KF25-Flanged Components and Accessories

Quickly Connect to KF25 Vacuum Systems
Adapters for Small-Diameter Tubing
Straight, Elbow, and Tee Fittings Available
Wing Nut Clamps and O-Ring Carriers

Application Idea

A KF25C050 Compression Fitting is used to add an empty glass cell to a KF-compatible vacuum system.

KF25WNC

KF25 Wing Nut Clamp

KF25E45

45° Elbow Fitting with KF25 Flanges

KF25C038

KF25 Flange to Ø0.38"
Compression Fitting Adapter

KF25T

Three-Way Tee Fitting with KF25 Flanges

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Overview
KF Flange Sizes
Application
Feedback

Features

Compatible with ISO 2861 Clamped-Type Quick-Release Couplings
Vacuum-Facing Parts Cleaned Ultrasonically
All Parts Double-Vacuum Bagged
To Request Other KF Flange Nominal Diameters, Please Contact Tech Support

KF flanges are ISO-standardized vacuum fittings that provide a vacuum seal by compressing a rubber O-ring between two mating surfaces. Also referred to as NW, QF, or DN flanges, KF flanges are ideal for quickly assembling vacuum systems thanks to their ease of use, maintenance, and reusability.

We offer KF25 Flange Compression Fitting Adapters to quickly connect glass and metal pipes to KF-compatible vacuum systems. For an application example using our KF25 Flanges and our empty glass cells, please see the Application tab. These adapters are constructed from 304 stainless steel for high durability and low outgassing. Options are available for connecting to pipes with imperial outer diameters from 1/4" to 1" and metric outer diameters from 6.0 mm to 25.0 mm. Please refer to the table in the KF25 Flange to Compresssion Fitting Adapters for compatible imperial and metric tube sizes. We also offer KF25-flanged straight, elbow, and tee fittings for use in assembling vacuum systems. Our straight fittings are available in 50 mm, 100 mm, and 250 mm lengths. Our elbow fittings are available with either a 45° or 90° angle.

Our wing nut clamps, O-ring carriers, and blank caps are available in KF16, KF25, and KF40 sizing. Key dimensions of these KF flanges can be found by visiting the KF Flange Sizes tab. Our in-house machine shop allows us to modify or customize components with any standard KF flange. To submit a request, please contact Tech Support.

Video Insight: Working with KF (QR) Vacuum Flange Components
An approach for working with KF flanges is demonstrated, and tips are shared for minimizing leak rates, extending lifetimes, and recognizing when to replace O-rings.

For best performance, a very thin coating of vacuum grease should be applied to any O-ring used in a KF system. This should be done by hand while wearing gloves, leaving just enough grease to cause the O-ring to shine. The O-ring and its mating surfaces should be visually inspected before installation to ensure there is no debris which may prevent proper contact and cause a leak. O-rings should be regularly inspected to spot dry rot, cracking, tearing, or accumulation of debris from normal operation. Replace any O-ring suspected of causing a leak.

Quick Links
Flange	Description	Flange	Description	Flange	Description
KF16	G1/4" Thread Adapter	KF25	Compression Fitting Adapters	KF40	200 - 1200 nm Fiber Feedthrough
	Compression Fitting Adapters		Straight, Elbow, and Tee Fittings		0.3 - 5.5 µm Fiber Feedthrough
	Double Ferrule Compression Fitting Adapter		O-Ring Carrier & Wing Nut Clamps		400 - 2400 nm Fiber Feedthrough
	O-Ring Carrier & Wing Nut Clamps		Blank Flanged Caps		O-Ring Carrier & Wing Nut Clamps
	Blank Flanged Caps		-		Blank Flanged Caps

KF Flange Sizes

KF flanges are defined by ISO 2861 Clamped-Type Quick-Release Couplings. This specification details the dimensions and tolerances for the five standard KF flange sizes: 10 mm, 16 mm, 25 mm, 40 mm, and 50 mm. These sizes are the nominal bore size of the flanged pipe. The name "KF" derives from the German phrase "Klein Flansche," literally translating to "small flange"; this name was used around the time of the flange's adoption by ISO and DIN, as flanges for pipes larger than 50 mm are more appropriately handled by flanges that deform metal gaskets to create a seal, like CF flanges.

