A bonded seal, also known as a Usit ring, Dowty seal, Dyna seal, Mega ring, Multi seal or sealing washer, is a static seal used to seal flange connections, bolt heads, hydraulic lines, screw couplings and locking nuts. The bonded seal prevents leakage of gases and liquids in hydraulic and pneumatic systems. The seal consists of a metal outer ring and a vulcanized elastomer inner ring that acts as a gasket. Thanks to this design, bonded seals are ideal for high-pressure applications and offer excellent resistance to bursting and leakage. Some versions feature a self-centering ring with an additional rubber lip on the inner diameter, which simplifies installation and improves sealing performance.
Metal rings:
Steel (zinc-plated, CR6-free), Stainless Steel 304, Stainless Steel 316, zinc-chromated
Elastomers:
NBR, Viton (FKM / FPM), EPDM, Silicone (VMQ), Fluorosilicone (FVMQ), HNBR, CR
Flange connections, hydraulic lines, screw couplings, pumps, compressors, engines, sealing and locking nuts, hydraulic systems, and high-pressure applications in industrial and mechanical engineering.
Bonded seals (also known as Dowty seals or Usit rings) are static seals used for sealing flanged joints on bolt heads, hydraulic lines, screw couplings, and cap or lock nuts.
Using bonded seals prevents leakage of gases or fluids in your application.
They are widely used in hydraulic systems, typically wherever a bolt head or fitting must be sealed.
Thanks to the metal outer ring, bonded seals are well-suited for high-pressure systems.
A bonded seal consists of a metal outer ring with a vulcanized, trapezoidal elastomer sealing lip on the inner diameter.
The correct compound depends on your application: which medium and temperature are involved?
Also consider operating pressure and duration. The metal ring defines the maximum pressure rating of the seal.
The most common metals are stainless steel and carbon steel; typical elastomers are NBR and Viton® (FKM/FPM).
Zinc-plated steel (Cr6-free), Stainless steel A2 (304) and A4 (316), zinc-chromated steel.
NBR, Viton® (FKM/FPM), EPDM, Silicone (VMQ), FVMQ, HNBR, CR.
Available with approvals such as KTW, WRC, NSF, WRAS, ACN, FDA, and R67-01 (LPG gas).
Bonded seals are available in multiple designs:
The most common variants are without centering and with centering.
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A centered bonded seal stays fixed to the bolt or coupling during assembly/disassembly—ideal for efficient mounting.
Select size based on the dimensions of the nut, screw, bolt, or flange to be sealed.
The seal designation typically follows the fastener/flange designation. Metric selections are also available.
Examples include: 1/8, 1/4, 5/16, 3/8, 1/2, 5/8, 11/16, 3/4, 7/8, 1, 1 1/16, 1 1/8, 1 1/4, 1 3/8, 1 1/2, 1 5/8, 1 3/4, 1 7/8, 2, 2 1/8, 2 1/4, 2 1/2, and more.
Examples include: M3, M4, M5, M6, M8, M10, M11, M12, M13, M14, M15, M16, M17, M18, M20, M21, M22, M24, M26, M27, M28, M30, M33, M36, M40.
Common types / references: SC, US, USS, SES, SCR, BS, TT, TM.
