increasing only the diameter of a washer will not avoid a plastic deformation of the soft part under the bolt head for instance. Further more if you consider for instance tubular parts for load transmission the distreibution on the threads is totally different and if one does not take it for real failures are around the corner and wait to manifest. I have seen such situations since I am working as consultant in special threaded assemblies. It is in fact much more complex than people think. It is not complicated but only complex.

Hi,

TO keep things simple, lets understand the requirement.

1. If the bolt is used only to hold and not to loosen, yes preload torque is good enough.

2. If you expect the application to have shear ( Cut-off), tensile ( pull ) load, that is when your yield and ultimate strength creeps in.....

From

Mr.Stupid's

Any comments welcome......