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Industrial MicroHydraulics

RoB Number: Definition


The ROB Number, a system of rating the relative resistance to blockage of safety screens, has been developed to minimize the guesswork and expensive testing formerly involved in selecting a screen.

The ROB Number was conceived to aid system designers in choosing the optimum screen for their conditions relative to other screen options available. The ROB Number is not used to predict absolute screen life.

The ROB Number system is based on a few assumptions.

  • The same contamination level applies for all screen options.
  • Fluid contamination level is not affected by having a safety screen in the system.
  • When particles block holes, the manner in which they block different size holes is essentially the same.
  • Particle distribution follows a log - log2 distribution.

ROB Number Definition

A fluid contamination level per MIL-STD-1246 Class 200 was chosen as the basis of comparison with the Rob Number defined as;

where n = 10 (4.9029 – 0.926 log2d)
d = hole size (µ)
n = # of holes in a screen
    A master screen of ROB = 1 is therefore defined as having 1000 holes all
100 µ in size.

The following tables give the Rob Number for single holes of a given diameter. To determine the Rob Number for a screen, multiply the single hole value by the number of holes in the screen.

Screen Rob # = Single orifice Rob # x # of holes.

The screen safety factor is defined as the ratio of the screen Rob Number to the Rob Number of the orifice it is protecting. You should choose a screen to provide the highest practical safety factor.

ROB Number Example

EXAMPLE: Which safety screen would be the better choice to protect a 0.030 diameter orifice: A 0.008 hole size screen with 850 holes, or a 0.015 hole size screen with 450 holes?

SOLUTION: From the table here, we find that a single orifice of 0.030 diameter has a Rob Number of 9.71.

Again using the table for 0.008 and 0.015 holes and multiplying by the respective number of holes gives the following results:

(in) (µ)
Orifice 0.030 762 1 9.7 1
Screen 1 0.008 203 850 14.4 1.5
Screen 2 0.015 381 450 131 13.5

The results show screen 2 to be the better choice.