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GUIDELINES FOR MONITORING THE QUALITY OF HYDRAULIC OIL

FOR TESTING OF USED HYDRAULIC OIL FOLLOWING GUIDELINES MAY BE FOLLOWED :

  • 1 Properties to be Checked : Following parameters of hydraulic oil should be monitored:

a.➢ Viscosity at 40° C

b.➢ Water content in ppm

c.➢ Suspended Particle count.

  • 2 Frequency of Analysis : Hydraulic oil should be tested every 1000 engine hours.
  • 3 Maximum Admissible Tolerances :

a.➢ Viscosity : The admissible tolerance of viscosity having up to + 10% of the original value i.e., +10% of viscosity Grade (like 61.2 to 74.8 for HLP 68N). If the viscosity exceeds the tolerance, the oil should be changed immediately after cleaning the tank.

b.➢ Water Content : The maximum permissible ingress of water in the hydraulic oil is 0.10% (1 lit of water per 1000 lit of hydraulic oil). If emulsified water exceeds the tolerance the oil should be changed immediately after cleaning the tank.

c.➢ Suspended Particle : The permitted suspended particles is different for different hydraulic component as tabulated below.

  • 4 Method for analysis of oil purity

a. Laboratory test of sample.

b. inline measuring instruments.

Inline measuring instruments are portable and can be installed either permanently or temporarily with the pressure line for the measuring and monitoring.

  • 5 Standard followed for measuring suspended particle :

As mentioned above degree of purity is measured as per ISO44O6:1999 or as NAS value standard detail of which are given below:

ISO4406:1999 standard determines the quantity of particles in a cumulative way. i.e. >4µm, > 6 µm >14µm by using particle counter and assigns characteristic (code) number to them. Contamination level of assignments to particle numbers is to facilitate the assessments of purity of fluid. 100 ml sample taken is defined in terms of number of suspended particles as per table below.

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Sudden breakdown in an oil system is often caused by large particles (>14µ) in the oil while slower, progressive faults, like wear and tear are caused by the smaller particles (4-6 µ).

Particle reference sizes are set to 4 µm, 6µm and 14µm in ISO 4406:1999 for classification of hydraulic oil.

The oil purity classes according to NAS 1638 is a differential representation of the number of particles within the 5 different size ranges i.e., 5 to 15 µm, 15 to 25 µm, 25 to 50 µm, 50 to 100 µm & >100 µm.

➢ NAS Grade : This is an indication of cleanness of the oil in the system and its values are given below:

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