A Simple Guide to Applying Cylindrical (GarMax, Polygon) Bearings

Written by Corey Rasmussen in Mechanical Design Last Updated August 19, 2020

As a design engineer, when I have pivoting members, one of my favorite bearings to use are fiber reinforced bearings. They are easy to apply and are very predictable.

Typical Cylindrical Bearing
Image courtesy of GGB

Fiber reinforced cylindrical bearings are round bearings with no moving parts. The inside diameter is coated with a low friction surface that is usually either nylon or Teflon. Carbon fiber is then wound on the outside to give ample strength.

To properly apply a cylinderical bearing an engineer must address all of these:

Selecting the size and wall thickness
Calculating the bearing pressure
Determine frictional forces
Calculate surface velocity
Estimate life cycle
Identify and minimize misalignment
Assembly Considerations
Common Issues

Let’s dive deeper:

Fiber reinforced cylindrical bearings are a subset of cylindrical bearings, but as mentioned, they have no moving parts. Typical cylindrical bearings have an inner and outer race, balls or rollers and seals. Having no moving parts makes the design more robust.

Are they useful in engineering design and if so, how useful?

As I look back on the last 15-years of my design career, cylindrical bearings stand out as being one of the most reliable and commonly used items in nearly all of my projects. They can be trusted to perform.

Technically, they are classified either as filament wound, self-lubricating bearings or simply by their brand names such as GarMax or Polygon. These bearings are unique because they have no moving parts.

The outside diameter is pressed into a bore and the pin is able to rotate on the inside diameter of the bearing. The bearings are formed by first putting a wear material like PFTE (Teflon) on the surface and then a winding material like carbon fiber or fiberglass on top. These bearings are a good choice for their:

High load capacity
Inexpensive pricing
High temperature capability
Minimal to no maintenance
Long life
Small form factor

The main reason to steer away from these is high surface velocity.

Like any other bearing, there are several important things to keep in mind in the design:

1. Selecting the Size and Wall Thickness

Cylindrical bearings are specified in 16th of an inch size and come in 1/8 and ¼” wall sizes (metric versions are also available). For example, if we were specifying a GarMax bearing of GM1620-16 the form is the:

Product code -GM
Inside Diameter, ID – 16 (1 inch)
Outside Diameter, OD – 20 (1.25 inch and thus a 1/8 wall)
Width – 16 (1 inch)

This is great because if you want to change to another type or supplier, you just change the product code using the formula above. It’s simple enough that you could extrapolate that to get other sizes.

Cylindrical Bearing Thickness

There are no hard guidelines here. I have found that if the bore size is fixed, lean toward a 1/8” wall bearing. This allows you to use a larger pin with more strength.

I learned this from a custom cylinder application were the loads were high and caused damage to the pin. Because of part interchangeability, we were not inclined to change the rod eye size of the cylinder, but we found that if we changed the bearing from a ¼” wall to a 1/8” wall we could use a ¼” larger pin.

This brought our design factors back up to allowable limits.

If the bore size is fixed, lean toward a 1/8” wall bearing. If the pin diameter is fixed I recommend a 1/4” wall bearing.

If the pin diameter is fixed and the bearings are pressed in a tube, I would recommend a 1/4” bearing.

Tubing is readily available in 1/8” steps of wall thickness and in DOM tubing (the good stuff) up to 5/8” thick. This allows me to machine my bore about 1/4” bigger than the ID on only a little bit deeper than the bearing depth on each side.

The thicker bearing prevents the need for cleaning up the tube’s stock ID between the bearings. As a result, I can machine this bore up to 1/8” off center (eccentric) and still not have any interference with the pin. This can be a big time saver in your machine shop.

2. Calculating the Bearing Pressure

Bearing Pressure (P) — Calculating the pressure for the bearing is important. First, find the area by taking the inside diameter and multiply it by the width. This number is a representative stress because the edges see no load and the center sees most of it. Let’s make our area one square inch.

We calculate the pressure by taking a force (1000 pounds), and divide it by the area (1 square inch) so our answer is 1000 PSI.

The bearing manufacturers have guidelines for the allowable pressures for many different applications. Generally they range 5000 psi 10,000 psi and some can be higher.

