Welded Gantries and Robot Pedestals
A gantry or a robot pedestal is bought to hold a position at the end of a reach. Everything about how it is specified follows from that, and almost none of it is about strength.
These structures rarely fail by breaking. They fail by deflecting, by twisting under a moving load, or by having a mounting face that was never as flat as the drawing implied.
What this application demands
- Deflection, not strengthThe design driver is stiffness under a load that moves. Going to a higher strength steel buys nothing here, because all structural steels share essentially the same elastic modulus. What works is section depth, closed box sections and bracing placed where the bending actually is.
- PerpendicularityAn upright out of square at the base is an error multiplied by the height. On a gantry this is usually the tolerance that actually matters and it is measured from a machined datum on the base, not from a welded edge.
- Mounting face flatnessA robot bolted to a face that is not flat is pre-loaded into a twist before it moves. The face is machined after stress relief and the flatness callout says so.
- Dynamic behaviourA structure that is stiff enough statically can still ring at the frequency the motion profile excites. If the application is fast and repetitive, the natural frequency is a design input and not an afterthought.
- Access for machiningTall welded structures are awkward to get onto a machine. Envelope, weight and crane capacity rule shops out faster than anything else on this part type.
What goes wrong
In roughly the order we see it on drawings.
Specifying strength when the problem is stiffness
Thinner high strength plate deflects more, not less. It is the classic wrong move on this part.
Datum on the welded base edge
Perpendicularity measured from a surface that moved during welding is a number nobody can hold or verify.
Open sections where a box was needed
Open sections are weak in torsion. A gantry that twists under an off centre load is usually an open section problem, not a material one.
Ignoring the machining envelope at design time
A 5 m upright that no local shop can machine in one setup is a design decision made without checking, and it is expensive to discover at quote stage.
What to put on the drawing
Every line here removes an assumption a shop would otherwise price as risk. Risk is always priced against you.
The full package is on the RFQ checklist, which is printable and remembers your ticks in your own browser.
- Deflection limit at the working point, stated as the actual requirement rather than a plate thickness.
- Perpendicularity of uprights referenced to a machined datum on the base.
- Flatness of every mounting face, with the condition after stress relief and finish machining.
- Positional tolerance on the robot or carriage mounting pattern.
- Natural frequency requirement where the motion is fast and repetitive.
- Overall envelope and finished weight, so shops can check the part fits their machine and crane.
Questions on this part type
Why does a higher strength steel not help a gantry?
Because the part is limited by deflection, and deflection depends on the elastic modulus and the section geometry. Every structural steel has essentially the same modulus, about 200 GPa, while yield strength varies by a factor of three. Higher strength lets you go thinner, and thinner deflects more.
Open or closed sections?
Closed box sections are far stiffer in torsion, which is usually what a gantry needs. Open sections are easier to weld and easier to access for machining, so the trade-off is real and it belongs in the design rather than in the quote.
How flat does a robot mounting face need to be?
Tight enough that the robot base is not pre-loaded into a twist when bolted down. The robot maker publishes a requirement, and that figure is what goes on the drawing with the condition attached.
Go deeper
- A36, S355 or A514: What the Grade Does to Your Weldment PriceHow structural steel grade changes welding, preheat, distortion and machining on a weldment, and when the higher strength plate is worth paying for.
- Why One Setup Is Cheaper Than Three on a Two Tonne FrameHow machine envelope, crane capacity and the number of setups drive the machining cost and the tolerance risk on a large weldment, and how to design for fewer setups.
- Putting Datums and Geometric Tolerances on a Weldment That MovesHow to apply datums, flatness, position and parallelism to a machined weldment, why welded and machined features need separate tolerance systems and where ISO 13920 fits.
- When a Welded Frame Needs Stress Relief, and When It Does NotThermal and vibratory stress relief for weldments: when residual stress will move your machined faces, what a PWHT cycle involves and where it belongs in the sequence.
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