Gantry Crane Failure Modes

There are a lot of ways to abuse a crane.  We won’t go into those, but let’s do take a minute on crane failures and what they look like.  In general, there are 3 main gantry crane failure modes.  Let’s look at them, then discuss contributors and solutions to make your crane more robust.

What Is A Failure Mode?

A quick way to think about it is the difference between a “Failure” and a “Failure Mode”.  A Failure is anytime something breaks, or doesn’t carry out the job.  (For example, this welding strength failure.)  In contrast, a Failure Mode, is a way of categorizing failures based on what or how it fails.  For this discussion on Gantry Crane Failure Modes, we’ll think about three:

  1. Failure of the top beam;
  2. A Failure of the legs;
  3. Failure of the lifting apparatus(s).

We’ll also talk about Contributors and Solutions with each of the modes to avoid a crane catastrophe.

Gantry Crane Failure Mode:  Top Beam Bending

gantry crane failureThis is sometimes called top beam folding.  I found this image online at:  The Practical Machinist.  I don’t know much about it, but I think it’s an awesome image showing a DIY crane gone wrong.

The crane construction is a nice idea in some respects, and it probably serves well for light things, but this time it’s a bit too far.  Classic gantry crane failure.

Note how the top beam folded very tightly around the lift strap.  Also, how the bending of the top beam then over-stressed and bent the top portion of the vertical leg.  This is a perfect example of exceeding the top beam capacity.  I’m glad are no injuries.

Contributors and Solutions (Top Beam):

Things that contribute to this kind of failure include these.  Solutions to make it better are also in the discussion.

  1. Beam Load Limits.  Yes, this is the obvious point, the load exceeds the limits, but that IS the point.  Know your limits, and stay well under them.  For a DIY crane like this, you might have to calculate the limits, and if you can’t and don’t have a friend that can, then don’t build the crane.
  2. I-Beam Crane TrolleyPoint Loading.  When there is only on strap or chain around a top beam, there are (in engineering speak) stress risers at that point.  Simply spreading out the load — like using a trolley — reduces the stress at any one place.  As a DIY hack, just put a chunk of wood, a 2×6 or something, on top of the beam, under the strap.  It won’t compensate for overloading the beam, but it will spread the load significantly.
  3. I-Beam ProfileBeam Type.  Some beam types are just better for carrying horizontal loads.  Thin-ish wall tubes (like the orange ones in the image) hold a lot of weight — when the weight is spread out and the ends are stable (see below).  But, an I-Beam is much more suited.  Please read the article on beam shapes.
  4. Beam Size.  In simple terms, the taller the beam, the more it can carry.  If you double the vertical dimension, you basically 4x the load capacity of the top beam.  (Don’t quote that number, because it’s a generalization.)  That’s not really true for the legs, but in a horizontal bending condition, taller is better.  Read the article on increasing load capacity for more perspective.
  5. Proper Beam End Support.  We’ll talk about this more later, but it is definitely a contributor.

Gantry Crane Failure Mode:  Leg Failure

There are a two ways a crane leg will typically fail.  Usually for a gantry crane, the failure occurs because the top of the leg doesn’t stay directly over the bottom.  (Think straight up and down.)  And, that is evident in the failure photo above.  When something happens to wiggle the top beam or move it somehow — either by a swinging load, or someone trying to drag it on it’s casters, or some other condition — the load in the leg becomes less than axial, and often that leads to failure.

For example, in the crane failure photo above, the folding of the main top beam pulled the legs (both) inward.  The folding also put a huge bending moment at the leg tops.  The relief of all that stress came when the top of the one leg also failed (bent).

The second, much less frequent failure is column buckling of the legs.  That means the leg becomes over stressed and collapses.  Most times that’s a bowing out and buckling of the leg, but on rare occasions it’s a collapse like stepping on a soda can.

To be complete, a third type of leg failure is when the gantry is on a track, and the track supports fail.  A concrete wall, for instance, but we won’t go into that.

Contributors and Solutions (Legs):

Leg failures are less common than top beam gantry crane failures.  However, as noted above in #5 above, legs that are not well supporting contribute to the top beam folding.  Either way, it’s a gantry crane failure.  Remember the weakest link — if the legs are weaker than the top beam, they’ll fail first.

Here are some things that can cause this, along with solutions.

