I-Beam Trolley Design For Gantry Cranes
– Learning With Engineering

What do you know about an I-Beam trolley?  They are usually a pretty simple device with 4 wheels and a hook bar.  There is not a lot to them – right?

I have had a few.  Mostly they are good, but maybe a few frustrating things?  So the question, if I design one for myself, can I improve the design enough to make it worth the effort?

This article is a little more personal than most, because it talks about my path in design.  It talks about what I set out to accomplish, then explains the engineering – and the learning – then the resulting trolley plans.  The real focus is on the lessons learned while designing a safer, stronger, and tighter DIY I-Beam trolley for a gantry crane.

I-Beam Trolley Design Goals

A new project always has a seed.  My motivating factors come from lifting things with a gantry crane in my shop.  Some goals are from the situation in my shop, trying to do better in the space I have.  If I had a larger shop, a taller shop, then some of these would not matter as much.

Even with that, many off-the-shelf I-Beam trolleys are designed for minimum cost, not for maximum performance, and I definitely want to flip that.

So, read on.  If your needs are different, then maybe your goals are different.  Yet, there is value in learning.  Here are the I-Beam trolley design goals.

1. Headroom Under The Beam

This is the space under the beam to lift something.  If the gantry crane has 8 ft clear under the beam, you can’t lift something 8 ft.  We must have an attaching device, then a lifting device, then hooks and rigging.  If we choose a chain fall, for instance, we may end up with 2 feet of headroom under the beam consumed before we even start lifting.

In this example, it leaves 6 ft to lift.  If our item is 3 ft tall, we might not lift high enough for a truck to pull under it.  To me, a short ceiling shop is often a frustration.

My crane is the Perfect Size Garage Crane from the plans we sell.  It is awesome.  Outside, the beam will go up to clear 10 ft.  In my shop the ceiling is 9 ft, which means I get 8 ft clearance under the crane.

First Goal:  Minimize trolley headroom required under the I-Beam.

2.  Real Strength and Safety

Around the shop we often joke “Yup, Safety 3rd.”  Of course, it is joking.  If it was true, I probably would not have all my body parts intact.  Yet, I like it when safety is so easy that we don’t have to think about it first.  Design it in, like these items of practical crane safety.

That also goes for things I design.  Make them right, with proper safety margins, then we don’t have to worry.

In engineering, “Safety Factors” are a way to compensate for discrepancies we cannot see, and for load variations we do not anticipate.  A safety factor of 4 means the part will theoretically hold 4 times the stated load.  In other words, a 3-ton lift will theoretically hold 24,000 lbs and not bend.  (6,000 lbs [3 tons] * 4 = 24,000 lbs.)

I use the example, because a safety factor of 4 is generally considered OK “for lifting”.  Assuming the material is ductile – meaning it will bend, well before it will fracture.  But, that is getting into the weeds.

We use a safety factor from Yield Strength, which is the point where something will bend permanently.  For us, when something bends, that is a failure, even if it does not crash to the ground.

In comparison, some people use a safety factor from breaking strength (Ultimate Strength).  This alternate interpretation gives a weaker part.  So, if our lifting loop looks big compared to similar parts on Amazon, that can be a difference.  Another difference may be a safety factor of 3 instead of 4, or maybe some vendors use stronger materials.

At Mechanical Elements we go by the first definition especially with items critical to safety, like an I-Beam trolley.

Second Goal:  The I-Beam trolley design must have full strength, with a proper safety factor.

3.  Load Spread

Wheels on most I-Beam trolleys are fairly close.  Some have the wheels as close as they can.  Why?  Often they specify the radius a trolley can turn.  (For an I-Beam that bends sideways, to go around a corner.)

The downside of having wheels close is concentration of load.  Since an I-Beam trolley on a straight gantry crane does not need to turn corners, then we can separate the wheels to spread the load along the I-Beam.

The animated image shows the effect on the I-Beam for close wheels compared to spreading the wheels.  A notable difference is the combined deflection – which gives more resistance to trolley rolling under heavy load.

There is definitely an effect, but it is not as pronounced as initially expected.

While separation is directionally correct, it comes at a cost.  The trolley side plates get larger, but it is more than width.  The load path of forces makes a wider V, which complicates the stress profile.  The simple solution is a thicker plate.  Another option is stronger material.

In the end, spreading the load of the trolley makes a difference to the I-Beam.  While it is a small-ish difference, it is directionally correct for making the trolley roll easier along the I-Beam.  The trade-off is higher cost and a little more weight.

Third Goal:  Spread the trolley wheels.

4.  An I-Beam Trolley With Mostly Off-The-Shelf Parts.

Have you ever wanted to change something on a tool – only to find out they don’t make the part anymore?  Or, to find it is a cheap foreign tool, and there is no way to get parts?  Yes, me too.  I don’t prefer a disposable society where we throw things away if something goes wrong.

