WORLD PUMPS ® All Ground Down - How Getting the Grinding Right Makes For Reliability Page 4 of 5

ALL GROUND DOWN-HOW GETTING THE

GRINDING RIGHT MAKES FOR RELIABILITY cont...

Table 2: Worst-Case Scenarios For Cumulative Tolerances On The Compression Of A 40 MM Ground Hose And Of A Non-Watson-Marlow Bredel Hose (Not Ground)
Dimensions (mm)	Watson-Marlow Bredel Hose SP(X)40	Non-Ground 40 mm Hose
Min. inner diameter, ID min	N/A	39.5
Max. inner diameter, ID max	N/A	40.5
Min. outer diameter, ID min	N/A	66.3
Max. outer diameter, ID max	N/A	67.7
Min. wall thickness, d min	13.0	12.9
Max. wall thickness, d max	13.4	14.1
2 x min. wall thickness, 2d min.	26.0	25.8
2x max. wall thickness, 2d max.	26.8	28.2
2d max - 2d min	0.8	2.4
Height of Bandaging*	N/A	1.1
Surplus of rubber (over compression)	0.8	3.5
	Data are from available specifications of Watson-Marlow Bredel and leaflets of other hose pump manufacturers. N/A: not applicable * Measured from a non-Watson-Marlow Bredel hose.

Smallest possible tolerances required

Keeping the importance of the weight amount of compression in mind and then going back to how the produced hoses look, the question must arise if such a hose can function optimally. Looking at the outer surface of a newly produced hose the print of the wrapping is visible. Height differences of this print are measured to values exceeding 1.0 mm. Secondly there is a tolerance on the inner as well as on the outer diameter of a hose. Adding up the tolerances gives a significant difference between a hose produced in the low tolerances with a smaller new one is as simple as it is effective: grinding the outer surface of the hose to a uniform wall thickness (Figure 8).

Figure 8. The outer surface of Watson-Marlow Bredel's hoses are ground to a uniform wall thickness.

The effects of the grinding process on the dimensional tolerances and the over-compression of the hose are shown in Table 2. In order to prevent backflow for any produced hose that is used, the compression should be high enough to compress a hose with a high tolerance inner diameter and a low tolerance outer diameter. The worst-case scenario is when a hose is used with the opposite dimensional tolerances. It shows that over-compression of non-ground hoses can have values from 2.4 up to peaks of 3.5 mm due to the height of the wrapping (Figure 9). When the data of Table 2 are coupled to Figure 7, it is found that the compression force of the ground Watson-Marlow Bredel 40 mm hose is approximately 4 to 5 kN. However, the compression force on the non-ground hoses can vary between 4 and 16 kN. This is very harmful for hose life. That ground hoses have a surface with a considerable reduction in friction is an extra, but significant, benefit.

Figure 9. The print left by the bandaging is clearly visible on this finished non-Watson-Marlow Bredel hose.

Watson-Marlow Bredel emphasizes the importance of grinding the rubber hoses used for hose pumps. This way only one geometrical tolerance is introduced, which can be controlled very accurately. A regular outer surface is created as well as a uniform wall thickness ensuring a constant compression force without peak values. The constant load on the hose and the pump results in their longevity, a synergy that symbolizes the quality of the Watson-Marlow Bredel products.