Robot Arm Design with FEA: How to Get the Stiffness, Payload and Accuracy You Need

When the tool tip ends up in the wrong place, the cause is usually a combination of small effects that add up along the arm:

• Link deflection: each link bends and twists under the payload, its own weight and the forces from motion.
• Joint compliance: gearboxes, bearings and joint housings are often more flexible than the links themselves, especially compact reducers under high torque.
• Thermal growth: motors and gearboxes warm up during operation and the structure grows with them.
• Backlash and play in the drive train.

FEA models the links and housings in detail and includes the joint stiffness from the gearbox and bearing datasheets, so you can see which part of the arm contributes most to the error and fix that first.

The worst case is usually the arm fully stretched out, carrying its maximum payload while braking hard. Checking only the nominal pose can hide the real problem.

• Loads: payload, the weight of the arm itself and the inertia forces from acceleration and deceleration, taken from the motion profile.
• Sections: hollow, closed sections give the most stiffness for their weight, in both bending and torsion.
• Materials: aluminium, steel and carbon fibre each trade weight, stiffness and cost differently, and FEA shows which one actually meets the deflection target.
• Weight: every gram saved near the tool tip reduces the torque needed at every joint behind it.

We took this approach in our structural analysis of a 6-DOF robotic drilling arm and its joints, where the whole arm and its bolted joints were checked together under the drilling loads.

Loads in a robot arm concentrate at the joints, so that is where stress, wear and loss of stiffness usually start.

• Joint housings and bearing seats need to be stiff enough to keep the bearings properly aligned under load.
• Bolted flanges between links have to stay clamped: FEA with contact shows whether the joint opens or slips under the worst load.
• Cable routing holes and cut-outs near joints create stress concentrations that are easy to miss without a detailed model.

In many applications the real limit on cycle time is not how fast the arm can move, but how long it takes to stop vibrating at the end of a move.

• Modal analysis finds the natural frequencies of the arm in each pose. The lowest one decides how quickly it settles, and it drops as the arm stretches out.
• Raising that frequency, by adding stiffness or removing mass far from the base, lets the controller move faster without overshoot.
• The results also help the controls team, who need the structural frequencies to tune the servo loops.

The end effector sits at the very end of the arm, so its weight costs payload and its flexibility costs accuracy.

• Grippers and tool mounts need to be light and stiff at the same time, which makes them ideal candidates for FEA-led design.
• Many are made from aluminium or from 3D-printed and moulded polymers such as PA12 or carbon-filled nylon, where choosing the right material matters.
• Gripper fingers flex thousands of times a day, so strain and fatigue are worth checking as well as strength.

Our FEA material comparison of polycarbonate, PA11, PA12 and carbon-filled PA12 and our FEA of a nylon injection-moulded part show the kind of polymer analysis that goes into these parts.

The structural results feed straight into motor and gearbox selection: the payload, the arm's inertia and the target accelerations set the torque each joint needs.

• A lighter, stiffer arm can often use smaller motors and gearboxes, which saves weight and cost at every joint.
• For custom actuators, electromagnetic FEA of an electric machine predicts torque, magnetic flux and losses before a prototype is wound.
• Thermal analysis checks that motors and drives stay within their temperature limits through a full duty cycle.

Collaborative robots work next to people, so how they behave in a collision matters as much as how accurately they move.

• ISO/TS 15066 gives the limits on contact forces and pressures that power- and force-limited cobots are designed around.
• Simulation helps you understand what happens in a contact, how padding and rounded shapes spread the load, and how the arm structure responds.
• For impacts and crashes, explicit dynamics is the right tool; we used it in our impact simulation in Ansys Explicit Dynamics.

• 3D CAD of the arm or end effector (STEP, IGES, Parasolid or native files), or drawings for our 3D CAD team to model.
• Payload, reach and the accuracy or deflection you need.
• The motion profile: speeds, accelerations and the poses you care about.
• Gearbox, bearing and motor datasheets for each joint.
• Materials for each part, and any test data you already have.

Cover photo: Homa Appliances on Unsplash