| INDUSTRY REFERENCE | SIZE REF. | THREAD | A | B | C | D | E | F | MIN. BURST PRESSURE (BAR) | |
| BSP | BOLT | +0,13 | +0,10 | +0,10 | +0,15 | +0,20 | ||||
| -0.00 | -0.10 | -0.10 | -0.15 | -0.20 | ||||||
| PP45-1 | 1 | – | 6BA | 6,35 | 4,09 | 3,05 |
0.20/ 0.45 |
1,22 |
– | 2150 |
| PP45-2 | 2 | – | 4BA | 7,26 | 5,26 | 4,12 | – | 1570 | ||
| PP45-3 | 3 | – | 2BA | 8,38 | 6,35 | 5,21 | – | 1375 | ||
| PP45-4 | 4 | – | 1/4 | 13,21 | 8 | 6,86 | 4,7 | 2450 | ||
| PP45-5 | 5 | – | 1/4 | 13,34 | 9,53 | 6,99 | 4,7 | 1700 | ||
| PP45-6 | 6 | – | 5/16 | 13,34 | 9,53 | 8,31 | 6,1 | 1700 | ||
| PP45-7 | 7 | – | 5/16 | 14,22 | 10,04 | 8,64 | 6,1 | 1750 | ||
| PP45-A | 20 | 1/8 | 3/8 | 15,88 | 11,84 | 10,37 |
0.25/ 0.51 |
2,03 |
8,56 | 1500 |
| PP45-8 | 8 | – | 0,4 | 18,36 | 12,45 | 11,26 | 8,56 | 1950 | ||
| PP45-9 | 9 | – | 7/16 | 19,05 | 13,08 | 11,69 | 8,8 | 1900 | ||
| PP45-B | 21 | 1/4 | 1/2 | 20,57 | 15,21 | 13,74 | 11,45 | 1550 | ||
| PP45-10 | 10 | – | 9/16 | 22,23 | 16,39 | 14,86 | 11,58 | 1575 | ||
| PP45-BB | 22 | – | 0,6 | 22,23 | 17,3 | 15,83 | 12,1 | 1310 | ||
| PP45-11 | 11 | – | 5/8 | 25,4 | 18,75 | 16,51 | 12,9 | 1550 | ||
| PP45-C | 23 | 3/8 | – | 23,8 | 18,75 | 17,28 | 14,96 | 1260 | ||
| PP45-12 | 12 | – | 11/16 | 25,4 | 19,69 | 18,16 |
2,5 |
14,5 | 1320 | |
| PP45-CC | 24 | – | 3/4 | 26,92 | 21,21 | 19,69 | 15,8 | 1260 | ||
| PP45-D | 25 | 1/2 | 13/16 | 28,58 | 23,01 | 21,54 | 18,64 | 1150 | ||
| PP45-E | 26 | 5/8 | 7/8 | 31,75 | 24,97 | 23,49 | 20,6 | 1250 | ||
| PP45-13 | 13 | – | 15/16 | 33,27 | 26,04 | 24,26 | 20,2 | 1275 | ||
| PP45-F | 27 | 3/4 | 1 | 34,93 | 28,53 | 27,05 | 24,13 | 1060 | ||
| PP45-FF | 28 | – | 1.1/16 | 38,61 | 30,61 | 27,82 | 22,9 | 1250 | ||
| PP45-14 | 14 | – | 1.1/8 | 36,58 | 30,86 | 29,33 | 23,9 | 900 | ||
| PP45-HH | 31 | 1 | 1.5/16 | 42,8 | 36,88 | 33,89 | 30,3 | 810 | ||
| PP45-G | 29 | 7/8 | 1.3/16 | 38,1 | 32,29 | 30,81 | 27,89 | 900 | ||
| PP45-15 | 15 | – | 1.1/4 | 41,4 | 35,69 | 32,64 |
3,38 |
27,1 | 800 | |
| PP45-H | 30 | 1 | 1.5/16 | 42,8 | 36,88 | 33,89 | 30,3 | 810 | ||
| PP45-16 | 16 | – | 1.3/8 | 44,45 | 38,99 | 35,94 | 29,5 | 700 | ||
| PP45-17 | 17 | – | 1.1/2 | 47,75 | 42,04 | 38,96 | 32,7 | 700 | ||
| PP45-J | 32 | 1.1/4 | 1.5/8 | 52,38 | 45,93 | 42,93 | 38,96 | 690 | ||
| PP45-18 | 18 | – | 1.3/4 | 57,15 | 48,39 | 45,34 | 37,9 | 875 | ||
| PP45-K | 33 | 1.1/2 | 1.7/8 | 58,6 | 51,39 | 48,44 | 44,86 | 690 | ||
| PP45-19 | 19 | – | 2 | 63,5 | 54,74 | 51,69 | 43,6 | 780 | ||
| PP45-L | 34 | 1.3/4 | 2.1/8 | 69,85 | 58,3 | 54,89 | 50,8 | 950 | ||
| PP45-M | 36 | 2 | – | 73,03 | 63,63 | 60,58 | 56,67 | 700 | ||
| PP45-MM | 37 | – | 2.1/2 | 77,72 | 67,44 | 64,39 | 55,4 | 750 | ||
| PP45-N | 38 | 2.1/4 | – | 79,5 | 69,98 | 66,68 | 62,8 | 670 | ||
