LET-LOK® type connectors2023-09-20T10:49:17+02:00

LET-LOK® type connectors

  • LET-LOK® type connectors

Instrumentation connectors double ferrule – general information

Precision pipe fittings with two compression ferrules were invented around 1945. They are designed for leak-free operation in extreme conditions: high pressure, vacuum, vibration and high temperatures. Two conical ferrules are pressed against the conical seat in the fitting body with a nut, gripping the tube at the same time. The cone of the back ferrule slides under the front ferrule, lifting it and pressing against the conical seat inside the fitting body. The tube gets slightly deformed from the compression load of the ferrules. This is a precise mechanism that holds the tube firmly in the fitting, ensuring perfect leak tightness at high pressure and vacuum, with no unnecessary stress in the joint. It is all-metal, so the maximum working temperature can be as high as 650ºC.

With all these features and benefits, the twin-ferrule fittings can be used in process engineering and instrumentation systems in the chemical, petrochemical and pharmaceutical industry, power plants, nuclear plants and other sectors where high performance is critical. Today, they are manufactured by many companies according to the same industry standard. However, their quality (leak tightness, durability, reliability, corrosion resistance etc.) depends on the precision and countless technology details and for seemingly same elements from different manufacturers is basically different.

złącza dwupierścieniowe precyzyjne


  1. body
  2. front ferrule
  3. back ferrule
  4. nut

1. Cutting and preparing tube

Cut the tube square and clean. Cut the tube with a pipe cutter with a suitable blade (cut with less tension, more rotations – to avoid deformation of the tube) or with a hacksaw.
If the hacksaw is used, clean and smooth off the inner and outer edges with a deburrer. Do not clamp the tube in a vice at the assembly spot to avoid deformation.

2. Inserting tube into fitting

The fitting is supplied ready for use, assembled with ferrules and nut, finger tight. Check if the nut is finger tight. Insert the tube end into the fitting until it rests against the bottom of the socket (important!).
For larger fittings ( larger than 1”, 25 mm) apply some anti-seize paste (picture on the right).

3. Correct tightening

Before tightening with a spanner, draw a mark on the nut and on the body. Tighten the nut with a spanner 1 and 1/4 turn for 1/4” – 6 mm sizes and larger. For 3/16” (4 mm) fittings and those in smaller sizes, only 3/4 turn is required.
Note: To tighten HM-LM(LC)767LM…, HM-LM(LC)767LP… port connectors and HM-LM(LC)7121L… blank plugs, apply 1/4 turn.

4. Inspection, stop collar

Check if the fittings are installed correctly by inspecting the width of the gap between the nut and the body using a go/no-go gauge (1/4”, 6 mm, 3/8”, 1/2”, 12 mm) or separate gauges for each size. If the gauge gets in – tighten up, if not – it is correct. A stop collar can also be used. Once the nut is tightened correctly, the collar will not move.

5. High safety installation

HIGH SAFETY installation is recommended for heavy duty conditions and high pressure. In such cases: check if the nut is finger tight; insert the tube into the socket until it bottoms in the seat; tighten the nut with a spanner until the tube stops rotating in the socket; make a mark on the nut and the body; tighten with a spanner 1 and 1/4 turn; use the narrow side of a go/no-go gauge – HIGH SAFETY SIDE.

6. Pre-assembly

If installation space is limited, LET-LOK® fittings should be pre-assembled using pre-assembly tools (separate for each tube diameter). The ferrules and nuts should be first installed on the ends of the tubes and then assembled by final tightening of the nut and the fitting (contact Tubes International).


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List of products:

Let-Lok® twin-ferrule fittings for imperial tubes

Let-Lok® twin-ferrule fittings for imperial tubes

Let-Lok® twin-ferrule fittings for metric tubes

Let-Lok® twin-ferrule fittings for metric tubes

One-Lok® single-ferrule fittings

One-Lok® single-ferrule fittings

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FAQ – Instrumentation fittings

What materials are twin-ferrules fittings made of?2023-02-23T11:09:08+01:00

Twin-ferrule fittings are made of AISI 316 steel as standard, but we can also supply fittings made of: brass, Hastelloy C-276, Monel 400, 6Mo steel and Super Duplex on request.

Are Hamlet LET-LOK fittings compatible with other brands of fittings?2023-02-23T11:08:09+01:00

LET-LOK fittings are interchangeable with Swagelok, Parker (A-LOK), DK-LOK, HY-LOK or Superlok products.

Can twin-ferrule fittings be used with pneumatic hoses?2023-02-23T11:08:20+01:00

Twin-ferrule fittings can be used with pneumatic hoses only if a 760LI reinforcing ferrule is fitted. There is also an option of fittings with PTFE compression ferrules, which can be used as an alternative to standard stainless steel compression ferrules on plastic hoses.

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