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Now let’s hear from Corey Rasmussen, owner and managing director of the Mentored Engineer.
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.
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.
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.
As a followup to MathCAD (Part 1), Corey continues to provide an overview of MathCAD for use in engineering calculations.
In this video, Corey gives an overview of MathCAD for use in engineering calculations.
When designing a piece of equipment, stability can be a significant issue. For stationary equipment, often securing the apparatus to the floor will ensure stability. However on a mobile piece of equipment, like a forklift or a crane, stability must be carefully considered along with its structural design.
There are four basic steps to determining stability: calculating the overturning moment, determining the stability envelope, calculating the stability moment and applying a margin. These steps can be applied to any object that has a risk for tipping over.
When considering stability, let’s define a few terms. First, what is the “stability envelope?” When looking at the unit from a top view, the stability envelope would be the outline of all the contact points with the ground. Be cautious, not all points are equal! For outriggers with a fixed foot, the furthest point away can be used. If there is a swivel foot, use the centerline of the pivot pin or spherical bearing. If tires are used, go to the center of the tire. In the case of a dually tire, go to the center of the dually. If a vehicle has a steering axle, do not use the tires. The suspension is very flimsy and a distance of only 12- 20 inches can be used without modifications.
Second, what is the “overturning moment?” The overturning moment is the sum of the moments containing everything on the unit that is directly connected to the load. In the example of a forklift, the overturning moment would include the weight of the mast as well as the lifted load.
Stability moment is the moment that resists the overturning moment. The difference between the stability moment and the overturning moment and is expressed as a percentage when compared to the stability moment
Everything that is not part of the overturning moment is part of the stability. Again with the forklift example, the stability moment would consist of the weight of the vehicle, the operator and fuel.
The final term is stability margin. This is usually measured in the extra capacity that can be lifted before instability occurs. These terms are related by the following equation.
The game for stability is simple; the stability moment must equal or be greater than the overturning moment for the product to be stable. It can be very easy if you know the loads and have a very rigid structure. Unfortunately, most situations are not this easy and more analysis is needed. There are several important things to consider when determining stability.
Loads – In most cases, the load is either a person or material and the force vector is pointed down. There are many times that the load is pulling to the side like if a man was up in an scissor lift and pulled sideways. A third cause of overturning moments is a rotating imbalance.
Unless guided by a standard, these side loads can be difficult to determine. Acceleration and deceleration are also loads that need to be considered.
Slope – Slope is a detriment in two ways. First, it can minimize the stability envelope which decreases the stability. Second, in most cases slope will increase the overturning moment by extending the distance the load is from the normal position
To calculate the overturning moment if it is on level ground. This will be usually be the cross product of weight of the components and the horizontal distance to where the load mounts. However, with side load and rotating imbalance, the force in question is usually sideways and needs to be evaluated by height.
The easiest way to determine the stability envelope is visually. We will demonstrate that with an example of a forklift. The stability envelope is drawn between the centers of the three tires. This is a good assumption because forklifts are very rigid usually without suspension and rarely have inflatable tires. The location of the stability moment has already been calculated and plotted on the chart.
Rigidity – Rigidity (or lack thereof) acts in the same way that slope does. If the structure is flimsy, deflection can create an artificial or internal slope that increases the overturning moment. Be cautious if your structure flexes too much. This may cause problems with excessive roll or increasing moment arms
How to determine the location of supports:
This is usually the least complicated step. We will just calculate the weight and center of gravity for all the involved components that are not part of the overturning load.
Once the Stability Moment and Overturning moment are known, calculating the stability margin is simple. To do that, use the formula below.
At this point, you would want to compare this to any internal or external standards your machine needs to adhere to.
Forklifts are a great example of stability. They are in use everywhere and there is a large impetus on the operator to make sure that loads are within acceptable ranges and operations on slopes are minimized.
In our example, we have a typical three wheeled forklift. We will assume the forklift (including mast), rider and fuel weighs 3000 lb and the center of gravity is 31.27″. Our mast is pivoted 13″ in front of the front tires. The load weighs 2000 lb and the center of gravity is 20 inches from the mast. We will first perform this analysis on flat ground.