| PP45-P | 39 | 2.1/2 | – | 90,17 | 79,38 | 76,08 | 72,2 | 680 | ||
| – | 79 | 3 | – | 101,47 | 92,84 | 89,09 | 85 | 550 | ||
| PP45-H | 9030 | 1 | – | 42,8 | 36,88 | 33,89 |
0.25/ 0.51 |
2,5 |
30,3 | 810 |
| PP45-J | 9032 | 1.1/4 | – | 52,38 | 45,93 | 42,93 | 38,96 | 690 | ||
| PP45-K | 9033 | 1.1/2 | – | 58,6 | 51,39 | 48,44 | 44,86 | 690 | ||
| PP45-M | 9036 | 2 | – | 73,03 | 63,63 | 60,58 | 56,67 | 700 | ||
| UNDUSTRY REFERENCE | SIZE REF. | A | B | C | D | E | F | MIN. BURST PRESSURE (BAR) |
| +0,13 | +0,10 | +0,10 | +0,25 | +0,10 | +0,20 | |||
| -0.00 | -0,10 | -0,10 | -0.00 | -0,10 | -0,20 | |||
| PPM4 | 202 | 7,00 | 5,40 | 4,50 |
0,30 |
1,00 |
3,40 | 1250 |
| PPM5 | 203 | 9,00 | 6,80 | 5,70 | 4,50 | 1400 | ||
| PPM5 | 204 | 10,00 | 7,40 | 5,70 | 4,50 | 1500 | ||
| PPM5.5 | 205 | 9,20 | 7,20 | 6,20 | 4,70 | 1220 | ||
| PPM6 | 206 | 10,00 | 8,00 | 6,70 | 4,70 | 1130 | ||
| PPM6 | 207 | 11,00 | 8,20 | 6,70 | 4,70 | 1510 | ||
| PPM6.7 | 210 | 10,20 | 8,60 | 7,30 | 5,77 | 1330 | ||
| PPM8 | 212 | 13,00 | 10,00 | 8,70 | 6,40 | 1330 | ||
| PPM8 | 213 | 14,00 | 10,40 | 8,70 | 6,40 | 1550 | ||
| PPM8.5 | 215 | 13,30 | 10,50 | 9,30 | 6,90 | 1200 | ||
| PPM10 | 216 | 15,88 | 12,00 | 10,35 |
0.40
0.40 |
2,00 | 8,56 | 1450 |
| PPM10 | 217 | 16,00 | 12,40 | 10,70 |
1,50 |
8,05 | 1350 | |
| PPM10 | 218 | 18,00 | 12,40 | 10,70 | 8,05 | 1880 | ||
| PPM11 | 219 | 16,30 | 12,70 | 11,40 | 9,80 | 1250 | ||
| PPM11 | 221 | 19,10 | 13,50 | 11,80 | 9,80 | 1770 | ||
| PPM12 | 222 | 18,00 | 14,30 | 12,70 | 9,73 | 1250 | ||
| PPM12 | 223 | 20,00 | 14,40 | 12,70 | 9,73 | 1680 | ||
| PPM13 | 225 | 22,00 | 15,40 | 13,70 | 10,80 | 1810 | ||
| PPM13.5 | 226 | 18,70 | 15,70 | 14,00 | 11,30 | 900 | ||
| PPM14 | 227 | 22,00 | 16,40 | 14,70 | 11,38 | 1510 | ||
| PPM16 | 229 | 24,00 | 18,40 | 16,70 | 13,41 | 1400 | ||
| PPM17 | 230 | 24,00 | 19,20 | 17,40 | 13,08 | 1150 | ||
| PPM17.5 | 231 | 24,70 | 20,10 | 18,00 | 13,60 | 1070 | ||
| PPM18 | 232 | 26,00 | 20,40 | 18,70 | 14,76 | 1275 | ||
| PPM20 | 233 | 28,00 | 22,50 | 20,70 | 16,76 | 1150 | ||
| PPM21 | 234 | 28,70 | 23,30 | 21,50 | 2,50 | 17,80 | 1080 | |
| PPM22 | 235 | 28,00 | 24,20 | 22,50 | 1,50 | 18,10 | 760 | |
| PPM22 | 236 | 30,00 | 24,40 | 22,70 | 2,00 | 18,74 | 1100 | |
| PPM22 | 237 | 30,00 | 24,40 | 22,70 | 3,00 | 18,74 | 1080 | |
| PPM24 | 238 | 32,00 | 26,40 | 24,70 |
2,00 |
20,11 | 1050 | |
| PPM26 | 239 | 35,00 | 28,40 | 26,70 | 22,30 | 1050 | ||
| PPM27 | 240 | 36,00 | 29,00 | 27,20 | 23,30 | 1130 | ||
| PPM30 | 242 | 39,00 | 33,00 | 31,00 | 25,70 | 860 | ||
| PPM33 | 243 | 42,00 | 35,80 | 33,70 | 28,70 | 900 | ||
| PPM33 | 244 | 43,00 | 36,40 | 34,30 | 28,70 | 880 | ||
| PPM36 | 245 | 46,00 | 38,80 | 36,70 | 31,10 | 880 | ||
