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The Best Design Reference Book – Omer Blodgett’s Design of Weldments
What Can I Do in High School to Become An Engineer?
On the first day of my high school physics class, our teacher stated, “If you don’t like solving problems, you shouldn’t be in this class.” Yes, the engineer’s main role is to solve problems. So if you want to be an engineer, you must love to solve problems.
To adequately prepare yourself for engineering school, you should take a college preparatory course load with a 3.0 GPA, have at least one STEM or community related extracurricular activity and have at least 680 Math and 530 EBRW scores on the SAT.
I own a shirt that says, “Engineer, solving problems you didn’t know you had in ways you can’t understand.” No matter what discipline of engineering you are in, you are solving a problem for someone. This is precisely what an engineer does, solves problems.
If you enjoy doing these following activities, you may want to pursue engineering as a career.
- Solving problems, puzzles, Rubik’s cubes etc.
- Thinking about new ways to perform tasks
- Building with Legos
- Identifying and optimizing efficiency in day-to-day tasks
- Like being challenged. (I often declare, “challenge accepted” at off the cuff comments by others.)
- Have a thirst to know who things work.
Having the above desires and traits won’t make you an engineer. It takes a lot of work to become an engineer. I know several people who would make great engineers because they couldn’t (or wouldn’t) take the classes.
Conversely, I do know other engineers who have a degree who can do all the math and science given to them, but cannot solve a “real world” problem on their own.
Engineering Career Paths
Most employers are looking for a bachelor’s of science in engineering degree from a college or university. This is a 4 yr degree (or 5, like me) and opens the door to most opportunities. You can also proceed to get a masters or doctorate if you like.
Another option is a 2 yr engineering technology associates degree. These degrees are intense and cram 4 years of math, science and engineering into 2 years. College admissions here are a little more relaxed as far as what courses need to be on your diploma.
I have known many very smart and capable engineers who went this route. Some of them even went on to get their professional engineering license.
Academic Course Requirements
Any ABET accredited engineering school is going to require a high school diploma. But not all of them are equal.
A student might have a 3.9 GPA but only taken the minimum classes needed to get a high school diploma. Another student my only have a 3.0 GPA but has taken 5 years of math and 3 years of science as well as several honors classes.
Most colleges will see the the latter student as superior to the former because of the increased course load.
College Prep vs Diploma Requirements
The table below shows the difference between most states requirements for a diploma and what colleges expect from a college prep curriculum.
| Subject | Years | Diploma Notes | Years | College Prep Notes |
| Math | 3 | Algebra and Geometry minimum | 4+ | Algebra 1, 2, Geometry, Trigonometry, Pre-Calculus, Calculus |
| Writing | 2 | One may be part of another class | 4 | 3 Literatures and composition |
| Language Arts | 4 | English or Foreign | – | |
| Science | 2 | – | 3+ | Biology, Chemistry, Physics (with labs) |
| Social Science | 2 | US history required | 2-3 | – |
| Electives | 1 | Art, music, vocational | 1 | Art, music, vocational |
| Physical Education | 2 | Daily exercise all four years, .5 credit per year | 2 | – |
| Health Education | 0.5 | 0.5 | ||
| Consumer Education | 0.25 | 0.25 | ||
| Challenging Electives | 0 | 1-3 | Communications, Computer Science, Statistics etc. |
You can see that there is an increase in English, Math, Science and History. In high school, I took 5 years of math and English. This required taking two English classes freshman year (literature and composition) and two math classes sophomore year (Algebra II / Trig and Geometry).
This sounds overwhelming, but I managed to be successful. After all, this curriculum was well established by the school and I was not the only one to do this level of coursework.
Required GPA
As you can probably guess, not only is taking the harder classes important, but you also must do well in them. It can be difficult to do both, but hey, you’re a problem solver.
Most colleges require a 3.0 GPA. Occasionally, you can have a lower GPA, but you will need to show that you are still in the top 25% of your class. This can be difficult to do so strive to get better grades and have the minimum GPA at time of graduation.
Since more of the engineering related subjects are taken later in high school, some colleges will look at your transcript and if your grades improve junior and senior year, you can sneak in with less than a 3.0.
