Through a novel seal construction and the use of self-lubricating elastomers, the VR seal of design SOA covers applications in the medium pressure range from 15 to 150 bar. In pumps and rotary unions it has hitherto been barely possible to use elastomer radial shaft seals reliably at high p·v values: high radial forces make the sealing material bite into the shaft surface, and high drag torques and high friction temperatures come on top of that.
The mechanical and physical overloading at high p·v values is caused by excessive gap extrusion between the pressure support and the shaft.
The sealing action at the lip
The operating principle of these springless radial shaft seals is that S-shaped diaphragm bodies are drawn over metal support bodies with precisely matched preload. The adjoining chamfered sealing lip works against the pressure direction of the medium and is supported at the rear by the integrated pressure disc.
When the diaphragm body is stretched over the metal body, a radial and a tangential force build up in diaphragm and sealing lip. These replace the spring force that generates the radial force in standard seals. The tangential force allows the radial force to be reduced to as little as a quarter of that of a standard seal. That is the essential design advantage:
- no running-in marks on the shaft, even without hardening and grinding
- usable on stainless steel shafts (V2A, V4A)
- low power loss, high speeds possible
- high pressure capability with high sealing performance
- long service life
Gap extrusion
Under pressure load every radial shaft seal is subject to extrusion at the sealing lip — the sealing material is drawn into the gap between shaft d1 and effective diameter Dw. The width of this gap is ds = (Dw − d1) / 2 and is the determining factor for the p·v value of the sealing point.
The effective diameter Dw is determined either by the seal design or by the component adjoining behind the sealing lip — housing, disc and the like.
Under pressure load a markedly higher radial force arises and, in addition, an axial force towards the side away from the pressure, because the material is drawn into the annular gap. With rotating shafts the torsion of the sealing lip geometry is added. This leads to high shear stresses in the material, which are a function of pressure, speed, coefficient of friction and annular gap.
It is therefore imperative to achieve a constant dimension ds around the whole circumference. The sealing lip must also be supported by a face standing at about 30° to the effective diameter, so that it does not bear against the shaft under pressure along its entire length.
If ds is too large or uneven, the material burns and the sealing lip shears off in service. The annular gap thus decides whether a seal withstands the specified pressure and circumferential speed or is destroyed.
In the VR sealing system the sealing gap is accounted for through the bore dimension of the support body. Higher and dynamically occurring pressure loads call for smaller gaps. Minimum values for ds under pressure load have been established for the VR shaft seals.
To take high pressures, ds has to be small and the pressure support behind the seal has to become more rigid and more precise. On the standard seal this support has hitherto been achieved with an additional disc behind the seal.
In the VR design SOA this pressure disc is integrated into the seal structure. The additional metal centring against the housing bore makes extremely low ds values attainable — the seal sits absolutely concentric to the shaft.
That is precisely what is needed to achieve the same ds dimension around the whole circumference. Only then is the stress distribution in the material under pressure constant around the circumference and no material extrusion occurs. Uneven stress conditions lead to premature failure through tangential cracks or shorten service life considerably.
Summary
Reducing the clearance between shaft and support ring and improving the elastomer with sliding additives led to a fundamentally new construction of the radial shaft seal. This design makes use in the medium pressure range possible, in pumps above all.
The design requirements for the support body dimension D3 are given in the Design guidelines.