Knowledge
Lead
With lead one describes a periodic circumferential structure on rotationally symmetric surfaces like shafts, bushings or flanges with a sealing function. Such a periodic structure can be continuous or interrupted. Usually this periodic structure is superimposed onto the stochastic roughness topography and has its origin during the manufacturing process in a slight misalignment between the workpiece and the tool or is due to the dressing process of the grinding wheel. Recently it has been proven that also relative vibrations between the workpiece and tool can cause such a harmful lead structure. The resulting lead angles range from 0 (zero lead), a few arc minutes to rough lead structures of a few degrees. When the shaft rotates the periodic lead structure may pump the oil below the sealing to the outside and from this an oil leakage results. An exchange of the seal will therefore be of no benefit.
In order to ensure a proper sealing function the overall system, comprising of:
- the rotary shaft seal
- the shaft surface and
- the sealing substance
must be investigated. This system possibly causes a pumping effect through the lubrication gap into the air space or liquid space (see fig.).
Therefore the requirement for the shaft surface is that it is more or less lead free. But there are indeed cases where a lead structure is desired in order to support the function of the sealing lip. In the end the seal manufacturer will recommend his requirements regarding the counter seal surface properties.
Selection of influence quantities in the sealing system
Shaft surface
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Sealing ring
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Sealing substance |
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lead
roughness
direction of rotation
manufacturing method
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geometry
material
pressure forceacting against shaft surface
friction moment under the sealing lip |
chemical composition
viscosity
temperature inside the lubrication gap |
The surface structure of the shaft surface must not vary significantly with regard to the condition for which the sealing ring has been designed.
Due to the rising pressure for better and in especially faster manufacturing procedures continuously new methods are being developed which are used besides the traditional methods like grinding and turning.
If the essential process parameters like:
- forward feed
- cutting speed
- cutting edge radius
- cutting material
are selected properly and the tool stability is sufficient one can manufacture shaft surfaces which show (almost) no periodic structures or at most a zero lead structure.
The most important shaft surface properties relating to the sealing function can be measured using a suitable area surface measuring technique and can be quantified by characteristic parameters.
The basis for these is the separation of the stochastic and periodic surface structure components which influence the sealing function essentially. This is done by applying suitable mathematical procedures like FFT and ACF. The calculation of the following parameters represent the state of the art;
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Lead parameters:
Lead slope S
Lead depth D
Lead angle
Period lenght p
Theoretical feed cross section Q |
If there is a lead angle different from zero an undesired feed effect of the oil comes into existence in the gap near the sealing lip. But even in the case of a zero lead structure a sufficient sealing effect is not necessarily guaranteed but is assumed to hold if certain lead parameters are within given limits.
The quality control of sealing surfaces is done by quantifying the lead structures;
1) - area measurement of the surface topography
2) - extraction of lead structures by suitable mathematical methods
3) - calculation/display/listing of suitable parameters
Application examples for harmful lead, zero lead and a lead free surface
Measurement protocol
Filtered topography
with lead |
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Filtered topography
with zero lead |
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Filtered topography
without lead |
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