If are excelling in some core classes, try to take some AP (Advanced Placement) or IB (International Baccalaureate) classes to help improve your GPA because they are on a 5.0 GPA scale where a B grade is now 4.0 instead of 3.0.
What do I do If I am Not on a College Prep Track?
Yes, there is hope for you to get into engineering school. First, you will want to plan the rest of your high school years to get as many solid math and science classes in. Preferably to pre-calculus and physics if not further.
If you are just a little short of credits, there is another option. Depending on what grade you are in, you can actually dual enroll in both high school and a local community college.
Dual Enrollment
Dual enrollment allows you to take a college physics class while in high school. And YES, you get credit for both! Super Sweet!
This allows you to take extra classes on nights and weekends so that your high school transcript is complete and you now have some college credit.
While you are doing this, you aren’t going to have much free time in high school. You will already be in 6 meaty high school courses and 1 night / weekend course. That is a full load!
If you are in dual enrollment for physics, you will want to have taken calculus first. Concurrent enrollment is acceptable, but you may be at a disadvantage because you are learning too many new concepts at once. This is because engineering programs require calculus based physics. There is no purpose in taking the class twice.
Summer School
Another option to catch up is summer school. Summer school is a great way to catch up. I recommend that you take a math class in summer school because they are probably offered and math courses need to be taken in order.
Math classes build on each other so each is a prerequisite for the next. Algebra I needs to be taken before Algebra II, but Algebra II can be taken with geometry and trigonometry. Calculus builds on pre-calculus which requires Algebra II.
When coming up with a schedule of classes, you will need to keep these prerequisites in mind. This is why I suggest doing extra math in summer school.
Community College
If you have already graduated or it is far too late to get on course, you will need to enroll in a local college or thanks to the internet, take online courses from a college far away.
This is a great option because allows you to increase your GPA and take many basic college courses much cheaper than at a 4 year college. When you have the basic requirements done, you can transfer to an engineering school when accepted. If you took the right classes, they should transfer as well.
I do want to caution you here. When going to a local college after high school, life gets in the way. The desire to make money, get married and have children can derail your plans if you don’t have laser like focus. Before going to a local college, make sure you have a well developed actionable plan so that life won’t easily get in the way.
Challenging Electives
Engineering schools want to see electives in your transcript. Depending on your school, you can look into electives that fall into the following general topics:
- Computer Assisted Design (CAD)
- Drawing / Blueprint Reading
- Computer Science
- Electronics
- Robotics
- Any engineering classes
- Advanced math / science (AP Calculus, AP Statistics, AP Physics)
Some of these courses may not be offered at your high school. If this is the case, look at dual enrollment options with your local community college.
SAT & ACT Scores
Taking standardized tests is a major headache and source of stress. Unfortunately, it is one of the hoops we need to jump through to prove we can do certain things. In the end, it is a character builder, so buck up and do it.
In general, universities are looking for higher math scores on the SAT than EBRW ( Evidence-Based Reading and Writing ). EBRW was simply called ‘verbal’ when I took the test, but I now find that ironic since there was nothing verbal about the test.
For the SAT, you should target around a 680 Math and 530 EBRW to have a wide selection of engineering programs to choose from. A score of 680 in math puts you in the top 90 percentile and 520 EBRW is in the 50 percentile.
However if you desire to go to a ‘name brand’ school, your math scores will need to be over 750.
As far as the ACT score your goal should be a 30 math and science and 21 for English and reading.
Preping for the SAT / ACT Tests
The SAT is a not test of your intelligence, but how well you take the SAT. Don’t get angry and try to be smarter by learning vocabulary and higher level math. Learn to take the test.
Image Courtesy of Alberto G on Flickr
So I know that everyone learns differently, but I took some of those weekly classes to prep for the SAT. I’m sure they helped, but I know that I did not put the time in to get better results out.
The SAT is a not test of your intelligence, but how well you take the SAT.
I recommend that before taking a class, you should get a hold of many practice tests and do one each weekend. Make sure they are timed. After each, score them and look for patterns.
I noticed that I always missed the last few problems in both math and ‘verbal’ so I left them blank. As a result my score improved.
The other benefit is that by taking many tests, you will manage your time better and become familiar with the test format.
If you do this and don’t see improvements in your score within four tests, go get some formal help.
