Dynamic sealing
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Dynamic sealing Paulstra, the real seal Paulstra, the real seal INDIVIDUAL WATERPROOF PACKAGING Innovation for your future

Dynamic sealing III - SELECTION OF A SEAL FOR A ROTATING SHAFT 111.1 Type of fluid to be sealed 7 I V - CONDITIONS FOR GOOD OPERATION IV.3 Eccentricity between the housing and the shaft 11 IV.5 Power absorbed due to friction 12 V.1 Assembly on a shaft without splines 13 V.2 Assembly on a shaft witht splines or a shoulder 13 V.3 PAULSTRA recommendationsforthe shape of the shaft 14 V.4 Axial positioning and alignement 14 V.5 Recommendations for the assembly tool 15 V.6 Lubrication during assembly 16 V.7 Reminder of the main principles of assembly 16 VI- CLASSIFICATION OF THE MAIN PROFILES OF...

I - GENERAL I.1 - What is a seal ? An element forms a sealing function when it prevents the passage of a fluid from a one enclosure to another. Such elements are called “Seals”. If the object is to prevent the flow of a fluid from an enclosure into a neighbouring enclosure the seal is called a single seal. If the seal must prevent the flow of another fluid which may be in the second enclosure into the first, the seal is called a double seal. If the two mechanical parts between which the leakage is likely to occur are fixed with relation to each other, the seal is called a static seal. If...

I.2 - Types of seals Many different methods have been or are still used for sealing such as: - labyrinth glands. - stuffing-boxes. - O-rings. - lip seals. - surface seals. • Labyrinth glands are frictionless seals. They do not provide total sealing and do not seal if completely immersed in the fluid. • Stuffing-boxes work by packing fibrous material which may or may not be braided tightly around a shaft by means of axial pressure applied by a screw cap or a flange tightened by a bolt. For many years, they have been the most common type of seals used. They produce a high frictional torque...

I.3 - Description of lip seals HOUSING Casing Metal Outer ring Fixing rim Hinge Spring Spring retaining lip Lip Cuff Heel Sealing ridge In outline, a seal for a rotating shaft consists of three essential parts: • The Outer ring • The elastomer • The spring - The Outer ring usually consists of a metal ring in stamped steel with a right-angled cross-section. - The elastomer is itself made up of 3 parts: • The casing • The cuff • The lip - The casing (from the front surface to the back of the seal) is the part of the elastomer which is bonded to the Outer ring. It can cover it more or less...

II - MATERIALS USED II.1 - Armatures Standard material : sheet steel of XE quality (AFNOR standard A 36 401) Special outer rings can be produced using other materials for special applications. II.2 - Springs Standard: Stabilised XC 70 steel On request: Z10 CN 18-09 stainless steel (AFNOR standard A 35 586). NOTA: All the PAULSTRA range of fluorinated elastomer seals fluorocarbon (FKM) are equiped with stainless steel springs. NITRILE (acrylo-nitrile butadiene) STANDARD MIXES * Temperature range This material is particularly resistant to the action of mineral oils and grease. Suitable in...

III - THE SELECTION OF A SEAL FOR A ROTATING SHAFT* III.1 - The type of fluid to be sealed The fluids in contact with each face of the seal can be gases or liquids which are more or less viscous even pasty (in the case of greases). They must not have too aggressive actions on the materials which make up the seal (the outer ring, spring and elastomer). III. 1.1 - ARMATURE AND SPRING The armature and spring of standard seals are steel, so they have a good resistance to all the chemical solvents which are currently used in industry with the exception of water and aqueous liquids which can...

Mechanical resistance The new brown colored fluorocarbon (FKM) formula presents a very low abrasivity and: - low shaft and lip wear - resistance to ageing Heat resistance For good performance an elastomeric seal must be used within its operating temperature range. The standard elastomeric mix is not only sensitive to high temperatures which harden it causing cracks and fissures, but also to intense cold which makes it hard and hardens it. The temperature which must be considered is that at the contact lip. It must be borne in mind that this gets much hotter than the ambient fluid, due to...

III.2 - Shaft speed The graph below gives an indication of the rotary or linear velocity of the shaft in relation to various elastomers which are permissible under normal conditions of use. Linear speed m/s Shaft diameter III.3 - Pressure The effective pressure to which a seal is submitted is the difference between the pressures of the fluids on each of its two sides (one of which is often the atmosphere). It is clear that the sealing lip should be found on the side which has the higher pressure. In theory, the lip seal for rotary shafts is not a pressure seal. However, most PAULSTRA seals...

IV - CONDITIONS FOR GOOD OPERATION IV.1 - The housing It is extremely important that there be no sharp edges. Our recommendations are shown on the figure below: Recommended shape of the housing: - for a covered seal: - for an external outer ring: avoid any burrs round off the angle surface condition depth > thickness of the seal + 0.5 chamfered entrance Note: if the housing is made of a material with a high coefficient of expansion, this must be taken into consideration when defining the interference (tightness) with the seal. The lack of a chamfer or too small a chamfer can cause: • A...

IV.2 - The shaft The PAULSTRA recommendations are as follows: • Tolerance on the diameter: h 11. • Surface state: R = 0.4 to 1.2 ED (so Ra Ӎ 0.2 to 0.5). • Hardness: if V ≤ 4 m/s: 45 HRC minimum (say 455 HV or 155 kg/mm2). if V > 4 m/s: 55 HRC minimum (say 625 HV or 195 kg/mm2). • Thickness of the treated zone: 0.3 mm minimum. • Circularity: 5 microns. • Neutrality: All machined surfaces have grooves from the machining process. If these grooves are inclined in relation to the axis of the shaft, they form a helix which will produce a hydrodynamic action. The bearing surfaces of a seal must...