Tensile strength Vs Shear-pin Material type? 1

[Mattberghofttt]

I have been asked to manufacture a shear pin out of some form of steel or iron. It will be used to break off, when a touch probe over travels its safe area on a CNC machine tool. I need a material that is brittle, so it will break when needed. And i need a material that will be strong enough to function as part of the touch probe option.
Does anyone know what kind of material that i could machine that would react the way that i need.

 

[kingnero]

The safety of my old-school lathe when thread-cutting, is a small brass pin. Strong enough to engage the coupling, brittle enough to break if the tool post would hit the tailstock.

You might incorporate a shear line (small cut) when using thicker pins...

 

[hydtools]

At what load or at what force do you want it to break?
How fast will the probe be traveling?
Single shear or double shear?
What over-travel will be permitted as the pin is sheared?

What reaction do you need? Lots of questions.

Ted

 

[Twoballcane]

Have you done any shear hand calcs yet? If so what did you find out?

Tobalcane
"If you avoid failure, you also avoid success."

 

[loafer09]

If the connection is designed correctly (no bending in the shear pin) you do not need to use a brittle material. Common shear pins/materials used in machinery are grade 2, 5, and 8 bolts, 316 stainless, brass/bronze, alloy steel dowel pins, S.S. dowel pins, etc.

 

[Mattberghofttt]

The part that I need to manufacture will be made to the dimensions of about 1/4" diameter with an M4 thread(which will thread into the machine housing)on the opposite end there will be a groove with a .04 full radius machined to approx diameter of .080 machined on that opposite end will be a stem about 3/32" diameter, this will fit inside of the touch probe sensor housing.If the machine touch probe comes in contact at the wrong position the stem where the radius is located will break, so damage to the probe sensor is minimal. Do you think that I need a material that is heat treated so it is breakable? I do not want the material to bend, I need it to break at the specified point!

 

[Twoballcane]

You will have to know the acceleration and weight of the probe sheeringthe pin; this will give you the sheer load. Once this is known, you can calc the shear stress from the pin. Then you will have to find a material that will have same or more than half of the ultimate strength compared to the shear stress you calculated.

Tobalcane
"If you avoid failure, you also avoid success."

 

[hydtools]

Non-heattreated material is breakable. You just need to exceed the shear strength at the groove area where you want the pin to break.

Ted

 

[Mattberghofttt]

There are some materials that do not break on a single impact. The material bends sometimes past 90 degrees without beakage, and sometimes you have to bend it 2 or 3 times to break it examples 12L14 steel 6061 aluminum. What i need is a material that has little to no elasticity, do you know of what kind of material would I be looking for?

 

[loafer09]

The pin won’t bend if you design the mating parts in such a way as to constrain bending. 6061 can be used as a shear pin, as long as you correctly design the connection. I’ve used bronze for shear pins, which is also very ductile, and works perfectly fine as a shear pin material.

 

[hydtools]

Glass
Ceramic
AISI 1095, harden by water quench, do not temper.

Ted

 

[EngineerTex]

All ductility occurs between the yield strength and the ultimate strength.

Look for material with a yield strength very close to its ultimate strength. Most high-quality/high-strength bolts meet this. Also, you can get similar performance out of certain mild carbon steels which have been extensively cold worked -- just verify that the yield strength is very close to its ultimate strength.

Also, you can get drill blanks made of tool steel from any catalog. Obtain the W-type (water quench) or 1095 stock as hydtools mentions. This is very easy to quench and temper after you machine it to the needed shape, just don't temper it. Since you're not looking for any extraordinary or exacting properties, just heat it with a torch - propane, MAPP or acetylene and keep it white hot for a minute or so, then dip it into a coffee can of water and leave it there until it's below 900F. Dip it in with the axis vertical so that you can reduce (not eliminate) the tendency for cracking. Now you have an extremely brittle pin. But be careful -- do not overtighten when you put it into your fixture. You may have a problem trying to retrieve it if you break it off inside your tapped hole.

Engineering is not the science behind building. It is the science behind not building.

 

[unclesyd]

Bases on many years experience I would not use 1095 in the
untempered state. We used 1095 drill rod to make small shear pins for our metering pump drives. Unless you have absolute control of the HT for the 1095 there is normally quite a bit variability in the quench. By tempering you take the majority of the variability seen as quenched version out of play. I don't know about you size but in
the smaller sizes a good number will crack longitudinally (die crack) if left untempered.
For Aluminum Pins we used various alloys, non that would age harden at RT, wire or rod. Our shear area was an undercut in the middle of the pin. The depth would be determine by shearing a few with an undercut of about 90% of nominal and from these values we would calculate the the require depth for this particular lot of material.
The key for getting a precise shear in any material is having a holding fixture harder than the pin, parallel faces with the gap as narrow as practical to prevent any bending with the resultant tensile stress.

 

[tbuelna]

Mattberghofttt,

Any material can be made to work. Designing and manufacturing a fuse pin that reliably shears at a consistent load point under possibly varying conditions is actually quite difficult. Surface finish, assembly tolerances, metallurgy, heat treatment, temperature, corrosion, number and type of load cycles the pin will see during its life before shearing, etc., all will have an impact on the reliability of your fuse pin design.

If you want a pin that fractures and breaks away, then you probably want a material that has a low elongation rate. If you want a fuse pin that fails more gradually and stretches before shearing, then you want a material with higher elongation.

Regardless, the key to getting consistent performance from your fuse pin is careful control of the pin's manufacture and assembly.

Good luck.
Terry