  1. Stiff Legs.  Truly, the top beam is up there high, and only the stiffness of the legs keep in place.  We frequently see legs that are braced perpendicular to the top beam, but not very often do we see them braced parallel to the main beam.  If you think about strong shapes, an open rectangle is not very strong except when the legs are perfectly vertical.  That gives emphasis to the need for stiff legs, and . . .
  2. Gantry Crane Example - Missing GussetsStiff Connections.  Related, to the above, beam connections are a critical area.  A stiff leg easily becomes much stiffer when gussets to the main beam are incorporated.  The gantry crane failure above as well as this crane shown (Cap 3 Tons) both illustrate the lack of a gusset.  Any side to side wobbling of the crane — even just pulling on one side to move it when loaded — can cause the joint to fail.
  3. High Forces.  Even when the materials are super strong, we can’t forget that these materials are under massive stress.  And, if it’s anywhere near the limit, just a little more in the wrong direction can bring it all tumbling down.  Youtube is full of examples.
  4. Crane is on sloping or uneven ground.  See Unsure Footing below.
  5. Related, When you build a crane, make it straight and true.  It’s like the Unsure Footing, but it’s not built straight and square, and it may never stand straight enough to carry the loads you intend.  When you build, take the time to do it right.
  6. Wheels.  This is a big issue, and it’s super common, so this point is in it’s own section below.
  7. Dynamic Loads.  Any load that moves around on the crane is potentially a contribution to failure.  This includes sliding loads, swinging loads, bouncing loads, attempts at moving a crane by pulling on one leg (thereby splaying the legs).  Swinging loads should be obvious.  Bouncing loads . . . the top beam is a very stiff spring.  If your load is “dropped” slightly — even if it’s just the popping movement of a chain or hook or cable  — can cause the load to bounce.  That puts extremely high forces on the system.

There’s a lot here to take in.  Hopefully I’m communicating that it’s a bunch of little things to keep the crane standing up.  It’s important to note that many of these compound to weaken a crane.  For instance, missing upper gussets, wheels, and a swinging load.  (It’s like the photo above with a slightly swinging load, and it all comes crashing down.)

Unsure Footing

I’m not sure how to label this one, but basically it means things are not straight and true.  For instance, using a gantry crane on a sloped driveway — the loads don’t go straight down through the legs.  The legs then carry a bending load too, so the total vertical load capacity becomes drastically less.  Another example is when the feet are not sitting parallel, creating a twist on the top beam.  How you place the crane matters.

Another aspect of sure footing is the security of the leg position.  To that end, we have a whole article section below about wheels.

Wheels:  Why We Love Them, &  Why We Must Respect Them.

Nearly all Gantry Cranes have some type of mobility.  On smaller cranes wheels are nearly universal, and why not, it’s so much better for crane positioning and/or moving the load.  However, they drastically decrease the load holding capacity of the vertical legs.  No, not because of the wheel capacity, but because they don’t give a stable platform for the crane.

The Basic Crane Leg

Gantry Crane Leg With No CastersThink about the crane leg setting flat on the ground.  Think of it as the “T”, upside down with two large gussets supporting it.  That’s typical of a small gantry.  See the image.

Arrows representing vectors in these images show the major forces.  Red arrows show force down through the leg from the top beam.  Purple arrows show the forces supporting the crane (from the ground).

The legs in this first simple case will not roll or slip or otherwise move when a significant load is on it.  It also directs the vertical loads coming down the leg directly into the ground.  Because the bottom is flat, it’s perfectly happy standing there straight up.

Crane Legs With Wheels

Gantry Crane Leg Forces With Casters on the BottomCompare that to the same crane leg above, but with casters added.  Forces are basically the same, except they also pass through the added wheels.  Of course, these wheels are plenty strong, that’s not the issue.

Wheels naturally want to roll, so only a little force will cause them to do it.  The load coming directly down the leg is NOT straight into the ground — it’s offset by the caster distance.  (See the side view.)  That means the leg naturally wants to tip over.  More explicitly, it wants to start tipping then roll out and fall.  It’s only because of the joint strength at the TOP (where the leg connects to the gantry beam) that the legs stay vertical.  These legs definitely will not naturally stand up on their own.

The Engineering

In engineering speak, we refer to legs with wheels as having additional degrees of freedom.  The leg is NOT solidly planted, so movement must be accounted for.

Having wheels on the legs allows the legs to more easily move or bend, which puts a lot more stress at the top beam joint.  If you must use casters, make sure you overdesign the legs and especially the joints, to take lots of additional stress.  Even if you orient the wheels so the leg forces are planar, if you move the crane, they will caster.  If you won’t be moving the crane when loaded, why have the wheels?

