Throughout my career I have had to apply thousands of pivoting pins. All have their unique loads and design criteria. They have ranged from pins that don’t rotate to pins that rotate on three surfaces and have loading in multiple directions.
After much research and weighing many options, I have settled on one type of design. My go to choice for a pivot pin is Turned Ground and Polished (TGP) Pre-Chromed Stressproof (ASTM A311 Class B). It has 100 ksi yield strength, good ductility, weldable and can be hardened to about 130 ksi. This will have a a welded lug for rotation lock on one end and a snap ring groove on the other.
The material and its hardness, surface finish and roughness need to be considered when designing a pivot pin. It is also critical how the pin is retained; using double retention with a rotational lock. Finally, you must consider how to assemble and disassemble the joint. A “bullet” guide can help here.
Let’s look into these components a bit deeper.
When choosing a material, three main things need to be considered: strength, hardness and surface finish. Each of these play a critical role in how the pin will perform in the application.
For pins, you want the material to be strong. A strong pin will reduce the required diameter leading to lower weight and easier installation.
The pin must also be hard or at least have the opportunity to be hardened. A hard pin will resist gouging, galling and indentation from roller or ball bearings. This means that carbon percentage (last 2 digits of the alloy number) are going to be higher (over 40 ie 1045 or 1144). If this is the case, you are going to have watch out for the material’s weldability. This will require special welding wire and possibility a pre-heat and controlled cool down. If you pin isn’t hard enough, look at induction hardening. This is a process where material is heated by electromagnetic induction and then quickly quenched. It increases the martensitic composition at the surface leading to increased hardness. If designing for a non-pressed spherical bearing, be sure to induction harden. In this case, the bearing will rotate between the pin and the ID of the spherical bearing. Hardening the pin will prevent galling.
Surface finish is a way of measuring how rough a surface is. For most applications with pins, you want a material to be smooth with surface finishes below 16Ra for most applications. Using Turned, Ground and Polished (TGP) or Drawn, Ground and Polished (DGP) materials give a tightly toleranced OD with a very smooth finish.
Pins need coatings to prevent them from rusting. A pin that has rusted into place is no fun to take out. Keep in mind that a well plated pin can rust to an unfinished bore. Be sure to minimize this by using an anti-seize compound upon assembly and repair. In my career, I have taken out many pins with an air hammer or a cutting torch. Not fun. I will discuss some of the most common finishes.
Also known as white zinc, this is probably one of the most common finishes out there. It is technically known as trivalent chrome zinc plating. The zinc plating does very little for corrosion protection and chrome is added to provide nearly about 30 hours of salt spray resistance. This plating is weak and I don’t recommend it for outdoor use.
Yellow Zinc plating
Probably one of the most common finishes out there. It is technically known as hexavalent chrome zinc plating. The zinc plating does very little for corrosion protection and chrome is added to provide nearly 100 hours of salt spray resistance. Unfortunately, hexavalent chrome has landed on RoHS’s (reduction of hazardous substances) list and is quickly becoming a thing of the past. You may remember that hexavalent chrome was what the movie Erin Brockiovich was based on. The finish is also soft and can scratch off easily.
This is a great plating and I recommend it.
Zinc phosphate plating
This is a good method of plating steel. The main drawbacks are that it cracks easily and cannot be hammered which can cause installation issues. Also, the salt spray rating isn’t great and lasts about as long as powder coating does. I recommend to stay away from this for outdoor use.
This is my personal favorite. Chrome offers great corrosion protection and a very smooth and hard surface. It can be bought as pre-chromed bar and easy use in several materials.
Also known as nitriding, this finish is applied by allowing chemicals to soak in to the surface of the steel. The process is not like electroplating in that the chemicals can get into every crack and crevice of the pin. Since it is a soaking process, it does not build thickness to the part, but impregnates it below the surface. Also, it leaves a nice black finish which is a thin oxidation film. As the pin wears on a bearing, this layer will flake off leaving a chrome like finish. It also hardens the surface a little. The main drawback to using nitrocarborization is the stress relieving that occurs during the soaking process. The process generally reduces the yield strength by about 15%. If this wasn’t the case, every pin I spec would have this finish. When deciding to use this process, be sure to have the material tested for yield strength, hardness and elongation. Use a coated and uncoated specimen to compare.
I recommend this for applications where good surface finish and wear is critical, but strength is not. Many times, you can make your pins larger in order to used this surface finish.