Extra Curricular Activities
Colleges want to see that you aren’t the person that only goes to school and gets good grades. They also want to see that you play well with others and have interests other than your studies.
Extra curricular activities look good on an application, but this will only get you 10% of the way. Don’t focus on this too much because your course work, grades and test scores will get you 90% of the way to an engineering program.
Sports – Maybe
I say ‘maybe’ because there are not many stellar athletes in the NFL that have engineering degrees. In fact, I had a friend in college who was on the college baseball team with a scholarship. During his sophomore year, his course load was so great that he couldn’t keep up with both engineering and baseball. He chose to drop baseball and focus on engineering.
My point is that sports requires a lot of time and so does a heavy course load. Most people can’t do both.
Don’t focus on extra curricular activities too much because your course work, grades and test scores will get you 90% of the way into an engineering program.
However, if you are in sports, it does show engineering programs several key things.
- You have self discipline
- You work well with a team
- You have good time management skills.
Other Activities
There are many other activities that show you are an engaged person in your community or school. Consider joining one or two of the following groups to round out your college applications.
- Any club that is STEM related
- Student body office
- Campus club like chess, computer, language, band
- Volunteer in your community / Kiwanis Club
- Boy’s and Girl’s Club
- Boy / Girl Scouts
- Robotics – Check out Vex robotics and how to start a Vex Robotics Team for Free!
- Community group for your hobby (search Facebook Groups)
- 4-H
- Model UN
- JROTC (Junior Reserve Officer’s Training Corps)
- Try getting an internship in the field you want to study. Consider it even if it is unpaid (I have had two internships where the only thing I learned was that I didn’t want to have that job.)
- Do something cool and unique that will beg the question on your college application. Example, I hiked the Appalachian Trail, camped in a blizzard or wrote an ebook.
Conclusion
From the moment you realize that you want to be an engineer, you need to figure out where you are and where you need to be. Almost none of us are in the perfect position to easily transition from where we are to our objective.
Don’t expect your transition to be easy. This will require work and lots of it. You are going to have to say ‘no’ to certain things that you want to do. Work and self discipline are great character builders.
You can do it. Come up with a plan of what needs to be done each semester or quarter in school and work the plan. Focus on the tasks that will get you 90% of the way to your goal. Work the plan until you reach your goal.
The Best Way to Make Difficult Decisions – A Decision Matrix
What is the Best Charge Pressure for My Accumulator?
Accumulators are wonderful devices that perform many functions. One function is to minimize pressure spikes from the water hammer effect. Many of us might actually have an accumulator attached to your home’s water system to prevent “banging” when shutting off the water.
Accumulators used as expansion tanks need to be charged at low pressures. Higher charge pressures are needed for load holding and applications with low duty cycles will be at medium pressure.
Shock Absorption
The situation above uses an accumulator, sometimes called and expansion tank with water systems, to absorb shock. This type of accumulator can handle high changes in volume over low pressure changes. Accommodating the large change in volume will minimize shocks to the system.
Accumulators for Load Holding
Another benefit of an accumulator is that it stores energy for future use. Perhaps you need to apply pressure to a cylinder for an extended period of time, but do not want to run your pump at idle. Many static tests are like this. You apply a load and leave it there for hours or days.
Adding an accumulator will allow you to store up pressurized fluid and then shut off the pump. You can program the pump to kick back on when the accumulator pressure is too low as well. Once it is charged, turn it back off.
Accumulators for low duty cycle machinery
Finally, you can use accumulators make your hydraulic system smaller by taking advantage of low duty cycle. Many roller coasters use hydraulics to power linear acceleration, which requires high pressure and flow rates to make them work.
The traditional solution is to size the system for the pressure and flow needed at launch. Let’s say we need 400 gpm at 1500 psi for 10 seconds. That is 350 hp! And a very big hydraulic system.
At that flow rate and launch time we need 67 gal per launch. (Don’t worry about the math or numbers here.)
Since there is 2 to 3 minutes in between launches, we can use that time to store up pressurized fluid in an accumulator. If we plan for a 10 second launch every two minutes (130 s), we can cut the flow rate to 31 gpm from the 400 gpm mentioned before. However, to have all the discharge fluid be above 1500 psi, we will need to run the pump to 2000 psi. Nonetheless, that reduces the power to 36 hp.
