Robot Structural Hardware, Explained Simply

The metal parts that hold a robot together - what they do, why small errors add up, and what to ask for when you order them.

What "structural hardware" means

Every robot is a set of moving joints connected by strong metal parts. Those parts have a plain job: hold everything in the right place while the robot moves, lifts and stops.

The common ones:

Joint housing - the block that holds a motor, a gearbox and a bearing together.

Motor mount - the plate that bolts the motor onto the structure.

Bearing seat or flange - the ring that keeps a shaft spinning in the right place.

Arm link - the beam between two joints.

Base bracket - the part that fixes the robot to the floor or the machine.

They look simple. Making them is not, because a robot's accuracy depends on them more than most people expect.

Why the joint matters more than the motor

Imagine a door with a slightly loose hinge. The door still opens, but the far edge moves a lot. A robot works the same way: a tiny error at one joint becomes a large error at the tool at the end of the arm.

So when a robot is inaccurate, the cause is often not the software or the motor. It is a housing that sits at a very slight angle, or a face that is not quite flat.

Small errors add up

Here is the part that surprises buyers. Four parts can each be "within tolerance" and still not fit together well.

Picture stacking four books. Each one is slightly rotated. By the fourth book, the corner has moved a long way.

A joint works like that. The error comes from four places:

1. The hole is slightly off position.

2. The hole is not perfectly square to the face it sits on.

3. The face is not perfectly flat.

4. The bearing fits a little loosely, or a little tightly.

None of these is a mistake on its own. Together, they decide whether the shaft runs true. This is why one or two features on the drawing deserve really tight limits, while the rest can stay normal.

Flat faces: why it matters

Two metal faces only touch where they are flat. If a housing is slightly bowed, it touches at the edges and leaves a gap in the middle.

What happens next is predictable: the joint flexes under load, vibrates in use, and the bolts slowly lose their grip.

Simple fixes at the design stage:

Say how flat the mounting face must be, not just how thick.

Machine both faces in the same setup if possible.

Remember that anodising adds a thin layer, which can change a fit.

Bearing seats: the hardest part of the job

A bearing is a ring that lets a shaft spin smoothly. It needs a seat that is round and straight.

Two problems are common:

Out of round. If a thin ring is clamped too hard in a chuck, it becomes slightly triangular. The bearing then sits unevenly.

Not lined up. If a shaft runs in two bearings, the two seats must point the same way. Making both in one clamping is the reliable way. Doing them in two separate setups is where most of the error comes from.

There is also temperature. A fit that is tight at room temperature can become loose when the joint warms up. For fast joints, ask the bearing supplier what fit they expect.

Lighter, but not weaker

Robot arms are usually hollowed out to save weight. Less weight at the end of the arm means less torque needed at every joint behind it.

But cutting material carelessly makes parts bend and vibrate.

Put pockets where the load is low, not where it looks good.

Keep walls and ribs following the load path.

Cut roughly first, then finish, so the part does not spring when the stress is released.

Keep the design roughly symmetrical, or it will warp.

Materials, in one line each

A common answer is aluminium for the body, steel inserts where screws go in and out.

Assembly habits that keep a joint tight

Use dowels for position, bolts for clamping. Bolts alone cannot hold a precise position.

Give threads enough depth in aluminium, or use an insert.

Separate different metals to avoid corrosion in humid air.

Write down the tightening force. "Tight by feel" loosens under vibration.

Try assembling before shipping. A part can measure perfectly and still not fit the part made somewhere else.

How we check the part

Checking every single dimension is slow and expensive, and it does not answer the real question: will it work in the assembly?

So we check what matters:

The important interfaces, using the same reference points as the drawing.

Flatness and squareness, not only size.

Fit with the mating part, or with a gauge that copies it.

A first-article report that shows how the part was measured, so the customer can repeat it.

What to send with your enquiry

Send the 3D model, the 2D drawing with tolerances and surface finish, the material, the surface treatment, the quantity and expected yearly use, the mating parts or assembly drawing, and your acceptance rules.

And if a joint has a critical interface, tell us which one. That single piece of information changes the machining plan more than anything else.

About the manufacturer

Shenzhen Strongd Model Technology Co., Ltd. makes robot structural hardware - joint housings, mounts, flanges and links - as well as precision parts for automotive, medical and consumer electronics customers. The factory in Shenzhen covers 40,000 m² and runs 85 CNC machining centres, 18 of them five-axis, with Hexagon CMM inspection and ISO 9001:2015 / IATF 16949:2016 certification. Robotics makes up 48% of revenue.

Website: https://www.strongdcnc.com/