How pressure is distributed in a bearing

3. Determine the Frictional Forces

Calculating the friction for the joint is also important. We will need to determine the frictional properties of the design and determine how much torque it is going to take; under load, to rotate the pin. And that’s critical if you’re going to size a cylinder or hydraulic motor that forces the motion. The general formula for calculating torque is equal to

T is torque in in-lbs., F is the force in lbs., and D is the diameter of the pin in inches. The diameter is divided by 2 for this reason: You are going to be rubbing on the bottom edge leaving a slight gap at the top. As a result, the pin will press at the center of the bearing at half the diameter.

The chart below is typical of cylindrical bearings. You can see from the chart below that as you increase pressure on the bearing, your coefficient of friction actually goes down.

This is a good thing because it means that as I load the bearing up more and more it actually gets easier to turn. But be cautions. If the bearing is oversized, pressures will be low (below 5000 psi), I can actually have a bearing and that is very hard to turn or worse it makes a lot of noise because low pressure leads to unwanted vibration creating unwanted noise.

I was once party to a design that had a bearing that was so loud it sounded like an irate whale. And you could hear it from half-a-mile away. We ran the calculations on it and found out that the pressures were around 1800 psi (very low). We narrowed up the bearing to increase the pressure and the noise was gone! So when you’re sizing bearings make sure that you look at the highest load and lowest load case.

4. Calculate Surface Velocity

This is the speed at which the pin surface sees movement. This is usually measured in feet per minute (fpm). So to convert from revolutions per minute (rpm), use the equation below. The 12 is a conversion from inches to feet so be sure to check your units.

Where V is the velocity in fpm, n is the rotation speed in rpm, and D is the diameter in inches. These types of bearings generally have a cut off speeds between 10 fpm and 25 fpm.

PV – The next consideration is a PV factor. It is simply the product of pressure and velocity. It is essentially an empirical method of estimating the bearing life. It balances the force and pressure to a limit. For example if our limit is 36, we could have a pressure of 4 and a velocity of 9 or a velocity of 4 and a pressure of 9. So we now have three limits on our bearing.

5. Estimating Life Cycle

Once we have the PV value, we then can calculate the estimated life cycle. What is life cycle? The life cycle is a function of multiple factors. Most of them can be looked up in the manufacturer’s charts.

Cyclic life factor (Q) — This is provided by the manufacturer based on the bearing type
High load factor (a_E) _—This is a derating factor if the pressure is on the high end of the spectrum
Temperature factor (a_T) — Heat dissipation is important. If the ambient temperature is elevated, a derating factor is needed.
Pin surface factor (a_M) –The manufacturer will give you a table to look up this factor. Generally a surface finish between 6 and 16 micro-inches is preferred.
Pin surface finish factor (a_S) — Even a little roughness on the pin surface can act like sand paper and de-rate the bearing life.
Bearing size factor (a_B) –This factor looks at the angular pressure area. This factor decreases as the bearing diameter increases because the used area of the bearing gets smaller and smaller as diameter increases.

As mentioned, there are charts and tables where all this information can be found. To calculate the lifecycle (in cycles) you combine all those elements as shown below.

6. Identify and Minimize Misalignment

It’s a bearing’s worst nightmare when things don’t line up! This can be remedied by proper machining, mechanical adjustments, or welding with the pin already in place.

Misalignment is measured in percent. A 0.2% misalignment is defined as 0.002” per inch of bearing. This level of misalignment is standard and no adjustments need to be made to the bearing pressure. A 0.6% misalignment is the highest I would ever consider allowing.

No matter what your P value is, your edge load stays constant. For example, if I have a bearing with diameter 2”, width 2” and loaded to 10 ksi, I will have an edge loading of 34 ksi. Quite a multiplier!

If you have a bearing that supports a cantilevered load, you are almost automatically at a 1% misalignment. That is a lot! You are essentially expecting 0.010” of deflection in the joint.

The edge loadings can easily double or triple the calculated P value. In the example above, a 1% misalignment would yield an edge loading of 45ksi. Ouch!

I highly recommend changing a cantilevered design to include two bearings that are spaced apart so that the bearing takes two axial loads rather than a moment load. You will also reduce the friction that will resist motion in the joint.

