Posted .

Bolts and Nuts frequently asked questionsIn our day-to-day operations we come across numerous questions from our customers. In this post we’ve compiled the most frequently asked questions and tried to answer them here.

How do you select a fastener size for a particular application?

The most important thing to ask yourself is what size and strength of the fastener you need for the job. So you have to determine the size, number, and strength required for your particular application.

Secondly, you need to determine the bolt material required for the prevailing environmental conditions. This could mean using a standard steel fastener with surface protection or using a material that is more naturally corrosion resistant such as stainless steel. The general underlying principle is to minimize the cost of the fastener whilst meeting the specification/life requirements of the application. Each situation must be considered on its merit. Obviously some detailed work is necessary to arrive at a detailed recommendation.

Is it okay to use a mild steel nut with a high tensile bolt?

Nut thickness standards have been drawn up on the basis that the bolt will always sustain tensile fracture before the nut will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which are partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. When specifying nuts and bolts it must always be ensured that the appropriate grade of nut is matched to the bolt grade.

The standard strength grade (or Property Class as it is known in the standards) for many industries is 8.8. On the head of the bolt, 8.8 should be marked together with a mark to indicate the manufacturer. The Property Class of the nut matched to a 8.8 bolt is a grade 8. The nut should be marked with a 8, a manufacturer’s identification symbol shall be at the manufacturer’s discretion.

Higher tensile bolts such as property class 10.9 and 12.9 have matching nuts 10 and 12 respectively. In general, nuts of a higher property class can replace nuts of lower property class (because as explained above, the ‘weakest link’ is required to be the tensile fracture of the bolt).

What are the benefits of fine threaded fasteners over coarse threaded fasteners?

The potential benefits of fine threads are:

  1. Size for size, a fine thread is stronger than a coarse thread . This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter).
  2. Fine threads have also less tendency to loosen since the thread incline is smaller and hence so is the off torque.
  3. Because of the smaller pitch they allow finer adjustments in applications that need such a feature.
  4. Fine threads can be more easily tapped into hard materials and thin walled tubes.
  5. Fine threads require less torque to develop equivalent bolt preloads.
  6. On the negative side:
  7. Fine threads are more susceptible to galling than coarse threads.
  8. They need longer thread engagements and are more prone to damage and thread fouling.
  9. They are also less suitable for high speed assembly since they are more likely to seize when being tightened.

Normally a coarse thread is specified unless there is an over-riding reason to specify a fine thread, certainly for metric fasteners, fine threads are more difficult to obtain.

What methods are available for calculating the appropriate tightening torque for a bolt?

A high bolt pre-load ensures that the joint is resistant to vibration loosening and to fatigue. In most applications, the higher the preload – the better (assuming that the surface pressure under the nut face is not exceeded that is).

The pre-load is related to the applied torque by friction that is present under the nut face and in the threads. The torque value depends primarily on the values of the under-head and thread friction values and so a single figure cannot be quoted for a given thread size.

The stress that is often quoted is often taken as the direct stress in the bolt as a result of the preload. It is normally calculated as preload divided by the stress area of the thread. Typical values vary between 50% to 80% of the yield strength of the bolt material, in many applications a figure of 75% of yield is used.

Does it matter whether you tighten the bolt head or the nut?

Normally it will not matter whether the bolt head or the nut is torqued. This assumes that the bolt head and nut face are of the same diameter. If they are not then it does matter.

Say the nut was flanged and the bolt head was not. If the tightening torque was determined assuming that the nut was to be tightened then if the bolt head was subsequently tightened instead then the bolt could be overloaded. Typically 50% of the torque is used to overcome friction under the tightening surface. Hence a smaller friction radius will result in more torque going into the thread of the bolt and hence being over tightened.

If the reverse was true – the torque determined assuming that the bolt head was to be tightened then if the nut was subsequently tightened – the bolt would be under tightened.

There is also an effect due to nut dilation that can, on occasion, be important. Nut dilation is the effect of the external threads being pushed out due to the wedge action of the threads. This reduces the thread stripping area and is more prone to happen when the nut is tightened since the tightening action facilitates the effect. Hence if thread stripping is a potential problem, and for normal standard nuts and bolts it is not, then tightening the bolt can be beneficial.

 

I can’t find the shear strength of a fastener in the specification, can you help?

Bolted shear joints can be designed as friction grip or direct shear. With friction grip joints you must ensure that the friction force developed by the bolts is sufficient to prevent slip between the plates comprising the joint. Friction grip joints are preferred if the load is dynamic since it prevents fretting.

With direct shear joints the shank of the bolts sustain the shear force directly giving rise to a shear stress in the bolt. The shear strength of a steel fastener is about 0.6 times the tensile strength. This ratio is largely independent of the tensile strength. The shear plane should go through the unthreaded shank of a bolt if not then the root area of the thread must be used in the calculation.

 

What is the best way to check the torque value on a bolt?

There are three basic methods for the checking of torques applied to bolts after their installation; namely, taking the reading on a torque gauge when:

  1. The socket begins to move away from the tightened position in the tightening direction. This method is frequently referred to as the “crack-on” method.
  2. The socket begins to move away from the tightened position in the un- tightening direction. This method is frequently referred to as the “crack-off” method.
  3. The fastener is re-tightened up to a marked position. With the “marked fastener” method the socket approaches a marked position in the tightening direction. Clear marks are first scribed on the socket and onto the joint surface which will remain stationary when the nut is rotated. (Avoid scribing on washers since these can turn with the nut.) The nut is backed off by about 30 degrees, followed by re-tightening so that the scribed lines coincide.

For methods 1. and 2. the break loose torque is normally slightly higher than the installation torque since static friction is usually greater than dynamic friction. In my opinion, the most accurate method is method 3 – however what this will not address is the permanent deformation caused by gasket creep. An alternative is to measure the bolt elongation (if the fastener is not tapped into the gearbox). This can be achieved by machining the head of the bolt and the end of the bolt so that it can be accurately measured using a micrometer. Checking the change in length will determine if you are losing preload.

The torque in all three methods should be applied in a slow and deliberate manner in order that dynamic effects on the gauge reading are minimized. It must always be ensured that the non- rotating member, usually the bolt, is held secure when checking torques. The torque reading should be checked as soon after the tightening operation as possible and before any subsequent process such as painting, heating etc. The torque readings are dependent upon the coefficients of friction present under the nut face and in the threads. If the fasteners are left to long, or subjected to different environmental conditions before checking, friction and consequently the torque values, can vary. Variation can also be caused by embedding (plastic deformation) of the threads and nut face/joint surface which does occur. This embedding results in bolt tension reduction and affects the tightening torque. The torque values can vary by as much as 20% if the bolts are left standing for two days.

 

If you still have some unanswered questions, please add them to the comments section and we’ll answer them the best way we know how.

Comments are closed.