DECHENFR
+49 2451 48208-0 Request a quotation
Design guidelines

The dimensions for your drawing

For the shaft running surface the steels usual in mechanical engineering are enough. Thanks to the low contact pressure of the VR seal, post-treatment can often be dispensed with — in many cases unhardened standard stainless steel will do. What you should nevertheless observe is set out here: all figures from our Catalogue 2024/25.

Discuss the seal sizingDownload the catalogue
Shaft

Counterface

The running surface decides the service life of the whole sealing system. The following requirements apply in accordance with DIN 3760 and DIN 3761.

Surface and tolerances

Mean roughness depth Rz1 – 5 µm
Mean roughness value Ra0,1 – 0,8 µm
Maximum roughness depth Rmax≤ 6.3 µm
Machining twist (Drall)free from machining twist
Diameter toleranceISO h11
RoundnessIT 8
Lead-in chamfer on the shaft1 – 2 mm at 30°, free from burrs
Concentricity, coaxialityto DIN 3760

More recent publications in sealing technology recommend Rz in the upper range, that is 3 – 5 µm. A running surface that is too smooth does not build up the lubricating film the sealing lip needs to survive.

Surface hardness

Simple applications25 – 30 HRC
Normal applicationsmin. 40 HRC
Dirt ingress from outside, or contaminated mediamin. 55 HRC

These figures are below what is usually demanded for standard shaft seals. The reason is the low contact pressure of the preloaded diaphragm: it puts less load into the shaft surface.

Roughness profile of the shaft running surface Section through the surface of the shaft running surface. Rz is the distance between the mean peak line and the mean valley line; Rmax is the distance between the highest peak and the deepest valley — here a single deep score. Rz Rmax Running track single score
Rz is the mean and Rmax the maximum roughness depth. A single deep score can render an otherwise perfect running track useless — which is why, next to Rz there is always an Rmax on the drawing as well.
Machining

Avoiding machining twist

Machining can produce a twist orientation — a structure like a very fine thread that pumps the medium out underneath the sealing lip.

Machining twist on the shaft running surface On the left a shaft with twist: the machining marks run at an angle to the circumferential direction and act like a micro-thread that pumps the medium out underneath the sealing lip. On the right a twist-free shaft: the marks run in the circumferential direction. with twist Shaft pumping action free from machining twist Shaft Sealing lip
The twist acts like a micro-thread. Depending on the direction of rotation it either supports the sealing action or works against it. If the pumping action of the shaft exceeds that of the seal, leakage occurs.

In applications with only one direction of rotation this behaviour can be used deliberately to reinforce the sealing action. Where the direction of rotation changes, that option does not exist — then the running surface has to be free from twist.

Plunge grinding

For a twist-free surface we recommend plunge grinding, that is grinding without axial feed. Three points matter if no twist is really to arise:

  • The speed ratio between grinding wheel and workpiece must not be a whole number.
  • The orientation transfers to the grinding wheel. Use multi-grit dressing tools with the smallest possible axial feed, or professional dressing rolls.
  • Spark out completely — allow enough time for it.

Other processes

  • Hard turning with a very low feed rate can be suitable. It depends on the individual case, which is why we do not recommend it without qualification.
  • Coatings on the shaft can increase the friction power of the whole system considerably. Coated shafts should be investigated beforehand.
  • No manual reworking. Scores orthogonal to the sealing surface must be avoided.
  • Do not polish shafts.

For sealing points under high thermal or mechanical load, good conditioning of the contact surface is essential. Talk to us so that we can work out the individual case together.

Housing

Bore and installation space

Besides the dynamic seal between sealing lip and shaft, a radial shaft seal also seals statically — via its outer casing against the receiving bore.

Housing bore and installation space Cut-away through the housing bore: bore diameter with ISO tolerance H8, a lead-in chamfer of at least 15 degrees at the mouth of the bore, corner radius r2 at the bottom of the bore, and the installation depths t1 and t2, each measured from the housing face. t1 t2 r2 15° Ø D H8 Housing Face
The lead-in chamfer is not cosmetic: without it the seal is not guided into the bore, and the diaphragm can shear off during pressing in. t1 and t2 are measured from the face.

Bore

Bore diameterISO tolerance H8
Roughness depth Rt≤ 16 µm (Ra ≤ 3 µm)
Mean roughness depth Rz4,0 – 8,0 µm
Lead-in chamfermin. 15°

The required surface quality is achieved by finishing. The bore must not be arbitrarily smooth — the outer casing needs the roughness in order to sit securely.

