The Concept of “Strong Enough”

I’m not sure how to categorize this Article.  It’s an answer, but more than that.  We get questions once in a while something like “My trailer is X long and I want to haul a Y.  Is 3 inch angle iron strong enough?”

Gantry Crane Not Strong EnoughHow do you know when your Trailer, Crane, Press, Axle, Hitch, Wall, or any other load carrying system is “Strong Enough”?  Honestly, I believe all the different forms of  “. . . is this strong enough?”  are very important questions.  Before using it, that’s something we should know.

Then, the other side.  How many times have you heard . . . “Oh Yeah, that looks strong enough.”

How do we know?  Or, at what point are we confident?  Is confidence enough?

Thinking And Reason

Think about it for a minute.  Let’s go stand on a railroad bridge together.  Ah yes, here we are looking out over a beautiful canyon!  Jump up and down, and the bridge feels super solid.  Yup, it’s good enough.

Train Bridge
Amazing image from www.frrandp.com

Then a train comes.  It’s hauling huge cars of coal, heaping up over the edges.  Those locomotives are massive!  As it comes closer, we feel the bridge shaking under our feet.  Is it strong enough?  If it is, then the train will pass by and it’s all a moot point.  If it is lacking, even by one ounce in all those tons, you and I go plunging to our deaths.

The word “enough” implies something.  Put it together with “strong” and you get a significant meaning.  As in the example of the railway bridge, we don’t really want “enough”, we want more.  Preferably, a lot more!

So it is with mechanical things of all sorts.  In the above image of a crane failure, the crane actually did lift the machine — for a bit.  (Customer submission.)  At first, it was enough, but something changed that went over the line, and it was no longer enough.  Fortunately no one was hurt, but it’s truly impossible to know where “enough” is – just before “not enough”.  “Not Strong Enough” is pretty obvious, but it’s usually too late.  That’s why we recommend margin — often called Safety Factor.

Let’s look at some examples.

Go Beyond Strong Enough

There are a lot of things we talk about on this website where “Not Strong Enough” has some serious consequences.  Unfortunately, many of these are really hard to see, so it’s pretty easy to go over the line without realizing it.  With Trailers, Cranes and Presses, going over that line has the potential of changing your life forever — and not in a good way.

Here are several areas to think about, along with our recommendations.  While there is a difference, the cost is typically minimal.  The interesting part to me, you probably won’t ever know if a small upgrade just saved your bacon.  The added safety can easily be the difference between “Nothing” or “Disaster”.

Crane Beams

— Like in the image above, when Gantry Crane Beams fail, it can be sudden and catastrophic.

We Recommend:  See that your gantry crane rating is well over the lifting requirement.  Then, know the limits, and the load, before lifting.  That’s one reason commercial cranes almost always have the weight limit printed in big letters right there on it.  Do that for your DIY crane too.  These kinds of failures don’t need to happen.  Next, do the simple things to make your crane robust.  Things like good gussets and making the lines of force always appropriate.

Lifting Apparatus

— This includes all the hardware for lifting.  The Trolley, Cable, Chain, Hooks, Links, Straps or whatever else you use to lift.

We Recommend:  Make sure the lifting apparatus with everything is rated higher than the things you’re lifting.  Ideally, higher than the Crane.  The weakest part is the controlling factor, so if your hook is rated for 1 Ton, and your crane is made for 2 Tons, your limit is still only 1 Ton.  Remember also, that you can lower the actual limit by using things in ways they were not intended — like tying a knot in a strap, or using a hook sideways.

Press Beams

— Just like for Cranes above, please know the limits, then stay within them.

We Recommend:  Make the prime mover the weakest thing in the press.  For instance, if the hydraulic cylinder can do 20 Tons, make sure everything else with the press is rated for more than that, maybe a lot more than that.

Trailer Frame

— When building a trailer from plans, the weight rating is almost always given.  Follow the plans, and you’ll be good.  However, there are many times when we prefer to customize by changing the plans a little to better fit our needs.  That’s great, and we encourage customizing within these bounds.

— For bigger changes and for trailers where you don’t have plans, make sure to properly Calculate the Beam Loading.

Analysis with Trailer Frame Materials and Design With Safety Factors
Through Synthesis, we do the Full Engineering for our plans so you can build with Confidence.