The components at this point will be much smaller and easier to work with. Only the hoses between the accumulators and the prime mover will be large.
How an Accumulator Works
An accumulator is a steel pressure vessel with two chambers. One chamber is attached to water line or hydraulic oil hose and the other is pressurized with gas.
The sections are separated by either a flexible rubber diaphragm or a piston that slides similar to a hydraulic cylinder.
The chamber that is pressurized with gas needs to be charged for the system to work. The pressure that it is charged to is called the “charge pressure.”
As hydraulic oil enters the other side of the the bladder or piston will move toward the opposite side compressing the gas. This movement isn’t linear as the pressure increases because the gas is compressible.
What Do I Charge the Accumulator With?
Most low pressure expansion tanks used with household water systems are filled with compressed air. This is done because compressed air is readily available in most households and the pressures are relatively low 20 to 60 psi (138 to 414 kPa).
Compressed air is not the best gas for this application, Nitrogen is! Nitrogen is the most prevalent component of our atmosphere so it is easily acquired. It is inert which is great because it won’t explode like oxygen. If it leaks out, there is no risk to humans like carbon monoxide or carbon dioxide.
Nitrogen also does not contain water vapor. Standard air will have both water vapors and oxygen. This with the combination of any mold or bacteria in the air (which there is) will start a nice Petri dish in your accumulator. No one wants that.
Also, as the temperature fluctuates, the water vapor may condense giving unpredictable performance under higher pressures.
For these reasons, hydraulic accumulators are charged with nitrogen gas.
Determining Charge Pressure
Determining the charge pressure of the accumulator is the most difficult part of using an accumulator. I’ll be honest because I struggle with it as well.
Since we are dealing with compressible, non-ideal gas, the calculation below is based on empirical data and not exact. The 95% used in the equation is an efficiency rating.
Where:
- D is the discharge volume
- P1 is the accumulator charge pressure
- P2 is the discharge pressure
- P3 is the system pressure or max pressure the accumulator is charged to and
- V is the accumulator total usable volume.
Since the equation is empirical, you should always design in more accumulator capacity than what is needed. At the very least, you should be able to increase the system pressure a little.
Compression Ratio
An accumulator has a compression limit based on the physical constraints of the design. This is called compression ratio and it is defined as the system pressure / charge pressure.
For bladder accumulators, this ratio is 4:1. For piston accumulators the ratio is higher at 6:1. If exceeded, this may cause rupture to the cylinder piston or the bladder
Minimum Pressure
To prevent damage to the accumulator, we need to keep the minimum pressure at or above the charge pressure. Doing this always keeps a little bit of oil the accumulator so that the bladder or piston is not resting on the internal stops. This may or may not be a requirement based on the specific manufacturer’s requirements.
Installing the accumulator on the pressure side of a pressure compensated pump is a good way to maintain minimum pressure on the accumulator.
This requirement of the minimum pressure is for normal use only. The accumulator can handle not being pressurized for shipping and maintenance purposes.
Since we are on the topic, I should also mention that there should be a safe way to empty the accumulators of all hydraulic pressure for service. This should not be loosening a fitting and collecting the oil in a pan. (That’s dangerous)
Ok, deep breath. We’ll go a little deeper to understand this better.
Let’s look at a plot to give a visual representation to how the accumulator behaves at multiple charge pressures. Each line is a different charge pressure and they are all capped off at the accumulator charge pressure. I chose a 1 gallon accumulator so if later you need more discharge volume you can simply scale the size of the accumulator.
Notice that all of the lines converge at the maximum pressure of the system (3000 psi). This is because in the equation, P2 equals P3 and the terms cancel out.
The curve is where the complexity in selecting a charge pressure exists to let’s do this the easy way with examples.
Expansion Tank Charge Pressure
In the case where we are protecting a system from the water hammer effect, we can add an expansion tank (small accumulator) to the line in question.
Our desire is to have lots of volume change with very little pressure change. This would be a near vertical line on the chart and the obvious selection would be a charge pressure of 250 psi.
We may even want to go with less pressure but if you do not have any charge, the volume may expand at too low of a pressure and not perform the intended duty. The other danger is that you will violate the 4:1 or 6:1 compression ratio.