Stability on Flat Ground
We can first to determine the overturning moment if it is on level ground. This will be equal to 2000 lb * (13in+20in) = 66,000 in-lb. Next, the stability moment is 3000 lb * 31.27 in = 93,810 in-lb. You can see that the unit is stable by a significant margin by 29.6%
Stability on 5° Slope
Now let’s see what happens when the operator drives down a 5° with the load 10 ft in the air. This makes the analysis more difficult. The overturning moment changes because the load is shifted out front. Mo = 2000 lb * (13 in +20 in +120 in sin (5°)) = 86,917 in-lb. This example still shows that the forklift is stable, but by a much narrower margin of 7.3%. Adding 160 lb to the load would make it unstable.
A More Visual Approach
This method of calculation works well when there is a clear tipping line (like the front tires). It is a little more complicated when the tipping line isn’t easy to spot or calculate. We see this happen when the forklift starts tilting side to side. We don’t necessarily know from inspection if it will tip over the side or front. For that we will use a second method that is more visible.
The method below uses a graphic method to see the stability margin. There are three plots shown. The gray plot simply shows the stability envelope. The blue plot shows where the CG of the load will be as the forklift is setup on 5° slopes in all directions. The orange plot shows the combined CG of the blue plot with the unit CG. You will notice that the area relocates and gets smaller. From here, finding stability is easy. Just check to make sure the orange plot falls entirely inside the gray stability envelope.
Add weight to the unit: Adding weight will help, but slowly. In cases where a load is suspended far from the nearest support, adding 6 -10 lb of weight might only allow for 1 lb additional load.
Add outriggers: This increases the stability envelope. This obviously will not work when the equipment carries the weight while in motion.
Check for proper tire inflation – A low tire can significantly reduce the stability envelope or allow for more roll. Inflate the tires and retest.
Design a rigid structure and suspension – Intend for the entire unit to flip at all at once when instability occurs. This minimizes roll and the potential for more overturning moment to be generated.
Bolster the suspension – Making improvements to the suspension can drastically improve machine performance. The more rigid things can be the better.
Thinking about stability in a graphical way can greatly simplify the calculations and remove some of the mystery. It will also help determine if there are regions of instability that are not obvious. Using the analysis tools mentioned, and the tips to increase stability should give you the tools to make sound decisions and make your design more stable.
One of the great benefits of working with steel is the ability to weld it without losing material strength. Since we are dealing with high temperatures when welding, the thermal effects can cause great problems with distortion. Most of the time this is an undesirable side effect and in many cases can be eliminated or at least minimized.
There are 13 basic methods of minimizing weld distortion in weldments. They are:
As an engineer, one of my roles was to inspect weldments that were out of tolerance. The second largest cause of problems were weldments that deformed when the weld was applied. (Parts welded in backwards or in the wrong place was number 1). Many of these parts needed to be scrapped which lead to stoppage in production and increased rework. Both are undesirable and avoidable.
The most important part of understanding and predicting distortion is to evaluate what happens in the welding process. When the liquid weld material at approximately 6500°F is added to the cooler parent material, the liquid material will form a shell around it still containing the liquid material inside. As the weld material cools, the shell becomes thicker and thicker until all the material becomes solid. As the cooling process continues, the weld material will contract and create new stresses in the parent material. (As a side note, when fully cooled, the weld material will be at its yield strength in tension.)
For example, if we have two plates arranged in a “T” joint and we weld a fillet on each side, we would expect the cooling weld material to deform the cross part of the “T” into more of a rounded top. Even if the cross part is held in position until the weldment is fully cooled, it will curve slightly when released. This is commonly known as elastic distortion (see suggestion 6)
It sounds simple enough, but simply making sure that your welds are sized for the desired load can remove a significant amount of distortion. Also, when joining plates, there is a variety of angles and spacing to choose from. Calculate the area required for each and choose the one with the least area.