Thorlabs offers from stock 16 mm, 25 mm, and 40 mm KF components and accessories. The general dimensions of our flanges can be seen by referencing the diagram and table below. Our in-house machine shop allows us to modify or customize components with any standard KF flange. To submit a request, please contact Tech Support.

General Diagram of a KF Flange ISO Dimension

Click to Enlarge
For Thorlabs' KF Flange Measurements please see the corresponding table to the left.

Equivalent Sizes				Thorlabs' KF Flange Measurements
Absolute Maximum Inner Diameter^a	KF Size	DN Size	NW Size	A	B	C
16 mm	KF16	DN16	NW16	1.18" (30.0 mm)	0.12" (3.0 mm)	0.10" - 0.11" (2.5 mm - 2.8 mm)
25 mm	KF25	DN25	NW25	1.58" (40.0 mm)	0.12" (3.0 mm)	0.10" - 0.11" (2.5 mm - 2.8 mm)
40 mm	KF40	DN40	NW40	2.17" (55.0 mm)	0.12" (3.0 mm)	0.11" (2.7 mm)

The absolute maximum inner diameter is the inner diameter of the largest pipe that can be welded to the flange. The item's inner diameter is often smaller than the absolute maximum inner diameter.

Creating a Vacuum Seal with a KF Flange

Thorlabs' KF25-Flanged components and accessories can be used to manage gas flow in and out of our empty borosilicate glass cells. Thorlabs' empty glass cells are terminated with one or two fill tubes that can be inserted into a compatible KF25 Series flange-to-compression-fitting adapter.

To input the glass cell into a KF25 vacuum system, follow the instructions below.

Select a KF25 flange-to-compression-fitting adapter that is compatible with the fill tube diameter of the glass cell.
Coat a cleaned and inspected KF25CR-F Centering O-Ring in vacuum grease and place it on a base post or pipe.
Place the flange-to-compression-fitting adapter on top of the KF25CR-F Centering O-Ring and secure them with the KF25WNC wing nut clamp.
Loosen and remove the knurled locking knob, O-ring, and seal sing of the flange-to-compression-fitting adapter so its mating surface can be inspected and the included O-ring can be coated in vacuum grease. This will create a vacuum seal when the cell is inserted into the system.
Slide the open end of the glass cell tube through the top (side with the Thorlabs engraving) of the knurled locking knob, seal ring (with the counterbore seat for the O-ring facing the adapter body), and O-ring (in that order) of the flange-to-compression-fitting adapter.
Slide the glass tube into the flange adapter body and then slide the O-ring, seal ring, and knurled locking knob down the tube onto the flange adapter body. Tighten the knurled locking knob by hand.

It is not recommended to insert the glass tube into a fully assembled KF Flange-to-compression-fitting adapter. The sharp edges of the glass tube may damage the O-ring thus causing the seal to fail.

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KF25 Flange to Compression Fitting Adapters

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Disassembled View of the KF25 Flange to Compression Fitting Adapter

Quickly Connect Glass and Metal Pipes to KF-Compatible Vacuum Systems
Options for a Variety of Pipe Outer Diameters (OD):
- 1/4" to 1" Imperial Sizes
- 6.0 mm to 25.0 mm Metric Sizes
304 Stainless Steel Construction for High Durability and Low Outgassing
Internal Viton^®* Fluoroelastomer O-Ring Included

KF25 flanges with compression fittings mount smooth-surfaced pipes to vacuum systems, including metal tubes, such as those used for gas lines, or glass tubes like those found on our empty glass cells or fused silica optical ports. Each fitting contains an internal Viton fluoroelastomer O-ring. When the knurled cap of the fitting is hand tightened, the O-ring compresses and forms a seal around the inserted pipe. For best results, a thin layer of vacuum grease should be applied to the internal O-ring; along with helping to create a vacuum seal, this grease allows an inserted pipe to slide along the O-ring without catching and causing damage. The knurled cap should not be overtightened, as excessive force could cause O-ring buckling, leading to leaks. The sizes of the internal O-rings are given in the table below. Each flange is shipped with a protective plastic cover.

*Viton is a registered trademark of The Chemours Company.