7. Assembly Considerations

Bore sizes – Over the years I’ve had a hard time reconciling the fact that the bearing can come in with an outside diameter tolerance of 0.002 to 0.004, but I have to hold my bore to 1/3 or ¼ of that. It’s not fair! Honestly, I’m not sure how it works mathematically. But it does! I’ve used thousands of these over the years in dozens of different applications and when machined to the recommended bore sizes you will not have any problems with assembly or undue wear. Opening up tolerances is usually a good cost savings so beware of attempts to change this.
Pin sizes – The same is true with the tolerances on pins. We get a little help on this one, though. If you have read my article on pins, you will see that I recommend the use of turned, ground and polished pre-chrome rod. The manufacturers know that the TG&P rod comprises a large portion of the pin industry and as a result, the TG&P tolerance matches the manufacturer’s recommendation (Wow!)
Chamfers – When installing a bearing, I recommend adding chamfers to both the pin and the bore. I recommend a 15°-30° x 1/16 (on the diameter) chamfer to ease alignment.
Bearing knockers – If you are assembling this bearing in high quantities, I recommend designing a bearing knocker. The knocker is simply a steel component that matched the ID of the bearing. It has at least one step to provide good contact on the edge of the bearing when pressure is applied. The matched ID prevents the bearing from having any localized buckling. A second step may be added if the bearing is to be recessed past the surface of the bore. You may need this to get past your leading chamfer on the bore.

8. Common Issues with Fiber Reinforced Cylinderical Bearings

As will all things, there are several things that are bound to happen when applying fiber reinforced bearings. Here are the most common.

No lead in chamfer – Add a lead in chamfer of at least 1/16” deep. I recommend a 15° angle so that the bearing has a smooth guide. You must also insert the bearing at least as far as the chamfer to make sure the bearing is properly supported.
Not cleaning out the bore – Frequently, paint overspray or other debris can mess with bore. It doesn’t take much to make a negative impact to the design. Use a flapper wheel tool to remove the paint, but don’t go crazy. You machined that hole for a reason!
Improper bearing installation – As mentioned, a slight press fit is recommended between the bore and the outside diameter of the bearing so it is going to take some force to install. The most common method is using a hammer. This one makes me cringe a bit. There are so many things that can go wrong here.
1. Even pressure distribution – There is no real way to make sure that a hammer blow is going to contact the edge evenly. This can partially be avoided by having the hammer strike a flat piece of wood larger than the bearing.
2. Buckling – This is a thin walled tube we are dealing with and it will buckle. Unlike steel, these bearings are anisotropic so when the compression loads are too high, the fibers will separate. At this point, the bearing must be replaced. As with all buckling, the best solution is to add more support over the length. I recommend having a bearing knocker (blue part below) that will add support all along the inside of the bearing. This nearly eliminates all buckling issues
There is an unknown amount of force that the hammer can produce. For large bearings, this can be properly sized, but for small bearings, you are likely to damage the bearing. I have one recommendation for this; use a bearing press. If you can’t get the piece to a bearing press, you can make one with a hollow cylinder (enerpac.com) and some all thread. Simply insert the all thread through the bore, bearing, knocker, cylinder and a few (thick) plates with holes in them. Add some nuts on the all thread until snug. Then use the cylinder to push the bearing into the bore.

Final Thoughts

Calculating cylindrical bearings can be a little tricky. To speed up the learning process, I have an example problem worked out for you.

I’ve also developed an online calculator for a quick check to see if your calculations are correct. It calculates pressure, velocity, torque and estimated life (including the factors). You will need to be able to verify and interpret the results for the brand of bearing you’re using but it will save you time by calculating most of the information.

In conclusion, cylindrical bearings are wonderful. They allow high loadings in tight spaces. They are largely chemical resistant, do not require lubrication and do not break the bank. When designing, seek these bearing products out! Remember, almost everything I design uses at least one of these bearings.

Enjoy designing!

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6 Easy Ways to Improve your Mechanical Design Skills

Written by Corey Rasmussen in Self Improvement Last Updated January 27, 2020

In this day and age, everybody wants the edge. Young engineers coming out of school want to have the skills necessary to be a great designer. However, engineers coming out of school lack not only experience, but technical ability as well. That’s okay.

Here are some ideas to help you get that edge quickly

Know that the obvious solution is not always the best
Brainstorm often
Ask yourself what the next step I can do
Leverage the strengths of others
Fail more
Be prepared for critique

College is where engineering fundamentals are taught and a love for learning is fostered; not where technical skills are developed and honed. Early on in your career, you may not have a clue about what specific area of engineering you’d like to enter into. Once you find an area that interests you, you can trust that these technical skills will be developed in your workplace. Surprisingly, though, in my experience, many engineers don’t rely on their technical skills to give them that edge, but rather on the scientific mindset they have developed. That’s our greatest asset! So with that in mind, here are some easy tips for becoming a better designer from the start.

