PTFE Liner ID Tolerance: How Much Clearance Does a Moving Cable Really Need?

There is no universal answer such as:

“Always leave 0.2 mm clearance.”

The correct clearance depends on the complete mechanical system.

That includes:

• cable diameter;
• liner ID tolerance;
• cable tolerance;
• ovality;
• bend radius;
• lubricant;
• temperature;
• contaminants;
• assembly compression.

A useful design therefore starts with a clearance window, not a nominal number.

Nominal clearance is not minimum clearance

Suppose:

Cable OD = A
Liner nominal ID = B

Nominal diametral clearance:

B − A

But production works at tolerance limits.

The more useful calculation is:

Minimum liner ID − Maximum cable OD

Then consider ovality and assembly deformation.

That is much closer to the real worst-case condition.

Why ultra-thin liners make this important

Aokeray manufactures Ultra-Thin-Wall PTFE Liners where very small walls and controlled lumen geometry are central to the existing product platform.

As wall decreases:

• external compression can matter more;
• handling becomes more important;
• ovality can consume usable clearance;
• measurement capability becomes more important.

For an automotive cable project, these manufacturing capabilities still require application-specific validation.

Too little clearance

Possible consequences:

• increased insertion force;
• higher sliding resistance;
• sensitivity to bends;
• greater effect from contamination;
• possible localized binding.

Too much clearance

Possible consequences:

• lateral wire movement;
• noise;
• less precise motion;
• variable contact;
• increased lost motion depending on cable architecture.

The objective is therefore controlled clearance, not maximum clearance.

Include ovality

Imagine a nominal ID of 2.00 mm.

If the lumen becomes oval, the minimum cross-section can be smaller than the nominal dimension implies.

For precision work, specify:

• ID;
• ID tolerance;
• ovality where required;
• measurement method.

Include the manufacturing process

The liner may change during:

• cutting;
• reeling;
• insertion;
• conduit forming;
• crimping;
• bending.

Measure both:

incoming liner

and

assembled housing

for critical programs.

Use several prototypes

Instead of approving one nominal ID immediately, prototype nearby clearance options.

For example:

• Option A: tighter;
• Option B: nominal;
• Option C: larger.

Then compare:

• assembly force;
• straight pull force;
• bent pull force;
• cycling;
• noise;
• mechanism accuracy.

This gives actual design evidence.

What tolerance should be on the drawing?

Ask:

1. Which dimension controls performance?
2. What is the worst-case cable OD?
3. What is the minimum acceptable lumen?
4. How much deformation occurs downstream?
5. How will ID be measured?

Then specify accordingly.

RFQ checklist

• cable minimum/maximum OD;
• target liner ID;
• ID tolerance;
• OD;
• wall;
• ovality requirement;
• cable length;
• bend radius;
• lubricant;
• conduit forming process;
• operating temperature;
• sample quantity.

Relevant pages

• Ultra-Thin-Wall PTFE Liner
• PTFE Etched Liner

FAQ

Is a tighter ID tolerance always better?

No. Tight tolerance adds cost and should support a functional requirement.

What clearance should I use?

Determine it from the complete cable system rather than using a universal rule.

Does wall thickness affect ID stability?

It can. Geometry, handling and downstream compression should be considered together.

Should the assembled cable lumen be measured?

For sensitive designs, it can provide more useful information than incoming tube inspection alone.