Installation space

DimensionRequirement
t1B × 0.85 (minimum)
t2B + 0.3 mm (minimum)
r20.7 mm (maximum)
Concentricity, coaxialityto DIN 3760

B is the width of the seal. You will find the width for your size in the Dimension finder. A corner radius that is too large prevents the ring from seating flat.

Sealing gap

ds, D3 and D4

With rotating shafts the sealing lip of a pressurised radial shaft seal twists. High shear stresses arise in the elastomer — a function of pressure, circumferential speed, coefficient of friction and annular gap.

Sealing gap ds between shaft and support body bore Half section through the sealing point: the annular gap ds is half the difference between the support body bore D3 and the shaft diameter D1. The pressure p acts on the sealing lip from the medium side; the radial force Fr presses it onto the shaft. The drawing is exaggerated — in practice ds is a matter of tenths of a millimetre. Ø D1 Ø D3 ds p Fr Shaft Housing ds = (D3 − D1) / 2
If the distance ds is too great, material burns and shearing of the sealing lip occur in service. ds is therefore the decisive factor, and one the designer can influence. Drawing exaggerated.

In the VR sealing system this distance is taken into account via the support body bore — either with dimension D3 or D4.

Standard version for stamped metal parts
DimensionSupport body bore
D3D1 + 1.0 mm
D4D1 + 0.5 mm

ds = (D3 − D1) / 2 — correspondingly ds = (D4 − D1) / 2. Higher and dynamic pressures require smaller distances ds — which ds your pressure still permits is shown by the diagram below. For small distances the stamped sheet-metal version gives way to the one-piece turned support body: the SOA design.

Standard dimension when nothing is stated

D3 = shaft Ø + 0.10 +0,05−0.00
Example: shaft Ø 30 mm → D3 = 30.10 +0,05−0.00

State D3 with your order if you need a different dimension.

Permissible sealing gap ds against pressure The higher the pressure, the smaller the annular gap ds has to be. Up to about 10 bar a support body with the D3 dimension is enough; up to about 20 bar the D4 dimension is needed; above that, a ds matched to the application. Below 0.10 millimetres VR recommends gunmetal RG7 as the support body material. Range I — support body with the D3 dimension, up to around 10 bar Range II — support body with the D4 dimension, up to around 20 bar Range III — ds matched to the application, gunmetal RG7 00,10,20,30,40,5 0102030 0123 ds (mm) bar MPa Range I D3 dimension Range II D4 dimension Range III matched ds
The higher the pressure, the smaller ds must be. After the pressure diagram in the VR Catalogue 2024/25.

Three ranges

RangeSupport bodyApplication
ID3 dimensionStandard range
IID4 dimensionincreased pressure load
IIIds matchedSpecial design — please consult us

Support body materials

Stamped support bodies are supplied in steel, stainless steel 1.4301 and special materials. Turned support bodies in gunmetal RG7, aluminium, stainless steel 1.4301, steel 1.0319 and in plastics such as PPS, PEEK, HDPE and PA6.

  • For distances ds < 0.10 mm we recommend gunmetal RG7, to rule out damage to the shaft.
  • For higher pressure loads the gap extrusion has to stay as small as possible — hence the turned support body of the SOA design.
  • For high-pressure applications, please coordinate your design with us early on.
Summary

What belongs on the drawing

ComponentFeatureRequirement
ShaftDiameterISO h11, roundness IT 8
Running trackRz 1 – 5 µm · Ra 0.1 – 0.8 µm · Rmax ≤ 6.3 µm
Machiningfree from twist, plunge-ground
Hardnessmin. 40 HRC · 55 HRC where dirt is present
Lead-in chamfer1 – 2 mm at 30°, free from burrs
HousingBoreISO H8
Bore surfaceRt ≤ 16 µm · Rz 4,0 – 8,0 µm
Lead-in chamfermin. 15°, free from burrs
Installation spacet1 ≥ B × 0.85 · t2 ≥ B + 0.3 mm · r2 ≤ 0.7 mm
SealSupport body borestate D3, otherwise shaft Ø + 0.10 +0,05−0.00
Operating dataPressure, speed, temperature, medium

This table is no substitute for a proper seal design. Send us your drawing — we will tell you which design fits and which ds your pressure calls for.

Is your sealing point anything but standard?

Send us the installation situation as a drawing or a model and we will tell you what is feasible. From a batch size of one.

Request a quotation Download the catalogue
Call us Request a quotation