Spring Lifted Foot for Safety

We like wheels, so, our solution is to put springs between the legs and wheels such that the springs compress and the legs stand firmly on the ground under load.  This solution gives roll-around freedom when empty (or lightly loaded), yet forces safety when loaded.  See the photo.  This approach is simple, and solves the safety problems.  (Blueprints for these are in all our Gantry Crane Plans.)

Gantry Crane Failure Mode:  Auxiliary Equipment

We often think of the Gantry Crane by itself, the legs and the overhead cross beam.  Those by themselves don’t often fail — unless there is something attached — the load — which is held by auxiliary equipment.  The lifting equipment is the 3rd gantry crane failure mode.

We won’t spend a lot of time here, but there are trolleys, straps, chains, cables, hooks, winches, gears, levers, come-a-longs, pulleys and a host of other things that can lift the load.  How they connect to the crane is crucial (as mentioned above), but also their own capacity and the interaction of you with these devices.  Make sure auxiliary equipment is rated for the loads, and make sure you know the weakest link.

One other thought about the auxiliary lifting equipment is your situation interacting with it.  For instance, the use of a come-a-long requires pulling on a lever which almost always causes the load to move.  Dynamics are discussed a little above, but keep in mind crane capacity goes down with movement.  That’s one part.

The other part is where you are for lifting.  If you have to climb up on something (come-a-long), or be right under the crane (chain fall) to operate it, then it’s much harder to see what’s happening with the crane in order to avoid issues before they happen.  Personally I prefer something like the Winching Pole to get away from the load during lift, and keep an eye on things generally.

The Gantry Crane Failure Perception

If you think some of these gantry crane failure modes are weird, please think about stressing things near their limit.  With huge forces already, it doesn’t take much to push them over the edge.  Unfortunately, they don’t usually give much warning.

The best practice is always to respect the crane and other lifting equipment.  By respect, I mean don’t push it to the limits, and follow the practical safety measures.  Be Smart, because in a fraction of a second you may destroy your work at a minimum, or totally change your life at the worst.  It’s just not worth it.

Disaster strikes in seconds, like in this gantry crane accident.

The Compounding Effects

It is always interesting to me as an engineer as I consult with people about failures.  Forensics, as we call it, is really figuring out what happened and why things die (fail).  Not very often is it just one thing.  Normally it’s a series of things that create the final problem.

Looking at the small crane image above (the one that says Cap 3 Tons).  I can see this one ready for problems just because there are several small things biasing a reduced capacity.  First, the wheels make it so the legs are not secure at the bottom.  Second, the beam is pretty high.  Longer legs = lower capacity.  Third, the top joints where the gantry beam connects to the legs is not very solid.  There is not a gusset there to stiffen that joint.

A ScenarioSo, the crane is placed and a full 6000 lb load is lifted with the chain fall. Then one of the guys grabs a leg and pulls on it hard to the side to move the crane.  That introduces motion, but the inertia of the load doesn’t immediately move.  In a second, the load starts to swing on the crane, which makes it easier for the guy to pull the leg (while the load is swinging his direction), so he pulls more.  The load then starts swinging in the opposite direction, with the guy still pulling on the leg, and suddenly the legs splay and it all comes crashing down.  There are no serious injuries, but the the splaying leg undercuts the guy pulling the crane.

It’s not just one thing, it’s a combination of things.

Ways to Counteract Failure

  • First, make sure the design of the crane far exceeds the max load.  The max load should be a fraction of the true calculated max.
  • Second, make the joints rigid with gussets or other mechanics.
  • Third, set the crane on the ground when lifting.  Use other, safer methods to move the load around.  Things like a cart are much safer.
  • Fourth, if you must move a loaded crane with wheels, connect the feet with a rigid bar.  It can clamp on, but this will avoid having the legs splay.
  • Fifth, do your absolute best to avoid load dynamics like bouncing or swinging.

The gantry crane is a system, and gantry crane failure is never out of the question.  Just be smart, and the likelihood of you being an eye witness of such a failure goes way down.

If you’re interested, here are 4 Pressing Issues along the same lines.

Good Luck With All Your Lifting

No one has share their thoughts...be the first!

No one has share their thoughts...be the first!

Leave a Comment

We Found These For You . . .

