Optimum Angle For Self Locking

What do you mean by "optimum angle"?

Do you understand the maximal angle at which the thread is still self-locking?

If you understand what is self-locking and you know what the terms of the equation mean then you can decide yourself about it. To give you a hint: a thread is self locking when the force component which act parallel to the thread direction is smaller than the friction force generated by the component normal to the sliding area of thread.

having the equation and this explanation you can find it. But explain what is for you "optimum".

thank you for answering me. ı mean maximum thread angle for self-locking yes u are right. i thought finding derivative of the equation and equalizing it to the zero i can find the angle but i couldnt find an equation related to angle after derivating it.

1- are you sure the equation is correct?

2- if you try 1st to understand the physics of self-locking you do not need a too complex mathematical approach since it is not required! Most of the explanations are in the short § I send to you.

3- I have the feeling that Stirling Stan is right: it is a home work and you came to a stall.

So that try to follow the scheme and may be try 1st the point 2 since if you do it in this order you will see if the equation is ok or not.

Self locking is physics considering that the friction a Coulomb type: two bodies pressed one on the other do not have a relative sliding movement as long as the transverse force is less the normal force x friction coefficient.

Reads like homework to me. Solving an equation as the objective rather than preventing the loosening or loss of parts! That being said then....

Special thread profiles can be used to improve self locking instead of finer pitch threads.

Critical applications do not rely on self locking threads.

The aircraft industry uses drilled holes in the bolts and castellated nuts wired to prevent loss.

Split, star, and similar lock-washers, are used for smaller sizes.

Thread locking compounds similar to "instant glue" are in widespread use today.

Reads like homework to me. Solving an equation as the objective rather than preventing the loosening or loss of parts!

I think psycho student is busted!

google "positive-locking fastener" to learn about effective locking methods.

Stirling Stan is right. locking is done by adhesives or by deformation of either the bolt or the nut. The safety wire through holes or castles is a safety measure if the true lock fails. Thread pitch or angles by themselves do not produce the locking. A good example of a deformation lock would be the U-bolts that hold the axles to leaf springs on busses or large trucks. The treads on the bolts have a zig-zag profile where the peaks and valleys come to a sharp edge. The nuts, however, are rounded on the edges. when the nuts are torqued on they deform the bolt thread edges creating the lock. That's why they can be re-torqued tighter but the u-bolts should never be reused.

I have the feeling that we should give an answer correlated to the question and not try show how much we know on a subject.

I could of course explain, if asked for, which are the theories concerning the loosening of a bolt or bolt-nut connection, why the different solutions are more or less acceptable (as for instance that the grower or split spring ring is the most inefficient "safety") and so on, but the question was ONLY limited to a "self-locking" of a thread.

Do you know for instance that the use of "glues" could lead to over-tightening? Do you know why?

Of course Stirling Stan was right in his list of possible solutions but this was correleted to the "ways and means of how keep an assy tight" not to the basic question of self-locking.

Now we can start the discussion :which is the most efficient way to keep an assembly preloaded under strong environmental actions?

Is there only one solution? Is there an optimal solution for every type of assembly?

How does temperature affects the different solutions? For instance at higher temperatures the "bonded" assemblies loose their preload due to the creep of the bonder. And the other ? Does corrosion affect the locking?

The solution you mention for the "U" bolts is not every where used and is not every where required and never the less the leaf springs are not lost.

You make a very good point nick name. Although most of us here normally have the best intentions in mind when answering questions, I find that rather frequently many of us begin to pass judgement on the posters common sense, rather than simply directly answer the question.

Not long ago, I asked if anyone knew where I could get wind load charts as pertaining to the sign industry. I needed charts that went beyond the square footage that is covered in the readily available charts. The replies I received were more lectures about why I shouldn't rely on charts for these things, but get someone "qualified" to do the calcs. Great advice, but not the answer to my query. This happens rather often here. And it's a bit demeaning. It's happened several times to me.

