Oil seals and gaskets: how to write size and material for an order

A shaft seal, what the shop floor calls an oil seal, is written with three numbers in millimeters: shaft diameter × bore diameter in the housing × width. For example, 45×62×10. That is enough for the seal to fit in place. For it to work, two more things are added to the size: the type (whether there is a dust lip) and the lip material. NBR suits oil and temperatures up to +100 °C, FKM suits heat up to +200 °C. An O-ring is written differently: inside diameter × cross-section.

Shaft seal: size

The order of numbers is the same at the large manufacturers. SKF writes, for example, "CR 45X62X10 HMS5 RG": shaft 45, housing bore 62, width 10, then the design and the material code. Freudenberg in its product cards writes "BA 30 x 50 x 8" in the same order.

Seal sizes are standardized: ISO 6194-1 sets nominal sizes and tolerances, the German DIN 3760 sets designs and sizes. So a 45×62×10 seal from different plants can go in the same place if the type and material match.

How to measure if the print has worn off:

• shaft diameter: with a caliper on the shaft where the lip runs, not on the old seal;
• bore diameter: on the housing;
• width: on the old seal or from the depth of the seat.

If the shaft is worn into a groove at the lip, a new seal will not last long on it. This shows when measuring.

Types: with and without a dust lip

Under DIN 3760 the main designs are marked with letters:

Manufacturers have their own names. At SKF the standard seals are HMS5 with a rubber outer surface, HMSA10 the same with an extra lip, CRW1 with a metal case. At Freudenberg BA is a standard seal with a rubber covering and a spring, BASL the same with a protective lip. We matched them to the DIN letters from the descriptions of the designs; there is no direct correspondence table in open catalogs.

A dust lip is needed if there is dust, dirt or water outside. In a cement or slate shop, or outdoors, a type with a dust lip is better.

Material and temperature

The ranges are from the SKF catalog and the Freudenberg data sheet. For NBR they agree. For FKM the lower limit is −20 °C at SKF and −30 °C at Freudenberg. The working temperature in oil can be lower than in air: for FKM in oils Freudenberg states +150 °C.

When heated above about +300 °C, FKM gives off hazardous fumes. Old FKM seals are not burned or heated with a torch.

O-rings

An O-ring is written as inside diameter d1 × cross-section diameter d2, in millimeters. For example, 50×3.55. The size series and tolerances are set by ISO 3601-1. General industrial cross-sections in this series: 1.8; 2.65; 3.55; 5.3; 7 mm.

There is also the inch series AS568 with numbers instead of sizes. If the package shows a number rather than a size, convert it with the manufacturer's table.

How to measure: the cross-section with a caliper on the uncompressed ring; the inside diameter on the groove or the shaft the ring sits on. An old ring is often stretched or flattened, so the groove size is more reliable.

Hydraulics: rod, piston, wiper

A hydraulic cylinder has three main seals: on the rod, on the piston, and a wiper where the rod exits. Sizes are written from the seat:

The housings for these seals are standardized: ISO 5597 for rod and piston, ISO 6195 for wipers. But the seals have different profiles: U-cup, set with back-up rings, compact piston seal. The profile of the replacement must match, otherwise the seal will not fit the same groove.

A hydraulic cylinder also needs a full repair kit: rod and piston seals, wiper, guide rings, O-ring. Everything is changed at once.

What to put in the request

1. The unit: gearbox shaft, pump, hydraulic cylinder.
2. Size: d × D × b for a shaft seal; d1 × d2 for an O-ring; groove dimensions for hydraulics.
3. Type: A, AS or equivalent; for hydraulics, the profile.
4. Medium and temperature: oil, water, dust outside; how many degrees.
5. For a shaft, speed; for a cylinder, working pressure.
6. A photo of the old seal in section or from the side.

How we do it

• From the dimensions and photos we look for seals at the unit's manufacturing plant or at a seal manufacturer.
• We send what we find before the order: size, type, material, working temperature.
• For the hydraulic cylinder and hydraulic pump from the catalog we select seals by model and seat dimensions.