Technical data

DESCRIPTION OF ROTARY SHAFT SEAL

WORKING PRINCIPLE

The area between the sealing edge and the shaft is the most importnat. The sealing effect is achieved by preloading the sealing lip, making its internal diameter slightly smaller than shaft diameter. The garter spring ensures constant mechanical pressure and maintains the radial force to the shaft, flattening the sealing edge to a defined width. Sealing is provided by surface tension of the hydrodynamic oil film between the seal flattened area the shaft.

Oil thickness must be between 1 and 3 μm to avoid leakage. The meniscus acts as an interface between the outside air and fluid. Any break in the meniscus will result in a leakage. This can occur if the shaft contains scratches along the seal path.

METAL CASE

The metal insert or case is used to give strength and rigidity to the seal. Normally it is made of cold rolled steel in accordance with DIN 1624.

To avoid rust or chemical attack, stainless steel can be used.
Chrome Nickel AISI 304 (DIN 1.4301-V4A)
Chrome Nickel Molybdenum AISI 316 (DIN 1.4401-V4A).

GARTER SPRING

The garter spring maintains the radial force exerted by the sealing lip around the shaft surface. Usually the products of harmonic spring steel wire SAH 1074 (DIN 17223) or stainless steel wire Chrome Nickel AISI 302 (DIN 1.4300)
For special application also stainless steel springs in AISI 316 (DIN 1.4401 - V4A) are available. All our standard shaft seals produced in FPM compound are fitted with stainless steel springs in AISI 302.

Installation & operation

SHAFT
The shaft hardness and surface finish are of primary importance for efficient sealing and for achieving a useful life. Basically the hardness should increase with increasing peripheral speed. According to DIN 3760 minimum hardness required is 45 HRC. At a peripheral speed of 4m/s the hardness should be 55 HRC and at 10m/s 60 HRC. Recommended hardness depth: 0.3mm if shafts are not fully hardened.

Lubrication is also very important.

Surface finish as specified by DIN 3760 must be Ra 0.2 to 0.8μm, R2 1 to 5μm with Rmax=6.3μm. Rougher surfaces generate higher friction, hence higher temeratures. Machining defects and scratches on the shaft must be avoided.

Even very small defects could be sufficient to increase the film thickness, eventually rupturing the meniscus and causing leakage. It is also importnat to avoid spiral grinding or marks, because they can cause a pumping effect and leakage.

Recommended machining tolerance is ISO h11 according to DIN 3760 (see table below).

Shaft diameter		Tolerance
from	to	h11
6	10	0 -0,090
10	18	0 -0,110
18	30	0 -0,130
30	50	0 -0,160
50	80	0 -0,190

Shaft diameter		Tolerance
from	to	h11
80	120	0 -0.220
120	180	0 -0.250
189	180	0 -0.290
250	250	0 -0.320
310	400	0 -0.360

 

Temperature

The temperature of the sealing lip is the medium temperature increased by the temperature caused by frictional heat.
The higher the effective operating temperature is, the faster the ageing of the elastomer will be, thus affecting the performance of the sealing lip and the shaft.
Frictional heat depends on seal design and material, peripheral speed, sealing lip preloading spring force, shaft design and surface finish, lubrication, medium, etc.

Premissible speeds in pressure-free state to DIN 3760

Pressure

In most applications there is no or little differential pressure. Where the rotary shaft seal is exposed to pressure, however, the sealing lip is pressed against the shaft, thus increasing temperature. In some cases the pressure can even cause overturning of the sealing lip!

Over 0,2 bar at higher peripheral speeds or over 0,5 bar at low peripheral speeds back up rings or special designed rotary shaft seals with sponger sealing lip and supporting metal insert must be used. For the latter we refer to PP-types (e.g.AS-P). Nevertheless, permissible overpressures with PP-type shaft seals are limited (see diagram below).

Rotary shaft seals AS-P Permissible Overpressure

 
On request we can supply shaft seals with special reinforced lip to withstand pressure over the indicated value.
If back up rings are installed, standard rotary shaft seals can be used. Hower, back up rings increase costs and often the neccesary space for installation is not available. Sometimes the use of back up rings is even not possible, since it requires a very accurate fitting as well as very low eccentricity of the shaft.

Special designed rotary shaft seels (PP-types) are therefore preferred, even if more accurate fitting and lower eccentricity of the shaft than normal cases is required.

Production and quality assurance

Our rotary shaft seals are manufactured according to  German standard DIN 3760 and Quality assurance standards  ISO 9001:2001.
ISO SRPS 9001:2008

All production phases are checked and all measurements are recorded and stored for eventual tracing.

Interference allowance and permissible eccentricity

In accordance with German Standard DIN 3760

Seal outer diameter d2		Interference allowance (1)		Tolerance on d2(2)
up	to	Types A, AUP	Types B, BUP, C, CUP	Types	A, AUP, B, BUP, C, CUP
up 50	to 80	+0,30 +0,15	+0,23 +0,13	0,25
up 50	to 80	+0,35 +0,20	+0,25 +0,15	0,35
up 80	to 120	+0,35 +0,20	+0,28 +0,18	0,50
up 120	to 180	+0,45 +0,25	+0,28 +0,18	0,65
up 180	to 300	+0,45 +0,25	+0,30 +0,20	0,80
up 300	to 500	+0,55 +0,30	+0,35 +0,23	1,80

(1) The average value for d₂ taken form a number of measurements shall not be  greater than the value specified for d₂ plus interference allowance.

(2) The tolerance on d₂ (i.e. d₂max - d₂min) is to be determined by taking three or more measurements equally spaced around the circumference.

Storage

Some storage precautions must be  taken in order to avoid deterioration of the material-Rotary shaft seals  should be stored in a dust free and dry atmosphere and they must be kept in their original wrapping which should only be opened just before installation - Samples should be repacked after inspection. Excessive humidity will deteriorate some elastomers as well as cause corrosive damage to metal casing and spring.

Do not drop rotary shaft seats on shelves or boxes, nor hang seals on hooks, wires or nails, since in either case the sealing lip can damaged. Seals should be stored in a horizontal position.

Seals should be used on a first-in-first-out basis to avoid ageing on the shelf. Avoid storage near sources of heat or near electrical equipments that may generate ozone. Also keep away from direct sun light.

Shaft seals interchange table