Everything you wanted to know about the screws
Do you miss a screw?
Screws, nuts, asparagus and threaded holes, our motorcycle is full of them. We are so accustomed to seeing them that we are familiar to us and we believe that we know everything about them: they squeeze and that's it, they perform their function by mechanically gathering two or more elements.
But under this simplicity a wide variety of shapes, threads, diameters and squeezes that should be known. Let's talk a little about it.
How do they work?
When I had to explain the operation of a screw to my mechanics students, I always used the same example: a screw is nothing more than a dock. A very rigid dock, but with some elasticity, that we tense when we squeeze it, and this tension is the one that creates the force that keeps together the pieces to be joined, and the screw in its place, thanks to the mechanical friction that occurs when performing The tightening.
In fact, while we tighten the screw stretches. We can easily check this by curling a certain torque of tightening a screw from which we have previously measured its length with a micrometer, squeezing it through some washers or pins, and measuring it again.
We will verify then that it has stretched a little, due to the tension we have applied: exactly like a dock.
Squeeze but, but without going
The most used way to measure the tightening or tension we exert on a screw is to do it by using a Dinamometric key.
This tool will allow us to screw the screw to a default tension, which will notify us by a sound "click",
A sound or vibration if it is electronic
or simply checking the needle in a dial.
This means that we will have the ability to control the tightening of that “dock”, so as not to leave it loose or tighten it too much.
Although in mechanical engineering there are a series of typical tightening values according to the diameter and material of each screw, it is best to follow the tightening indications of the manufacturer manual of the vehicle in which we are working. Sometimes these indications consist of a tightening torque + a certain angle of rotation of the screw, such as in the bolts of the Harley-Davidson engines, the pulleys or the primary chain compensator.
This tightening method was popularized during World War II, beginning to be used in combat aircraft engines. It seems to be more accurate when not depending so much on friction and simple tightening with dynamometric, but requires a angle meter To obtain the desired precision.
Shapes and sizes
Generally, when designing an engine, the use of the smallest screw/nut is taken into account in possible diameter and length, both thinking about the lightness of the components and (especially) in the minimum cost for the function to be performed.
As a general rule, the maximum force that a screw can make when a part of its diameter is threaded, that is, if a screw has a 5mm diameter, it would suffice with a 5mm deep nut to obtain the maximum force of the same .
This rule does not work exactly with aluminum, bronze, and other non -ferrous materials. For example, screws that hold your Harley's primary lid need to thread at least twice their diameter to correctly perform their function.
The screws are classified by their elastic force or degree, the higher stress the screw can withstand.
Metric or inches?
Here is another of the great trouble when we face the screws of our American, English, European, Japanese machines ...
Generally Anglo -Saxon marks use the imperial measurement system (in inches), and in Europe and much of the rest of the world the decimal metric system is used. Due to this duality we have screws and nuts of both systems, and on top of them there are the steps of fine and thick thread.
As a general rule, fine thread steps are used in applications subject to great stress regardless of the material used, and the thickest thread steps in more common applications. Thick thread is also usually used when the female part to thread is weaker than the male part, and vice versa with fine thread.
If all this seems little to you, you have to know that there are also the screws with "left" thread, which are squeezed and loosen in the opposite direction to the conventional. They are often used in rotating parts, helping the turn not to release the screw, although I personally believe that motor designers use them to go crazy to mechanics ...
Fixations
Many times, especially when the mechanical pieces are subject to vibrations (does something sound to you?) It is necessary
and also with a system widely used in competition and aeronautics, the Safety wire.
It is important to use them again in the same place where they were, if someone has put them there, they are necessary.
On other occasions the manufacturer will recommend the use of fijatornillos, which are nothing more than anaerobic sealants, that is, a liquid that in the absence of air solidifies holding the screw in its place. Important to know that there are different support capabilities, according to screw size and the type of work you have to develop.
Conclusion
After all this information, you will surely understand the importance of tightening the screws and nuts of your motorcycle using the tightening pairs and procedures recommended by the manufacturer's manual, and that is that next to a good dynamometric key and the “Loctite” boat The workshop manual is one of the tools that should not be missing in any garage, serious or amateur.
Frank Burguera