The next thing to focus on is pin retention and the first question to ask if whether or not the pin will rotate in normal operations. I don’t know why, but if it can rotate, it will and always on the wrong or unintended surface. If it does not rotate, like in an extension cylinder application, using retention methods that prevent translation (sliding) may suffice.
Examples of this are snap rings, cookies, or cross drilled holes. If the pin does rotate, effort will need to be made to force the pin to rotate on the bearings.
Examples of these are pins with welded flags, tuning fork retention plates and buckeyes (or banjos). My favorite is the weld on flag plate. If the bolt holding the tuning fork or buckeye falls out, it may be lost leading to unintended pin movement and down time to replace.
Contrary, a flagged pin would be retained. The buckeye can also be subject break in cases with large, highly load articulations. Because of this, I have converted most legacy buckeye designs to a welded flag design.
In all cases strive to have double retention in each pin. Double retention requires two things to fail before the pin can be removed.
The final thing to consider is assembly and disassembly. Be sure to start with the pin design. If using a flagged pin, I recommend turning down a step in the bar stock part of the pin for the flag plate to slip over. This accomplishes an easy way for the welder to locate the flag plate.
Second, this makes a stronger joint requiring the weld to be pulled through the plate in order to fail. Looking at the leading edge is also important. We can all realize that a sharp edge on the edge of a pin doesn’t allow for easy assembly. Most pin designs utilize a small 45° chamfer at the end.
These are easy to manufacturer and relatively easy to install. It also allows for a shorter overall length. Where possible, I recommend a 30° chamfer with a fillet on each edge. Modern CNC lathes make this complex shape easy to manufacture.
When used with fiber backed bearings, this leading edge will prevent tear out in the event of misalignment at install.
If the overall length doesn’t allow for the longer chamfer, try using a “bullet”. Named for its shape, a bullet is a temporary tool used to insert the pin and then removed. The bullet will have the 30° chamfer (or less) with fillets and simply bolts into the holes used to mount the cookie retention plate.
The final thing to consider in assembly is to consider how the parts will be pinned in reality. Is there enough room to insert the pin? Do I need to cut a hole to allow for a driver bar? Is there enough room to swing a hammer without contacting expensive or easily breakable items? Is there access to cranes or other lifting equipment so that this can be assembled safely?
Complete the thought process for disassembly. Can I remove the pin easily? Can I attach a slide hammer to ease disassembly?
My go to choice for a pivot pin is Turned Ground and Polished (TGP) Pre-Chromed Stressproof (ASTM A311 Class B). It has 100 ksi yield strength, good ductility, weldable and can be hardened to about 130 ksi. This will have a a welded lug for rotation lock on one end and a snap ring groove on the other.
As you can see, there is a lot that goes into pin design and we didn’t even talk about loadings! The goal of this article is to allow you to confidently design a pin that will give many years of reliable use…the first time. Good luck in your design.
2 thoughts on “A Simple Guide to Designing Structural Pivot Pins that Last”
I disagree about the statement that “Stressproof” material has good ductility and weldability.
Stressproof has chemistry like 1144 steel which has a high Sulphur content which gives it poor weldability and the elongation at the higher strength level is below 10% even down to 7% on larger diameters.
I personally always consider anything with an elongation below 10% brittle.
When I years ago was talked into using Stressproof pins because of machinability, which ended up being welded to shipping crates for engines as alignment pins, I learned my lessons the hard way.
The welds failed during transporting the crates in the back of semi-trailers full of engines……..
Another time we machined some axles for racing sulkies out of Stressproof, which were formed, and failed on the track because of ductility.
Well that is my dealings with Stressproof for what it is worth.
Thanks for the response. There is a lot of good insight!
You are right about the high sulphur content elongation of stress-proof. Ductility is one of the things that make steel a great material to design with and 10% elongation is considered the line between brittle and ductile.
If you do decide to use stress proof, you will need to get mill specs and tell your vendor that it needs to be a least 10% elongation. Most users don’t ask for this so the lower ductility bars will go to other customers.
In your case, using such a low ductility pin for a shipping crate was probably asking for problems. Transportation can be harder on the structures than when they are used. I can see why a 7% elongation pin might fail. Increasing the ductility can ease this issue. When you do that the strength will go down and the pin diameter will get bigger costing you about the same amount. (I speculate that stressproof was chosen because it needed to be a certain diameter.)
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