This relates to #1 in that it is not only the size of the weld, but also the parent material that is being welded that causes distortion. Reducing the voltage or current can minimize distortion. Also, increasing the travel speed can minimize distortion.
Heating up the entire weld area can be an easy method to eliminate distortion since the contraction of the weld will be the same as the parent material. This prevents any residual tension or compression stresses from forming.
Like preheating, stress relieving can be used to eliminate distortion. If the weldment is put back in the fixture and heated, it will relieve any residual stresses from the welding process. It is important that during the stress relieving process, the weldment is constrained to its desired condition. Simply heat treating it without constraints may make it deform more. Also, be careful not to stress relieve at a temperature high enough to change the temper in the steel.
A number of small passes will deform more than several large passes. This is mainly a consideration with thick materials that require complete joint penetration.
Holding the part in place during the entire welding and cool down phase will force the weldment to maintain its locked shape. However, when released, there will be a slight amount of elastic distortion. Elastic distortion is where a relatively small force can produce a large deflection. This principle is illustrated by the picture of the ruler where the force applied by the finger is very small compared to the deflection it produces. If a weldment is welded in a fixture and allowed to cool, it may spring to a slightly deflected state when released. In most cases, the part could be put back into the fixture and conform to the desired shape.
If you know that the distortion is going to happen, let it. When fixturing the weldment components, purposely put them together so at the end of the welding process, the distortion makes it straight. In the T example above, slightly bending the cross plate in the center could produce a mostly straight cross plate.
Adding large metal blocks near the weld on both sides will reduce the amount of distortion because it will act as a heat sink by pulling heat from the weld and redistribute it in the parent material allowing for even cool down. Copper is probably the best material for this. It has a higher specific heat than aluminum and will not become welded to the steel. Try several configurations to find the one that works best.
Welding on or symmetrical about the neutral axis can eliminate distortion. Make sure that all the weldment components are tacked together thoroughly before beginning. If the welding cannot be symmetrical, perhaps the weld size could be increased or decreased to make it symmetrical. The figure to the left shows that balancing the area of the weld from the neutral axis can be used to keep the weldment from bending. Look for opportunities where welds can be removed if they do not allow for symmetrical welding. Keep in mind that this may also be accomplished by adding only a few components at a time and welding in stages. This process will take longer to produce, but will lead to far more usable weldments.
Perhaps welding at the neutral axis is just not possible. Try tacking two (or more) parts together so that the welding is done symmetrically around the neutral axis. There are three main drawbacks to this process. First, the weldment must be cool to the touch before they can be separated. Second, the tacks holding the components together must be broken and ground smooth. Finally, this process doesn’t fit into most lean manufacturing processes unless there are multiple weldments that are produced at the same time.
Using a rosebud nozzle on a torch, heat can be applied to the weldment in certain specific places to straighten a weldment. I’ll caution you that while this does work, there is no real method to it. In my experience, trial and error is the best method to tackle this. If what was heated gives you worse results, try heating the opposite side. Document your findings so that the process can be repeatable. An interesting alternate use of heat can be to make a straight beam curved without applying a load.
Local sections of a weldment may suffer from local buckling or large amounts of distortion. For these two types of distortion, the best solution is the increase the torsional resistance or critical buckling strength (See Omer Blodgett’s Design of Weldments Section 3.6 and 2.12) In the case of a tall I-beam with a thin web, welding may cause the web to locally buckle in a wavy configuration or twist. welding many vertical stiffeners can reinforce the web and prevent localized buckling
If work is started on one end and moves consistently toward the other end, the components will end up pulling away from each other. Tacking the part from end to end will prevent them from pulling apart. Also, sequencing the weld location can minimize the heat input.
In conclusion, weld distortion is predictable and there are ways to minimize it. The more you study and apply the principles mentioned here, the more that weld deformation and distortion can be designed out before a prototype is even built. Applying these principles will save money in scrapped product and fixtures and hours of unproductive time spent evaluating out of tolerance parts.