1. The obvious solution is not always the best

2. Brainstorm often

Let’s face it; we don’t have all the answers! Good ideas come from others imputing ideas into your work and vice versa. If you are like me, you might want to shoot down an idea that seems unworkable. Don’t do that! Resist the temptation to say anything negative about another idea. At the start of a brainstorming session, if every “bad” idea is taken off the table immediately, people stop contributing. And that’s the worst thing that could happen. So, please don’t do that or if you see someone else doing that, stop them. Yes, those ideas may be bad, but I have seen many times over the years, that those bad ideas can be shaped and formed into good ideas eventually. When multiple people share their input, they can each bring a new viewpoint and build on the idea. That’s teamwork!

I recommend a group of 4 to 8 people with at least 2 people from other departments. This gives balance and promotes different perspectives.

3. What’s the next thing I can do?

This question governs my design process. My mind doesn’t seem to follow any known logical paths when I am designing. I am constantly shifting back and forth between modeling and calculations and then I’m working on an entirely different section of the design and then circle back again. It may seem unstructured and frankly, bizarre, but the truth is very simple; I get stuck. And often. You will, too. When I get stuck, I simply ask myself, what is the next thing I CAN do?

For example, today I was modeling a bolted design and needed to know what bolt size and plate thickness to use. I started working the calculations and determined that I needed more information from the model. So I modeled. My modeling got to a point where I needed more information from a part that was all the way on the other side of the design. Before I knew it, I had my model in order and could finally come back to sizing my bolted joint. I discovered the next thing I could do.

In the midst of a design project there are times when you are really stuck. You’ve hit a brick wall. Perhaps you have trouble determining what your next tangible action could be. In these cases I recommend windshield time. This is the time you spend on your drive to and from work looking through your windshield. Your brain is somewhat on autopilot (but keep your eyes on the road) and your mind is reaching into its subconscious. (Ever pass a traffic light and struggle to remember if it was green or not?) That’s when new ideas often come. I highly suggest that you immediately, but safely, write down or record these new ideas. You don’t want to risk losing this groundbreaking new idea! You may need to pull over first. If you have an idea when you are asleep, get up and write it down. Don’t fool yourself into thinking you’ll remember it in the morning, because you won’t. Yes, it’s inconvenient, but definitely worth it.

4. Leverage the strengths of others

In case you didn’t know, you don’t know everything. Woo hoo! I can’t imagine the burden of knowing everything. I remember when I was a teenager and I knew everything, but I digress. Yes, you don’t know everything, but you can use that humbling truth to your advantage. Find someone you trust to be your mentor and glean from their knowledge and experience. Now that I have been mentoring for a while, I thoroughly enjoy when young engineers ask me for guidance. I relish the opportunity to be able to impart what I know to the next generation. Don’t be fearful to ask for help, because mentors are mentors because they want to help you succeed. Be willing to ask for help and you will see your own knowledge and experience grow. A little warning here, your mentor is going to push you and it will not be pleasant. Know that they are grooming you so that someday you will be a mentor.

Another great advantage is leveraging the power of a team. In a previous role, I worked with a woman whose skills were completely opposite from mine. She enjoyed paperwork and getting things organized. I enjoyed the technical aspects of design and am less…er….organized. This relationship was very productive, and as a team, we launched one of the most profitable projects in the company’s history. Knowing the strengths and weaknesses within your design team and leveraging them can really propel a project forward.

5. Fail more

Ouch! This is a tough one. Many of us were raised with the notion that failure is bad. Yes, it is bad when bearing the brunt of it, but don’t let that get in the way of learning from it. Here are two important examples of constructive failures:

The Exxon Valdez oil spill of 1989 was to-date one of the largest oil spills ever, at 10.8 million gallons. It had a profoundly negative effect on the environment. However, it brought about huge and lasting improvements related to the transportation of oil. When the news of the spill broke, most newspapers reported that the captain was intoxicated and single-handedly caused this disaster.