Fourth Goal:  For a new I-Beam trolley, make it easy to modify or repair.

Things change.  I had a 5″ I-Beam and trolley for years.  When I sold the crane to build an 8″ I-Beam upgrade, the trolley did not fit.  Bummer for me, so I gave it to the person who bought the crane.  I then got a new trolley.

The next one does not work well, so instead of buying yet another, I will build a better one.  The new I-Beam trolley design will use off-the-shelf parts as much as practical.  If a wheel goes bad, buy another one.  If I need it wider, change the shaft (which is a long bolt).  When I need a smaller shaft (lower strength for a smaller hook), change the bolt and spacers.

While the new trolley does have some special parts, they are main parts that will move with the width, and can space with adjustments.  Design for easy maintenance is not hard, but we see it too infrequently – IMHO.

Fifth Goal:  The design must be straightforward to build DIY and use off-the-shelf parts where practical.

Engineering For Capacity

One of the first things to know when designing an I-Beam trolley:  How much weight will it hold?

This key specification drives everything in the design.  Looking again at the goals, one is strength and safety.  So, this trolley must support the full named load.  A 3-ton trolley must hold 6000 lbs all day without ever a complaint.  Likewise a 2 ton trolley with 4000 lbs.

Because we are talking about DIY plans, we can’t say they are “For Lifting”.  There are too many things we cannot control – like the actual material choices, actual parts purchased, etc..  If you use parts or materials that are not to spec, then the safety margin is gone, and “For Lifting” becomes questionable.

Loading For An I-Beam Trolley?

The design of the I-Beam trolley is pretty simple.  2 wheels fastened to a plate.  One plate and wheel assembly on each side of the I-Beam, connected by a single main bolt in the center.  The sets of 2 wheels will equalize to the beam by pivoting slightly around the main bolt.  This concept is well proven in industry with many similar trolleys.  We will do it a little different, but the concept is the same.

But, how thick should the plate be?  My initial guess was wrong, and not by a little.

In analysis of the first design, I saw some really high stress!  It surprised me so much that I had to go back and check my work.  After validation, the analysis still says the first choice will fail.  Ouch!

Make it thicker.  The initial design is the 3-ton trolley.  That means 6000 lbs – 3000 lbs each side.  The forces apply at the wheels, then to the ends of the short shafts.  Because the trolley wheels cantilever from the plate, the forces create bending stress in the plate where the bolts attach.

So, we have the vertical forces to support the load, and also bending forces.  Fortunately, our fun FEA tools allow us to visualize the stress distribution.

Stress Distribution In The I-Beam Trolley Side Plate

Please see the animated image of the side plate.  There are a few things here to learn.

1. Note the V shape stress pattern.  Of course, it shows how loads carry from the wheels to the main center bolt.  It is as predicted.
2. Note the stress immediately under the wheels, and over the center bolt.  (Pink areas that stay.)  These are primarily vertical, and define the tips of the V pattern.  Yet, they are primarily vertical, and stay vertical even as stress in the V changes.
3. Changes in stress are big with changes in shape at first, but diminishing returns set in quickly.
4. Stresses make the plate want to deflect.  The image does not show deflection, but it wants to bend perpendicular to the arms of the V, which want to misalign the wheels.
5. Also not shown, a wider stance creates more stress, and worse wheel misalignment.
6. Thicker material reduces both stress and deflection, which in turn reduces wheel misalignment.

These are all interesting points, but the real takeaways are:

1. Shape of the plate matters.
2. Width of the trolley matters.  (It is a trade.  Spreading the wheels makes less stress in the I-Beam (see above), but trades for more stress in the trolley plates.)
3. Material thickness matters, not just for stress reduction, but also for deflection and alignment.  Deflection is something that simply going to super high strength steel will not solve.

The biggest surprise in design is how thick the plates must be to function well for lifting.

The Effect Of Safety Margins

When we design for a safety margin (also called safety factor), we are actually making it extra strong.  The extra strength ensures margin against the unexpected.

Look at the I-Beam trolley roller bolts for example.  The roller is on a shaft, which is a bolt, going through the trolley side plate.  The bearing sleeve on one side, and the nut on the other side contact the plate.

The roller applies a vertical force (red arrow F, roller not shown), which translates to forces above and below the hole, opposites, by the sleeve and the nut.  (Blue Arrows C.)  These forces create bending stress in the plate.

Let’s say the stress caused by arrows C is 20,000 psi.  We look at materials and say yes, a standard steel like A36 (36,000 psi yield) will handle 20,000 psi.  Great.  Oh, but if we define a safety factor of 4, then we need a material to handle 4 times 20,000 psi = 80,000 psi.

Stress goes up quickly with thinner material.  Thicker material separates the forces and decreases stress, in a diminishing effect.  A thicker washer and larger bearing sleeve also reduce stress – again as a diminishing effect.  In the end, this stress becomes our limiting factor.

These are nice, round numbers for discussion purposes only, but they highlight the point.

