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?”

This is a continued post from [ 🔗Past post: #1 Bearings ].

💠 Recap: 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.

💠 Recap: 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.

＊＊＊

💠 2. Bushings

🔹 Moving & Non-Moving Surfaces (Bushings)

🔩 Bushings 🔩
🔴 Moving surface → Inner ⌀ (Interface to shaft)
🟡 Fixed surface → Outer ⌀ (Interface to housing hole)

✅ Correct design:
Shaft rotates inside bushing, or bushing rotates around shaft.
❌ Incorrect design:
Bushing rotates in housing.

🔹 Tolerance Fits (Bushing)

📌 Non-Moving Surfaces

The bushing is often pressed into the housing hole by force.
Sometimes it’s also placed into the housing by thermal installation.
This is done by first cooling down the bushing (making it smaller) and slightly warming up the housing hole (making it bigger).
Then, the bushing is placed into the housing hole, expanding the bushing into a fixed position.

• If using pressing force, use interference fits (see manufacturer datasheet)
• (Thermal installation often has different design guidelines depending on your company.
  -> Check within your company design guideline, or consult bushing manufacturer.)

📌 Moving Surfaces

• Use sliding / rolling fits

✅ Manufacturer datasheet will tell you the needed fits and assembly instructions.

✅ Choose bushings that have liners at moving areas.
Liners are coatings which are extremely slipper, often made from PTFE material or sometimes Teflon (like your non-stick pan).

❌ Movements on blank bronze material (no coatings / liners) will still face a lot of friction, thus not very recommendable.

❌ Not recommendable to deviate from datasheet recommended fits.
Manufacturer fits have much better performance after assembly, and will give you minimal surprising damages.

💠 Recommended Design Resources

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

• SKF
• RBC Bearings
• Schaeffler

Although these are famous bearing manufacturers, they also produce a lot of bushing products.
You can check their design guideline section, to know more about how to best design your joints.