Charge Pressure for Load Holding
Load holding applications are quite common and generally they don’t require very much flow because position doesn’t change. Most of your fluid loss will be from leakage internal to directional valves.
For this type of system, you will need to have a high charge pressure. Say that our application will charge the accumulator to 2500 psi and shut off the pump. Our load needs to be held at 2500 psi. (We will need a pressure reducing valve to maintain the needed pressure.)
One the graph above, if I use a 500 psi charge pressure, I can only store about 7 in3 of fluid between 2500 psi and 3000 psi. Not anywhere close enough for most requirements.
However, if I change my charge pressure to 2500 psi, I can now store about 37 in3; an incredible difference.
Now if we tweak our requirements a little, we can gain more advantage. I want to hold my load at 2000 psi. If my accumulator charge pressure is still 2000 psi but I only apply 2500 psi of hydraulic fluid to the accumulator, I increase my discharge volume.
To calculate this we will need to see what volume is available at each pressure. At 2000 psi, there is 73.2 in3; at 2500, there is 29.3 in3. The available discharge is the 43.9 in3 (73.2 – 29.3).
Determining the Charge Pressure for Low Duty Cycle Accumulators
In our roller coaster linear acceleration example above, we demonstrated how long periods of rest can be used to continuously store fluid for sudden release. The main benefit of this was smaller components and a more level loading of the system.
For these applications to be successful, you will want your function to operate at lower pressure. I recommend designing your system to be less than 50% of the system pressure. This would be no more than 1500 psi on a 3000 psi system.
For this application, you also want the charge pressure to match the functions design pressure. In the linear acceleration example, the accumulator should be charged to 1500 psi to give the best performance.
How the System Pressure Effects Volume
With a system that needs 67 gallons to perform a specific function, the system pressure before discharge is related to the size of the accumulator needed. At 1500 psi, there is 109.7 in3 available.
The table below shows what the discharge would be for an accumulator charged to 1500 psi at different operating pressures.
| Pressure (psi) | Discharge (in3 ) | Discharge Available (in3) | Size Needed (gal) |
| 2000 | 54.9 | 54.8 | 282 |
| 2500 | 21.9 | 87.8 | 176 |
| 3000 | 0 | 109.7 | 141 |
At the surface, it looks like going with a 3000 psi operating pressure makes the most sense because for 1000 psi additional pressure, I can cut my accumulator size in half. Pretty sweet.
On small systems, it probably makes sense to go with the higher pressure. However, on large systems, operating at a higher pressure may not be cost effective due to electricity and component costs.
Can I Charge to a Pressure Higher Than My System Pressure?
The short answer is No. As mentioned previously, the hydraulic pressure should always be at least the charge pressure. This is to prevent internal damage to the piston or bladder.
Since that is the case for the minimum operating pressure, it should be the same for the maximum operating pressure.
Conclusion
After analyzing three cases of accumulator usage, we have determined that accumulator charge pressure is different for each use.
Accumulators used as expansion tanks need to be charged at low pressures. Higher charge pressures are needed for situations with load holding and applications with low duty cycles will be at medium pressure.
You Won’t Like This Important FEA Tip
How to Design Great Wear Pads for Sliding Systems
With the wide spread use of sliding systems in industry, I thought it would be beneficial to create a design guide for wear pads for sliding applications. Sliding applications is one aspect of my designs that started off very poor and is now pretty solid. I would like to share these tips with you.
A well designed sliding system will address all the following items thoroughly.
- Wear Pad Retention
- Contact Stress
- Sliding Speed and Duty Cycle
- Torsional Loadings
- Wear Pad Placement
- Planning for Manufacturing Tolerance
Wear pad design is critical for the success of your sliding project. The first thing to address is chamfers on the slide pad. There should be a lead in chamfer of at least 1/8 inch and no steeper than than 45° in the direction of motion. The wear pad should also be longer (in the direction of travel) than it is wide.
Narrower pads give an advantage in several ways. First they can reduce the number of fasteners needed to retain the pad. Second, you can get better material per sheet usage.
Finally and most importantly, you can place the loads where you want them better. If your inner member is a structural tube with corner radii, placing two pads on the outside is a far better design than one large pad along the middle. There is no benefit gained by pressing on the middle of a hollow tube.