I'd like to suggest that we all try to keep this in mind. I, like most people here, don't really need lectures to insinuate how dumb we are. We all can't possibly know the reasons the question is being asked. Advice is fine if it's included in addition to the answer. But not just judgements. Merely the fact that someone finds CR4 might suggest that the person asking the question has some degree of intelligence and common sense. At least enough not to be patronized. I'll try harder not to be guilty of this myself.

Am I wrong with this line of thought?

Happy Holidays!

Now ... back to self locking threads ....

That's not to say that I don't suspect Physco is trying to get his homework done here

Hi Mr Experience,

Thank you for your moral support, it shows that I am not the only one to have this feeling. People ask us because they expect us to know more than they do. It is very often the case and I read extremely interesting inputs. But I think that we have to be more matter of fact.

If one of us wants to start a philosophical discussion he has the opportunity to do it a part from the question which was asked.

Best Holidays for you too,

Hi,

again on the same subject. I am asking myself if we read the questions or not. The last comment shows the same problem: is a mean to secure the pre-load an answer to the question of an optimal self-locking angle ?

On my opinion clearly not.

The Norlock system is based on a principle which has been (partly) used by B&S several years before the Norlock. I think that without the ramps - which make the system so attractive- the indentations on the contact areas of the washers have the most important effect. Without them the ramp will not have same effect.

yes i am student and that was my homework and i couldnt do it. i was looking for answers to my question and some of u help me to understand what was dealing with. In addition to this i think homeworks are given us to learn something about the topic i think i learn something from this homework thank you for helping me ...

Consider the Nord-lock fastener.

http://www.nordlock.de/web/1_23_37.htm

Just a word of caution here:

Vibration of the ramp interface will usually drastically reduce the coefficient of friction. Thus, the mechanism may begin to slip even though it was thought to be well within the "locked" zone. Once slipping begins the dynamic coefficient of friction becomes relevant ...and this is usually lower than static friction. Therefore, a vibration induced slippage may continue even after the vibration has subsided.

You are right and wrong at the same time.

Friction is reduced not by all vibrations, only by those generating very small sliding in the contact plane of the assembly components. Usually the pre-load is important and the associated masses small enough to avoid a separation of the parts or an important loss of pre-load if the vibrations occur in the axis of the bolt. When vibrations are transverse the mentioned sliding lead to a maximal friction in the sliding direction and to nil friction in the perpendicular direction so that part of the thread is not any more secure in tangential direction : the "apparent" friction coefficient is smaller even if the true coefficient stays the same. When the bolt is preloaded due to the ramp, when the tightening is finished and the tool removed, the pre-load generates a torque which tries to free the bolt/nut, it is directed in the direction contrary to the tightening one. This torque turns slightly the bolt/nut if it is bigger than the friction remnant torque and reduces the pre-load. When (if) vibrations stop the assembly is less preloaded as before. Even if the friction reaches the same value as before (static friction will occur if the relative movement is nil for a time) since the parts do not move any more with respect to each other the load is less and the risks of separation are bigger. If again vibrations appear the process goes on till the pre-load is small enough for the assembly to part. But since pre-load is small the full load in axial direction is on the bolt which will break even if it does not fall out of the assembly.

Bolted assemblies are applied physics, all is simple!

When considering the pre-torque required, the difference between the temperature during maintenance and the operating temperature should be considered.

On large steam turbines the casing flange bolts are internally heated to a predetermined temperature and then torqued to the specification. This avoids loss of tension on the bolt, or allows for it, and helps to reduce gaulling that might otherwise occur at the high compression interface between the threads.

Is this very interesting information related in any way with the optimal self-locking angle? Please explain where and how since I do not see it.

