Mechanism Design: Moving & Non-Moving Surfaces (#1 Bearings)

When designing movable joints and parts in a mechanism, one of the first important thing to make sure we are clear on is:

“What are the moving surfaces, and what are the non-moving surfaces?”

💠 Typical Joint

Joints typically use either

  1. Bearings (Spherical, ball bearing, roller bearing, needle bearing…)
  2. Bushes / Bushings

Let’s talk about what should be the moving and non-moving surfaces of these two options.

💠 But Why Do We Care?

In all movable joints, you have a strong difference between

  • places where the movement is smooth like butter (moving surfaces), and
  • places where there should be no movement at all (non-moving surfaces).

Let’s say, your non-moving surfaces start to have micro-movements, because you chose a fit that is too lose (Hole too big, bearing too small).

-> This can lead to:

  • Premature wear or microcracks
  • Fretting corrosion
  • Excess friction, blocked motion

-> This can lead to damages in places where you didn’t anticipate it to break.

-> Very expensive, unplanned repair on big parts (costing $10k – $30k).
Or even worse – The joint fails unexpectedly.

Making this design choice clear from the start will help you have a safer and more reliable joint design.

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Let’s start the discussions with bearings.

💠 1. Bearings

🔹 Moving & Non-Moving Surfaces (Bearing)

Moving surfaces should be within the bearing — not in any interface to other mating parts.

-> Localize motion within the bearing itself.

🔩 Rotating bearings (spherical, ball, roller…) 🔩
🔴 Moving surfaces → should be inside the bearing
🟡 Everything else → should stay fixed in place
→ Inner ⌀ = Fixed to shaft
→ Outer ⌀ = Fixed to housing hole

Bearings are designed to have extremely low friction when rotating within itself.
Therefore, it’s very non-effective, or even dangerous, to have moving surfaces outside of the bearing.

🔹 How to Fixate Non-Moving Surfaces? (Bearing)

📌 Inner race

  • Use interference (press) fits
  • Typically: larger shaft + smaller inner diameter

📌 Outer race

  • Use interference fits
  • Reinforce with staking / swaging / retaining rings

💠 Recommended Design Resources

These companies provide high-quality mechanical bearings and engineering design documentation:

  • SKF
  • RBC Bearings
  • Schaeffler

You can check their design guideline section, to know more about how to best design your joints.

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🔗 Read next:
#2 Bushings