Again, “Safety Factor” is a way to compensate for things we cannot control.  Everything should have an appropriate safety factor.  A safety factor of 4 is commonly accepted for lifting, so we follow it.  Especially when lifting, it is better to be extra safe.  And the only real challenge is the need for higher strength materials – which are available.

(If you are wondering, the safety factor for lifting people is even higher, like 6.)

The Effect Of A Wide Trolley

We spoke above about the goals in separating the rolling wheels of the trolley.  That does make the trolley wide, which helps the I-Beam (though not as much as expected).  The take-away is the deflection under the wheels which make the trolley harder to roll.  Close wheels more than separated wheels.

Since aluminum deflects a lot more under the same load than steel, and since mine is an aluminum I-Beam, I want the trolley width.

One of the negative outcomes of the wide trolley is the wide V of stress (see the animated graphic above).  Why is this a problem?  The material bending wants to follow perpendicular to the V legs.  As you can imagine, that also wants to skew the wheels so they are not in the same plane.  It is like adding a little steering to each wheel – one turns in and one turns out a little.

All steels, from the low strength  to the high strength have roughly the same Modulus of Elasticity, E.  That means for the same forces, both the weak and strong steels will deflect about the same amount.  True, the weak steel will permanently bend or break sooner, but up to that point, both will deflect roughly the same.

That means our high strength I-Beam trolley plates will hold more, but still have the same deflection to deal with.  The only real answer for less deflection is increased stiffness – which means ribs, added structure, or thicker material.

Thicker material is not only a way of decreasing stress, it also makes things stiffer.  (We know this empirically, but it is worth spelling it out because it is the solution to both material problems.)  Also, thicker material does not have to be the super high strength steel, which solves a 3rd problem because lower strength steel is less expensive.

The New I-Beam Trolley Plans

If you want to build a trolley designed with these principles, see the I-Beam Trolley Plans.  Final designs of the I-Beam trolley include both a 3-ton and 2-ton version with plans available.

The path to the final design was certainly not as straightforward as anticipated.  While the design looks simple enough, there is a lot in the design that does not show.  Here are a few samples.

Design Areas Of Exploration:

1. Ribs.  – The practical side of rib placement is not clean when the canvas is small and large bolts dominate.  Required wrench space limits placement, and the need to weld ribs to the plate seriously limits material choices.  (Most high strength steels do not weld well.)
2. Thickening.  – Making side plates ever thicker has diminishing returns for strength.  It also affects trolley weight.  – A heavy trolley is not bad, but we want it manageable for DIY.
3. Shape.  – This is quite fascinating.  Some shape changes make a big difference, then some unexpectedly small.  Many shapes were explored.
4. Multi-layer.  – As a way of adding thickness only where it is needed most, we explored laminations.  It works nice, but the practical side of joining layers gets in the way – again, because of welding high strength steel.
5. Wider / Narrower.  – We like the idea of a wide stance (described above), but going wider increases both deflection and stress.  We wanted it wider, but the designs we settled on seem a nice balance.
6. Material Strength.  – Using high strength material is ultimately the path to better margins.  There is no substitute for thickness up to a point, but material strength is required, no matter.  There is certainly a balance for strength and thickness, so our plans give choices for thickness and define the required strength for each thickness choice.
7. Lightening Holes.  – To compensate for thick material, we experimented with various hole shapes to lighten the plates.  All of these increase stress (because they interrupt the surfaces).  In the end, lightening holes were nixed to reduce stress.

Settling On The I-Beam Trolley Designs

In design there is always a balance of competing needs.  See the Triangle of Achievable Engineering.  For this gantry crane trolley, we want function first, with high quality.  That means it will not be cheap.  Cost is always a factor, but for this design we decided sacrificing a little on cost so the trolley meets the goals is worth it.

The goals for the project are all functional.  So, function first.

1. Minimize the consumed headroom under the beam.
2. A trolley to carry a true, full 3 tons.  (2-tons for the second design.)
3. Wheels spread farther for better rolling and to better distribute load on the I-Beam.
4. An I-Beam trolley you can build with mostly off-the-shelf parts.
5. The design must be straightforward to build DIY.
6. Easy adjustment for various I-Beam types and sizes.

Then, as a matter of philosophy, if I invest the effort to build it, then it needs to be good, and last a long time.  So, quality is the second priority.

What did we learn?  Key engineering lessons from designing a DIY I-Beam trolley are these:

1. Plate thickness is super important to control both stress and deflection.
2. High strength steel is required as a trade-off so thickness does not become excessive.
3. Wider wheel spacing helps the beam, but it also increases plate stress.
4. A safety factor in line with “For Lifting” standards drives many aspects of the design.

Anyway, I did the design originally for me.  Now the plans are also available for you.  If a quality I-Beam trolley is on your list of things to build, try these plans.

In a future article (and maybe a video), we will show the first trolley build.

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