Every linear motion design starts with the same choice: How do you convert rotary motor motion into linear travel? The two most common answers are the Lead Screw (simple, cheap, friction-based) and the Ball Screw (complex, expensive, rolling-based). Making the wrong choice here is costly. Use a lead screw where you need precision, and you get backlash. Use a ball screw in a vertical lift without a brake, and your load crashes to the floor. In this guide, we compare them side-by-side. Table of Contents 1. The Physics: Sliding vs. Rolling 2. Efficiency & The "Back-Driving" Danger 3. Accuracy and Backlash 4. Selection Table Advertisement 1. The Physics: Sliding vs. Rolling The fundamental difference is friction. Lead Screws rely on Sliding Friction . The nut (often bronze or plastic) slides directly against the steel screw threads. This generates heat and wear. Ball Screws re...
Figure 1: Precision-machined PTFE components, highlighting the material's machinability and diverse applications in engineering.
The King of Chemical Resistance: PTFE
PTFE (Polytetrafluoroethylene), most famously known by the DuPont trade name Teflon®, is an engineering plastic that stands in a league of its own. It is the slipperiest solid material known to science and is virtually immune to chemical attack.
While common in non-stick cookware, its true engineering value lies in extreme industrial environments—from cryogenic rocket seals to high-temperature semiconductor manufacturing equipment.
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Key Engineering Properties
1. Extreme Temperature Range
PTFE has one of the widest operating temperature ranges of any plastic: -200°C to +260°C.
It remains flexible at liquid nitrogen temperatures (making it ideal for cryogenics) and does not melt until nearly 327°C.
2. Universal Chemical Resistance
PTFE is inert to almost all chemicals, acids, and solvents. It is only attacked by molten alkali metals (like sodium) and elemental fluorine at high temperatures. This makes it the standard material for Chemical Processing Valves and Corrosive Fluid Piping.
3. Low Friction (Tribology)
With a coefficient of friction typically between 0.05 and 0.10, PTFE is self-lubricating. It eliminates "stick-slip" motion in sliding mechanisms, making it perfect for slide plates and machine tool ways.
The Achilles Heel: Cold Flow (Creep)
Engineering Warning:
Despite its amazing properties, Virgin (pure) PTFE has a major mechanical weakness: Cold Flow.
Under a continuous load (like a tightened bolt on a flange), PTFE will slowly deform or "squish" over time, even at room temperature. This leads to leaks in gaskets and loss of tolerance in bearings. Solution: Use Filled Grades.
Despite its amazing properties, Virgin (pure) PTFE has a major mechanical weakness: Cold Flow.
Under a continuous load (like a tightened bolt on a flange), PTFE will slowly deform or "squish" over time, even at room temperature. This leads to leaks in gaskets and loss of tolerance in bearings. Solution: Use Filled Grades.
Advanced Materials: Filled PTFE Grades
To stop PTFE from deforming, engineers blend additives into the resin. Choosing the right filler is critical for performance.
1. Glass-Filled PTFE (15-25% Glass)
Best for: Chemical resistance and compressive strength.
Glass fibers significantly reduce creep (cold flow) and increase wear resistance. It is the industry standard for valve seats and gaskets.
Warning: Glass is abrasive and can eat away at soft mating shafts.
2. Carbon-Filled PTFE (25% Carbon)
Best for: Dry running bearings and high-speed seals.
Carbon improves thermal conductivity (helping dissipate heat) and makes the material conductive (anti-static). It has excellent wear life in water applications.
3. Bronze-Filled PTFE (40-60% Bronze)
Best for: Hydraulic seals and heavy-load guides.
Bronze adds massive compressive strength and thermal conductivity. It is widely used in hydraulic cylinder wear rings.
Warning: Not suitable for corrosive chemical environments (acids will attack the bronze).
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Machining & Processing Tips
Machining PTFE is easy, but holding tight tolerances is hard.
- Softness: The material is so soft it can squash in the chuck jaws. Use collets or soft jaws with light pressure.
- Thermal Expansion: PTFE grows rapidly when heated. If you machine it fast and measure it while hot, it will be undersized when it cools down. Use lots of coolant and measure at room temperature.
- Burrs: PTFE tends to feather and burr. Sharp tools are mandatory.
Typical Industrial Applications
- Aerospace: Wire insulation (due to dielectric strength and fire resistance).
- Oil & Gas: Downhole seals that resist sour gas (H2S) and high heat.
- Semiconductor: High-purity piping for transporting ultrapure water and etching acids.
- Automotive: Power steering seals and transmission thrust washers.
Summary Table
| Property | Virgin PTFE | Glass-Filled |
|---|---|---|
| Friction | Lowest | Moderate |
| Compressive Strength | Low (High Creep) | High (Low Creep) |
| Chemical Resistance | Excellent | Good (Glass resists acid) |
| Wear Life | Poor | Excellent |
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