Item #	Compatible Pipe OD		Included O-Ring Dimensions
Item #	Imperial	Metric	OD	ID	Thickness	Dash #^a
KF25C025	1/4"	6.0 mm and 6.4 mm	0.38" (9.6 mm)	0.24" (6.1 mm)	0.07" (1.8 mm)	010
KF25C031	-	8.0 mm	0.44" (11.2 mm)	0.30" (7.6 mm)	0.07" (1.8 mm)	011
KF25C038	3/8"	10.0 mm	0.50" (12.8 mm)	0.36" (9.2 mm)	0.07" (1.8 mm)	012
KF25C050	1/2"	13.0 mm	0.63" (16.0 mm)	0.49" (12.4 mm)	0.07" (1.8 mm)	014
KF25C075	3/4"	19.0 mm	0.88" (22.4 mm)	0.74" (18.8 mm)	0.07" (1.8 mm)	018
KF25C100	1.00"	25.0 mm	1.13" (28.6 mm)	0.99" (25.2 mm)	0.07" (1.8 mm)	022

Society of Automotive Engineers (SAE) Dash Number as Defined in Standard AS568: Aerospace Size Standard for O-rings

KF25 Flange Fittings

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KF25E90 Elbow Fitting Dimensions

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KF25E45 Elbow Fitting Dimensions

Straight, Elbow, and Tee Fittings for KF25 Vacuum Systems
Tube Diameters:
- Straight Fittings 0.87" (22.1 mm) ID, 0.99" (25.1 mm) OD
- Elbow and Tee Fittings 0.87" (22.1 mm) ID, 1.00" (25.4 mm) OD
304 Stainless Steel Construction for High Durability and Low Outgassing

We offer straight, elbow, and tee fittings with KF25 flanges for use when assembling vacuum systems. Details on the angles of the elbow fittings can be found by clicking on the diagrams to the right. Each flange is protected with a plastic cover during shipping.

KF25 Flange-Centering O-Ring Carrier and Wing Nut Clamp

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Item #	Included O-Ring Dimensions
Item #	OD	ID	Thickness	Dash #^a
KF25CR-F	1.52"	1.10"	0.21"	320

Society of Automotive Engineers (SAE) Dash Number as Defined in Standard AS568: Aerospace Size Standard for O-rings

KF25CR-F Centering O-Ring Carrier for KF25 Flanges
- Holds O-Ring in Place Between KF Flanges
- Creates Vacuum Seal When Compressed Against KF Mating Surfaces
- Includes Fluorocarbon O-Ring
KF25WNC Wing Nut Clamp for KF25 Flanges
- Secures Two KF25 Flanges Separated by an O-Ring
- Fast Hand Tightening; No Tools Required

This animation illustrates a step-by-step procedure for coupling KF flanges.

Thorlabs offers a centering O-ring and a wing nut fastening clamp for mating KF25 flanges to a vacuum system. The angled sides of the wing nut clamp compress the O-ring between the flanges, quickly creating a vacuum seal. The wing nut clamp, which has M5 x 0.8 threads, requires only hand tightening, allowing parts to be quickly swapped in and out of a KF-flanged system.

For best performance, a very thin coating of vacuum grease should be applied to the O-ring before installation. This should be done by hand while wearing gloves, leaving just enough grease to cause the O-ring to shine. The O-ring and its mating surfaces should be visually inspected before installation to ensure there is no debris which may impede proper contact and cause a leak. O-rings should be regularly inspected to spot dry rot, cracking, tearing, or accumulation of debris from normal operation. Replace any O-ring that is suspected of causing a leak. Additionally, be aware that under vacuum pressure, the KF connection may undergo additional compression, causing the wing nut clamp to loosen. Some additional tightening may be performed, but be careful not to overtighten the clamp, as the wing nut may become difficult to remove or jammed when the system returns to atmospheric pressure. Do not remove a wing nut clamp while a system is under vacuum.

We also offer KF16 wing nut clamps and O-rings, as well as KF40 wing nut clamps and O-rings.

Blank KF25-Flanged Caps

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304 Stainless Steel Construction for High Durability and Low Outgassing
Blocks Unused Vacuum Ports

Stainless steel KF flange blank caps can be used to protect a vacuum assembly that is not attached to a vacuum system, to block a port during leak testing, or to cap off a port that is not being used on a vacuum chamber body. Each cap is shipped with a protective plastic cover.

We also offer KF16 blank flange caps and KF40 blank flange caps.