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rotate
06-07-2009, 10:35 PM
I have shear pin which is about 3mm in diameter and 25mm in length. It's used to connect a hydraulic shaft which causes rotation motion to cause the convertible top to swing open.

I'm wondering what material to machine it out of. I'm sure the original is steel but I'm not sure whether it just mild steel or hardened. I'm curious, are shear pins typically made of mild steel or hardened steel?

I would test the original material with a file, but I not longer have this piece. It went flying when I was using a punch to push it out :(

doctor demo
06-07-2009, 10:46 PM
My advice would be mild steel , or aluminum.
Hardened steel will most likely cause some thing else to fail.

Steve

MickeyD
06-07-2009, 11:18 PM
I have had good luck with brass. Take a 1/8th brazing rod and turn it down if you don't have any brass drops laying around.

Boucher
06-07-2009, 11:31 PM
There are some pressure regulator pop offs that use a common nail for the shear pin. That might work if the size range corresponds.

Circlip
06-08-2009, 04:43 AM
Check the original, even if it means buying from the main agent, SOME shear pins are "Spirol", "Roll" or "Selluck"?? pins.

Regards Ian.

rotate
06-08-2009, 08:42 AM
Check the original, even if it means buying from the main agent, SOME shear pins are "Spirol", "Roll" or "Selluck"?? pins.


You got me worried. Can you explain the significance of these possibilities?

JCHannum
06-08-2009, 09:25 AM
I suspect the pins described are British equivalents of roll pins. They are are usually hardened and not intended to shear.

Shear pins are mechanical fuses, and generally are of mild steel. It is a good idea to undercut the pin in the area where the shear will occur as this will prevent the ends from "smearing" and making the pin difficult to remove. The undercut can also serve to determine the shear strength.

It is better to start with a too weak shear pin and increase the size or hardness until it holds at an acceptable level than starting with a too strong pin and risk damaging some other component in the system when it fails to do its intended job.

lazlo
06-08-2009, 09:51 AM
I have a packet of the original 40-year old shear pins that shipped with my Clausing 5914, and they're aluminum.

Circlip
06-08-2009, 11:12 AM
Yep, the shear pin on my MaxiMat screw cutting shaft is also aluminininium, but the shear/drive pin on one of the metal cutting chop saws I had to repair was a "Spirol", "Roll" or "Selluck" pin.

I tried with the alternative names I know of, cos I realise the little bu**ers undergo a name or sex change when they're transported over the Atlantic.

All the above (SAME parts, different names) are made from spring steel sheet, rolled into a tube and hardened and tempered. So just to set your mind at rest, before screwing the unit :-

Check the original, even if it means buying one from the main agent.


Regards Ian.

JCHannum
06-08-2009, 11:41 AM
Ian, don't confuse a shear pin with a drive pin. Roll pins are designed specifically not to shear. There is a difference in the application.

Circlip
06-08-2009, 01:50 PM
Preciate what you say Jim, but a roll pin can satisfy both situations, had the "Whatever" pin been a solid hardened one in the chop saw it wouldn't have sheared. Have seen them used in many Drive applications and due to the construction allow a certain amount of Shock tolerance that's why I advocated to buy a dealer original. Chances are, as the original has sheared, the ends could have "Smeared", we don't know cos Rotate lost it. How much is it gonna cost?? If it's $200, once he's SEEN it, :D

Regards Ian

rotate
06-08-2009, 02:13 PM
I'm certain that it's a shear pin and it's designed specifically to shear when there's something blocking the tonneau cover of the convertible. I know this to be the case because that's what the dealership calls it and the pin sheared because I accidentally had a box on top of the tonneau cover which caused it to shear.

When I was hammering out the part I did noticed that one end of the pin was slightly tapered.

I would get the part from the dealer but they are back ordered (2 week delivery) and I need my top down asap. :)

Roughly speaking what is the shear strength of mild steel v.s. brass? Just a very rough figure? I find Google pretty useless for some basic information like this. Thanks everyone.