We Recommend:  Know your trailer, and know your load.  Get a stronger trailer if needed.  Also, make sure the weight distribution is right — not too heavy in the front or back.

Trailer Hitch & Hardware

— Know your Hitch Class Ratings, then stay within them.  That means knowing your trailer weights, vehicle capacity, tongue weight, etc..

We Recommend:  This is often an easy place to make a step up for very little cost.  Go stronger than needed.

Safety Chains

— This is a topic we’ve covered in several articles.  They are a hassle, it’s true, yet there is no excuse for putting wimpy Safety Chains on.

We Recommend:  The cost and hassle for going a little stronger is negligible, so why not.  Make them more than “Strong Enough”, and attach them right.  Most likely you won’t need them, but the consequences are huge if you ever do.

Tires

— For trailers, the Tires need to be:  1) rated for a greater capacity than the axle;  2) in good shape (no dry rot, etc.).  For the most part, if the tires are not old, or pushed to the limit, you probably won’t need a spare tire.  (Of course, that does not include road hazards, or axle misalignments, etc.)

We Recommend:  Get tires with at least 10% extra capacity over the axle, then keep an eye on your tires for wear, damage, and sunlight cracking.

— We can say the same thing for using Casters around the shop, too.  A little extra capacity, and they will serve you well.

Bolts

— We have articles about bolts.  Start with Bolts 101 for a good overview.

Examples of Bolt Styles

We Recommend:  For DIY projects, use Grade 5 or better for any bolts that really matter.  When life or limb are involved, go with Grade 8 or better.  Or go with one size larger bolts than you first think.  For instance, use 3/8″ bolts instead of 5/16″ if  you wonder “is it strong enough?”

Aluminum

Building with Aluminum requires some extra care.  While the Aluminum is great for many things, it does require some added skill and some accommodations because it fatigues easier, and welds are not as easy to get right.

We Recommend:  If your aluminum welding project is subject to fatigue, make sure you have the experience before relying on it.  Even many professional welders won’t do aluminum because it requires expertise in getting it right.  It can be great when done well, so get help if needed.

Working With Standards

For many things it’s pretty easy to know if it’s strong enough just by looking at the standards.  Tires are a great example because they have load rating and speed rating standards that are pretty universal.  Just read the info and stay within the specs.

While that is generally true with Hitch Class Ratings too, sometimes parts don’t have the class.  However, if they are load critical parts, they always have a load rating.  That is also true with crane and press parts that you buy.  Things like the chain-fall, the trolley, or the press hydraulic cylinder, for instance.  In these cases the standard is the label with the load rating, and that’s how we know it’s strong enough.

Standards also exist for things like Trailer Axles.  While there are exceptions, trailer components generally fall into rough load categories, like 2000# capacity, 3500#, or 10,400# capacity to name just a few.  Trailers, and trailer plans also follow these standards because they follow the axles.  Hopefully, the plans you get are fully engineered like ours so they actually work at the standards.  Our job, when towing a trailer, is to know the capacity, and to stay within it.  That includes total trailer weight, tongue weight, and capacity of the tow vehicle.

I have to inject a note here.  It seems like standards should be standards, but they are not always treated that way.  When you buy a trailer off  a lot, for instance, you’d think it will function.  See this article about a fixing a bent tongue or this YouTube video about a bent tongue.  There are plenty of other failures too.  It’s sad that manufacturers don’t follow the standards or don’t understand the engineering.  Be aware.

Do The Engineering

Standards work well for parts we buy, but what about parts we make?  How do we know they are actually strong enough?

As a DIY’er, I have to admit I often just guess.  I know a lot about strength, so when I’m fabricating something small, I just over-kill it.  If it’s a critical part, or if I am not exactly sure if it’s strong enough, I do the engineering.

Both the experience to make an educated judgement, and the skill to do the engineering come with time.  By calculating beam loads over and over, and by breaking things by testing to be sure they are right, you gain that experience.  So if you don’t know, look up the info to find out.  Or, ask a super smart friend for help.

One of the easiest ways to be sure is to compare to things we know.  For example, you want to build a trailer, buy the plans that are a little bigger or a little stronger, then follow that lead.  I’ve had many people buy our 16′ trailer plans then make the trailer at 15′ using their own design, but following the beam sizes in our plans.  That will get them close enough to have confidence in the build.  Don’t go the other way — to 17′ for instance — because that’s uncharted territory.