Wear Pad Retention
Screw Retention Methods
Most wear pads are retained by screws. I highly recommend using this method on all your slide pads. The other main retention method is using an adhesive which may fail and prevents easy replacement of the pad. So, use screws.
Make sure there are at least 2 fasteners per pad and push the fasteners a close to the edge as possible.
Perform a calculations on the wear pad screws to ensure that they have a design factors consistent with the equations below. If there a persistent problem insists, look into the design. You may have an interference or other issue that is causing higher loads.
There are two main ways to use screws. You can either insert the screw from the wear side of the wear pad (say that 5x fast) and add a nut or have the screw insert from the other side. Both have their advantages and disadvantages.
Assembling Screw From the Wear Side of the Pad
Inserting the screw from the from the wear side of the pad is great for two reasons. First, you can evenly distribute the load to the wear pad using a countersink fastener. This is important so that the wear pad has more contact area and won’t pull out of the material. Second, it allows you to have predictable head clearance on your fastener.
Unfortunately, you will need to access the head of the screw in order to change the wear pad. This can be an issue for service since you many need to remove the entire inner section to have access to your screw heads.
The other main draw back is that countersunk set screws can get clogged with debris making it much easier to strip the head.
It may be necessary to fasten your wear pads in this manner if your inner section is too small to gain access to from the inside or if you need to fasten into a tapped hole.
Assembling Screw From the Rear of the Wear Pad
The other alternative is to assemble a standard hex capscrew from the rear or back of the slide pad. This is my preferred method, but it has it’s drawbacks as well.
Do do this, we will need to add a retention method into the design of the pad. There are some steel hex inserts that can be pressed into round holes cut in the wear pads. The inserts have a tapped hole in them for the screws.
However, I prefer to use weld nuts because they won’t strip in the soft plastic like like the hex nuts inserts do. Using a small bearing press, you can press weld nuts into a hole that is slightly smaller than the weld nut. You can also add a few drops of super glue if you don’t trust your press fit.
I like this method because it removes the countersunk socket screw from the design and therefore access to the top of the wear pad. This translates into easy change out of wear pads for 95% of applications. A side benefit is that you don’t have screw threads sticking out to catch your sleeve or cut you.
Assembling a standard hex head capscrew from the rear of the slide pad is my preferred method.
The major downside is that you will need to plan for clearance on the screw more carefully. This roughly translates into the weld nut being recessed 0.25″ (6.3 mm) from the wear side of the pad. I get to 1/4″ because there should be at least 1/16″ (1.6mm) for wear and screw thread sticking out. Most wear pads will be retained by screws that are less than 1.00″ (25mm) and come in increments of 1/8″ (3.1mm) for when we need to set up in size. The sum of these numbers is 1/4″.
Assembly can be an issue especially if shim pads are needed. The assembler will need to know when it is necessary to increase or decrease the screw length. Be sure to stock fasteners in 1/8″ (3.1 mm) length increments.
Entrapped Retention
Also known as U-shaped retention, entrapped retention is a method that allows for a larger bearing area for the wear pad to contact. We do this by adding another piece to the assembly. As shown below, the wear pad will have a side profile in a U-shape (hence the name) where a metal, usually steel, bar will fit. The steel bar is retained by two or more screws.
You will notice that the wear pad is not direct fastening of the wear pad to the anything. It is held in by gravity so it should only be used in applications were there is constant contact and gravity will keep it in place. Futhermore, you will want the width of the U to be just larger than the steel plate for easy assembly. However, you don’t want it too big in order to minimize impact loading when the direction of travel reverses. Usually 1/16″ (1.6 mm) is adequate to accomplish both. Much more and you can start hearing chatter as well.
This type of joint is used for highly loaded joints where you cannot insert enough screws to get your bearing area to an acceptable level.
Screw in “Puck” Design
The screw in design is my least favorite. I personally have never tried it and have very little experience with it. The puck design is used for less loaded bearings which are usually on the side of the extension.
The concept is simple: thread the outside diameter of a piece of round UHMW or Nylon. The diameter may be 2 (50mm) to 3″ (75mm) to get enough bearing area. You can then machine a smaller hex on one end to turn it. A less expensive option is to drill two holes in the end and use a spanner wrench to turn the puck.