Circlip
06-08-2009, 02:35 PM
Remember what Colour (Color) the bit you knocked out was ???:D

rotate
06-08-2009, 03:27 PM
Remember what Colour (Color) the bit you knocked out was ???:D

Black. How does this matter?

juergenwt
06-08-2009, 05:02 PM
I second JC., Shear pins are from brass or aluminum. Hardened Roll pins are used as drive pins, never as shear pins. If you have a tapered pin it serves as a locating or as a drive pin. Never as a shear pin

Rich Carlstedt
06-08-2009, 05:52 PM
I will third what JC said.
Calculating shear loads is very involved.
Check your Machinery's Handbook under "Strenght of Material"
to get an idea of what you want. They have a section on taper pins and shear.


Basically what you need to know is that all :
Steels shear at 75% of tensile (T)
Aluminum is about 75%
Brass is about 100%

So the cross-sectional area at both ends of the pin- times 2 (ends) - times tensile strength - times 75% = the shear load.
so a steel pin (soft) 1/4 inch in diameter has :
(.05) x 2 x 60,000 (T) x 75%= 4,415 pounds to shear.
Now if the shaft is 1 inch it will take "X" amount of torque to shear
But if the shaft is 2 inches at shear point, you double the required force to shear. So you see, the points of shear,
relative to diameter have a dramatic effect as well. Diameter, material, and distance from center of rotation are all needed to figure it out.

I am not trying to be contentious here, but I have never heard of a roll pin used for shearing, only as coupling devices.
The problem with roll pins is that you cannot calculate the shear point of the pin, because of FOUR variables.
First, the orientation of the 'layers" will affect shear
(when a roll pin is in the hole, it does not have a equal numjber of layers all the way around )
Second, "shear" for a flexible coupling is very difficult for calculation
( tables for roll pin use will express load "capacity" which is the MINIMUM capability of the pin.)
No engineering manuals exist that express how to calculate it
Third, not all roll pins are of the same spring material , which means 'T" is a unknown.
Fourth. It is very poor practice to use shear pins that allow, or even promote scoring/galling of the mating surfaces.
A hardened pin will tear up the coupling surfaces and make it impossible to remove the broken pin.

Do they use harden pins ? you ask ?
Yes, when the designer fails to allow a big enough pin diameter, or a installer
thinks it isn't strong enough. Both are mistakes in my opinion, having had the experience of fixing those issues in the past.
If you find one in an application, it is a mistake.

Shear pin users and all standard references all point to straight or taper pin use, using standard material for ease of calculation.
The simple method pointed out earlier by someone is to use aluminum, and if it shears, go to brass, and then to mild steel.
When that fails, you need to ream to a larger size, not screw up the couplings

Rich

Circlip
06-09-2009, 03:37 AM
Reason I asked what colour was is because Brass is yellow, Alumin. is whiteish and Steel is grey. Rich has solved the problem for you, if the 3mm pin won't work bore it out to say 6mm or even 10mm and you'll never have problems with it again. Original designer must have not allowed for someone putting excess load on the system.

Sadly in the REAL world, not every design uses the best material for the job.

Regards Ian.

Carm
06-09-2009, 04:59 AM
Rich, that was an informative precis. Thanks!

rotate
06-11-2009, 01:47 PM
Machined a 4mm shear pin out of brass and put it in. After just 2 operations, the pin sheared.

I machined it out of mild steel, and this time I over sized by 0.02mm and put a slit down the middle so that it will be held in there firmly. Pressed it in and now it's working :)

I'm glad that I took the conservative approach first. Thanks everyone. I can now enjoy the sun.

Lew Hartswick
06-11-2009, 10:08 PM
I second JC., Shear pins are from brass or aluminum. Hardened Roll pins are used as drive pins, never as shear pins. If you have a tapered pin it serves as a locating or as a drive pin. Never as a shear pin

Excpetion taken to the last sentance. The SHEAR pin on the Clausing/
Metosa lead screw is TAPERED. It is brass though. I've had to make a
few replacements for the school shop. :-)
...lew...

tony ennis
06-11-2009, 10:53 PM
I'm glad that I took the conservative approach first. Thanks everyone. I can now enjoy the sun.

Hmmm keep a few spares in the car. :)