Fasteners, Bearings, and Alignment

A shaft can be perfectly machined and still bind if its bearings are mounted out of line.

This is one of those boring mechanical details that decides whether the robot feels precise or terrible.

Bearings support motion

A bearing constrains a shaft while allowing rotation with relatively low friction. It also transfers loads between the shaft and structure.

That means the bearing mount becomes part of the load path.

Two bearings supporting one shaft should generally agree about where the shaft axis is. If the mounts force the bearings into conflicting positions, the shaft may:

• bind;
• create extra friction;
• wear components;
• load the motor;
• distort the frame.

Fasteners do more than "hold parts together"

Bolted joints depend on:

• hole position;
• clamping force;
• material thickness;
• joint geometry;
• vibration;
• whether the bolt is loaded in tension, shear, or both.

A loose fastener changes the system over time. It can turn a precise assembly into a moving joint that the design never intended.

Thread-locking methods, locking nuts, proper washers, and torque practices exist because vibration is not a theoretical problem on robots.

Tolerance stack-up

Suppose three parts each place a hole with a possible error of ±0.5 mm.

The final assembly error is not necessarily ±0.5 mm. Several tolerances can combine.

That matters when:

• two shafts must remain parallel;
• a chain needs alignment;
• a sensor must point at a target;
• an arm must fit between frame members.

A design that only works when every part is perfect is not robust.

Alignment diagnostic

Imagine a wheel shaft is hard to turn by hand after installation.

Do not immediately blame the bearing.

A controlled check could be:

1. loosen one bearing mount;
2. test whether the shaft turns more freely;
3. inspect spacing and parallelism;
4. check for bent shaft or side load;
5. retighten while preserving alignment.

One controlled change tells you something. Randomly swapping parts does not.

Assembly note

Choose one rotating robot assembly and draw:

shaft → bearings → mounts → frame

Mark where alignment is established and where tolerance can accumulate. Add one inspection that could catch the problem before the motor is powered.