Although he was drinking, he was not on duty at the time of the accident. I’ve personally seen the causal map for this incident. After ongoing investigation, it was discovered that, in fact, there were roughly twenty root causes for the accident. Here are a few causes of the accident coupled with improvements made:

The ship was leaving the port at night with bad visibility – now large ships only leave during the day.
Computers (very new technology at the time) were newly implemented and replaced the human job of navigation.
The pilot of the ship was piloting alone because the navigator was replaced by a computer.
There were large areas of the harbor that were not visible by radar and the ship was not seen – More radar coverage was added and other harbors were analyzed for similar issues.
The ship was a single hull so that if the hull was punctured, oil would leak out even if the damage was minimal. Today, most tanker ships have two hulls so that the outer hull can suffer damage while keeping the oil safe.

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The other example of a constructive failure is the Tacoma Narrows Bridge in Washington. Known as “Galloping Girdie,” this suspension bridge would twist violently in the wind until it collapsed on November 7, 1940 after being open for only four months. A suspension bridge spanning such a great distance at that time was new and fairly untested. Apparently, solid beams supporting the roadway, which did not allow the wind to pass peaceably, was the design’s fatal flaw. In fact, on windy days, the wind would get “hooked” inside under the roadway and violently lift one side of the bridge at a time. After the collapse, wind tunnel testing showed that a solid web beam was not acceptable in suspension bridge design. As a result, suspension bridges today are only made with open lattice frames.

Please don’t get the impression that I am asking you to purposely try to fail in this manner. These were examples of huge failures. Small failures are good. You will learn more from a single failure that you will from a thousand successes. When you fail, welcome it, dig in and learn something.

A final thought is to know when your design idea is a failure. You may have been working on it for a few weeks now and you see that it has one or more fundamental flaws. Brainstorm and ask for help on it, but be ready to abandon this design.

6. Be prepared for critique – say thank you when you get input/feedback

Let me say this up front–this is a hard one. You may be tempted to say something like…”But I’ve worked for weeks on this idea and now someone is going to say bad things about my baby? Not acceptable!” Designing something is very personal because any design is a reflection of us. Over the years, I have been able to identify who designed something based on characteristics I’ve seen in other designs of theirs.

Design reviews can be the worst time for an engineer. And it can be extremely humbling. I once had a design review where a question from left field on slide 3, totally derailed the rest of the presentation (about 15 slides). My design wouldn’t meet a critical, but unknown requirement, so I acknowledged the short coming and admitted that this design would be changing dramatically. I remained calm and didn’t argue. I spent the remainder of the time quickly pointing out on each of the remaining slides all the things that would now need to be redesigned. It was humbling and yet memorable experience.

Whether your design is reviewed by five of your peers or by two dozen upper level management personnel, it is a daunting situation. So you need to be prepared for the coming critique. View it as constructive criticism that could potentially build on your idea rather than seeing it as a personal attack. There is no way to get around it; people will be asking you to defend every aspect of the design. Just relax and take a deep breath. You know your design better than anyone else. Here are some other tips to help with critique.

People’s words can hurt – Sometimes they don’t even know they upset you. Don’t get upset and give them the benefit of doubt.
Be prepared, know your design inside and out – If there is an area of the design that isn’t finished or you don’t know how to proceed, let others know up front. Don’t try to defend something you aren’t that attached to. I will often lead a segment of the review with, “I really don’t like this idea, but it is the best thing that I have come up with.” You’ve now turned the tables and taken a weak area of the design and allowed others to be involved in the process. They will remember that.
Know when to defend your decision and when not to – Seems simple, but it isn’t. Don’t get caught up in the emotion of the situation.
Say thank you – This one’s important. Saying thank you after every question/remark shows that you value the input of others in your design. It’s not easy at first, but quickly becomes habit. It’s a sign of a mature and seasoned engineer.
Take notes – Show that you are serious about the input by writing it down. If it is a large group or a short amount of time, get someone else to take notes. Be sure to follow up with any unanswered questions in a timely manner.

As I applied these six principles in my engineering career, I saw that the quality of my work and my creativity improved dramatically. I was also able to take on more responsibility and become a resource to my team. I challenge you to implement these in your life so that you can enjoy the same growth that I did.