| PPM39 | 246 | 51,00 | 41,90 | 40,00 | 2,50 | 34,10 | 1020 | |
| PPM42 | 247 | 53,00 | 44,40 | 42,70 |
3,00 |
36,50 | 940 | |
| PPM48 | 248 | 59,00 | 50,80 | 48,70 | 41,90 | 800 | ||
| PPM52 | 250 | 64,50 | 56,40 | 53,30 | – | 710 | ||
| PPM88 | 254 | 101,35 | 92,10 | 89,09 | 3,25 | – | 510 |
| PAMARGAN REFERENCE | ALTERNATIVE REF. | A | B | C | D | E | F | MIN. BURST PRESSURE (BAR) |
| +0,13 | +0,10 | +0,10 | +0,25 | +0,10 | +0,20 | |||
| -0.00 | -0,10 | -0,10 | -0.00 | -0,10 | -0,20 | |||
| PPFM3 | 301 | 7,50 | 5,00 | 3,60 |
0,30 |
1,00 |
– | 1950 |
| PPFM4 | 302 | 9,00 | 6,00 | 4,60 | 3,40 | 2000 | ||
| PPFM5 | 303 | 10,00 | 7,00 | 5,60 | 4,50 | 1780 | ||
| PPFM6 | 304 | 11,00 | 8,00 | 6,60 | 4,70 | 1680 | ||
| PPFM6 | 306 | 11,40 | 8,40 | 7,00 | 4,70 | 1540 | ||
| PPFM8 | 307 | 13,00 | 10,00 | 8,60 | 6,40 | 1330 | ||
| PPFM10 | 310 | 17,00 | 12,10 | 10,70 |
1,50 |
8,56 | 1730 | |
| PPFM11 | 312 | 18,10 | 13,20 | 11,80 | 9,80 | 1600 | ||
| PPFM12 | 313 | 19,00 | 14,10 | 12,70 | 9,73 | 1530 | ||
| PPFM13 | 315 | 20,10 | 15,20 | 13,80 | 0,40 | 10,80 | 1440 | |
| PPFM14 | 316 | 21,00 | 16,10 | 14,70 | 0,30 | 11,38 | 1370 | |
| PPFM16 | 317 | 23,00 | 18,10 | 16,70 | 13,41 | 1240 | ||
| PPFM17 | 318 | 23,70 | 18,80 | 17,40 | 0,40 | 13,08 | 1200 | |
| PPFM18 | 320 | 27,00 | 20,40 | 18,70 | 0,30 |
2,00 |
14,76 | 1450 |
| PPFM20 | 321 | 29,00 | 22,40 | 20,70 | 16,76 | 1340 | ||
| PPFM21 | 323 | 30,00 | 23,40 | 21,70 |
0,40 |
17,80 | 1290 | |
| PPFM22 | 324 | 31,00 | 24,40 | 22,70 | 18,74 | 1240 | ||
| PPFM23 | 325 | 32,00 | 25,40 | 23,70 | 19,30 | 960 | ||
| PPFM24 | 326 | 33,00 | 26,40 | 24,70 | 20,11 | 1160 | ||
| PPFM26 | 327 | 35,30 | 28,70 | 27,00 | 22,30 | 870 | ||
| PPFM27 | 328 | 36,00 | 29,40 | 27,70 | 23,30 | 1060 | ||
| PPFM28 | 329 | 36,00 | 30,30 | 28,60 | 24,80 | 730 | ||
| PPFM30 | 331 | 39,00 | 32,40 | 30,70 | 25,70 | 970 | ||
| PPFM33 | 332 | 42,00 | 35,40 | 33,70 | 28,70 | 900 | ||
| PPFM36 | 333 | 48,00 | 39,60 | 37,00 |
2.50 |
31,10 | 1010 | |
| PPFM39 | 334 | 51,00 | 42,60 | 40,00 | 34,10 | 950 | ||
| PPFM42 | 335 | 54,00 | 45,60 | 43,00 | 36,50 | 890 | ||
| PPFM45 | 336 | 57,00 | 48,60 | 46,00 | 39,50 | 860 | ||
| PPFM48 | 337 | 60,00 | 51,60 | 49,00 | 41,90 | 790 |
| NITRILE (NBR) |
HYDROGENATED NITRILE (HNBR) |
FLUOROCARBON (FKM) |
SILICONE (VMQ) | FLUOROSILICONE (FVMQ) | ETHYLENE PROPYLENE (EPDM) | CHLOROPRENE (CR) | |
| General Characteristics | |||||||
| Hardness range IRHD | 40-90 | 40-85 | 60-90 | 40-80 | 40-80 | 80-90 | 40-80 |
| Continuous high temperature limit | 100∞C | 135∞C | 200∞C | 200∞C | 175∞C | 125∞C | 140∞C |
| Low temperature capability | -30∞C | -40∞C | -25∞C | -60∞C | -60∞C | -30∞C | -55∞C |
| Dynamic service/Abrasion resistance | Excellent | Excellent | Very Good | Poor | Poor | Fair | Very Good |
