What Are Straight Compression Fittings?
Straight compression fittings join rigid tube in a straight run without welding, soldering, or flaring: a nut drives ferrules onto the tube so the joint seals mechanically. This page covers the steel and stainless steel dual ferrule family, built for lubrication trunk lines and hydraulic hard tube where pressure, temperature, and vibration rule out softer materials. For low-pressure grease lines in brass, see the brass straight fitting page instead.
Specifications
| Item | Declared Specification |
|---|---|
| Imperial tube OD | 1/8″, 3/16″, 1/4″, 3/8″, 1/2″, 5/8″, 3/4″, 1″ |
| Metric tube OD | 3, 4, 6, 8, 10, 12, 16, 20, 25 mm |
| Materials | 316/316L stainless steel, brass, carbon steel |
| Working pressure (stainless) | 1/4″: 6,000 PSI; 1/2″: 4,800 PSI; 1″: 3,100 PSI |
| Temperature range | -65°F to 1200°F (-54°C to 649°C) |
| Thread forms | NPT, BSPT, BSPP, SAE, JIC, metric |
| Ferrule system | Dual ferrule: front ferrule seals, back ferrule grips |
| Tubing compatibility | Seamless or welded cold-drawn tube, copper tube, aluminum tube |
For media that exceed the corrosion resistance of stainless steel, Monel 400 variants are part of the range; confirm media compatibility before ordering.
How the Dual Ferrule Joint Works
The two ferrules share the work. As the nut tightens, the front ferrule presses into the body seat and forms the pressure seal, while the back ferrule bites into the tube wall and supplies mechanical grip and vibration resistance. Because sealing and gripping are separate, the joint resists the tube slippage that single-ferrule designs can show under vibration.
Sizing follows the exact tube outside diameter, never a nominal or thread size: a 1/4″ fitting is made for tube measuring exactly 0.250″ OD. The grease line tubing and fittings selection guide covers measuring tube and identifying threads.
Steel, Stainless, or Brass
Carbon steel suits indoor hydraulic and lubrication lines where corrosion exposure is low. Stainless steel is the choice for corrosive or washdown environments and for the highest declared pressures and temperatures. Brass seals at lower torque, but its pressure and temperature ceilings sit well below steel, so it belongs on low-pressure grease lines; for small-bore runs, see the brass compression sleeve tube fittings. When pressure, temperature, or environment pushes past brass, move up to the dual ferrule steel family.
Applications
- Hydraulic power and return lines on stationary and mobile equipment
- Hard-pipe trunk lines in centralized lubrication systems
- Process instrumentation and pressure-sensing lines
- Pneumatic control lines on factory automation equipment
- General process piping that needs a demountable metal-to-metal joint
Assembly and Reassembly
Cut the tube square, remove burrs, and insert it fully against the body stop. Tighten the nut finger-tight, then wrench-tighten 1-1/4 turns for tube OD up to 1/2″, or 3/4 turn for 5/8″ to 1″ sizes. Marking the nut before tightening makes the rotation easy to verify.
Know the limits before committing. Ferrules bite into the tube wall, so repeatedly remaking the same joint reduces sealing reliability; fit new ferrules when remaking a much-used connection. Stainless ferrules are too hard for nylon or other soft tube and will cut rather than grip, so soft lines need hose-barb or push-in fittings instead. Thread families do not cross: a 1/8 NPT port does not accept a 1/8 BSP fitting. If a dimension is uncertain, send tube and thread details through the quote request page before ordering.
Frequently Asked Questions
Should I choose steel dual ferrule or brass compression fittings?
Decide by pressure, temperature, and environment. Steel and stainless dual ferrule fittings cover high-pressure hydraulic and lubrication hard tube and corrosive or washdown service. Brass compression fittings suit low-pressure grease lines where easy assembly matters more than pressure headroom.
How do I select the correct size?
Measure the tube outside diameter with calipers and match the fitting to that exact figure; a 1/4″ fitting fits only 0.250″ OD tube. Never size by the thread or a nominal pipe name.
What is the difference between single and dual ferrule designs?
A single ferrule must seal and grip at once. The dual ferrule design splits the work: the front ferrule seals against the body seat, and the back ferrule grips the tube, which improves holding power under vibration.
Can I reuse a fitting after disassembly?
The body and nut can be reused, but the ferrules deform permanently during first assembly. After repeated remakes, sealing reliability drops, so install new ferrules when remaking a joint that has been apart several times.
How tight should the nut be?
Finger-tight first, then 1-1/4 turns for tube OD up to 1/2″, or 3/4 turn for 5/8″ to 1″ sizes. Over-tightening damages the ferrules and the tube without improving the seal.
Which tubing types are compatible?
Seamless or welded cold-drawn tube, plus copper and aluminum tube, within the declared size range. Soft plastic tube such as nylon is not compatible with metal ferrules; use push-in or barbed fittings there.
Are NPT and BSP threads interchangeable?
No. The thread forms and pitches differ, and forcing one into the other damages both parts and seals poorly. Confirm the thread family on the port before ordering.
Can stainless fittings be used on nylon lubrication tube?
No. Stainless ferrules are harder than nylon and cut the tube instead of gripping it. Small nylon grease lines need purpose-made push-in or compression sleeve fittings.
What thread forms are available?
NPT, BSPT, BSPP, SAE, JIC, and metric threads cover the common port forms; match form and size to the equipment port.
Who supplies these fittings and how do I order?
IsoHiTech Lubrication Systems was founded in 2023, engineering since 2006. For sizing help or a quotation, email inquiry@isohitech.com or contact the team with your tube and thread details.