A 10 Minute Guide to Understanding Steel

Written by Corey Rasmussen in Basics Last Updated January 27, 2020

In this video, Corey gives an overview of using various steels.

https://youtu.be/E6XYxbyOBnk

What You Should Know About Selecting Steel for your Project (https://youtu.be/E6XYxbyOBnk)

Mohr’s Circle Simplified – What You Really Need to Know

Written by Corey Rasmussen in Basics Last Updated August 19, 2020

In this video, Corey simplifies how to use Mohr’s Circle for general calculations.

https://youtu.be/DTv5HsJEr_Y

The Best 4 Ways to Improve Torsional Beam Performance

Written by Corey Rasmussen in Basics,Mechanical Design Last Updated September 28, 2022

I recently stayed at a very nice hotel and out at the pool there were some plastic rocking chairs. I sat in one and rocked for a few minutes and noticed something odd. In addition to the chair rocking, there was a vibration where one side of the chair was leading the other.

The cause of this was obvious; the floor was uneven. This sparked my interest because I had never noticed this in any other, mainly wooden, rocking chair. Not being content, I had my wife sit in the rocking chair next to me and she noticed the vibration as well.

Even on vacation, I cannot turn off my mind so I flipped the chair over and immediately noticed what caused the issue. The sections that made up the chair were open sections in the shape of a U and would deflect much more than an equivalent wooden rocker when in torsion. That extra deflection, allowed the vibration I noticed to occur. In this article, I will discuss multiple ways analyze and improve torsional resistance in structures so that you will design a better rocking chair.

In order to design a better section for torsional deflection consider using closed sections like bars or tubes; avoid plates and structural shapes. You can also box in open sections, thicken walls and apply bracing.

1. Open vs Closed Sections

One great challenge of engineering is designing structures for torsional loading. This can be an issue for two reasons: first, calculating the stress in open sections can be rather difficult. Second, the deflection of a section can be excessive and ambiguous to calculate.

Let’s examine why designing for torsional loads are much harder than bending. In the, I beam shown, tension and compressive stresses flow through the top and bottom members. The deflection will be very predictable since it is governed by the equation:

Where M is the moment applied, E is the Young’s modulus, I is the Area moment of inertia and v’’ is the second derivative of the deflection. Using this equation, you can use your calculus skills to integrate and setup your boundary conditions etc. I prefer to look these up in a table like everyone else. My reference of preference is Blodgett’s Design of Welded Structures.

Now, if we take the same I-beam and apply a torsional load to it, logically we would expect it to have a large amount of deflection. But why? The reason for this is that there is nowhere for the torsional stress to flow. In bending, the web of the beam takes the shear load in between them. If you remember the first few days of your Mechanics of Materials class, you will recall that shear stress is maximum at 45° from the longitudinal axis where it essentially becomes a normal stress.

Consider applying torsional stress to the ideal section, a tube. This is because the torsional stress has some where to flow. Imagine taking a tube and cutting it at a 45° angle from its axis. (Think of a candy cane stripe.) Now flatten it out. What do you have? It is a flat bar. Since shear stress is normal stress offset by 45°, normal stress flows freely through the flat bar.

With torsional loading, there are two types of sections, open and closed. An I-beam is an example of an open section and the tube is a closed section. Obviously there can be combinations and modified versions of both. Closed sections are preferable for torsional loading because there is a very clear path for the shear stress to flow in. The generic closed section torsional formula is based on the “average enclosed area.” The larger the enclosed area, the better the torsional resistance. There are trade-offs with using a closed section. In the table below, three sections with equal areas are compared. You can see that you can shape the material to be good in bending or torsion, but you will have to compromise to get one that is good in both. An I-beam is great in bending and conversely, the round tube is great in torsion.

You can also see that the rectangular tube is a decent compromise and is readily available. From quick inspection, you can see that material had to be taken from the flanges of the W beam to form a second vertical that creates the wonderful closed section.

When you can use a closed section, do so. But what happens when you just have to use an open section. Two very important questions come to mind. How much will it twist? How do I make this section better?

Good news! Both can be answered.

2. Box in Portions of Open Sections

For the former question, I wanted to point out the excellent work of Omer Blodgett in his book, “Design of Weldments” put out by Lincoln Electric (Section 3.6). If you do not own a copy of this, get one. It is a priceless tool every mechanical engineer should have. In the torsion section of the book, Blodgett describes how torsional resistance can be calculated using some formulas. He then correlates the calculated deflections with real deflections. For most open sections you find, you can use his torsional resistance method to calculate the stiffness of your section by combining smaller components that he provides information on. For example, Blodgett describes an I-beam as three flat plates. From these equations, we can easily estimate the twist and see what changes will make it better (or worse). Since this is an empirical principle, it is not 100% accurate, but will give a reasonable representation.

