ALL GROUND DOWN-HOW GETTING THE

GRINDING RIGHT MAKES FOR RELIABILITY cont...

Watson-Marlow Bredel believes strongly that ground hoses are of higher quality.  Since quality is an important issue for Watson-Marlow Bredel, all hoses are ground before they leave the factory.

Up till now the hoses were ground according to traditional methods.  It is only thanks to the experience and the dedication of the operators of the company’s original machinery that the hoses were ground within satisfactory tolerances.  Convinced as the company is of the significance of the accurate grinding of hoses, an investment of 2.2 million was made to design and build completely new, fully automated, grinding machinery according to its specifications (Figure 5).  In this article it will be outlined why it is important to grind the hoses before they are mounted in a hose pump. 

Constant quality control

The production of a patented Watson-Marlow Bredel hose is a multi-step process.  First of all, the specific rubber compounds are prepared by mixing them in an internal mixer controlled by a computer.   This way the right amount of ingredients is added and the right mixing conditions are assured.  Control is carried out afterwards by checking some key properties of the rubber compound, these being the processing behavior ( Mooney viscosity), curing behavior (rheogram) and mechanical properties – the hardness and density of the vulcanized compound.  Only after the rubber compound has passed these tests is clearance given for its use in the production department.   

Figure 3.  the 'heart' of the hose pump.

The construction of the hose starts with extruding a homogeneous resilient rubber inner layer upon typically a 40-metre mandrel.  The compound used for the inner layer determines the area of application of the finished hose, since this material needs to be chemically resistant towards the medium to be pumped.  The available compounds and their area of application are listed in Table 1.  Then the reinforcement layers are fitted by winding dyed nylon cords around the inner layer, each of the four layers separated by a thin layer of rubber preventing the cords of the individual layers touching each other (Figure 6).  This reinforcement is needed to increase the maximum operating pressure and to secure the hose’s recovery. 

Figure 4.  Finite element analysis (FEA) of a hose under moderate compression

The construction of the cords compared to the rubber, inaccurate construction can result in dimensional changes of the hose under dynamic loading and internal pressure.  These unnecessary deformations of the hose cause a shorter lifetime, capacity loss or backflow.  The final hose construction step is the extrusion of the outer layer, which is tough, but still has very good dynamic properties.  During all these construction steps there is regular checking on the geometry to be sure that the production is still within the specified tolerance.   

The hose is now ready for the vulcanization.  Before the actual vulcanization reaction the hose is wrapped with a bandage to prevent the rubber from flowing away at the beginning of this reaction.  Since the vulcanization of rubber is a foam-determining reaction, the surface roughness of the bandaging is printed upon the outer surface of the finished hose.  Finally the hoses are sawn into the right lengths. 

A part of the finished vulcanized hose will go to the laboratory for the final control checks.   At this stage the mechanical properties of the rubber, the bonding between the cords and the rubber as well as all the geometrical characteristics of the hose are checked.  Only when the results of this control conform to the specifications are the hoses sent to the grinding division.

Figure 5.  Watson-Marlow Bredel Hose Pumps has recently invested in new, fully automatic grinding machinery

Other hose manufacturers producing reinforced hoses often do so in a somewhat different fashion.  But the most important similarity in all hose manufacturing processes, as far as the scope of this article is concerned, is that bandaging is used during the vulcanization process and that a print is therefore left behind on the finished hoses.