Adjustments:

Now the biggest questions of all.  How do you adjust for time?  How do you compensate for unknowns?

Time?  Think about things that happen over time like fatigue, or rust, or wear.  Wood is the best example.  You build a fence and it’s awesome.  A few years later, a simple wind blows it down.  The change is degradation over time.  That is pronounced with examples like wood, but it’s also true in different ways with other materials.

Other unknowns are things like a bouncing or swinging load on a Gantry Crane.  If we know the dynamics, we can calculate, but all the ways a product can be abused are just not known.  Since we know there will be some, we compensate by adding margin to handle (some) extra loading that we can’t anticipate.

While there are calculations for fatigue, that is beyond the scope of this article.  We mention it here so it’s not forgotten.  Yes, there is always more to think about.

What Level Of “Strong Enough” Is Really Strong Enough?

The first step in building confidence in know something is strong enough, is to consider the consequences of not.  In Engineering we call this FMEA (Failure Mode Effects Analysis).  It’s basically asking the questions:  “What happens if it fails?” And, “What can make it fail?”  Effectively, what happens if it is NOT strong enough?

What is the item for?  Is life at stake?  How about limbs or serious injury?  Or will equipment or property damage be the outcome of failure?  Or, maybe there’s really not much to loose.

Thinking about what can make something fail is often tricky.  For a trailer, something simple like a big pot hole is too often forgotten.  Make sure you include all the normal possible situations in your calculations.

The amount of Margin (Safety Factor) involved depends on the consequences of failure.  The more that’s at risk, the more margin you need.  There is always a balance between “strong enough” and “safe” and “overkill”, so take some time and make sure you get it right.  The key here is don’t assume, and don’t just guess.

Good Luck With Your Projects!

Comments

0 Comments
Inline Feedbacks
View All Comments

We Found These For You . . .

Product
24 Foot Deck-Over Trailer

Great Trailer Plans for a very tough, 8.5' wide x 24' Long Deck-Over trailer. Plans have many options like 14,000 lbs or 16,000 lbs total capacity, ramp length and load rating, plus a lot more.

Product
6x14 Utility Trailer Plans

Get your 6x16 Utility Trailer Plans here, then build your own 7000 lb. capacity tandem axle workhorse.  These plans include all the details and instructions.  Build it, and make it better than you can buy it.

Article
Build it with Torsion Axle Trailer Plans
New Product Announcement:  The Mechanical Elements line of DIY Trailer Blueprints has just expanded with some new sizes for Torsion Axle Trailer Plans.  You ask, then we work to deliver.

Read The Article

Article
Drill Matching Bolt Holes
So many times we need to make matching holes to bolt something on.  You purchased something to mount on your trailer or your workbench or some other project, but now you need the exact

Read The Article

Product
Up to 50 Ton Hydraulic Press

Do-It-Yourself Shop Press Plans designed for super high capacity, like 20-50 Ton with a Hydraulic ram or Jack.  Choose these plans for times where perpendicularity and parallelism are important.

Article
Low & Lean Trailer Suspension
A unique trailer suspension offering a low profile, tuning possibilities, and more. By design, the concept offers complete tandem axle load sharing via a common coil spring which is a big plus. Though different than a torsion axle suspension, this…

Read The Article

Article
Trailer Plans Download @ Mechanical Elements
At Mechanical Elements, we encourage customers to modify and customize the plans.  That’s how you build the perfect project.  So, what things can you customize?  And what shouldn’t?

Read The Article

Article
Various Wheel Bolt Patterns
What is the bolt pattern on your trailer wheels?  For the most part it doesn’t matter because you’re not changing wheels that often, yet when it does matter, it really matters.  Here is some good info to measure and calculate

Read The Article

Article
Harbor Freight Improvement
You’ve seen them, the inexpensive bolt-together Harbor Freight trailer kits.  While they pack a lot of function, they unfortunately are often something like a rattle trap.  (Not always, of course.)  While they have a cheap rap, they sell a lot

Read The Article

Article
Trailer Wheel Bolts
A friend asked me this question, along with the companion “How Tight?”  The last thing we want is to lose a wheel — because it will not only delay our trip, but a wheel coming loose can do serious damage…

Read The Article