The latter question can be answered by understanding that closed sections don’t have to be closed the entire way. Let me explain by making a parallel to a section in bending. Going back to I-beam example. If I put a small hole (Figure 1) in the web of the beam, the shear stress will simply flow around it. It makes a stress concentration. If I make the hole a slot it (Figure 2), there may be more of a disruption in the flow, but the beams is still going to work well in bending. Now, if I make the slot longer (Figure 3), at some slot length, the whole I beam section will stop functioning as a unit. This will result in bending stresses in the flanges (not just tension or compression) and the section is no longer capable of working as intended.

Figure 1

Figure 2

3. Apply Bracing to Open Sections

The same is true in torsion, having a closed section for part of the way will improve the rigidity of the beam. Let’s say that our I-beam is cantilevered with an offset vertical load on the opposite end. On the mounted end, it is boxed in for the first 10%. The boxed end stiffens the entire length (not as much as a fully boxed section) because it forces the flanges to bend upon their strong axis. Since torsion flows in a constant direction, the flanges will bend in opposite directions. In figure 4, you can imagine a torsional load bending the top flange away from us and the bottom flange towards us. Figures 5 and 6 show alternate methods of boxing in a partial section.

Figure 4 – Boxed in end section improves torsional rigidity by forcing the flanges to bend as well as twist.

Figure 5

Figure 6

What I want you to understand is this: a section doesn’t have to be perfect or uniform the entire length to do what it needs to. A slot in an I-Beam is acceptable for bending and a semi-boxed in section used in torsion can work as well. I do want to point out that some of these methods may create rather larger stress concentrations where the sections change. Be aware of this and be sure to taper plates to alleviate large stress concentrations.

4. Increasing the Thickness

As a last resort, increasing the thickness of a plate will increase torsional stiffness. This comes at a large weight penalty so use this sparingly. Please note that the largest shear stress on a plate will occur at the center of the longest side. You may be able to add more material by welding a thinner plate in this location. This will minimize some of the added weight.

Conclusion

Designing for torsion can be daunting, but with practice, your mind will start to breakdown the sections. It will first happen by spotting open and closed sections. Then it will break open sections into calculable components. Pretty soon, you will love the creativity you can use in open section design. Remember to ask yourself: How does the stress flow at 45°. Can I enclose this section? How about part of it? It isn’t as tough as it seems once you know what to look for.

Now go design your better rocking chair!

The Best Way to Design for Ergonomics

Written by Corey Rasmussen in Basics Last Updated January 27, 2020

What is “ergonomics?” Simply put, ergonomics is the study of people’s efficiency in their working environment.

The best way to design a machine for ergonomics is to put yourself in the roles that the people interface with the machine. Some are obvious like the operator, but others are not. Maintenance is one that is often overlooked

As engineers, we realize that most things we design will interface with human beings at some point. In other words, people will be using the products we develop. How easily, safely and efficiently will people be able to use our products? For the electrical engineer it might be creating access to a touch screen computer that is “user friendly” for all. For the mechanical engineer, it may be allowing enough access for people to service parts located inside a tube.

Humans come in all sorts of shapes and sizes, so designing for all of them is definitely a challenge and one to consider carefully. For example, designing a computer touch screen interface is easy for a small person with petite fingers to use. However, when a person with larger fingers tries to use the same system, the wrong keys are continually pushed. Have you experienced this? Think about how frustrating it must be for a 6-foot tall male to text on any small scale device.

Safety is one of the key benefits to excellent ergonomics. If the operator is not forced to work in awkward positions and is performing the job comfortably, increased safety is ensured. Awkward positions can drain one physically and mentally, and can even lead to a loss of balance resulting in injury.

Increased productivity is another benefit to good ergonomics. When the operator is able to perform a job comfortably without becoming fatigued, he can usually work longer requiring fewer breaks.

When designing with ergonomics in mind, it’s helpful to know all about the human body. There are many good reference books out there that chart all sorts of useful information about reach, body strength and comfort level. For example, did you know that placing high impact, high load jobs at waist level allows the operator to leverage the natural strength of their body? Or the grip strength of a person’s hand is cut by more than half when the wrists are bent at 90 degrees towards each other? Facts like these are all readily available. See my resources page for recommended resources.