| Compression set resistance | Very Good | Excellent | Very Good | Excellent | Very Good | Very Good | Good |
| Fluid Compatibility | |||||||
| Acid Inorganic | Fair | Good | Good | Good | Good | Excellent | Fair/Good |
| Acid Organic | Good | Good | Good | Excellent | Good | Good | Good |
| Ageing Oxygen | Fair | Good | Very Good | Excellent | Excellent | Excellent | Very Good |
| Ageing Ozone | Fair/Poor | Good | Very Good | Excellent | Excellent | Excellent | Very Good |
| Ageing Weather | Poor | Good | Very Good | Excellent | Excellent | Excellent | Very Good |
| Air | Fair | Excellent | Very Good | Excellent | Very Good | Excellent | Good |
| Alcohols | Very Good | Excellent | Fair | Very Good | Very Good | Very Good | Very Good |
| Aldehydes | Fair/Poor | Fair | Poor | Good | Poor | Very Good | Fair/Poor |
| Alkalis | Fair/Poor | Fair | Good | Very Good | Good | Very Good | Good |
| Amines | Poor | Poor | Poor | Good | Poor | Good | Very Good |
| Animal oils | Excellent | Excellent | Very Good | Good | Excellent | Very Good | Good |
| Esters Alkyl Phosphate (Skydrol) | Poor | Poor | Poor | Good | Fair/Poor | Fair/Good | Poor |
| Esters Aryl Phosphate | Fair/Poor | Fair | Excellent | Good | Very Good | Fair | Fair/Poor |
| Esters Silicate | Good | Good | Excellent | Poor | Very Good | Fair | Fair |
| Ethers | Poor | Poor | Poor | Poor | Fair | Poor | Poor |
| Gas permeability | Good | Good/Excellent | Good | Poor | Poor | Good | Good |
| Hydrocarbon fuels Aliphatic | Excellent | Excellent | Excellent | Fair | Excellent | Poor | Fair |
| Hydrocarbon fuels Aromatic | Good | Good | Excellent | Poor | Very Good | Poor | Fair/Poor |
| Hydrocarbons Halogenated | Fair/Poor | Poor | Excellent | Poor | Very Good | Good | Poor |
| Ketones | Poor | Poor | Poor | Poor | Fair/Poor | Very Good | Poor |
| Lubricating oils High aniline | Excellent | Excellent | Excellent | Very Good | Very Good | Poor | Good |
| Lubricating oils Low aniline | Very Good | Excellent | Excellent | Fair | Poor | Poor | Fair/Poor |
| Silicone oils | Excellent | Excellent | Excellent | Good | Excellent | Very Good | Excellent |
| Vegetable oils | Excellent | Excellent | Excellent | Excellent | Excellent | Very Good | Good |
| Water/steam | Good/Excellent | Excellent | Fair | Fair | Fair | Excellent | Fair |
Other Usit ring / washer types: Parker fitting seals (Stat-O-Seal), Thread Seal, UNC, UNF, 600, 610, 750, 800.
Manufacturers for bonded seal / Usit ring / Multiseal / Dowty seal: Trelleborg (Busak & Shamban), Dowty, Parker, Dichtomatik, ERIKS, Merkel, NAK, NOK.
Order your Bonded Seals directly in our webshop.
Also see: Oil seal webshop • Freudenberg Simmerrings
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