Locking problems and solutions of stainless steel screws
What does lockout mean? Lockout, also known as bite, refers to the bolt and the nut in the fastening process occurs bite hold phenomenon, generally occurs between the stainless steel bolt and stainless steel nut, so also known as stainless steel screw lockout or stainless steel screw bite.
What happens when a lock-up occurs
1.Electric (or pneumatic) tool installation
1.Quick installation with power tools (or pneumatic tools, same below).
Even in a free state (i.e. not under pressure), the rapid installation of 304 screws with 304 nuts (or 316 screws with 316 nuts) using power tools is bound to produce lock-up. As for the probability of lock-up, and power tool speed, the faster the speed, the higher the probability of lock-up, the slower the speed the lower the probability, in some extreme cases such as power tools repeatedly and quickly screwed in and out, and power tools to install lock nuts (including nylon locking, metal locking, flattening lock nuts, etc.), can be up to 100% lock-up. (This is an example of locking of a power tool fitted with a stainless steel locking nylon nut on a stainless steel screw)
However, there is no lock-up problem if the installation is done manually, which also shows the correlation between the installation of power tools and the occurrence of lock-up.
2. High strength torsion, unbalance, tilt and uncentric mounting
A typical example of high-strength torsion and unbalanced mounting is the mounting of flanges.
The flange, as a key support piece for the equipment, has a large installation force. Whether there is an operation manual or not, I have seen too many workers always screwing to death until they can't; at the same time, the flange is installed along the flange edge, and there is an imbalance in the process of screw tightening, the above two points lead to a correspondingly greater chance of locking
The flange is a representative case of lock-up, there are similar hidden dangers in the installation environment such as
Tilted installation.
The screw mouth and the nut are not correctly aligned (i.e.: different centres.) (i.e.: different centres).
What is the reason
The core reasons are twofold.
1.stickiness
2.heat
The first point is that "stickiness" is a unique property of 304 (and 316) stainless steel, how to understand the concept of "stickiness", we can imagine and good dough compared to a stone, it is obvious that the dough is sticky.
Physical parameters: ductility, (ductility is a mechanical property of a substance that indicates the ability of a material to deform plastically before it is subjected to a force that produces a rupture) The following is a screenshot of a table of the chemical composition of 304 stainless steel.
As a comparison, the most common material used for the production of carbon steel fasteners has been selected:10B21. Here is a screenshot of the composition table for 10B21.
I have marked it in red. I have collated this into the following comparison table (ductility of 304 stainless steel vs. 10B21).
Material | 304 stainless steel (in %) | 10B21 carbon steel (in %) | Difference |
Elongation after break % | 62 | 27 | 230% |
Shrinkage after break % | 78 | 60 | 30% |
The above table shows that 304 stainless steel is far more ductile than 10B21 carbon steel. We don't need to learn about it, but according to our common sense we can appreciate: among the metals we are familiar with, gold, silver and copper (especially pure copper) have high ductility, while from tungsten steel, high-speed steel ductility is definitely low.
Are you close to concluding that the harder the metal, the less ductile it is, and the softer the metal, the more ductile it is? There is a correlation, but not quite, which is of course a topic for another day and will not be discussed here. In any case, 304 is more ductile.
In fact, if you are a teacher of processing stainless steel materials, you must have this experience: precisely because of its viscous and ductile characteristics, 304/316 materials are machined with extruded crumpled or spring-like chips, while other materials such as brass or carbon steel show crumbly or filamentous chips during the turning process. Here we are producing a stainless steel nut (the oil jet has been removed for the photo):
Its chip removal is not easily dislodged, and processing even requires dedicated personnel to clean up the chip removal.
After talking about ductility, let's take a look at the thermal conductivity of stainless steel. Here is a comparison table of the thermal conductivity of stainless steel, carbon steel and copper:
Material | Stainless steel | Carbon steel | Copper |
Thermal conductivity | 16.2 | 45 | 383 |
The table above shows that stainless steel is not conducive to thermal conductivity, copper is good, and carbon steel is second.
If you understand the adhesion and thermal conductivity of stainless steel, then you have basically guessed the reason why lock-ups occur, which is:
The stickiness of stainless steel itself determines that the bolts and nuts bite together easily, and the heat generated by friction promotes this phenomenon. It instantly the whole process goes like this:
When a stainless steel fastener is locked in place
The pressure and heat generated between the teeth destroys and erases the chromium oxide layer between them
Direct blocking and shearing at one of the contact points of the fluting
Adhesion then occurs
Adhesion occurs continuously (usually within one full turn of the tooth diameter), forming a face
Stainless steel fasteners are completely locked in place
It can no longer be removed or locked.
The solution
The solution to the bite of stainless steel screws is not only a unilateral need of the fastening manufacturer, it undoubtedly requires the reference and improvement of the user.
As a supplier.
1. The common solution at present is thermal conductivity and lubrication treatment. The surface of the thread is coated with heat-conducting oil and lubricant and dried to speed up heat dissipation and reduce the heat generated by friction, and one can be applied when the screw is used with the nut.
2 Some companies apply a layer of wax to the surface, the principle is the same.
As a user.
1. it is necessary to reconsider the feasibility of power tools to find a balance between ensuring efficient installation and reducing the chances of lock-up. turning down the speed of the power tool is perhaps the most intuitive way to do this.
This is generally applied to bolts and nuts for high strength installations, such as the flange connections mentioned above, but of course this solution will have oil on the thread surface.
3. Of course, it is also necessary to avoid rough installation, as mentioned above, "screwing to death" is never acceptable, with a torque spanner to prevent overuse.
Copper, aluminium, titanium screws locking
In fact, copper screws are also easy to lock up, just compared to stainless steel screws is not so prominent, the better the quality of copper screws the easier it is to lock up, often because good quality copper nuts than other high copper content, and some poor quality copper screws with high lead content and less copper content. The high ductility of copper causes it to lock phenomenon, but copper has a huge advantage, its thermal conductivity is very good, can be timely heat dissipation, so a large degree of submission and avoid the locking
Aluminium and titanium screws also have this problem
But carbon steel bolts do not lock up. The reason that carbon steel bolts break straight away but do not lock up is that carbon steel is much less ductile than stainless steel.
So is ductility an advantage or a disadvantage? As a mechanical property, it is undoubtedly an advantage. In short, as long as its threshold is not exceeded, stainless steel can withstand longer stretching rates, which is undoubtedly better in vibrating environments, and longer fatigue life. It is only as a lock-up that this property takes the blame.