The Ergonomics of a Log Splitter

Now, fellow engineer, let’s get our hands dirty. For the remainder of this article, I will be doing a case study on my log splitter to see how ergonomics plays into the design. As you can see, my log splitter is in need of an ergonomic upgrade! Let’s look at loading a large and heavy log. The operator (me) would need to stand up and rotate the jib into position above the log. This is an unpowered manual process and there isn’t a good location to push between the shoulders and waist. In this figure you can see that I can only grab the cylinder rod or lifting chain. These are both bad choices because they move. Adding a handle bar could resolve this concern ergonomically.

The next ergonomic issue is placement. When the load is on the left side of the machine (standing at tongue looking at axle), the operator is required to duck under the jib or in between the jib and cylinder to operate the jib cylinder. Both of these options are dangerous because the operator is bent over and at risk for injury or worse. To eliminate this, there are three good options.

First, the valve could be relocated to a better position. Secondly, it is possible to add another control valve could so that operation from both sides is easy. A third option is to add a longer throw to the lever as shown. This is an easier idea to implement, but the valve may not be designed to withstand the added load of the handle. This could manifest itself in the valve centering spring failure caused by road travel fatigue.

Once the log is in place, the normal working area can be considered. A brief inspection shows that sitting is the proper body posture for this machine. For good ergonomic design, the log should be able to be handled from this sitting position. When making the first cut of a large log, if you aren’t paying attention, one half can fall on the other side of the machine. This requires the operator to bend over the machine and pick it up putting undue stress on your back. This is an obvious ergonomic and safety concern. Perhaps a shelf could be added to the machine to support the wood. If both sides of the machine are to be used, the designer should make this part removable, or design it to swing or slide into position.

Notice the sitting stance of the operator when using the log splitter. In order to operate the valve, and handle the log coming off the knife, a very wide arm span is required. Even for a tall person, this would lead to increased fatigue. Moving the valve closer to the operator would mitigate this issue. The picture below shows the operator operating the valve with a lever. As a result, he is able to move closer to the knife and reach the out-feed table with a much narrower stance. The operator can easily reach for a log (to his left or across the machine), position it in front of the knife, split the wood using the improved lever all while controlling the log with one hand. This positioning should lead to decreased fatigue as well as increased productivity.

The gathering of fresh wood up to the log splitter is a mixed ergonomic issue. There can only be so much wood stacked around the machine for the operator to grab while sitting. At some point, the operator will need to stand up and bring more material to split. The benefit of this ergonomically is that it requires the operator to move around, stretch and take a break. However, the operator may have just gotten into a rhythm and has to be interrupted to acquire more wood to split. A simple solution is to have a second person bring wood to the operator.

The final ergonomic (and safety issue) with the log splitter involves the exhaust system on the engine. This engine has the exhaust pointed right to where the operator would sit on this side. The fumes are too noxious to operate from this side. Perhaps designing another exhaust configuration to point this in another direction would work. (Be sure to prevent the exhaust from being sucked right back into the air intake. This would lead to poor performance.) Another alternative is to add a baffle that forces the exhaust to go in another direction. I will be looking into this as an option for my log splitter.

When designing for ergonomics, the following is important to keep in mind. There are many areas that can be easily identified looking at a 3D model. However, some areas of ergonomic improvement can’t often be seen until the prototype is made. This may be far too late to change problem areas! Building a mockup out of cardboard is one of the best ways to foresee ergonomic issues. I believe that cardboard is one of the best materials for this because it is cheap, readily available and can be easily formed into a variety of shapes.

Here are 10 basic ergonomic questions that can be answered during the design phase of the project. Precious time and money can be saved if these questions are answered early on. You don’t want to find yourself launching a new product that is unsellable.

What is the basic flow of material through the machine?
When does the operator have to interface with the material?
What position is the operator in during these times?
Are there times when the operator has to operate two or more functions? Can these be eliminated or reduced?
Can the operator see the effect of what is being controlled? Can mirrors, windows, cameras, etc be added to eliminate this?
Does the operator wear gloves? Do the controls allow easy access with gloves on?
Can a touch screen computer work with gloves on?
What routine maintenance needs to be done such as lubrication? Is there access to these areas?
Are access holes large enough for an operator wearing gloves?
Is the area properly lit?
If the apparatus is to be used at night, is the lighting too bright or the wrong color of light? Will the light ruin the operator’s night vision?

In conclusion, designing for ergonomics is a necessary and challenging part of the design. With the simple processes outlined in this article, you can be proactive when designing with ergonomics in mind.

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