The mating component will have the same thread as the puck and you screw it in until the depth you want. One neat thing about this design is that as the puck wears, you can screw it in a little further thus replenishing the wear pad.
The major problem with this is that the pucks have an tendency to loosen easily and fall out. You then are at risk for metal on metal galling. To minimize this risk, I recommend that you have at least two pucks per joint.
You can also use retention methods such as thread locking compounds. Be sure they are plastic compatible. You can also try jam nuts with low torque or a plate that will fit over the hex and prevent it from rotating.
Contact Stress
The pressure on a standard linear bearing is super simple to calculate using the traditional formula pressure = force / area. It is important to use only the area in contact with the other surface. As a result, the area will be reduced by the lead in chamfer and holes for retaining screws.
You’re 3D CAD software should be able to calculate your contact area by selecting the surface. Easy peasy.
In general, I look for the maximum contact pressure to be 10% of the tensile strength or a 10:1 design factor. I do this because I assume that only 50% of the pad is actually in contact with the mating surface and then add a 5:1 design factor so that the wear rate is minimal.
Low Bearing Pressure
Noise is always an area of concern with slide pads. With sliding wear pads, the noise usually comes from a wear pad that makes light contact.
Noise usually comes from a wear pad that makes light contact
The Whale (Sometimes I still hear it in my sleep)
At a previous job, we had an articulating cylinder (rotational motion) that had two cylindrical bearings. One of the bearings was directly under the load needed to articulate the cylinder. The other bearing was equal sized but had nearly no load on it.
As a result, the no-load bearing sounded like a irate whale and could be heard from a quarter mile away. No joke…it was loud!
The solution was two part. First, move both bearings laterally a little so that the load was distributed better. Second we narrowed up the no-load bearing so that the pressure increased. The noise vanished!
Recommended Minimum Design Pressure
I tell the story so that you think about bearings in terms of the maximum and minimum loads while moving. As mentioned, I look for the maximum contact pressure to be 10% of the tensile strength or a 10:1 design factor. I do this because I assume that only 50% of the pad is actually in contact with the mating surface and then add a 5:1 design factor so that the wear rate is minimal.
I also look for the minimum load on the bearing to be 2% of the tensile strength or 1/5th of the maximum allowable contact load. Keeping this minimum load in mind will increase your chances that noise will not be an issue.
Contact Stress Through Tubes with Corner Radii
If you are using a steel square tube with corner radii in your sliding joint, you are starting at a disadvantage. Since the internal radii will be 2 – 3 times the wall thickness, you are automatically causing the corner and side of the tube to take moment. This will limit the amount of normal force that the tube section can take due to localized buckling.
Ideally you want the load from the pad to be able to be translated directly into the vertical member of the tube. This is the best design for stress flow because it eliminates localized buckling.
There are basically two courses of action to increase the strength. First you can thicken the tube wall. Doubling the thickness of the wall will quadruple the strength of the corner. However, it adds a lot of weight so it might be impractical.
The other option is to add a plate on the contact side of the tube. This adds less weight than thickening the tube wall. Sometimes it will allow you to thin the wall up leading to a reduced weight.
The downside is that you will need to weld the plate solid for the entire length where contact is made. This can be a lot of weld and you will probably need to worry about weld distortion. One way to avoid this is to make them in pairs with the added plates touching.
Plug welds should also be added down the length of the section to allow the added plate to act more like it is one solid piece with the tube. A plug weld should have a diameter of at least 1.5 times the thickness of the added plate. Be sure to grind it flush so that you don’t have high spots for the wear pad to run over.
Sliding Speed and Duty Cycle
Wear pads are friction members. If extending under load, they produce heat! Since they are usually plastic, they can melt pretty easily. They need time to cool off in between cycles. If your PV (Pressure * Velocity) is higher than the material allows you can usually get away with it if your duty cycle is less than a 25%. Otherwise you will need to switch to roller bearings.
Also, the maximum speed of the tube is critical. Depending on your material speeds of up to 10 fpm to 25 fpm are acceptable.
Some manufacturers of wear pad materials will publish the maximum pressure, velocity and PV ratings. Experimentation in your application will be necessary.