Thanks

The method you mention for the bolt tightening is to day replaced by the hydraulic tensioners which are more precise and do the job faster. The method was developed in order to avoid the dispersion of the pre-load due to friction scattering is but very slow. the difference in temperature between maintenance and operation could affect the joint if the expansion coefficients of the parts (flanges) and bolt are different. In general both are of steel so that it has less influences.

It is related to the pre-torque requirement. The co-efficient of linear expansion means that the length of the bolt changes with the increase in temperature under operating conditions. Bolt grows longer – pre-torque reduced – less compression between threads – more room for vibration – increased probability of reduced locking tension.

Is the bolt isolated from the flange? Is the flange temperature not variable in operation?

Your explanation is valid ONLY if the bolt will expand MORE than the flanges. The reality is that the bolt is heated by the heat coming from the flanges which are in direct contact with the hot source. So that if you consider the temperature gradient due to the transfer from the inside to the outside the bolt is even (slightly) at a lower temperature than the flanges, so that in fact with respect to the maintenance the pre-load increases in operation !

But any way I do not see the connection to the optimal self-locking ANGLE.

Linea expansion is dependant on length. It is measured as increase in length per unit length per degree increase in temperature.

A bolt or stud must be longer than the flange it is to secure and while the length between the nuts (Securing faces) is the same as the flange thickness (near enough) that part of the thread held within the nut provides additional units of length for growth.

Generally steels (Bolts/Studs) expand at a greater rate than cast irons (flanges). This differential expansion should be checked for the specific materials involved.

I sincerely appreciate you knowledge but I still do not understand the relationship with the OPTIMAL self-locking angle. By the way many flanges in high pressure applications are made of steel and welded. It is true that for steam turbines the "boxes" are cast nodular iron. But this an aspect the assembly professionals consider when they compute the heating temperature. The necessary time for heating and the fact that one depends on dilatation coefficients not always very well known lead to the use of tensioners.

Please explain how this assembly technology is related to the thread angle, it could be of great interest for me.

I also want to know more than I know today so that new ideas are always welcome.

thank u for all commenting on the topic. i have learned the answer of my question and ı want to share it to u.I think u know the answer already but ı couldnt ask it to u directly what ı want to learn.Anyway the optimum thread angle is 45 degrees and it is becouse the frictional coefficient must be smaller than the tangent of the angle and for additional 45 degrees is the optimum but screws are manufactured with a thread angle of 8 or 10 degrees becouse if so we will lower the force that is applied to the screw.

Sorry to say that you are TOTALLY wrong! A 45° angle will never, under the real friction conditions, be self locking. May be you do not have the same definition for self locking as usual.

Try again and if you make the right sketch and analyze it the right way you will come, I hope for you, to the correct result.

Good luck

45 degrees is not the right answer for any self locking thread in materials that I know of. It is also going to be interesting cutting this thread on a lathe. What material or materials are you proposing to use in this fastener?

So for machine screw threads we typically use 7 degrees as the maximum pitch angle that will not back drive without a friction modification caused by vibration. Obviously less angle will have lower sensitivity. The previous comments regarding finer pitches being for higher forces or lower turning effort are only partially correct. Shear area and minor diameter generally increases with the finer threads, increasing the loads the fasteners can sustain. They do produce higher clamping forces too which does make it easier to apply the torque required to preload the bolt correctly to maximize its fastening performance. If you have thread with pitch angles higher that 7 degrees they will not stay tight no matter how much you try to torque them. Impulse loading will cause these threads to back off. An example of this is the behavior of double lead threads. Double lead threads will typically have pitches that produce more than 7 degrees of lead angle and they do not work for static positioning. You have to provide alternative locking means in order to prevent movement similar to what you have to do when using ball screws and nuts. Besides vibration, shock loads will also produce movement even on very shallow pitch angles. That is why fasteners used on punch presses and other equipment subject to shock loading end up have fasteners that are secured by pinning or by safety wires if they are critical joints. Conveyors and crushers have similar issues with bolted connections. Even the fasteners that are modified to