Product
6x16 Utility Trailer Plans

DIY projects designed for you.  These plans show how to construct a 6 ft x 14 ft tandem axle utility trailer with 7000 lb total capacity.  Use the trailer for work or to haul your big toys.

Product

Walking-Beam style trailer suspension plans are engineered for mid-size trailers.  The design combines benefits of torsion axles with the excellent tandem load sharing of twin pivot beams.  2000 - 4400 lbs Capacity.

Article
Trailer Lights and Wires Feature
On new trailer builds, and on existing rigs, the trailer lights and wires must all function.  Here are some tips on the subject so if you find the electrical a little confusing or intimidating, read on.

Read The Article

Product
6x11, 2 Place, ATV Trailer Plans

When you need full space for 2 large ATV’s, or other off-road vehicles, this is your trailer.  Plans give options for ramps, tie-downs, and other functions.  Build this for robust action to last through the years.

Article
Finishing or Trailer Frame Painting
(And finishing other DIY Projects.) Now you’ve invested all that time and effort into building an awesome trailer, how should you finish it?   Of course we don’t want it to rust.  And, we want it to last. 

Read The Article

Article
Aluminum Trailer Frame Underside
Aluminum is such a great material.  It’s light, strong, and looks sweet to boot.  So, why is there a debate about Aluminum Trailer Frames comparing to others like steel?  Or comparing for beams like with gantry cranes?

Read The Article

Article
Folding Trailer Tongue Finished
Intrigued by the cool potential of saving storage space, increased security, and removal of a shin knocker to step over, this new folding trailer tongue adds greater utility to a trailer.

Read The Article

Article
Replace Trailer Tires
Don’t mess around with your safety or unexpected trip interruptions.  The Best Reason to replace old or worn trailer tires is personal sanity, because who wants to deal with it on the side of the road?

Read The Article

Article
Steel for the Next DIY Build
This is what the beginning of a new DIY build looks like.  Of course, there’s the planning stages, the design stages and finally the acquisition stages where all the raw materials begin to come in.  With all of that behind…

Read The Article

Article
What Tires For My Trailer?
There are a million tires out there with a ton of classifications and designations.  What tires do I choose for my trailer?  That is not a silly question at all.

Read The Article

Article
Understanding Bolt Choice
This is Page 2, continuing the Bolts 101 article.  Here we discuss choices for bolts for an application.  The previous post, Page 1 of Bolts 101, gives a ton of background info, so we recommend reading that first,

Read The Article

Product
6 x 12 Utility Trailer Plans

Blueprints for a Strong and Stable 6x12 utility trailer.  This single axle, 3500# capacity trailer with many options is ideal for a million uses.  We provide great DIY Plans for builders who want the best.

Article
Secure and Flat Trailer Frame Setup
Just how do you setup for building a trailer frame?  Seems simple enough, just layout all the frame members and start welding, Right?  Well, mostly, but there are some additional steps that will help it turn out better.

Read The Article

Product
5x10.5, 2 Place, ATV Trailer Plans

Our smallest ATV and off road vehicle specific trailer plans.  Great for taking 2 ATV’s out, yet small enough for towing without a big truck.  The size also makes for easier storage between the fun trips.

Article
Overslung or Underslung Trailer Springs
Because you asked . . . What are the advantages of Overslung – or – Underslung trailer springs?  Let’s discuss it.  The concepts are pretty easy to see with some good graphics.

Read The Article

Article
Nuts and Bolts Project Fasteners
Ever wonder why fasteners like nuts and bolts are so expensive?  You would think something so standard and manufactured in such high volumes would be cheap, but in most places, they’re actually pretty expensive.  Why?

Read The Article

Article
Custom Changes To The Plans
I can’t find quite the right plans for the trailer I want to build.  Can I make changes to the plans at Mechanical Elements to fit my needs?

Read The Article

Article
Analysis with Trailer Frame Materials and Design With Safety Factors
This post is a question from an engineering student wondering about trailer frame materials selection and safety factor.  His analysis shows a material thickness that “does the job” is much less than his welder associates recommend.  Wondering about why, he…

Read The Article

Article
How Much Will It Cost To Build?
Before starting a project we always want to know — or at least have some idea — how much will it cost to build?  If you’re like me, resources are limited, so knowing how much you’re going to sink into…

Read The Article

Article
Trailer Sway Video Review
Understanding Trailer Dynamics like Sway is a really important.  If you look for it, there are many articles and some great YouTube videos discussing and demonstrating the many factors and issues.

Read The Article

Leave a Comment