For example, UHMW has a limiting PV of 750 psi * ft /min with a maximum velocity of 15 ft/min. Nylatron GSM boasts a much higher PV limit of 5500 psi ft-min
Torsional Loadings
Many extension aerial lifts have buckets that rotate from side to side causing a torsional load to transmit through the sliding joint. Whether or not you like it, there will be some amount of torsion on every extension joint.
On round extension systems, there will need to be a separate system to handle this. Commonly, a rectangular plate will be welded or bolted to the end of the extension that is inside the outer tube. Wear pads can be placed on this plate to take the loading.
With rectangular extension systems, there is built in resistance to torsion. There are many ways to approach how to do this, but we need to always look at the worst case scenario.
A good sliding joint will have the the torsional loading add to only the highest loaded pad on each end of the reaction. In most cases, the extension is taller than it is wide. This means that the the moment arm for resisting torsion will react on the narrow dimension of the extension.
For example, the picture below has the two pads that will take the torsional load highlighted. One is on the outer reaction to the lower left and the other is on the inner reaction on the upper right. If the design had two wear pads, assume the pad is evenly loaded. If it is a single pad, assume the load can be offset to 1/6 to 1/4 the pad length off center.
This is the worst case. I like it because you will never go wrong here with insufficient retention or higher than expected loads. Sure other wear pads, like those on the sides, may take more load, but we are good engineers and plan on the worst case.
Wear Pad Placement
When planning your wear pad placement, you want to have them as close to the end as possible. The benefits are threefold: you minimize normal forces, which minimizes friction, which maximizes bearing life. Boom!
As an added benefit, placing the wear pad flush with the end will greatly help when aligning holes. This is a critical step in making your assemblers happy because they will be able to hold the pad comfortably.
If using a hollow inner tube, you will want to get the contact forces to the corners of the tube. This minimizes the bending load seen on the tube wall and reduces weight. The best way to accomplish this is by splitting the wear pad into two pieces and placing then as close to the corners as possible.
Planning for Manufacturing Tolerance
Shims
Most sliding systems are not designed to be perfect. This will show up when you mass produce the design. We need to have a plan going in. For most of us, this plan will contain shims. Great!
However, when we often fall short is engineering in a condition where we can still be too tight when there are no shims. Oops!
I have been burned on this one. Luckily it was found in the prototype phase and we were able to correct it before production started.
The solution is simple; always plan on using at least one shim on each pad. I usually plan on 1/16″ (1.6 mm) to 10 ga (0.1345in / 3.4 mm) per side. This gives me plenty of room to adjust the shims as needed.
Crooked Sliding
Many times, you will find that the inner tube is crooked in the outer tube. Not only does this look bad, you can actually load up your bearings higher than expected.
If you think about it, this will always happen to some level; even if you need a microscope to see it. This is why I recommend using the 10:1 design factor. The good news is that the wear pads will wear in the high contact levels and distribute the load to more of the pad over time.
One solution to this is to have machined shims that are faced at an angle (wedge shaped). This is very expensive to manufacture but will work. Another solution is to have two different thickness shims under the wear pad. This works best for shims that have at least four screws and with small changes in thickness.
Assembly
This is where most young engineers blow it, myself included. You have to have a plan for assembly! Here are some things to consider your wear pad design.
- Not thinking through how the unit is assembled – Oh boy has this burned me! Tube in tube joints usually have a specific way that they need to be assembled. You need to go through each step in the assembly process one component at a time and insure that each step can be completed. I’ve known many people who can do this in their head. For the rest of us, hiding parts in 3D CAD will do the same job.
- Not leaving room to tighten fasteners – There is nothing more frustrating than tightening a fastener by flipping a combination wrench over each time or only hearing one click on a socket wrench. Design in more room please!
- Reaching inside tube really far – If you need to reach inside a tube more than 18″ to insert a part, something is wrong with your design. I do admit that I have had the occasional design where a bolt spring was needed to install a carriage bolt. It is one thing to reach in to install a bolt, but another to have to hold it during torquing.
- Wear pad replacement – Your wear pads may need to be replaced from time to time. As you are assembling this, ask “How many parts need to be removed to swap out the wear pads.” It was always a requirement for me that the wear pads could be replaced by only removing a cylinder pin or other component. I liked to be able to slide the inner tube out the opposite end so that those wear pads could be replaced easily.